Merge #4769

4769: Federated search r=ManyTheFish a=dureuill

# Pull Request

## Related issue
Fixes #4747 

[Usage](https://meilisearch.notion.site/v1-10-federated-search-698dfe36ab6b4668b044f735fb40f0b2)

## What does this PR do?
- multi-search now allows a top-level federation object. When not `null`, the results of multi-search are modified to be a single list of results rather than a list of a list of results
- changed lifetimes around tokenizer et al. to be able to make hits one by one rather than using a vector
- adds `roaring` to Meilisearch itself. As the federated search happens at the Meilisearch level (reuses the search functions declared at the Meilisearch level + merge happens after the hits were created), `RoaringBitmap`s are needed to track the candidates: hits that were seen,  all candidates.
- Refactor `make_hits` to allow for an individual, optimized `make_hit` 
- Score details comparison no longer fail when sorting on different field names or target point (for geo)

Co-authored-by: Louis Dureuil <louis@meilisearch.com>

Cargo.lock

@@ -3394,6 +3394,7 @@ dependencies = [
  "rayon",
  "regex",
  "reqwest",
+ "roaring",
  "rustls 0.21.12",
  "rustls-pemfile 1.0.4",
  "segment",

meilisearch-types/src/deserr/mod.rs

@@ -192,6 +192,7 @@ merge_with_error_impl_take_error_message!(ParseOffsetDateTimeError);
 merge_with_error_impl_take_error_message!(ParseTaskKindError);
 merge_with_error_impl_take_error_message!(ParseTaskStatusError);
 merge_with_error_impl_take_error_message!(IndexUidFormatError);
+merge_with_error_impl_take_error_message!(InvalidMultiSearchWeight);
 merge_with_error_impl_take_error_message!(InvalidSearchSemanticRatio);
 merge_with_error_impl_take_error_message!(InvalidSearchRankingScoreThreshold);
 merge_with_error_impl_take_error_message!(InvalidSimilarRankingScoreThreshold);

meilisearch-types/src/error.rs

@@ -238,6 +238,11 @@ InvalidIndexLimit                     , InvalidRequest       , BAD_REQUEST ;
 InvalidIndexOffset                    , InvalidRequest       , BAD_REQUEST ;
 InvalidIndexPrimaryKey                , InvalidRequest       , BAD_REQUEST ;
 InvalidIndexUid                       , InvalidRequest       , BAD_REQUEST ;
+InvalidMultiSearchFederated           , InvalidRequest       , BAD_REQUEST ;
+InvalidMultiSearchFederationOptions   , InvalidRequest       , BAD_REQUEST ;
+InvalidMultiSearchQueryPagination     , InvalidRequest       , BAD_REQUEST ;
+InvalidMultiSearchQueryRankingRules   , InvalidRequest       , BAD_REQUEST ;
+InvalidMultiSearchWeight              , InvalidRequest       , BAD_REQUEST ;
 InvalidSearchAttributesToSearchOn     , InvalidRequest       , BAD_REQUEST ;
 InvalidSearchAttributesToCrop         , InvalidRequest       , BAD_REQUEST ;
 InvalidSearchAttributesToHighlight    , InvalidRequest       , BAD_REQUEST ;
@@ -512,6 +517,12 @@ impl fmt::Display for deserr_codes::InvalidSearchSemanticRatio {
     }
 }
 
+impl fmt::Display for deserr_codes::InvalidMultiSearchWeight {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "the value of `weight` is invalid, expected a positive float (>= 0.0).")
+    }
+}
+
 impl fmt::Display for deserr_codes::InvalidSimilarId {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(

meilisearch/Cargo.toml

@@ -102,6 +102,7 @@ tracing-subscriber = { version = "0.3.18", features = ["json"] }
 tracing-trace = { version = "0.1.0", path = "../tracing-trace" }
 tracing-actix-web = "0.7.11"
 build-info = { version = "1.7.0", path = "../build-info" }
+roaring = "0.10.2"
 
 [dev-dependencies]
 actix-rt = "2.10.0"

meilisearch/src/analytics/mock_analytics.rs

@@ -42,7 +42,7 @@ pub struct MultiSearchAggregator;
 
 #[allow(dead_code)]
 impl MultiSearchAggregator {
-    pub fn from_queries(_: &dyn Any, _: &dyn Any) -> Self {
+    pub fn from_federated_search(_: &dyn Any, _: &dyn Any) -> Self {
         Self
     }
 

meilisearch/src/analytics/segment_analytics.rs

@@ -34,8 +34,8 @@ use crate::routes::indexes::documents::{DocumentEditionByFunction, UpdateDocumen
 use crate::routes::indexes::facet_search::FacetSearchQuery;
 use crate::routes::{create_all_stats, Stats};
 use crate::search::{
-    FacetSearchResult, MatchingStrategy, SearchQuery, SearchQueryWithIndex, SearchResult,
+    FacetSearchResult, FederatedSearch, MatchingStrategy, SearchQuery, SearchQueryWithIndex,
-    SimilarQuery, SimilarResult, DEFAULT_CROP_LENGTH, DEFAULT_CROP_MARKER,
+    SearchResult, SimilarQuery, SimilarResult, DEFAULT_CROP_LENGTH, DEFAULT_CROP_MARKER,
     DEFAULT_HIGHLIGHT_POST_TAG, DEFAULT_HIGHLIGHT_PRE_TAG, DEFAULT_SEARCH_LIMIT,
     DEFAULT_SEMANTIC_RATIO,
 };
@@ -1095,22 +1095,33 @@ pub struct MultiSearchAggregator {
     show_ranking_score: bool,
     show_ranking_score_details: bool,
 
+    // federation
+    use_federation: bool,
+
     // context
     user_agents: HashSet<String>,
 }
 
 impl MultiSearchAggregator {
-    pub fn from_queries(query: &[SearchQueryWithIndex], request: &HttpRequest) -> Self {
+    pub fn from_federated_search(
+        federated_search: &FederatedSearch,
+        request: &HttpRequest,
+    ) -> Self {
         let timestamp = Some(OffsetDateTime::now_utc());
 
         let user_agents = extract_user_agents(request).into_iter().collect();
 
-        let distinct_indexes: HashSet<_> = query
+        let use_federation = federated_search.federation.is_some();
+
+        let distinct_indexes: HashSet<_> = federated_search
+            .queries
             .iter()
             .map(|query| {
+                let query = &query;
                 // make sure we get a compilation error if a field gets added to / removed from SearchQueryWithIndex
                 let SearchQueryWithIndex {
                     index_uid,
+                    federation_options: _,
                     q: _,
                     vector: _,
                     offset: _,
@@ -1142,8 +1153,10 @@ impl MultiSearchAggregator {
             })
             .collect();
 
-        let show_ranking_score = query.iter().any(|query| query.show_ranking_score);
+        let show_ranking_score =
-        let show_ranking_score_details = query.iter().any(|query| query.show_ranking_score_details);
+            federated_search.queries.iter().any(|query| query.show_ranking_score);
+        let show_ranking_score_details =
+            federated_search.queries.iter().any(|query| query.show_ranking_score_details);
 
         Self {
             timestamp,
@@ -1151,10 +1164,11 @@ impl MultiSearchAggregator {
             total_succeeded: 0,
             total_distinct_index_count: distinct_indexes.len(),
             total_single_index: if distinct_indexes.len() == 1 { 1 } else { 0 },
-            total_search_count: query.len(),
+            total_search_count: federated_search.queries.len(),
             show_ranking_score,
             show_ranking_score_details,
             user_agents,
+            use_federation,
         }
     }
 
@@ -1180,6 +1194,7 @@ impl MultiSearchAggregator {
         let show_ranking_score_details =
             this.show_ranking_score_details || other.show_ranking_score_details;
         let mut user_agents = this.user_agents;
+        let use_federation = this.use_federation || other.use_federation;
 
         for user_agent in other.user_agents.into_iter() {
             user_agents.insert(user_agent);
@@ -1196,6 +1211,7 @@ impl MultiSearchAggregator {
             user_agents,
             show_ranking_score,
             show_ranking_score_details,
+            use_federation,
             // do not add _ or ..Default::default() here
         };
 
@@ -1214,6 +1230,7 @@ impl MultiSearchAggregator {
             user_agents,
             show_ranking_score,
             show_ranking_score_details,
+            use_federation,
         } = self;
 
         if total_received == 0 {
@@ -1238,6 +1255,9 @@ impl MultiSearchAggregator {
                 "scoring": {
                     "show_ranking_score": show_ranking_score,
                     "show_ranking_score_details": show_ranking_score_details,
+                },
+                "federation": {
+                    "use_federation": use_federation,
                 }
             });
 

meilisearch/src/error.rs

@@ -25,6 +25,10 @@ pub enum MeilisearchHttpError {
     DocumentNotFound(String),
     #[error("Sending an empty filter is forbidden.")]
     EmptyFilter,
+    #[error("Using `federationOptions` is not allowed in a non-federated search.\n Hint: remove `federationOptions` from query #{0} or add `federation: {{}}` to the request.")]
+    FederationOptionsInNonFederatedRequest(usize),
+    #[error("Inside `.queries[{0}]`: Using pagination options is not allowed in federated queries.\n Hint: remove `{1}` from query #{0} or remove `federation: {{}}` from the request")]
+    PaginationInFederatedQuery(usize, &'static str),
     #[error("Invalid syntax for the filter parameter: `expected {}, found: {1}`.", .0.join(", "))]
     InvalidExpression(&'static [&'static str], Value),
     #[error("A {0} payload is missing.")]
@@ -86,6 +90,12 @@ impl ErrorCode for MeilisearchHttpError {
             MeilisearchHttpError::DocumentFormat(e) => e.error_code(),
             MeilisearchHttpError::Join(_) => Code::Internal,
             MeilisearchHttpError::MissingSearchHybrid => Code::MissingSearchHybrid,
+            MeilisearchHttpError::FederationOptionsInNonFederatedRequest(_) => {
+                Code::InvalidMultiSearchFederationOptions
+            }
+            MeilisearchHttpError::PaginationInFederatedQuery(_, _) => {
+                Code::InvalidMultiSearchQueryPagination
+            }
         }
     }
 }

meilisearch/src/routes/multi_search.rs

@@ -10,12 +10,14 @@ use serde::Serialize;
 use tracing::debug;
 
 use crate::analytics::{Analytics, MultiSearchAggregator};
+use crate::error::MeilisearchHttpError;
 use crate::extractors::authentication::policies::ActionPolicy;
 use crate::extractors::authentication::{AuthenticationError, GuardedData};
 use crate::extractors::sequential_extractor::SeqHandler;
 use crate::routes::indexes::search::search_kind;
 use crate::search::{
-    add_search_rules, perform_search, RetrieveVectors, SearchQueryWithIndex, SearchResultWithIndex,
+    add_search_rules, perform_federated_search, perform_search, FederatedSearch, RetrieveVectors,
+    SearchQueryWithIndex, SearchResultWithIndex,
 };
 use crate::search_queue::SearchQueue;
 
@@ -28,46 +30,86 @@ struct SearchResults {
     results: Vec<SearchResultWithIndex>,
 }
 
-#[derive(Debug, deserr::Deserr)]
-#[deserr(error = DeserrJsonError, rename_all = camelCase, deny_unknown_fields)]
-pub struct SearchQueries {
-    queries: Vec<SearchQueryWithIndex>,
-}
-
 pub async fn multi_search_with_post(
     index_scheduler: GuardedData<ActionPolicy<{ actions::SEARCH }>, Data<IndexScheduler>>,
     search_queue: Data<SearchQueue>,
-    params: AwebJson<SearchQueries, DeserrJsonError>,
+    params: AwebJson<FederatedSearch, DeserrJsonError>,
     req: HttpRequest,
     analytics: web::Data<dyn Analytics>,
 ) -> Result<HttpResponse, ResponseError> {
-    let queries = params.into_inner().queries;
-
-    let mut multi_aggregate = MultiSearchAggregator::from_queries(&queries, &req);
-    let features = index_scheduler.features();
-
     // Since we don't want to process half of the search requests and then get a permit refused
     // we're going to get one permit for the whole duration of the multi-search request.
     let _permit = search_queue.try_get_search_permit().await?;
 
+    let federated_search = params.into_inner();
+
+    let mut multi_aggregate = MultiSearchAggregator::from_federated_search(&federated_search, &req);
+
+    let FederatedSearch { mut queries, federation } = federated_search;
+
+    let features = index_scheduler.features();
+
+    // regardless of federation, check authorization and apply search rules
+    let auth = 'check_authorization: {
+        for (query_index, federated_query) in queries.iter_mut().enumerate() {
+            let index_uid = federated_query.index_uid.as_str();
+            // Check index from API key
+            if !index_scheduler.filters().is_index_authorized(index_uid) {
+                break 'check_authorization Err(AuthenticationError::InvalidToken)
+                    .with_index(query_index);
+            }
+            // Apply search rules from tenant token
+            if let Some(search_rules) = index_scheduler.filters().get_index_search_rules(index_uid)
+            {
+                add_search_rules(&mut federated_query.filter, search_rules);
+            }
+        }
+        Ok(())
+    };
+
+    auth.map_err(|(mut err, query_index)| {
+        // Add the query index that failed as context for the error message.
+        // We're doing it only here and not directly in the `WithIndex` trait so that the `with_index` function returns a different type
+        // of result and we can benefit from static typing.
+        err.message = format!("Inside `.queries[{query_index}]`: {}", err.message);
+        err
+    })?;
+
+    let response = match federation {
+        Some(federation) => {
+            let search_result = tokio::task::spawn_blocking(move || {
+                perform_federated_search(&index_scheduler, queries, federation, features)
+            })
+            .await;
+
+            if let Ok(Ok(_)) = search_result {
+                multi_aggregate.succeed();
+            }
+
+            analytics.post_multi_search(multi_aggregate);
+            HttpResponse::Ok().json(search_result??)
+        }
+        None => {
             // Explicitly expect a `(ResponseError, usize)` for the error type rather than `ResponseError` only,
             // so that `?` doesn't work if it doesn't use `with_index`, ensuring that it is not forgotten in case of code
             // changes.
             let search_results: Result<_, (ResponseError, usize)> = async {
                 let mut search_results = Vec::with_capacity(queries.len());
-        for (query_index, (index_uid, mut query)) in
+                for (query_index, (index_uid, query, federation_options)) in queries
-            queries.into_iter().map(SearchQueryWithIndex::into_index_query).enumerate()
+                    .into_iter()
+                    .map(SearchQueryWithIndex::into_index_query_federation)
+                    .enumerate()
                 {
                     debug!(on_index = query_index, parameters = ?query, "Multi-search");
 
-            // Check index from API key
+                    if federation_options.is_some() {
-            if !index_scheduler.filters().is_index_authorized(&index_uid) {
+                        return Err((
-                return Err(AuthenticationError::InvalidToken).with_index(query_index);
+                            MeilisearchHttpError::FederationOptionsInNonFederatedRequest(
-            }
+                                query_index,
-            // Apply search rules from tenant token
+                            )
-            if let Some(search_rules) = index_scheduler.filters().get_index_search_rules(&index_uid)
+                            .into(),
-            {
+                            query_index,
-                add_search_rules(&mut query.filter, search_rules);
+                        ));
                     }
 
                     let index = index_scheduler
@@ -81,10 +123,11 @@ pub async fn multi_search_with_post(
                         })
                         .with_index(query_index)?;
 
-            let search_kind = search_kind(&query, index_scheduler.get_ref(), &index, features)
+                    let search_kind =
+                        search_kind(&query, index_scheduler.get_ref(), &index, features)
+                            .with_index(query_index)?;
+                    let retrieve_vector = RetrieveVectors::new(query.retrieve_vectors, features)
                         .with_index(query_index)?;
-            let retrieve_vector =
-                RetrieveVectors::new(query.retrieve_vectors, features).with_index(query_index)?;
 
                     let search_result = tokio::task::spawn_blocking(move || {
                         perform_search(&index, query, search_kind, retrieve_vector)
@@ -116,7 +159,11 @@ pub async fn multi_search_with_post(
 
             debug!(returns = ?search_results, "Multi-search");
 
-    Ok(HttpResponse::Ok().json(SearchResults { results: search_results }))
+            HttpResponse::Ok().json(SearchResults { results: search_results })
+        }
+    };
+
+    Ok(response)
 }
 
 /// Local `Result` extension trait to avoid `map_err` boilerplate.

meilisearch/src/search/federated.rs

@@ -0,0 +1,629 @@
+use std::cmp::Ordering;
+use std::collections::BTreeMap;
+use std::fmt;
+use std::iter::Zip;
+use std::rc::Rc;
+use std::str::FromStr as _;
+use std::time::Duration;
+use std::vec::{IntoIter, Vec};
+
+use actix_http::StatusCode;
+use index_scheduler::{IndexScheduler, RoFeatures};
+use meilisearch_types::deserr::DeserrJsonError;
+use meilisearch_types::error::deserr_codes::{
+    InvalidMultiSearchWeight, InvalidSearchLimit, InvalidSearchOffset,
+};
+use meilisearch_types::error::ResponseError;
+use meilisearch_types::milli::score_details::{ScoreDetails, ScoreValue};
+use meilisearch_types::milli::{self, DocumentId, TimeBudget};
+use roaring::RoaringBitmap;
+use serde::Serialize;
+
+use super::ranking_rules::{self, RankingRules};
+use super::{
+    prepare_search, AttributesFormat, HitMaker, HitsInfo, RetrieveVectors, SearchHit, SearchKind,
+    SearchQuery, SearchQueryWithIndex,
+};
+use crate::error::MeilisearchHttpError;
+use crate::routes::indexes::search::search_kind;
+
+pub const DEFAULT_FEDERATED_WEIGHT: f64 = 1.0;
+
+#[derive(Debug, Default, Clone, Copy, PartialEq, deserr::Deserr)]
+#[deserr(error = DeserrJsonError, rename_all = camelCase, deny_unknown_fields)]
+pub struct FederationOptions {
+    #[deserr(default, error = DeserrJsonError<InvalidMultiSearchWeight>)]
+    pub weight: Weight,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, deserr::Deserr)]
+#[deserr(try_from(f64) = TryFrom::try_from -> InvalidMultiSearchWeight)]
+pub struct Weight(f64);
+
+impl Default for Weight {
+    fn default() -> Self {
+        Weight(DEFAULT_FEDERATED_WEIGHT)
+    }
+}
+
+impl std::convert::TryFrom<f64> for Weight {
+    type Error = InvalidMultiSearchWeight;
+
+    fn try_from(f: f64) -> Result<Self, Self::Error> {
+        if f < 0.0 {
+            Err(InvalidMultiSearchWeight)
+        } else {
+            Ok(Weight(f))
+        }
+    }
+}
+
+impl std::ops::Deref for Weight {
+    type Target = f64;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+#[derive(Debug, deserr::Deserr)]
+#[deserr(error = DeserrJsonError, rename_all = camelCase, deny_unknown_fields)]
+pub struct Federation {
+    #[deserr(default = super::DEFAULT_SEARCH_LIMIT(), error = DeserrJsonError<InvalidSearchLimit>)]
+    pub limit: usize,
+    #[deserr(default = super::DEFAULT_SEARCH_OFFSET(), error = DeserrJsonError<InvalidSearchOffset>)]
+    pub offset: usize,
+}
+
+#[derive(Debug, deserr::Deserr)]
+#[deserr(error = DeserrJsonError, rename_all = camelCase, deny_unknown_fields)]
+pub struct FederatedSearch {
+    pub queries: Vec<SearchQueryWithIndex>,
+    #[deserr(default)]
+    pub federation: Option<Federation>,
+}
+#[derive(Serialize, Clone, PartialEq)]
+#[serde(rename_all = "camelCase")]
+pub struct FederatedSearchResult {
+    pub hits: Vec<SearchHit>,
+    pub processing_time_ms: u128,
+    #[serde(flatten)]
+    pub hits_info: HitsInfo,
+
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub semantic_hit_count: Option<u32>,
+
+    // These fields are only used for analytics purposes
+    #[serde(skip)]
+    pub degraded: bool,
+    #[serde(skip)]
+    pub used_negative_operator: bool,
+}
+
+impl fmt::Debug for FederatedSearchResult {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let FederatedSearchResult {
+            hits,
+            processing_time_ms,
+            hits_info,
+            semantic_hit_count,
+            degraded,
+            used_negative_operator,
+        } = self;
+
+        let mut debug = f.debug_struct("SearchResult");
+        // The most important thing when looking at a search result is the time it took to process
+        debug.field("processing_time_ms", &processing_time_ms);
+        debug.field("hits", &format!("[{} hits returned]", hits.len()));
+        debug.field("hits_info", &hits_info);
+        if *used_negative_operator {
+            debug.field("used_negative_operator", used_negative_operator);
+        }
+        if *degraded {
+            debug.field("degraded", degraded);
+        }
+        if let Some(semantic_hit_count) = semantic_hit_count {
+            debug.field("semantic_hit_count", &semantic_hit_count);
+        }
+
+        debug.finish()
+    }
+}
+
+struct WeightedScore<'a> {
+    details: &'a [ScoreDetails],
+    weight: f64,
+}
+
+impl<'a> WeightedScore<'a> {
+    pub fn new(details: &'a [ScoreDetails], weight: f64) -> Self {
+        Self { details, weight }
+    }
+
+    pub fn weighted_global_score(&self) -> f64 {
+        ScoreDetails::global_score(self.details.iter()) * self.weight
+    }
+
+    pub fn compare_weighted_global_scores(&self, other: &Self) -> Ordering {
+        self.weighted_global_score()
+            .partial_cmp(&other.weighted_global_score())
+            // both are numbers, possibly infinite
+            .unwrap()
+    }
+
+    pub fn compare(&self, other: &Self) -> Ordering {
+        let mut left_it = ScoreDetails::score_values(self.details.iter());
+        let mut right_it = ScoreDetails::score_values(other.details.iter());
+
+        loop {
+            let left = left_it.next();
+            let right = right_it.next();
+
+            match (left, right) {
+                (None, None) => return Ordering::Equal,
+                (None, Some(_)) => return Ordering::Less,
+                (Some(_), None) => return Ordering::Greater,
+                (Some(ScoreValue::Score(left)), Some(ScoreValue::Score(right))) => {
+                    let left = left * self.weight;
+                    let right = right * other.weight;
+                    if (left - right).abs() <= f64::EPSILON {
+                        continue;
+                    }
+                    return left.partial_cmp(&right).unwrap();
+                }
+                (Some(ScoreValue::Sort(left)), Some(ScoreValue::Sort(right))) => {
+                    match left.partial_cmp(right) {
+                        Some(Ordering::Equal) => continue,
+                        Some(order) => return order,
+                        None => return self.compare_weighted_global_scores(other),
+                    }
+                }
+                (Some(ScoreValue::GeoSort(left)), Some(ScoreValue::GeoSort(right))) => {
+                    match left.partial_cmp(right) {
+                        Some(Ordering::Equal) => continue,
+                        Some(order) => return order,
+                        None => {
+                            return self.compare_weighted_global_scores(other);
+                        }
+                    }
+                }
+                // not comparable details, use global
+                (Some(ScoreValue::Score(_)), Some(_))
+                | (Some(_), Some(ScoreValue::Score(_)))
+                | (Some(ScoreValue::GeoSort(_)), Some(ScoreValue::Sort(_)))
+                | (Some(ScoreValue::Sort(_)), Some(ScoreValue::GeoSort(_))) => {
+                    let left_count = left_it.count();
+                    let right_count = right_it.count();
+                    // compare how many remaining groups of rules each side has.
+                    // the group with the most remaining groups wins.
+                    return left_count
+                        .cmp(&right_count)
+                        // breaks ties with the global ranking score
+                        .then_with(|| self.compare_weighted_global_scores(other));
+                }
+            }
+        }
+    }
+}
+
+struct QueryByIndex {
+    query: SearchQuery,
+    federation_options: FederationOptions,
+    query_index: usize,
+}
+
+struct SearchResultByQuery<'a> {
+    documents_ids: Vec<DocumentId>,
+    document_scores: Vec<Vec<ScoreDetails>>,
+    federation_options: FederationOptions,
+    hit_maker: HitMaker<'a>,
+    query_index: usize,
+}
+
+struct SearchResultByQueryIter<'a> {
+    it: Zip<IntoIter<DocumentId>, IntoIter<Vec<ScoreDetails>>>,
+    federation_options: FederationOptions,
+    hit_maker: Rc<HitMaker<'a>>,
+    query_index: usize,
+}
+
+impl<'a> SearchResultByQueryIter<'a> {
+    fn new(
+        SearchResultByQuery {
+            documents_ids,
+            document_scores,
+            federation_options,
+            hit_maker,
+            query_index,
+        }: SearchResultByQuery<'a>,
+    ) -> Self {
+        let it = documents_ids.into_iter().zip(document_scores);
+        Self { it, federation_options, hit_maker: Rc::new(hit_maker), query_index }
+    }
+}
+
+struct SearchResultByQueryIterItem<'a> {
+    docid: DocumentId,
+    score: Vec<ScoreDetails>,
+    federation_options: FederationOptions,
+    hit_maker: Rc<HitMaker<'a>>,
+    query_index: usize,
+}
+
+fn merge_index_local_results(
+    results_by_query: Vec<SearchResultByQuery<'_>>,
+) -> impl Iterator<Item = SearchResultByQueryIterItem> + '_ {
+    itertools::kmerge_by(
+        results_by_query.into_iter().map(SearchResultByQueryIter::new),
+        |left: &SearchResultByQueryIterItem, right: &SearchResultByQueryIterItem| {
+            let left_score = WeightedScore::new(&left.score, *left.federation_options.weight);
+            let right_score = WeightedScore::new(&right.score, *right.federation_options.weight);
+
+            match left_score.compare(&right_score) {
+                // the biggest score goes first
+                Ordering::Greater => true,
+                // break ties using query index
+                Ordering::Equal => left.query_index < right.query_index,
+                Ordering::Less => false,
+            }
+        },
+    )
+}
+
+fn merge_index_global_results(
+    results_by_index: Vec<SearchResultByIndex>,
+) -> impl Iterator<Item = SearchHitByIndex> {
+    itertools::kmerge_by(
+        results_by_index.into_iter().map(|result_by_index| result_by_index.hits.into_iter()),
+        |left: &SearchHitByIndex, right: &SearchHitByIndex| {
+            let left_score = WeightedScore::new(&left.score, *left.federation_options.weight);
+            let right_score = WeightedScore::new(&right.score, *right.federation_options.weight);
+
+            match left_score.compare(&right_score) {
+                // the biggest score goes first
+                Ordering::Greater => true,
+                // break ties using query index
+                Ordering::Equal => left.query_index < right.query_index,
+                Ordering::Less => false,
+            }
+        },
+    )
+}
+
+impl<'a> Iterator for SearchResultByQueryIter<'a> {
+    type Item = SearchResultByQueryIterItem<'a>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let (docid, score) = self.it.next()?;
+        Some(SearchResultByQueryIterItem {
+            docid,
+            score,
+            federation_options: self.federation_options,
+            hit_maker: Rc::clone(&self.hit_maker),
+            query_index: self.query_index,
+        })
+    }
+}
+
+struct SearchHitByIndex {
+    hit: SearchHit,
+    score: Vec<ScoreDetails>,
+    federation_options: FederationOptions,
+    query_index: usize,
+}
+
+struct SearchResultByIndex {
+    hits: Vec<SearchHitByIndex>,
+    candidates: RoaringBitmap,
+    degraded: bool,
+    used_negative_operator: bool,
+}
+
+pub fn perform_federated_search(
+    index_scheduler: &IndexScheduler,
+    queries: Vec<SearchQueryWithIndex>,
+    federation: Federation,
+    features: RoFeatures,
+) -> Result<FederatedSearchResult, ResponseError> {
+    let before_search = std::time::Instant::now();
+
+    // this implementation partition the queries by index to guarantee an important property:
+    // - all the queries to a particular index use the same read transaction.
+    // This is an important property, otherwise we cannot guarantee the self-consistency of the results.
+
+    // 1. partition queries by index
+    let mut queries_by_index: BTreeMap<String, Vec<QueryByIndex>> = Default::default();
+    for (query_index, federated_query) in queries.into_iter().enumerate() {
+        if let Some(pagination_field) = federated_query.has_pagination() {
+            return Err(MeilisearchHttpError::PaginationInFederatedQuery(
+                query_index,
+                pagination_field,
+            )
+            .into());
+        }
+
+        let (index_uid, query, federation_options) = federated_query.into_index_query_federation();
+
+        queries_by_index.entry(index_uid.into_inner()).or_default().push(QueryByIndex {
+            query,
+            federation_options: federation_options.unwrap_or_default(),
+            query_index,
+        })
+    }
+
+    // 2. perform queries, merge and make hits index by index
+    let required_hit_count = federation.limit + federation.offset;
+    // In step (2), semantic_hit_count will be set to Some(0) if any search kind uses semantic
+    // Then in step (3), we'll update its value if there is any semantic search
+    let mut semantic_hit_count = None;
+    let mut results_by_index = Vec::with_capacity(queries_by_index.len());
+    let mut previous_query_data: Option<(RankingRules, usize, String)> = None;
+
+    for (index_uid, queries) in queries_by_index {
+        let index = match index_scheduler.index(&index_uid) {
+            Ok(index) => index,
+            Err(err) => {
+                let mut err = ResponseError::from(err);
+                // Patch the HTTP status code to 400 as it defaults to 404 for `index_not_found`, but
+                // here the resource not found is not part of the URL.
+                err.code = StatusCode::BAD_REQUEST;
+                if let Some(query) = queries.first() {
+                    err.message =
+                        format!("Inside `.queries[{}]`: {}", query.query_index, err.message);
+                }
+                return Err(err);
+            }
+        };
+
+        // Important: this is the only transaction we'll use for this index during this federated search
+        let rtxn = index.read_txn()?;
+
+        let criteria = index.criteria(&rtxn)?;
+
+        // stuff we need for the hitmaker
+        let script_lang_map = index.script_language(&rtxn)?;
+
+        let dictionary = index.dictionary(&rtxn)?;
+        let dictionary: Option<Vec<_>> =
+            dictionary.as_ref().map(|x| x.iter().map(String::as_str).collect());
+        let separators = index.allowed_separators(&rtxn)?;
+        let separators: Option<Vec<_>> =
+            separators.as_ref().map(|x| x.iter().map(String::as_str).collect());
+
+        // each query gets its individual cutoff
+        let cutoff = index.search_cutoff(&rtxn)?;
+
+        let mut degraded = false;
+        let mut used_negative_operator = false;
+        let mut candidates = RoaringBitmap::new();
+
+        // 2.1. Compute all candidates for each query in the index
+        let mut results_by_query = Vec::with_capacity(queries.len());
+
+        for QueryByIndex { query, federation_options, query_index } in queries {
+            // use an immediately invoked lambda to capture the result without returning from the function
+
+            let res: Result<(), ResponseError> = (|| {
+                let search_kind = search_kind(&query, index_scheduler, &index, features)?;
+
+                let canonicalization_kind = match (&search_kind, &query.q) {
+                    (SearchKind::SemanticOnly { .. }, _) => {
+                        ranking_rules::CanonicalizationKind::Vector
+                    }
+                    (_, Some(q)) if !q.is_empty() => ranking_rules::CanonicalizationKind::Keyword,
+                    _ => ranking_rules::CanonicalizationKind::Placeholder,
+                };
+
+                let sort = if let Some(sort) = &query.sort {
+                    let sorts: Vec<_> =
+                        match sort.iter().map(|s| milli::AscDesc::from_str(s)).collect() {
+                            Ok(sorts) => sorts,
+                            Err(asc_desc_error) => {
+                                return Err(milli::Error::from(milli::SortError::from(
+                                    asc_desc_error,
+                                ))
+                                .into())
+                            }
+                        };
+                    Some(sorts)
+                } else {
+                    None
+                };
+
+                let ranking_rules = ranking_rules::RankingRules::new(
+                    criteria.clone(),
+                    sort,
+                    query.matching_strategy.into(),
+                    canonicalization_kind,
+                );
+
+                if let Some((previous_ranking_rules, previous_query_index, previous_index_uid)) =
+                    previous_query_data.take()
+                {
+                    if let Err(error) = ranking_rules.is_compatible_with(&previous_ranking_rules) {
+                        return Err(error.to_response_error(
+                            &ranking_rules,
+                            &previous_ranking_rules,
+                            query_index,
+                            previous_query_index,
+                            &index_uid,
+                            &previous_index_uid,
+                        ));
+                    }
+                    previous_query_data = if previous_ranking_rules.constraint_count()
+                        > ranking_rules.constraint_count()
+                    {
+                        Some((previous_ranking_rules, previous_query_index, previous_index_uid))
+                    } else {
+                        Some((ranking_rules, query_index, index_uid.clone()))
+                    };
+                } else {
+                    previous_query_data = Some((ranking_rules, query_index, index_uid.clone()));
+                }
+
+                match search_kind {
+                    SearchKind::KeywordOnly => {}
+                    _ => semantic_hit_count = Some(0),
+                }
+
+                let retrieve_vectors = RetrieveVectors::new(query.retrieve_vectors, features)?;
+
+                let time_budget = match cutoff {
+                    Some(cutoff) => TimeBudget::new(Duration::from_millis(cutoff)),
+                    None => TimeBudget::default(),
+                };
+
+                let (mut search, _is_finite_pagination, _max_total_hits, _offset) =
+                    prepare_search(&index, &rtxn, &query, &search_kind, time_budget)?;
+
+                search.scoring_strategy(milli::score_details::ScoringStrategy::Detailed);
+                search.offset(0);
+                search.limit(required_hit_count);
+
+                let (result, _semantic_hit_count) = super::search_from_kind(search_kind, search)?;
+                let format = AttributesFormat {
+                    attributes_to_retrieve: query.attributes_to_retrieve,
+                    retrieve_vectors,
+                    attributes_to_highlight: query.attributes_to_highlight,
+                    attributes_to_crop: query.attributes_to_crop,
+                    crop_length: query.crop_length,
+                    crop_marker: query.crop_marker,
+                    highlight_pre_tag: query.highlight_pre_tag,
+                    highlight_post_tag: query.highlight_post_tag,
+                    show_matches_position: query.show_matches_position,
+                    sort: query.sort,
+                    show_ranking_score: query.show_ranking_score,
+                    show_ranking_score_details: query.show_ranking_score_details,
+                };
+
+                let milli::SearchResult {
+                    matching_words,
+                    candidates: query_candidates,
+                    documents_ids,
+                    document_scores,
+                    degraded: query_degraded,
+                    used_negative_operator: query_used_negative_operator,
+                } = result;
+
+                candidates |= query_candidates;
+                degraded |= query_degraded;
+                used_negative_operator |= query_used_negative_operator;
+
+                let tokenizer = HitMaker::tokenizer(
+                    &script_lang_map,
+                    dictionary.as_deref(),
+                    separators.as_deref(),
+                );
+
+                let formatter_builder = HitMaker::formatter_builder(matching_words, tokenizer);
+
+                let hit_maker = HitMaker::new(&index, &rtxn, format, formatter_builder)?;
+
+                results_by_query.push(SearchResultByQuery {
+                    federation_options,
+                    hit_maker,
+                    query_index,
+                    documents_ids,
+                    document_scores,
+                });
+                Ok(())
+            })();
+
+            if let Err(mut error) = res {
+                error.message = format!("Inside `.queries[{query_index}]`: {}", error.message);
+                return Err(error);
+            }
+        }
+        // 2.2. merge inside index
+        let mut documents_seen = RoaringBitmap::new();
+        let merged_result: Result<Vec<_>, ResponseError> =
+            merge_index_local_results(results_by_query)
+                // skip documents we've already seen & mark that we saw the current document
+                .filter(|SearchResultByQueryIterItem { docid, .. }| documents_seen.insert(*docid))
+                .take(required_hit_count)
+                // 2.3 make hits
+                .map(
+                    |SearchResultByQueryIterItem {
+                         docid,
+                         score,
+                         federation_options,
+                         hit_maker,
+                         query_index,
+                     }| {
+                        let mut hit = hit_maker.make_hit(docid, &score)?;
+                        let weighted_score =
+                            ScoreDetails::global_score(score.iter()) * (*federation_options.weight);
+
+                        let _federation = serde_json::json!(
+                            {
+                                "indexUid": index_uid,
+                                "queriesPosition": query_index,
+                                "weightedRankingScore": weighted_score,
+                            }
+                        );
+                        hit.document.insert("_federation".to_string(), _federation);
+                        Ok(SearchHitByIndex { hit, score, federation_options, query_index })
+                    },
+                )
+                .collect();
+
+        let merged_result = merged_result?;
+        results_by_index.push(SearchResultByIndex {
+            hits: merged_result,
+            candidates,
+            degraded,
+            used_negative_operator,
+        });
+    }
+
+    // 3. merge hits and metadata across indexes
+    // 3.1 merge metadata
+    let (estimated_total_hits, degraded, used_negative_operator) = {
+        let mut estimated_total_hits = 0;
+        let mut degraded = false;
+        let mut used_negative_operator = false;
+
+        for SearchResultByIndex {
+            hits: _,
+            candidates,
+            degraded: degraded_by_index,
+            used_negative_operator: used_negative_operator_by_index,
+        } in &results_by_index
+        {
+            estimated_total_hits += candidates.len() as usize;
+            degraded |= *degraded_by_index;
+            used_negative_operator |= *used_negative_operator_by_index;
+        }
+
+        (estimated_total_hits, degraded, used_negative_operator)
+    };
+
+    // 3.2 merge hits
+    let merged_hits: Vec<_> = merge_index_global_results(results_by_index)
+        .skip(federation.offset)
+        .take(federation.limit)
+        .inspect(|hit| {
+            if let Some(semantic_hit_count) = &mut semantic_hit_count {
+                if hit.score.iter().any(|score| matches!(&score, ScoreDetails::Vector(_))) {
+                    *semantic_hit_count += 1;
+                }
+            }
+        })
+        .map(|hit| hit.hit)
+        .collect();
+
+    let search_result = FederatedSearchResult {
+        hits: merged_hits,
+        processing_time_ms: before_search.elapsed().as_millis(),
+        hits_info: HitsInfo::OffsetLimit {
+            limit: federation.limit,
+            offset: federation.offset,
+            estimated_total_hits,
+        },
+        semantic_hit_count,
+        degraded,
+        used_negative_operator,
+    };
+
+    Ok(search_result)
+}

meilisearch/src/search/mod.rs

@@ -1,6 +1,6 @@
 use core::fmt;
 use std::cmp::min;
-use std::collections::{BTreeMap, BTreeSet, HashSet};
+use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 use std::str::FromStr;
 use std::sync::Arc;
 use std::time::{Duration, Instant};
@@ -31,6 +31,11 @@ use serde_json::{json, Value};
 
 use crate::error::MeilisearchHttpError;
 
+mod federated;
+pub use federated::{perform_federated_search, FederatedSearch, Federation, FederationOptions};
+
+mod ranking_rules;
+
 type MatchesPosition = BTreeMap<String, Vec<MatchBounds>>;
 
 pub const DEFAULT_SEARCH_OFFSET: fn() -> usize = || 0;
@@ -257,11 +262,13 @@ pub struct HybridQuery {
     pub embedder: Option<String>,
 }
 
+#[derive(Clone)]
 pub enum SearchKind {
     KeywordOnly,
     SemanticOnly { embedder_name: String, embedder: Arc<Embedder> },
     Hybrid { embedder_name: String, embedder: Arc<Embedder>, semantic_ratio: f32 },
 }
+
 impl SearchKind {
     pub(crate) fn semantic(
         index_scheduler: &index_scheduler::IndexScheduler,
@@ -358,7 +365,7 @@ impl SearchQuery {
     }
 }
 
-/// A `SearchQuery` + an index UID.
+/// A `SearchQuery` + an index UID and optional FederationOptions.
 // This struct contains the fields of `SearchQuery` inline.
 // This is because neither deserr nor serde support `flatten` when using `deny_unknown_fields.
 // The `From<SearchQueryWithIndex>` implementation ensures both structs remain up to date.
@@ -373,10 +380,10 @@ pub struct SearchQueryWithIndex {
     pub vector: Option<Vec<f32>>,
     #[deserr(default, error = DeserrJsonError<InvalidHybridQuery>)]
     pub hybrid: Option<HybridQuery>,
-    #[deserr(default = DEFAULT_SEARCH_OFFSET(), error = DeserrJsonError<InvalidSearchOffset>)]
+    #[deserr(default, error = DeserrJsonError<InvalidSearchOffset>)]
-    pub offset: usize,
+    pub offset: Option<usize>,
-    #[deserr(default = DEFAULT_SEARCH_LIMIT(), error = DeserrJsonError<InvalidSearchLimit>)]
+    #[deserr(default, error = DeserrJsonError<InvalidSearchLimit>)]
-    pub limit: usize,
+    pub limit: Option<usize>,
     #[deserr(default, error = DeserrJsonError<InvalidSearchPage>)]
     pub page: Option<usize>,
     #[deserr(default, error = DeserrJsonError<InvalidSearchHitsPerPage>)]
@@ -417,12 +424,33 @@ pub struct SearchQueryWithIndex {
     pub attributes_to_search_on: Option<Vec<String>>,
     #[deserr(default, error = DeserrJsonError<InvalidSearchRankingScoreThreshold>, default)]
     pub ranking_score_threshold: Option<RankingScoreThreshold>,
+
+    #[deserr(default)]
+    pub federation_options: Option<FederationOptions>,
 }
 
 impl SearchQueryWithIndex {
-    pub fn into_index_query(self) -> (IndexUid, SearchQuery) {
+    pub fn has_federation_options(&self) -> bool {
+        self.federation_options.is_some()
+    }
+    pub fn has_pagination(&self) -> Option<&'static str> {
+        if self.offset.is_some() {
+            Some("offset")
+        } else if self.limit.is_some() {
+            Some("limit")
+        } else if self.page.is_some() {
+            Some("page")
+        } else if self.hits_per_page.is_some() {
+            Some("hitsPerPage")
+        } else {
+            None
+        }
+    }
+
+    pub fn into_index_query_federation(self) -> (IndexUid, SearchQuery, Option<FederationOptions>) {
         let SearchQueryWithIndex {
             index_uid,
+            federation_options,
             q,
             vector,
             offset,
@@ -454,8 +482,8 @@ impl SearchQueryWithIndex {
             SearchQuery {
                 q,
                 vector,
-                offset,
+                offset: offset.unwrap_or(DEFAULT_SEARCH_OFFSET()),
-                limit,
+                limit: limit.unwrap_or(DEFAULT_SEARCH_LIMIT()),
                 page,
                 hits_per_page,
                 attributes_to_retrieve,
@@ -480,6 +508,7 @@ impl SearchQueryWithIndex {
                 // do not use ..Default::default() here,
                 // rather add any missing field from `SearchQuery` to `SearchQueryWithIndex`
             },
+            federation_options,
         )
     }
 }
@@ -864,15 +893,7 @@ pub fn perform_search(
             used_negative_operator,
         },
         semantic_hit_count,
-    ) = match &search_kind {
+    ) = search_from_kind(search_kind, search)?;
-        SearchKind::KeywordOnly => (search.execute()?, None),
-        SearchKind::SemanticOnly { .. } => {
-            let results = search.execute()?;
-            let semantic_hit_count = results.document_scores.len() as u32;
-            (results, Some(semantic_hit_count))
-        }
-        SearchKind::Hybrid { semantic_ratio, .. } => search.execute_hybrid(*semantic_ratio)?,
-    };
 
     let SearchQuery {
         q,
@@ -919,8 +940,13 @@ pub fn perform_search(
         show_ranking_score_details,
     };
 
-    let documents =
+    let documents = make_hits(
-        make_hits(index, &rtxn, format, matching_words, documents_ids, document_scores)?;
+        index,
+        &rtxn,
+        format,
+        matching_words,
+        documents_ids.iter().copied().zip(document_scores.iter()),
+    )?;
 
     let number_of_hits = min(candidates.len() as usize, max_total_hits);
     let hits_info = if is_finite_pagination {
@@ -988,6 +1014,22 @@ pub fn perform_search(
     Ok(result)
 }
 
+pub fn search_from_kind(
+    search_kind: SearchKind,
+    search: milli::Search<'_>,
+) -> Result<(milli::SearchResult, Option<u32>), MeilisearchHttpError> {
+    let (milli_result, semantic_hit_count) = match &search_kind {
+        SearchKind::KeywordOnly => (search.execute()?, None),
+        SearchKind::SemanticOnly { .. } => {
+            let results = search.execute()?;
+            let semantic_hit_count = results.document_scores.len() as u32;
+            (results, Some(semantic_hit_count))
+        }
+        SearchKind::Hybrid { semantic_ratio, .. } => search.execute_hybrid(*semantic_ratio)?,
+    };
+    Ok((milli_result, semantic_hit_count))
+}
+
 struct AttributesFormat {
     attributes_to_retrieve: Option<BTreeSet<String>>,
     retrieve_vectors: RetrieveVectors,
@@ -1033,19 +1075,72 @@ impl RetrieveVectors {
     }
 }
 
-fn make_hits(
+struct HitMaker<'a> {
-    index: &Index,
+    index: &'a Index,
-    rtxn: &RoTxn<'_>,
+    rtxn: &'a RoTxn<'a>,
-    format: AttributesFormat,
+    fields_ids_map: FieldsIdsMap,
-    matching_words: milli::MatchingWords,
+    displayed_ids: BTreeSet<FieldId>,
-    documents_ids: Vec<u32>,
+    vectors_fid: Option<FieldId>,
-    document_scores: Vec<Vec<ScoreDetails>>,
+    retrieve_vectors: RetrieveVectors,
-) -> Result<Vec<SearchHit>, MeilisearchHttpError> {
+    to_retrieve_ids: BTreeSet<FieldId>,
-    let fields_ids_map = index.fields_ids_map(rtxn).unwrap();
+    embedding_configs: Vec<milli::index::IndexEmbeddingConfig>,
-    let displayed_ids =
+    formatter_builder: MatcherBuilder<'a>,
-        index.displayed_fields_ids(rtxn)?.map(|fields| fields.into_iter().collect::<BTreeSet<_>>());
+    formatted_options: BTreeMap<FieldId, FormatOptions>,
+    show_ranking_score: bool,
+    show_ranking_score_details: bool,
+    sort: Option<Vec<String>>,
+    show_matches_position: bool,
+}
 
-    let vectors_fid = fields_ids_map.id(milli::vector::parsed_vectors::RESERVED_VECTORS_FIELD_NAME);
+impl<'a> HitMaker<'a> {
+    pub fn tokenizer<'b>(
+        script_lang_map: &'b HashMap<milli::tokenizer::Script, Vec<milli::tokenizer::Language>>,
+        dictionary: Option<&'b [&'b str]>,
+        separators: Option<&'b [&'b str]>,
+    ) -> milli::tokenizer::Tokenizer<'b> {
+        let mut tokenizer_builder = TokenizerBuilder::default();
+        tokenizer_builder.create_char_map(true);
+        if !script_lang_map.is_empty() {
+            tokenizer_builder.allow_list(script_lang_map);
+        }
+
+        if let Some(separators) = separators {
+            tokenizer_builder.separators(separators);
+        }
+
+        if let Some(dictionary) = dictionary {
+            tokenizer_builder.words_dict(dictionary);
+        }
+
+        tokenizer_builder.into_tokenizer()
+    }
+
+    pub fn formatter_builder(
+        matching_words: milli::MatchingWords,
+        tokenizer: milli::tokenizer::Tokenizer<'_>,
+    ) -> MatcherBuilder<'_> {
+        let formatter_builder = MatcherBuilder::new(matching_words, tokenizer);
+
+        formatter_builder
+    }
+
+    pub fn new(
+        index: &'a Index,
+        rtxn: &'a RoTxn<'a>,
+        format: AttributesFormat,
+        mut formatter_builder: MatcherBuilder<'a>,
+    ) -> Result<Self, MeilisearchHttpError> {
+        formatter_builder.crop_marker(format.crop_marker);
+        formatter_builder.highlight_prefix(format.highlight_pre_tag);
+        formatter_builder.highlight_suffix(format.highlight_post_tag);
+
+        let fields_ids_map = index.fields_ids_map(rtxn)?;
+        let displayed_ids = index
+            .displayed_fields_ids(rtxn)?
+            .map(|fields| fields.into_iter().collect::<BTreeSet<_>>());
+
+        let vectors_fid =
+            fields_ids_map.id(milli::vector::parsed_vectors::RESERVED_VECTORS_FIELD_NAME);
 
         let vectors_is_hidden = match (&displayed_ids, vectors_fid) {
             // displayed_ids is a wildcard, so `_vectors` can be displayed regardless of its fid
@@ -1056,6 +1151,9 @@ fn make_hits(
             (Some(map), Some(vectors_fid)) => map.contains(&vectors_fid),
         };
 
+        let displayed_ids =
+            displayed_ids.unwrap_or_else(|| fields_ids_map.iter().map(|(id, _)| id).collect());
+
         let retrieve_vectors = if let RetrieveVectors::Retrieve = format.retrieve_vectors {
             if vectors_is_hidden {
                 RetrieveVectors::Hide
@@ -1066,8 +1164,6 @@ fn make_hits(
             format.retrieve_vectors
         };
 
-    let displayed_ids =
-        displayed_ids.unwrap_or_else(|| fields_ids_map.iter().map(|(id, _)| id).collect());
         let fids = |attrs: &BTreeSet<String>| {
             let mut ids = BTreeSet::new();
             for attr in attrs {
@@ -1101,63 +1197,69 @@ fn make_hits(
             &fields_ids_map,
             &displayed_ids,
         );
-    let mut tokenizer_builder = TokenizerBuilder::default();
+
-    tokenizer_builder.create_char_map(true);
-    let script_lang_map = index.script_language(rtxn)?;
-    if !script_lang_map.is_empty() {
-        tokenizer_builder.allow_list(&script_lang_map);
-    }
-    let separators = index.allowed_separators(rtxn)?;
-    let separators: Option<Vec<_>> =
-        separators.as_ref().map(|x| x.iter().map(String::as_str).collect());
-    if let Some(ref separators) = separators {
-        tokenizer_builder.separators(separators);
-    }
-    let dictionary = index.dictionary(rtxn)?;
-    let dictionary: Option<Vec<_>> =
-        dictionary.as_ref().map(|x| x.iter().map(String::as_str).collect());
-    if let Some(ref dictionary) = dictionary {
-        tokenizer_builder.words_dict(dictionary);
-    }
-    let mut formatter_builder = MatcherBuilder::new(matching_words, tokenizer_builder.build());
-    formatter_builder.crop_marker(format.crop_marker);
-    formatter_builder.highlight_prefix(format.highlight_pre_tag);
-    formatter_builder.highlight_suffix(format.highlight_post_tag);
-    let mut documents = Vec::new();
         let embedding_configs = index.embedding_configs(rtxn)?;
-    let documents_iter = index.documents(rtxn, documents_ids)?;
+
-    for ((id, obkv), score) in documents_iter.into_iter().zip(document_scores.into_iter()) {
+        Ok(Self {
+            index,
+            rtxn,
+            fields_ids_map,
+            displayed_ids,
+            vectors_fid,
+            retrieve_vectors,
+            to_retrieve_ids,
+            embedding_configs,
+            formatter_builder,
+            formatted_options,
+            show_ranking_score: format.show_ranking_score,
+            show_ranking_score_details: format.show_ranking_score_details,
+            show_matches_position: format.show_matches_position,
+            sort: format.sort,
+        })
+    }
+
+    pub fn make_hit(
+        &self,
+        id: u32,
+        score: &[ScoreDetails],
+    ) -> Result<SearchHit, MeilisearchHttpError> {
+        let (_, obkv) =
+            self.index.iter_documents(self.rtxn, std::iter::once(id))?.next().unwrap()?;
+
         // First generate a document with all the displayed fields
-        let displayed_document = make_document(&displayed_ids, &fields_ids_map, obkv)?;
+        let displayed_document = make_document(&self.displayed_ids, &self.fields_ids_map, obkv)?;
 
         let add_vectors_fid =
-            vectors_fid.filter(|_fid| retrieve_vectors == RetrieveVectors::Retrieve);
+            self.vectors_fid.filter(|_fid| self.retrieve_vectors == RetrieveVectors::Retrieve);
 
         // select the attributes to retrieve
-        let attributes_to_retrieve = to_retrieve_ids
+        let attributes_to_retrieve = self
+            .to_retrieve_ids
             .iter()
             // skip the vectors_fid if RetrieveVectors::Hide
-            .filter(|fid| match vectors_fid {
+            .filter(|fid| match self.vectors_fid {
                 Some(vectors_fid) => {
-                    !(retrieve_vectors == RetrieveVectors::Hide && **fid == vectors_fid)
+                    !(self.retrieve_vectors == RetrieveVectors::Hide && **fid == vectors_fid)
                 }
                 None => true,
             })
             // need to retrieve the existing `_vectors` field if the `RetrieveVectors::Retrieve`
             .chain(add_vectors_fid.iter())
-            .map(|&fid| fields_ids_map.name(fid).expect("Missing field name"));
+            .map(|&fid| self.fields_ids_map.name(fid).expect("Missing field name"));
+
         let mut document =
             permissive_json_pointer::select_values(&displayed_document, attributes_to_retrieve);
 
-        if retrieve_vectors == RetrieveVectors::Retrieve {
+        if self.retrieve_vectors == RetrieveVectors::Retrieve {
             // Clippy is wrong
             #[allow(clippy::manual_unwrap_or_default)]
             let mut vectors = match document.remove("_vectors") {
                 Some(Value::Object(map)) => map,
                 _ => Default::default(),
             };
-            for (name, vector) in index.embeddings(rtxn, id)? {
+            for (name, vector) in self.index.embeddings(self.rtxn, id)? {
-                let user_provided = embedding_configs
+                let user_provided = self
+                    .embedding_configs
                     .iter()
                     .find(|conf| conf.name == name)
                     .is_some_and(|conf| conf.user_provided.contains(id));
@@ -1170,21 +1272,21 @@ fn make_hits(
 
         let (matches_position, formatted) = format_fields(
             &displayed_document,
-            &fields_ids_map,
+            &self.fields_ids_map,
-            &formatter_builder,
+            &self.formatter_builder,
-            &formatted_options,
+            &self.formatted_options,
-            format.show_matches_position,
+            self.show_matches_position,
-            &displayed_ids,
+            &self.displayed_ids,
         )?;
 
-        if let Some(sort) = format.sort.as_ref() {
+        if let Some(sort) = self.sort.as_ref() {
             insert_geo_distance(sort, &mut document);
         }
 
         let ranking_score =
-            format.show_ranking_score.then(|| ScoreDetails::global_score(score.iter()));
+            self.show_ranking_score.then(|| ScoreDetails::global_score(score.iter()));
         let ranking_score_details =
-            format.show_ranking_score_details.then(|| ScoreDetails::to_json_map(score.iter()));
+            self.show_ranking_score_details.then(|| ScoreDetails::to_json_map(score.iter()));
 
         let hit = SearchHit {
             document,
@@ -1193,7 +1295,38 @@ fn make_hits(
             ranking_score_details,
             ranking_score,
         };
-        documents.push(hit);
+
+        Ok(hit)
+    }
+}
+
+fn make_hits<'a>(
+    index: &Index,
+    rtxn: &RoTxn<'_>,
+    format: AttributesFormat,
+    matching_words: milli::MatchingWords,
+    documents_ids_scores: impl Iterator<Item = (u32, &'a Vec<ScoreDetails>)> + 'a,
+) -> Result<Vec<SearchHit>, MeilisearchHttpError> {
+    let mut documents = Vec::new();
+
+    let script_lang_map = index.script_language(rtxn)?;
+
+    let dictionary = index.dictionary(rtxn)?;
+    let dictionary: Option<Vec<_>> =
+        dictionary.as_ref().map(|x| x.iter().map(String::as_str).collect());
+    let separators = index.allowed_separators(rtxn)?;
+    let separators: Option<Vec<_>> =
+        separators.as_ref().map(|x| x.iter().map(String::as_str).collect());
+
+    let tokenizer =
+        HitMaker::tokenizer(&script_lang_map, dictionary.as_deref(), separators.as_deref());
+
+    let formatter_builder = HitMaker::formatter_builder(matching_words, tokenizer);
+
+    let hit_maker = HitMaker::new(index, rtxn, format, formatter_builder)?;
+
+    for (id, score) in documents_ids_scores {
+        documents.push(hit_maker.make_hit(id, score)?);
     }
     Ok(documents)
 }
@@ -1309,7 +1442,13 @@ pub fn perform_similar(
         show_ranking_score_details,
     };
 
-    let hits = make_hits(index, &rtxn, format, Default::default(), documents_ids, document_scores)?;
+    let hits = make_hits(
+        index,
+        &rtxn,
+        format,
+        Default::default(),
+        documents_ids.iter().copied().zip(document_scores.iter()),
+    )?;
 
     let max_total_hits = index
         .pagination_max_total_hits(&rtxn)
@@ -1482,10 +1621,10 @@ fn make_document(
     Ok(document)
 }
 
-fn format_fields<'a>(
+fn format_fields(
     document: &Document,
     field_ids_map: &FieldsIdsMap,
-    builder: &'a MatcherBuilder<'a>,
+    builder: &MatcherBuilder<'_>,
     formatted_options: &BTreeMap<FieldId, FormatOptions>,
     compute_matches: bool,
     displayable_ids: &BTreeSet<FieldId>,
@@ -1540,9 +1679,9 @@ fn format_fields<'a>(
     Ok((matches_position, document))
 }
 
-fn format_value<'a>(
+fn format_value(
     value: Value,
-    builder: &'a MatcherBuilder<'a>,
+    builder: &MatcherBuilder<'_>,
     format_options: Option<FormatOptions>,
     infos: &mut Vec<MatchBounds>,
     compute_matches: bool,

meilisearch/src/search/ranking_rules.rs

@@ -0,0 +1,823 @@
+use std::collections::HashMap;
+use std::fmt::Write;
+
+use itertools::Itertools as _;
+use meilisearch_types::error::{Code, ResponseError};
+use meilisearch_types::milli::{AscDesc, Criterion, Member, TermsMatchingStrategy};
+
+pub struct RankingRules {
+    canonical_criteria: Vec<Criterion>,
+    canonical_sort: Option<Vec<AscDesc>>,
+    canonicalization_actions: Vec<CanonicalizationAction>,
+    source_criteria: Vec<Criterion>,
+    source_sort: Option<Vec<AscDesc>>,
+}
+
+pub enum CanonicalizationAction {
+    PrependedWords {
+        prepended_index: RankingRuleSource,
+    },
+    RemovedDuplicate {
+        earlier_occurrence: RankingRuleSource,
+        removed_occurrence: RankingRuleSource,
+    },
+    RemovedWords {
+        reason: RemoveWords,
+        removed_occurrence: RankingRuleSource,
+    },
+    RemovedPlaceholder {
+        removed_occurrence: RankingRuleSource,
+    },
+    TruncatedVector {
+        vector_rule: RankingRuleSource,
+        truncated_from: RankingRuleSource,
+    },
+    RemovedVector {
+        vector_rule: RankingRuleSource,
+        removed_occurrence: RankingRuleSource,
+    },
+    RemovedSort {
+        removed_occurrence: RankingRuleSource,
+    },
+}
+
+pub enum RemoveWords {
+    WasPrepended,
+    MatchingStrategyAll,
+}
+
+impl std::fmt::Display for RemoveWords {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let reason = match self {
+            RemoveWords::WasPrepended => "it was previously prepended",
+            RemoveWords::MatchingStrategyAll => "`query.matchingWords` is set to `all`",
+        };
+        f.write_str(reason)
+    }
+}
+
+pub enum CanonicalizationKind {
+    Placeholder,
+    Keyword,
+    Vector,
+}
+
+pub struct CompatibilityError {
+    previous: RankingRule,
+    current: RankingRule,
+}
+impl CompatibilityError {
+    pub(crate) fn to_response_error(
+        &self,
+        ranking_rules: &RankingRules,
+        previous_ranking_rules: &RankingRules,
+        query_index: usize,
+        previous_query_index: usize,
+        index_uid: &str,
+        previous_index_uid: &str,
+    ) -> meilisearch_types::error::ResponseError {
+        let rule = self.current.as_string(
+            &ranking_rules.canonical_criteria,
+            &ranking_rules.canonical_sort,
+            query_index,
+            index_uid,
+        );
+        let previous_rule = self.previous.as_string(
+            &previous_ranking_rules.canonical_criteria,
+            &previous_ranking_rules.canonical_sort,
+            previous_query_index,
+            previous_index_uid,
+        );
+
+        let canonicalization_actions = ranking_rules.canonicalization_notes();
+        let previous_canonicalization_actions = previous_ranking_rules.canonicalization_notes();
+
+        let mut msg = String::new();
+        let reason = self.reason();
+        let _ = writeln!(
+            &mut msg,
+            "The results of queries #{previous_query_index} and #{query_index} are incompatible: "
+        );
+        let _ = writeln!(&mut msg, "  1. {previous_rule}");
+        let _ = writeln!(&mut msg, "  2. {rule}");
+        let _ = writeln!(&mut msg, "  - {reason}");
+
+        if !previous_canonicalization_actions.is_empty() {
+            let _ = write!(&mut msg, "  - note: The ranking rules of query #{previous_query_index} were modified during canonicalization:\n{previous_canonicalization_actions}");
+        }
+
+        if !canonicalization_actions.is_empty() {
+            let _ = write!(&mut msg, "  - note: The ranking rules of query #{query_index} were modified during canonicalization:\n{canonicalization_actions}");
+        }
+
+        ResponseError::from_msg(msg, Code::InvalidMultiSearchQueryRankingRules)
+    }
+    pub fn reason(&self) -> &'static str {
+        match (self.previous.kind, self.current.kind) {
+            (RankingRuleKind::Relevancy, RankingRuleKind::AscendingSort)
+            | (RankingRuleKind::Relevancy, RankingRuleKind::DescendingSort)
+            | (RankingRuleKind::AscendingSort, RankingRuleKind::Relevancy)
+            | (RankingRuleKind::DescendingSort, RankingRuleKind::Relevancy) => {
+                "cannot compare a relevancy rule with a sort rule"
+            }
+
+            (RankingRuleKind::Relevancy, RankingRuleKind::AscendingGeoSort)
+            | (RankingRuleKind::Relevancy, RankingRuleKind::DescendingGeoSort)
+            | (RankingRuleKind::AscendingGeoSort, RankingRuleKind::Relevancy)
+            | (RankingRuleKind::DescendingGeoSort, RankingRuleKind::Relevancy) => {
+                "cannot compare a relevancy rule with a geosort rule"
+            }
+
+            (RankingRuleKind::AscendingSort, RankingRuleKind::DescendingSort)
+            | (RankingRuleKind::DescendingSort, RankingRuleKind::AscendingSort) => {
+                "cannot compare two sort rules in opposite directions"
+            }
+
+            (RankingRuleKind::AscendingSort, RankingRuleKind::AscendingGeoSort)
+            | (RankingRuleKind::AscendingSort, RankingRuleKind::DescendingGeoSort)
+            | (RankingRuleKind::DescendingSort, RankingRuleKind::AscendingGeoSort)
+            | (RankingRuleKind::DescendingSort, RankingRuleKind::DescendingGeoSort)
+            | (RankingRuleKind::AscendingGeoSort, RankingRuleKind::AscendingSort)
+            | (RankingRuleKind::AscendingGeoSort, RankingRuleKind::DescendingSort)
+            | (RankingRuleKind::DescendingGeoSort, RankingRuleKind::AscendingSort)
+            | (RankingRuleKind::DescendingGeoSort, RankingRuleKind::DescendingSort) => {
+                "cannot compare a sort rule with a geosort rule"
+            }
+
+            (RankingRuleKind::AscendingGeoSort, RankingRuleKind::DescendingGeoSort)
+            | (RankingRuleKind::DescendingGeoSort, RankingRuleKind::AscendingGeoSort) => {
+                "cannot compare two geosort rules in opposite directions"
+            }
+            (RankingRuleKind::Relevancy, RankingRuleKind::Relevancy)
+            | (RankingRuleKind::AscendingSort, RankingRuleKind::AscendingSort)
+            | (RankingRuleKind::DescendingSort, RankingRuleKind::DescendingSort)
+            | (RankingRuleKind::AscendingGeoSort, RankingRuleKind::AscendingGeoSort)
+            | (RankingRuleKind::DescendingGeoSort, RankingRuleKind::DescendingGeoSort) => {
+                "internal error, comparison should be possible"
+            }
+        }
+    }
+}
+
+impl RankingRules {
+    pub fn new(
+        criteria: Vec<Criterion>,
+        sort: Option<Vec<AscDesc>>,
+        terms_matching_strategy: TermsMatchingStrategy,
+        canonicalization_kind: CanonicalizationKind,
+    ) -> Self {
+        let (canonical_criteria, canonical_sort, canonicalization_actions) =
+            Self::canonicalize(&criteria, &sort, terms_matching_strategy, canonicalization_kind);
+        Self {
+            canonical_criteria,
+            canonical_sort,
+            canonicalization_actions,
+            source_criteria: criteria,
+            source_sort: sort,
+        }
+    }
+
+    fn canonicalize(
+        criteria: &[Criterion],
+        sort: &Option<Vec<AscDesc>>,
+        terms_matching_strategy: TermsMatchingStrategy,
+        canonicalization_kind: CanonicalizationKind,
+    ) -> (Vec<Criterion>, Option<Vec<AscDesc>>, Vec<CanonicalizationAction>) {
+        match canonicalization_kind {
+            CanonicalizationKind::Placeholder => Self::canonicalize_placeholder(criteria, sort),
+            CanonicalizationKind::Keyword => {
+                Self::canonicalize_keyword(criteria, sort, terms_matching_strategy)
+            }
+            CanonicalizationKind::Vector => Self::canonicalize_vector(criteria, sort),
+        }
+    }
+
+    fn canonicalize_placeholder(
+        criteria: &[Criterion],
+        sort_query: &Option<Vec<AscDesc>>,
+    ) -> (Vec<Criterion>, Option<Vec<AscDesc>>, Vec<CanonicalizationAction>) {
+        let mut sort = None;
+
+        let mut sorted_fields = HashMap::new();
+        let mut canonicalization_actions = Vec::new();
+        let mut canonical_criteria = Vec::new();
+        let mut canonical_sort = None;
+
+        for (criterion_index, criterion) in criteria.iter().enumerate() {
+            match criterion.clone() {
+                Criterion::Words
+                | Criterion::Typo
+                | Criterion::Proximity
+                | Criterion::Attribute
+                | Criterion::Exactness => {
+                    canonicalization_actions.push(CanonicalizationAction::RemovedPlaceholder {
+                        removed_occurrence: RankingRuleSource::Criterion(criterion_index),
+                    })
+                }
+
+                Criterion::Sort => {
+                    if let Some(previous_index) = sort {
+                        canonicalization_actions.push(CanonicalizationAction::RemovedDuplicate {
+                            earlier_occurrence: RankingRuleSource::Criterion(previous_index),
+                            removed_occurrence: RankingRuleSource::Criterion(criterion_index),
+                        });
+                    } else if let Some(sort_query) = sort_query {
+                        sort = Some(criterion_index);
+                        canonical_criteria.push(criterion.clone());
+                        canonical_sort = Some(canonicalize_sort(
+                            &mut sorted_fields,
+                            sort_query.as_slice(),
+                            criterion_index,
+                            &mut canonicalization_actions,
+                        ));
+                    } else {
+                        canonicalization_actions.push(CanonicalizationAction::RemovedSort {
+                            removed_occurrence: RankingRuleSource::Criterion(criterion_index),
+                        })
+                    }
+                }
+                Criterion::Asc(s) | Criterion::Desc(s) => match sorted_fields.entry(s) {
+                    std::collections::hash_map::Entry::Occupied(entry) => canonicalization_actions
+                        .push(CanonicalizationAction::RemovedDuplicate {
+                            earlier_occurrence: *entry.get(),
+                            removed_occurrence: RankingRuleSource::Criterion(criterion_index),
+                        }),
+                    std::collections::hash_map::Entry::Vacant(entry) => {
+                        entry.insert(RankingRuleSource::Criterion(criterion_index));
+                        canonical_criteria.push(criterion.clone())
+                    }
+                },
+            }
+        }
+
+        (canonical_criteria, canonical_sort, canonicalization_actions)
+    }
+
+    fn canonicalize_vector(
+        criteria: &[Criterion],
+        sort_query: &Option<Vec<AscDesc>>,
+    ) -> (Vec<Criterion>, Option<Vec<AscDesc>>, Vec<CanonicalizationAction>) {
+        let mut sort = None;
+
+        let mut sorted_fields = HashMap::new();
+        let mut canonicalization_actions = Vec::new();
+        let mut canonical_criteria = Vec::new();
+        let mut canonical_sort = None;
+
+        let mut vector = None;
+
+        'criteria: for (criterion_index, criterion) in criteria.iter().enumerate() {
+            match criterion.clone() {
+                Criterion::Words
+                | Criterion::Typo
+                | Criterion::Proximity
+                | Criterion::Attribute
+                | Criterion::Exactness => match vector {
+                    Some(previous_occurrence) => {
+                        if sorted_fields.is_empty() {
+                            canonicalization_actions.push(CanonicalizationAction::RemovedVector {
+                                vector_rule: RankingRuleSource::Criterion(previous_occurrence),
+                                removed_occurrence: RankingRuleSource::Criterion(criterion_index),
+                            });
+                        } else {
+                            canonicalization_actions.push(
+                                CanonicalizationAction::TruncatedVector {
+                                    vector_rule: RankingRuleSource::Criterion(previous_occurrence),
+                                    truncated_from: RankingRuleSource::Criterion(criterion_index),
+                                },
+                            );
+                            break 'criteria;
+                        }
+                    }
+                    None => {
+                        canonical_criteria.push(criterion.clone());
+                        vector = Some(criterion_index);
+                    }
+                },
+
+                Criterion::Sort => {
+                    if let Some(previous_index) = sort {
+                        canonicalization_actions.push(CanonicalizationAction::RemovedDuplicate {
+                            earlier_occurrence: RankingRuleSource::Criterion(previous_index),
+                            removed_occurrence: RankingRuleSource::Criterion(criterion_index),
+                        });
+                    } else if let Some(sort_query) = sort_query {
+                        sort = Some(criterion_index);
+                        canonical_criteria.push(criterion.clone());
+                        canonical_sort = Some(canonicalize_sort(
+                            &mut sorted_fields,
+                            sort_query.as_slice(),
+                            criterion_index,
+                            &mut canonicalization_actions,
+                        ));
+                    } else {
+                        canonicalization_actions.push(CanonicalizationAction::RemovedSort {
+                            removed_occurrence: RankingRuleSource::Criterion(criterion_index),
+                        })
+                    }
+                }
+                Criterion::Asc(s) | Criterion::Desc(s) => match sorted_fields.entry(s) {
+                    std::collections::hash_map::Entry::Occupied(entry) => canonicalization_actions
+                        .push(CanonicalizationAction::RemovedDuplicate {
+                            earlier_occurrence: *entry.get(),
+                            removed_occurrence: RankingRuleSource::Criterion(criterion_index),
+                        }),
+                    std::collections::hash_map::Entry::Vacant(entry) => {
+                        entry.insert(RankingRuleSource::Criterion(criterion_index));
+                        canonical_criteria.push(criterion.clone())
+                    }
+                },
+            }
+        }
+
+        (canonical_criteria, canonical_sort, canonicalization_actions)
+    }
+
+    fn canonicalize_keyword(
+        criteria: &[Criterion],
+        sort_query: &Option<Vec<AscDesc>>,
+        terms_matching_strategy: TermsMatchingStrategy,
+    ) -> (Vec<Criterion>, Option<Vec<AscDesc>>, Vec<CanonicalizationAction>) {
+        let mut words = None;
+        let mut typo = None;
+        let mut proximity = None;
+        let mut sort = None;
+        let mut attribute = None;
+        let mut exactness = None;
+        let mut sorted_fields = HashMap::new();
+
+        let mut canonical_criteria = Vec::new();
+        let mut canonical_sort = None;
+
+        let mut canonicalization_actions = Vec::new();
+
+        for (criterion_index, criterion) in criteria.iter().enumerate() {
+            let criterion = criterion.clone();
+            match criterion.clone() {
+                Criterion::Words => {
+                    if let TermsMatchingStrategy::All = terms_matching_strategy {
+                        canonicalization_actions.push(CanonicalizationAction::RemovedWords {
+                            reason: RemoveWords::MatchingStrategyAll,
+                            removed_occurrence: RankingRuleSource::Criterion(criterion_index),
+                        });
+                        continue;
+                    }
+                    if let Some(maybe_previous_index) = words {
+                        if let Some(previous_index) = maybe_previous_index {
+                            canonicalization_actions.push(
+                                CanonicalizationAction::RemovedDuplicate {
+                                    earlier_occurrence: RankingRuleSource::Criterion(
+                                        previous_index,
+                                    ),
+                                    removed_occurrence: RankingRuleSource::Criterion(
+                                        criterion_index,
+                                    ),
+                                },
+                            );
+                            continue;
+                        }
+                        canonicalization_actions.push(CanonicalizationAction::RemovedWords {
+                            reason: RemoveWords::WasPrepended,
+                            removed_occurrence: RankingRuleSource::Criterion(criterion_index),
+                        })
+                    }
+                    words = Some(Some(criterion_index));
+                    canonical_criteria.push(criterion);
+                }
+                Criterion::Typo => {
+                    canonicalize_criterion(
+                        criterion,
+                        criterion_index,
+                        terms_matching_strategy,
+                        &mut words,
+                        &mut canonicalization_actions,
+                        &mut canonical_criteria,
+                        &mut typo,
+                    );
+                }
+                Criterion::Proximity => {
+                    canonicalize_criterion(
+                        criterion,
+                        criterion_index,
+                        terms_matching_strategy,
+                        &mut words,
+                        &mut canonicalization_actions,
+                        &mut canonical_criteria,
+                        &mut proximity,
+                    );
+                }
+                Criterion::Attribute => {
+                    canonicalize_criterion(
+                        criterion,
+                        criterion_index,
+                        terms_matching_strategy,
+                        &mut words,
+                        &mut canonicalization_actions,
+                        &mut canonical_criteria,
+                        &mut attribute,
+                    );
+                }
+                Criterion::Exactness => {
+                    canonicalize_criterion(
+                        criterion,
+                        criterion_index,
+                        terms_matching_strategy,
+                        &mut words,
+                        &mut canonicalization_actions,
+                        &mut canonical_criteria,
+                        &mut exactness,
+                    );
+                }
+
+                Criterion::Sort => {
+                    if let Some(previous_index) = sort {
+                        canonicalization_actions.push(CanonicalizationAction::RemovedDuplicate {
+                            earlier_occurrence: RankingRuleSource::Criterion(previous_index),
+                            removed_occurrence: RankingRuleSource::Criterion(criterion_index),
+                        });
+                    } else if let Some(sort_query) = sort_query {
+                        sort = Some(criterion_index);
+                        canonical_criteria.push(criterion);
+                        canonical_sort = Some(canonicalize_sort(
+                            &mut sorted_fields,
+                            sort_query.as_slice(),
+                            criterion_index,
+                            &mut canonicalization_actions,
+                        ));
+                    } else {
+                        canonicalization_actions.push(CanonicalizationAction::RemovedSort {
+                            removed_occurrence: RankingRuleSource::Criterion(criterion_index),
+                        })
+                    }
+                }
+                Criterion::Asc(s) | Criterion::Desc(s) => match sorted_fields.entry(s) {
+                    std::collections::hash_map::Entry::Occupied(entry) => canonicalization_actions
+                        .push(CanonicalizationAction::RemovedDuplicate {
+                            earlier_occurrence: *entry.get(),
+                            removed_occurrence: RankingRuleSource::Criterion(criterion_index),
+                        }),
+                    std::collections::hash_map::Entry::Vacant(entry) => {
+                        entry.insert(RankingRuleSource::Criterion(criterion_index));
+                        canonical_criteria.push(criterion)
+                    }
+                },
+            }
+        }
+
+        (canonical_criteria, canonical_sort, canonicalization_actions)
+    }
+
+    pub fn is_compatible_with(&self, previous: &Self) -> Result<(), CompatibilityError> {
+        for (current, previous) in self.coalesce_iterator().zip(previous.coalesce_iterator()) {
+            if current.kind != previous.kind {
+                return Err(CompatibilityError { current, previous });
+            }
+        }
+        Ok(())
+    }
+
+    pub fn constraint_count(&self) -> usize {
+        self.coalesce_iterator().count()
+    }
+
+    fn coalesce_iterator(&self) -> impl Iterator<Item = RankingRule> + '_ {
+        self.canonical_criteria
+            .iter()
+            .enumerate()
+            .flat_map(|(criterion_index, criterion)| {
+                RankingRule::from_criterion(criterion_index, criterion, &self.canonical_sort)
+            })
+            .coalesce(
+                |previous @ RankingRule { source: previous_source, kind: previous_kind },
+                 current @ RankingRule { source, kind }| {
+                    match (previous_kind, kind) {
+                        (RankingRuleKind::Relevancy, RankingRuleKind::Relevancy) => {
+                            let merged_source = match (previous_source, source) {
+                                (
+                                    RankingRuleSource::Criterion(previous),
+                                    RankingRuleSource::Criterion(current),
+                                ) => RankingRuleSource::CoalescedCriteria(previous, current),
+                                (
+                                    RankingRuleSource::CoalescedCriteria(begin, _end),
+                                    RankingRuleSource::Criterion(current),
+                                ) => RankingRuleSource::CoalescedCriteria(begin, current),
+                                (_previous, current) => current,
+                            };
+                            Ok(RankingRule { source: merged_source, kind })
+                        }
+                        _ => Err((previous, current)),
+                    }
+                },
+            )
+    }
+
+    fn canonicalization_notes(&self) -> String {
+        use CanonicalizationAction::*;
+        let mut notes = String::new();
+        for (index, action) in self.canonicalization_actions.iter().enumerate() {
+            let index = index + 1;
+            let _ = match action {
+                PrependedWords { prepended_index } => writeln!(
+                    &mut notes,
+                    "    {index}. Prepended rule `words` before first relevancy rule `{}` at position {}",
+                    prepended_index.rule_name(&self.source_criteria, &self.source_sort),
+                    prepended_index.rule_position()
+                ),
+                RemovedDuplicate { earlier_occurrence, removed_occurrence } => writeln!(
+                    &mut notes,
+                    "    {index}. Removed duplicate rule `{}` at position {} as it already appears at position {}",
+                    earlier_occurrence.rule_name(&self.source_criteria, &self.source_sort),
+                    removed_occurrence.rule_position(),
+                    earlier_occurrence.rule_position(),
+                ),
+                RemovedWords { reason, removed_occurrence } => writeln!(
+                    &mut notes,
+                    "    {index}. Removed rule `words` at position {} because {reason}",
+                    removed_occurrence.rule_position()
+                ),
+                RemovedPlaceholder { removed_occurrence } => writeln!(
+                    &mut notes,
+                    "    {index}. Removed relevancy rule `{}` at position {} because the query is a placeholder search (`q`: \"\")",
+                    removed_occurrence.rule_name(&self.source_criteria, &self.source_sort),
+                    removed_occurrence.rule_position()
+                ),
+                TruncatedVector { vector_rule, truncated_from } => writeln!(
+                    &mut notes,
+                    "    {index}. Truncated relevancy rule `{}` at position {} and later rules because the query is a vector search and `vector` was inserted at position {}",
+                    truncated_from.rule_name(&self.source_criteria, &self.source_sort),
+                    truncated_from.rule_position(),
+                    vector_rule.rule_position(),
+                ),
+                RemovedVector { vector_rule, removed_occurrence } => writeln!(
+                    &mut notes,
+                    "    {index}. Removed relevancy rule `{}` at position {} because the query is a vector search and `vector` was already inserted at position {}",
+                    removed_occurrence.rule_name(&self.source_criteria, &self.source_sort),
+                    removed_occurrence.rule_position(),
+                    vector_rule.rule_position(),
+                ),
+                RemovedSort { removed_occurrence } => writeln!(
+                    &mut notes,
+                    "   {index}. Removed rule `sort` at position {} because `query.sort` is empty",
+removed_occurrence.rule_position()
+                ),
+            };
+        }
+        notes
+    }
+}
+
+fn canonicalize_sort(
+    sorted_fields: &mut HashMap<String, RankingRuleSource>,
+    sort_query: &[AscDesc],
+    criterion_index: usize,
+    canonicalization_actions: &mut Vec<CanonicalizationAction>,
+) -> Vec<AscDesc> {
+    let mut geo_sorted = None;
+    let mut canonical_sort = Vec::new();
+    for (sort_index, asc_desc) in sort_query.iter().enumerate() {
+        let source = RankingRuleSource::Sort { criterion_index, sort_index };
+        let asc_desc = asc_desc.clone();
+        match asc_desc.clone() {
+            AscDesc::Asc(Member::Field(s)) | AscDesc::Desc(Member::Field(s)) => {
+                match sorted_fields.entry(s) {
+                    std::collections::hash_map::Entry::Occupied(entry) => canonicalization_actions
+                        .push(CanonicalizationAction::RemovedDuplicate {
+                            earlier_occurrence: *entry.get(),
+                            removed_occurrence: source,
+                        }),
+                    std::collections::hash_map::Entry::Vacant(entry) => {
+                        entry.insert(source);
+                        canonical_sort.push(asc_desc);
+                    }
+                }
+            }
+            AscDesc::Asc(Member::Geo(_)) | AscDesc::Desc(Member::Geo(_)) => match geo_sorted {
+                Some(earlier_sort_index) => {
+                    canonicalization_actions.push(CanonicalizationAction::RemovedDuplicate {
+                        earlier_occurrence: RankingRuleSource::Sort {
+                            criterion_index,
+                            sort_index: earlier_sort_index,
+                        },
+                        removed_occurrence: source,
+                    })
+                }
+                None => {
+                    geo_sorted = Some(sort_index);
+                    canonical_sort.push(asc_desc);
+                }
+            },
+        }
+    }
+    canonical_sort
+}
+
+fn canonicalize_criterion(
+    criterion: Criterion,
+    criterion_index: usize,
+    terms_matching_strategy: TermsMatchingStrategy,
+    words: &mut Option<Option<usize>>,
+    canonicalization_actions: &mut Vec<CanonicalizationAction>,
+    canonical_criteria: &mut Vec<Criterion>,
+    rule: &mut Option<usize>,
+) {
+    *words = match (terms_matching_strategy, words.take()) {
+        (TermsMatchingStrategy::All, words) => words,
+        (_, None) => {
+            // inject words
+            canonicalization_actions.push(CanonicalizationAction::PrependedWords {
+                prepended_index: RankingRuleSource::Criterion(criterion_index),
+            });
+            canonical_criteria.push(Criterion::Words);
+            Some(None)
+        }
+        (_, words) => words,
+    };
+    if let Some(previous_index) = *rule {
+        canonicalization_actions.push(CanonicalizationAction::RemovedDuplicate {
+            earlier_occurrence: RankingRuleSource::Criterion(previous_index),
+            removed_occurrence: RankingRuleSource::Criterion(criterion_index),
+        });
+    } else {
+        *rule = Some(criterion_index);
+        canonical_criteria.push(criterion)
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum RankingRuleKind {
+    Relevancy,
+    AscendingSort,
+    DescendingSort,
+    AscendingGeoSort,
+    DescendingGeoSort,
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct RankingRule {
+    source: RankingRuleSource,
+    kind: RankingRuleKind,
+}
+
+#[derive(Debug, Clone, Copy)]
+pub enum RankingRuleSource {
+    Criterion(usize),
+    CoalescedCriteria(usize, usize),
+    Sort { criterion_index: usize, sort_index: usize },
+}
+
+impl RankingRuleSource {
+    fn rule_name(&self, criteria: &[Criterion], sort: &Option<Vec<AscDesc>>) -> String {
+        match self {
+            RankingRuleSource::Criterion(criterion_index) => criteria
+                .get(*criterion_index)
+                .map(|c| c.to_string())
+                .unwrap_or_else(|| "unknown".into()),
+            RankingRuleSource::CoalescedCriteria(begin, end) => {
+                let rules: Vec<_> = criteria
+                    .get(*begin..=*end)
+                    .iter()
+                    .flat_map(|c| c.iter())
+                    .map(|c| c.to_string())
+                    .collect();
+                rules.join(", ")
+            }
+            RankingRuleSource::Sort { criterion_index: _, sort_index } => {
+                match sort.as_deref().and_then(|sort| sort.get(*sort_index)) {
+                    Some(sort) => match sort {
+                        AscDesc::Asc(Member::Field(field_name)) => format!("{field_name}:asc"),
+                        AscDesc::Desc(Member::Field(field_name)) => {
+                            format!("{field_name}:desc")
+                        }
+                        AscDesc::Asc(Member::Geo(_)) => "_geo(..):asc".to_string(),
+                        AscDesc::Desc(Member::Geo(_)) => "_geo(..):desc".to_string(),
+                    },
+                    None => "unknown".into(),
+                }
+            }
+        }
+    }
+
+    fn rule_position(&self) -> String {
+        match self {
+            RankingRuleSource::Criterion(criterion_index) => {
+                format!("#{criterion_index} in ranking rules")
+            }
+            RankingRuleSource::CoalescedCriteria(begin, end) => {
+                format!("#{begin} to #{end} in ranking rules")
+            }
+            RankingRuleSource::Sort { criterion_index, sort_index } => format!(
+                "#{sort_index} in `query.sort` (as `sort` is #{criterion_index} in ranking rules)"
+            ),
+        }
+    }
+}
+
+impl RankingRule {
+    fn from_criterion<'a>(
+        criterion_index: usize,
+        criterion: &'a Criterion,
+        sort: &'a Option<Vec<AscDesc>>,
+    ) -> impl Iterator<Item = Self> + 'a {
+        let kind = match criterion {
+            Criterion::Words
+            | Criterion::Typo
+            | Criterion::Proximity
+            | Criterion::Attribute
+            | Criterion::Exactness => RankingRuleKind::Relevancy,
+            Criterion::Asc(s) if s == "_geo" => RankingRuleKind::AscendingGeoSort,
+
+            Criterion::Asc(_) => RankingRuleKind::AscendingSort,
+            Criterion::Desc(s) if s == "_geo" => RankingRuleKind::DescendingGeoSort,
+
+            Criterion::Desc(_) => RankingRuleKind::DescendingSort,
+            Criterion::Sort => {
+                return either::Right(sort.iter().flatten().enumerate().map(
+                    move |(rule_index, asc_desc)| {
+                        Self::from_asc_desc(asc_desc, criterion_index, rule_index)
+                    },
+                ))
+            }
+        };
+
+        either::Left(std::iter::once(Self {
+            source: RankingRuleSource::Criterion(criterion_index),
+            kind,
+        }))
+    }
+
+    fn from_asc_desc(asc_desc: &AscDesc, sort_index: usize, rule_index_in_sort: usize) -> Self {
+        let kind = match asc_desc {
+            AscDesc::Asc(Member::Field(_)) => RankingRuleKind::AscendingSort,
+            AscDesc::Desc(Member::Field(_)) => RankingRuleKind::DescendingSort,
+            AscDesc::Asc(Member::Geo(_)) => RankingRuleKind::AscendingGeoSort,
+            AscDesc::Desc(Member::Geo(_)) => RankingRuleKind::DescendingGeoSort,
+        };
+        Self {
+            source: RankingRuleSource::Sort {
+                criterion_index: sort_index,
+                sort_index: rule_index_in_sort,
+            },
+            kind,
+        }
+    }
+
+    fn as_string(
+        &self,
+        canonical_criteria: &[Criterion],
+        canonical_sort: &Option<Vec<AscDesc>>,
+        query_index: usize,
+        index_uid: &str,
+    ) -> String {
+        let kind = match self.kind {
+            RankingRuleKind::Relevancy => "relevancy",
+            RankingRuleKind::AscendingSort => "ascending sort",
+            RankingRuleKind::DescendingSort => "descending sort",
+            RankingRuleKind::AscendingGeoSort => "ascending geo sort",
+            RankingRuleKind::DescendingGeoSort => "descending geo sort",
+        };
+        let rules = self.fetch_from_source(canonical_criteria, canonical_sort);
+
+        let source = match self.source {
+            RankingRuleSource::Criterion(criterion_index) => format!("`queries[{query_index}]`, `{index_uid}.rankingRules[{criterion_index}]`"),
+            RankingRuleSource::CoalescedCriteria(begin, end) => format!("`queries[{query_index}]`, `{index_uid}.rankingRules[{begin}..={end}]`"),
+            RankingRuleSource::Sort { criterion_index, sort_index } => format!("`queries[{query_index}].sort[{sort_index}]`, `{index_uid}.rankingRules[{criterion_index}]`"),
+        };
+
+        format!("{source}: {kind} {rules}")
+    }
+
+    fn fetch_from_source(
+        &self,
+        canonical_criteria: &[Criterion],
+        canonical_sort: &Option<Vec<AscDesc>>,
+    ) -> String {
+        let rule_name = match self.source {
+            RankingRuleSource::Criterion(index) => {
+                canonical_criteria.get(index).map(|criterion| criterion.to_string())
+            }
+            RankingRuleSource::CoalescedCriteria(begin, end) => {
+                let rules: Vec<String> = canonical_criteria
+                    .get(begin..=end)
+                    .into_iter()
+                    .flat_map(|criteria| criteria.iter())
+                    .map(|criterion| criterion.to_string())
+                    .collect();
+
+                (!rules.is_empty()).then_some(rules.join(", "))
+            }
+            RankingRuleSource::Sort { criterion_index: _, sort_index } => canonical_sort
+                .as_deref()
+                .and_then(|canonical_sort| canonical_sort.get(sort_index))
+                .and_then(|asc_desc: &AscDesc| match asc_desc {
+                    AscDesc::Asc(Member::Field(s)) | AscDesc::Desc(Member::Field(s)) => {
+                        Some(format!("on field `{s}`"))
+                    }
+                    _ => None,
+                }),
+        };
+
+        let rule_name = rule_name.unwrap_or_else(|| "default".into());
+
+        format!("rule(s) {rule_name}")
+    }
+}

meilisearch/tests/auth/tenant_token_multi_search.rs

@@ -310,6 +310,23 @@ macro_rules! compute_authorized_single_search {
                     tenant_token,
                     key_content
                 );
+
+                // federated
+                let (response, code) = server.multi_search(json!({"federation": {}, "queries" : [{"indexUid": "sales", "filter": $filter}]})).await;
+                assert_eq!(
+                    200, code,
+                    "{} using tenant_token: {:?} generated with parent_key: {:?}",
+                    response, tenant_token, key_content
+                );
+                assert_eq!(
+                    // same count as the search is federated over a single query
+                    $expected_count,
+                    response["hits"].as_array().unwrap().len(),
+                    "{} using tenant_token: {:?} generated with parent_key: {:?}",
+                    response,
+                    tenant_token,
+                    key_content
+                );
             }
         }
     };
@@ -375,6 +392,25 @@ macro_rules! compute_authorized_multiple_search {
                     tenant_token,
                     key_content
                 );
+
+                let (response, code) = server.multi_search(json!({"federation": {}, "queries" : [
+                    {"indexUid": "sales", "filter": $filter1},
+                    {"indexUid": "products", "filter": $filter2},
+                ]})).await;
+                assert_eq!(
+                    code, 200,
+                    "{} using tenant_token: {:?} generated with parent_key: {:?}",
+                    response, tenant_token, key_content
+                );
+                assert_eq!(
+                    response["hits"].as_array().unwrap().len(),
+                    // sum of counts as the search is federated across to queries in different indexes
+                    $expected_count1 + $expected_count2,
+                    "{} using tenant_token: {:?} generated with parent_key: {:?}",
+                    response,
+                    tenant_token,
+                    key_content
+                );
             }
         }
     };
@@ -433,6 +469,24 @@ macro_rules! compute_forbidden_single_search {
                     "{} using tenant_token: {:?} generated with parent_key: {:?}",
                     response, tenant_token, key_content
                 );
+
+                let (mut response, code) = server.multi_search(json!({"federation": {}, "queries" : [{"indexUid": "sales"}]})).await;
+                if failed_query_index.is_none() && !response["message"].is_null() {
+                    response["message"] = serde_json::json!(null);
+                }
+                assert_eq!(
+                    response,
+                    invalid_response(failed_query_index),
+                    "{} using tenant_token: {:?} generated with parent_key: {:?}",
+                    response,
+                    tenant_token,
+                    key_content
+                );
+                assert_eq!(
+                    code, 403,
+                    "{} using tenant_token: {:?} generated with parent_key: {:?}",
+                    response, tenant_token, key_content
+                );
             }
         }
     };
@@ -494,6 +548,27 @@ macro_rules! compute_forbidden_multiple_search {
                     "{} using tenant_token: {:?} generated with parent_key: {:?}",
                     response, tenant_token, key_content
                 );
+
+                let (mut response, code) = server.multi_search(json!({"federation": {}, "queries" : [
+                    {"indexUid": "sales"},
+                    {"indexUid": "products"},
+                ]})).await;
+                if failed_query_index.is_none() && !response["message"].is_null() {
+                    response["message"] = serde_json::json!(null);
+                }
+                assert_eq!(
+                    response,
+                    invalid_response(failed_query_index),
+                    "{} using tenant_token: {:?} generated with parent_key: {:?}",
+                    response,
+                    tenant_token,
+                    key_content
+                );
+                assert_eq!(
+                    code, 403,
+                    "{} using tenant_token: {:?} generated with parent_key: {:?}",
+                    response, tenant_token, key_content
+                );
             }
         }
     };

meilisearch/tests/search/mod.rs

@@ -132,6 +132,79 @@ static NESTED_DOCUMENTS: Lazy<Value> = Lazy::new(|| {
     ])
 });
 
+static FRUITS_DOCUMENTS: Lazy<Value> = Lazy::new(|| {
+    json!([
+        {
+            "name": "Exclusive sale: green apple",
+            "id": "green-apple-boosted",
+            "BOOST": true
+        },
+        {
+            "name": "Pear",
+            "id": "pear",
+        },
+        {
+            "name": "Red apple gala",
+            "id": "red-apple-gala",
+        },
+        {
+            "name": "Exclusive sale: Red Tomato",
+            "id": "red-tomatoes-boosted",
+            "BOOST": true
+        },
+        {
+            "name": "Exclusive sale: Red delicious apple",
+            "id": "red-delicious-boosted",
+            "BOOST": true,
+        }
+    ])
+});
+
+static VECTOR_DOCUMENTS: Lazy<Value> = Lazy::new(|| {
+    json!([
+      {
+        "id": "A",
+        "description": "the dog barks at the cat",
+        "_vectors": {
+          // dimensions [canine, feline, young]
+          "animal": [0.9, 0.8, 0.05],
+          // dimensions [negative/positive, energy]
+          "sentiment": [-0.1, 0.55]
+        }
+      },
+      {
+        "id": "B",
+        "description": "the kitten scratched the beagle",
+        "_vectors": {
+          // dimensions [canine, feline, young]
+          "animal": [0.8, 0.9, 0.5],
+          // dimensions [negative/positive, energy]
+          "sentiment": [-0.2, 0.65]
+        }
+      },
+      {
+        "id": "C",
+        "description": "the dog had to stay alone today",
+        "_vectors": {
+          // dimensions [canine, feline, young]
+          "animal": [0.85, 0.02, 0.1],
+          // dimensions [negative/positive, energy]
+          "sentiment": [-1.0, 0.1]
+        }
+      },
+      {
+        "id": "D",
+        "description": "the little boy pets the puppy",
+        "_vectors": {
+          // dimensions [canine, feline, young]
+          "animal": [0.8, 0.09, 0.8],
+          // dimensions [negative/positive, energy]
+          "sentiment": [0.8, 0.3]
+        }
+      },
+    ])
+});
+
 #[actix_rt::test]
 async fn simple_placeholder_search() {
     let server = Server::new().await;

milli/src/score_details.rs

@@ -425,9 +425,6 @@ pub struct Sort {
 
 impl PartialOrd for Sort {
     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
-        if self.field_name != other.field_name {
-            return None;
-        }
         if self.ascending != other.ascending {
             return None;
         }
@@ -466,9 +463,6 @@ pub struct GeoSort {
 
 impl PartialOrd for GeoSort {
     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
-        if self.target_point != other.target_point {
-            return None;
-        }
         if self.ascending != other.ascending {
             return None;
         }

milli/src/search/new/matches/mod.rs

@@ -46,7 +46,7 @@ impl<'m> MatcherBuilder<'m> {
         self
     }
 
-    pub fn build<'t>(&'m self, text: &'t str) -> Matcher<'t, 'm> {
+    pub fn build<'t>(&self, text: &'t str) -> Matcher<'t, 'm, '_> {
         let crop_marker = match &self.crop_marker {
             Some(marker) => marker.as_str(),
             None => DEFAULT_CROP_MARKER,
@@ -105,19 +105,19 @@ pub struct MatchBounds {
     pub length: usize,
 }
 
-/// Structure used to analize a string, compute words that match,
+/// Structure used to analyze a string, compute words that match,
 /// and format the source string, returning a highlighted and cropped sub-string.
-pub struct Matcher<'t, 'm> {
+pub struct Matcher<'t, 'tokenizer, 'b> {
     text: &'t str,
-    matching_words: &'m MatchingWords,
+    matching_words: &'b MatchingWords,
-    tokenizer: &'m Tokenizer<'m>,
+    tokenizer: &'b Tokenizer<'tokenizer>,
-    crop_marker: &'m str,
+    crop_marker: &'b str,
-    highlight_prefix: &'m str,
+    highlight_prefix: &'b str,
-    highlight_suffix: &'m str,
+    highlight_suffix: &'b str,
     matches: Option<(Vec<Token<'t>>, Vec<Match>)>,
 }
 
-impl<'t> Matcher<'t, '_> {
+impl<'t, 'tokenizer> Matcher<'t, 'tokenizer, '_> {
     /// Iterates over tokens and save any of them that matches the query.
     fn compute_matches(&mut self) -> &mut Self {
         /// some words are counted as matches only if they are close together and in the good order,