Introduce a function to retrieve the facet level range docids

src/heed_codec/facet/facet_level_value_f64_codec.rs

@@ -0,0 +1,62 @@
+use std::borrow::Cow;
+use std::convert::TryInto;
+
+use crate::facet::value_encoding::f64_into_bytes;
+
+// TODO do not de/serialize right bound when level = 0
+pub struct FacetLevelValueF64Codec;
+
+impl<'a> heed::BytesDecode<'a> for FacetLevelValueF64Codec {
+    type DItem = (u8, u8, f64, f64);
+
+    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
+        let (field_id, bytes) = bytes.split_first()?;
+        let (level, bytes) = bytes.split_first()?;
+
+        let left = bytes[16..24].try_into().ok().map(f64::from_be_bytes)?;
+        let right = bytes[24..].try_into().ok().map(f64::from_be_bytes)?;
+
+        Some((*field_id, *level, left, right))
+    }
+}
+
+impl heed::BytesEncode<'_> for FacetLevelValueF64Codec {
+    type EItem = (u8, u8, f64, f64);
+
+    fn bytes_encode((field_id, level, left, right): &Self::EItem) -> Option<Cow<[u8]>> {
+        let mut buffer = [0u8; 32];
+
+        // Write the globally ordered floats.
+        let bytes = f64_into_bytes(*left)?;
+        buffer[..8].copy_from_slice(&bytes[..]);
+
+        let bytes = f64_into_bytes(*right)?;
+        buffer[8..16].copy_from_slice(&bytes[..]);
+
+        // Then the f64 values just to be able to read them back.
+        let bytes = left.to_be_bytes();
+        buffer[16..24].copy_from_slice(&bytes[..]);
+
+        let bytes = right.to_be_bytes();
+        buffer[24..].copy_from_slice(&bytes[..]);
+
+        let mut bytes = Vec::with_capacity(buffer.len() + 2);
+        bytes.push(*field_id);
+        bytes.push(*level);
+        bytes.extend_from_slice(&buffer[..]);
+        Some(Cow::Owned(bytes))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use heed::{BytesEncode, BytesDecode};
+    use super::*;
+
+    #[test]
+    fn globally_ordered_f64() {
+        let bytes = FacetLevelValueF64Codec::bytes_encode(&(3, 0, -32.0, 32.0)).unwrap();
+        let (name, level, left, right) = FacetLevelValueF64Codec::bytes_decode(&bytes).unwrap();
+        assert_eq!((name, level, left, right), (3, 0, -32.0, 32.0));
+    }
+}

src/heed_codec/facet/facet_level_value_i64_codec.rs

@@ -0,0 +1,43 @@
+use std::borrow::Cow;
+use std::convert::TryInto;
+
+use crate::facet::value_encoding::{i64_from_bytes, i64_into_bytes};
+
+pub struct FacetLevelValueI64Codec;
+
+impl<'a> heed::BytesDecode<'a> for FacetLevelValueI64Codec {
+    type DItem = (u8, u8, i64, i64);
+
+    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
+        let (field_id, bytes) = bytes.split_first()?;
+        let (level, bytes) = bytes.split_first()?;
+
+        let left = bytes[..8].try_into().map(i64_from_bytes).ok()?;
+        let right = if *level != 0 {
+            bytes[8..].try_into().map(i64_from_bytes).ok()?
+        } else {
+            left
+        };
+
+        Some((*field_id, *level, left, right))
+    }
+}
+
+impl heed::BytesEncode<'_> for FacetLevelValueI64Codec {
+    type EItem = (u8, u8, i64, i64);
+
+    fn bytes_encode((field_id, level, left, right): &Self::EItem) -> Option<Cow<[u8]>> {
+        let left = i64_into_bytes(*left);
+        let right = i64_into_bytes(*right);
+
+        let mut bytes = Vec::with_capacity(2 + left.len() + right.len());
+        bytes.push(*field_id);
+        bytes.push(*level);
+        bytes.extend_from_slice(&left[..]);
+        if *level != 0 {
+            bytes.extend_from_slice(&right[..]);
+        }
+
+        Some(Cow::Owned(bytes))
+    }
+}

src/heed_codec/facet/facet_value_f64_codec.rs

@@ -1,50 +0,0 @@
-use std::borrow::Cow;
-use std::convert::TryInto;
-
-use crate::facet::value_encoding::f64_into_bytes;
-
-pub struct FacetValueF64Codec;
-
-impl<'a> heed::BytesDecode<'a> for FacetValueF64Codec {
-    type DItem = (u8, f64);
-
-    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
-        let (field_id, buffer) = bytes.split_first()?;
-        let value = buffer[8..].try_into().ok().map(f64::from_be_bytes)?;
-        Some((*field_id, value))
-    }
-}
-
-impl heed::BytesEncode<'_> for FacetValueF64Codec {
-    type EItem = (u8, f64);
-
-    fn bytes_encode((field_id, value): &Self::EItem) -> Option<Cow<[u8]>> {
-        let mut buffer = [0u8; 16];
-
-        // Write the globally ordered float.
-        let bytes = f64_into_bytes(*value)?;
-        buffer[..8].copy_from_slice(&bytes[..]);
-
-        // Then the f64 value just to be able to read it back.
-        let bytes = value.to_be_bytes();
-        buffer[8..].copy_from_slice(&bytes[..]);
-
-        let mut bytes = Vec::with_capacity(buffer.len() + 1);
-        bytes.push(*field_id);
-        bytes.extend_from_slice(&buffer[..]);
-        Some(Cow::Owned(bytes))
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use heed::{BytesEncode, BytesDecode};
-    use super::*;
-
-    #[test]
-    fn globally_ordered_f64() {
-        let bytes = FacetValueF64Codec::bytes_encode(&(3, -32.0)).unwrap();
-        let (name, value) = FacetValueF64Codec::bytes_decode(&bytes).unwrap();
-        assert_eq!((name, value), (3, -32.0));
-    }
-}

src/heed_codec/facet/facet_value_i64_codec.rs

@@ -1,28 +0,0 @@
-use std::borrow::Cow;
-use std::convert::TryInto;
-
-use crate::facet::value_encoding::{i64_from_bytes, i64_into_bytes};
-
-pub struct FacetValueI64Codec;
-
-impl<'a> heed::BytesDecode<'a> for FacetValueI64Codec {
-    type DItem = (u8, i64);
-
-    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
-        let (field_id, buffer) = bytes.split_first()?;
-        let value = buffer.try_into().map(i64_from_bytes).ok()?;
-        Some((*field_id, value))
-    }
-}
-
-impl heed::BytesEncode<'_> for FacetValueI64Codec {
-    type EItem = (u8, i64);
-
-    fn bytes_encode((field_id, value): &Self::EItem) -> Option<Cow<[u8]>> {
-        let value = i64_into_bytes(*value);
-        let mut bytes = Vec::with_capacity(value.len() + 1);
-        bytes.push(*field_id);
-        bytes.extend_from_slice(&value[..]);
-        Some(Cow::Owned(bytes))
-    }
-}

src/heed_codec/facet/mod.rs

@@ -1,7 +1,7 @@
-mod facet_value_f64_codec;
-mod facet_value_i64_codec;
+mod facet_level_value_f64_codec;
+mod facet_level_value_i64_codec;
 mod facet_value_string_codec;
 
-pub use self::facet_value_f64_codec::FacetValueF64Codec;
-pub use self::facet_value_i64_codec::FacetValueI64Codec;
+pub use self::facet_level_value_f64_codec::FacetLevelValueF64Codec;
+pub use self::facet_level_value_i64_codec::FacetLevelValueI64Codec;
 pub use self::facet_value_string_codec::FacetValueStringCodec;

src/search.rs

@@ -4,6 +4,7 @@ use std::fmt;
 
 use anyhow::{bail, ensure, Context};
 use fst::{IntoStreamer, Streamer};
+use heed::types::DecodeIgnore;
 use levenshtein_automata::DFA;
 use levenshtein_automata::LevenshteinAutomatonBuilder as LevBuilder;
 use log::debug;
@@ -11,7 +12,7 @@ use once_cell::sync::Lazy;
 use roaring::bitmap::RoaringBitmap;
 
 use crate::facet::FacetType;
-use crate::heed_codec::{CboRoaringBitmapCodec, facet::FacetValueI64Codec};
+use crate::heed_codec::{facet::FacetLevelValueI64Codec, CboRoaringBitmapCodec};
 use crate::mdfs::Mdfs;
 use crate::query_tokens::{QueryTokens, QueryToken};
 use crate::{Index, DocumentId};
@@ -238,29 +239,105 @@ impl<'a> Search<'a> {
         // We create the original candidates with the facet conditions results.
         let facet_candidates = match self.facet_condition {
             Some(FacetCondition::Operator(fid, operator)) => {
-                use std::ops::Bound::{Included, Excluded};
+                use std::ops::Bound::{self, Included, Excluded};
                 use FacetOperator::*;
-                // Make sure we always bound the ranges with the field id, as the facets
-                // values are all in the same database and prefixed by the field id.
-                let range = match operator {
-                    GreaterThan(val) => (Excluded((fid, val)), Included((fid, i64::MAX))),
-                    GreaterThanOrEqual(val) => (Included((fid, val)), Included((fid, i64::MAX))),
-                    LowerThan(val) => (Included((fid, i64::MIN)), Excluded((fid, val))),
-                    LowerThanOrEqual(val) => (Included((fid, i64::MIN)), Included((fid, val))),
-                    Equal(val) => (Included((fid, val)), Included((fid, val))),
-                    Between(left, right) => (Included((fid, left)), Included((fid, right))),
-                };
 
-                let mut candidates = RoaringBitmap::new();
+                fn explore_facet_levels(
+                    rtxn: &heed::RoTxn,
+                    db: &heed::Database<FacetLevelValueI64Codec, CboRoaringBitmapCodec>,
+                    field_id: u8,
+                    level: u8,
+                    left: Bound<i64>,
+                    right: Bound<i64>,
+                    candidates: &mut RoaringBitmap,
+                ) -> anyhow::Result<()>
+                {
+                    let mut left_found = left;
+                    let mut right_found = right;
 
-                let db = self.index.facet_field_id_value_docids;
-                let db = db.remap_types::<FacetValueI64Codec, CboRoaringBitmapCodec>();
-                for result in db.range(self.rtxn, &range)? {
-                    let ((_fid, _value), docids) = result?;
-                    candidates.union_with(&docids);
+                    let range = {
+                        let left = match left {
+                            Included(left) => Included((field_id, level, left, i64::MIN)),
+                            Excluded(left) => Excluded((field_id, level, left, i64::MIN)),
+                            Bound::Unbounded => Bound::Unbounded,
+                        };
+                        let right = Included((field_id, level, i64::MAX, i64::MAX));
+                        (left, right)
+                    };
+
+                    for (i, result) in db.range(rtxn, &range)?.enumerate() {
+                        let ((_fid, _level, l, r), docids) = result?;
+                        match right {
+                            Included(right) if r > right => break,
+                            Excluded(right) if r >= right => break,
+                            _ => (),
+                        }
+
+                        eprintln!("{} to {} (level {})", l, r, _level);
+                        candidates.union_with(&docids);
+                        // We save the leftest and rightest bounds we actually found at this level.
+                        if i == 0 { left_found = Excluded(l); }
+                        right_found = Excluded(r);
+                    }
+
+                    // Can we go deeper?
+                    let deeper_level = match level.checked_sub(1) {
+                        Some(level) => level,
+                        None => return Ok(()),
+                    };
+
+                    // We must refine the left and right bounds of this range by retrieving the
+                    // missing part in a deeper level.
+                    // TODO we must avoid going at deeper when the bounds are already satisfied.
+                    explore_facet_levels(rtxn, db, field_id, deeper_level, left, left_found, candidates)?;
+                    explore_facet_levels(rtxn, db, field_id, deeper_level, right_found, right, candidates)?;
+
+                    Ok(())
                 }
 
-                Some(candidates)
+                // Make sure we always bound the ranges with the field id, as the facets
+                // values are all in the same database and prefixed by the field id.
+                let (left, right) = match operator {
+                    GreaterThan(val)        => (Excluded(val),      Included(i64::MAX)),
+                    GreaterThanOrEqual(val) => (Included(val),      Included(i64::MAX)),
+                    LowerThan(val)          => (Included(i64::MIN), Excluded(val)),
+                    LowerThanOrEqual(val)   => (Included(i64::MIN), Included(val)),
+                    Equal(val)              => (Included(val),      Included(val)),
+                    Between(left, right)    => (Included(left),     Included(right)),
+                };
+
+                let db = self.index
+                    .facet_field_id_value_docids
+                    .remap_key_type::<FacetLevelValueI64Codec>();
+
+                let biggest_level = match fid.checked_add(1) {
+                    Some(next_fid) => {
+                        // If we are able to find the next field id we ask the key that is before
+                        // the first entry of it which corresponds to the last key of our field id.
+                        let db = db.remap_data_type::<DecodeIgnore>();
+                        match db.get_lower_than(self.rtxn, &(next_fid, 0, i64::MIN, i64::MIN))? {
+                            Some(((id, level, _left, _right), _docids)) if fid == id => Some(level),
+                            _ => None,
+                        }
+                    },
+                    None => {
+                        // If we can't generate a bigger field id, it must be equal to 255 and
+                        // therefore the last key of the database must be the one we want.
+                        match db.remap_data_type::<DecodeIgnore>().last(self.rtxn)? {
+                            Some(((id, level, _left, _right), _docids)) if fid == id => Some(level),
+                            _ => None,
+                        }
+                    },
+                };
+
+                match biggest_level {
+                    Some(level) => {
+                        let mut candidates = RoaringBitmap::new();
+                        explore_facet_levels(self.rtxn, &db, fid, level, left, right, &mut candidates)?;
+                        Some(candidates)
+                    },
+                    None => None,
+                }
             },
             None => None,
         };

src/subcommand/infos.rs

@@ -2,9 +2,9 @@ use std::path::PathBuf;
 use std::{str, io};
 
 use anyhow::Context;
-use crate::Index;
 use heed::EnvOpenOptions;
 use structopt::StructOpt;
+use crate::Index;
 
 use Command::*;
 
@@ -225,12 +225,52 @@ fn most_common_words(index: &Index, rtxn: &heed::RoTxn, limit: usize) -> anyhow:
     Ok(wtr.flush()?)
 }
 
+fn facet_values_iter<'txn, DC: 'txn>(
+    rtxn: &'txn heed::RoTxn,
+    db: heed::Database<heed::types::ByteSlice, DC>,
+    field_id: u8,
+    facet_type: crate::facet::FacetType,
+) -> heed::Result<Box<dyn Iterator<Item=heed::Result<(String, DC::DItem)>> + 'txn>>
+where
+    DC: heed::BytesDecode<'txn>,
+{
+    use crate::facet::FacetType;
+    use crate::heed_codec::facet::{
+        FacetValueStringCodec, FacetLevelValueF64Codec, FacetLevelValueI64Codec,
+    };
+
+    let iter = db.prefix_iter(&rtxn, &[field_id])?;
+    match facet_type {
+        FacetType::String => {
+            let iter = iter.remap_key_type::<FacetValueStringCodec>()
+                .map(|r| r.map(|((_, key), value)| (key.to_string(), value)));
+            Ok(Box::new(iter) as Box<dyn Iterator<Item=_>>)
+        },
+        FacetType::Float => {
+            let iter = iter.remap_key_type::<FacetLevelValueF64Codec>()
+                .map(|r| r.map(|((_, level, left, right), value)| if level == 0 {
+                    (format!("{} (level {})", left, level), value)
+                } else {
+                    (format!("{} to {} (level {})", left, right, level), value)
+                }));
+            Ok(Box::new(iter))
+        },
+        FacetType::Integer => {
+            let iter = iter.remap_key_type::<FacetLevelValueI64Codec>()
+                .map(|r| r.map(|((_, level, left, right), value)| if level == 0 {
+                    (format!("{} (level {})", left, level), value)
+                } else {
+                    (format!("{} to {} (level {})", left, right, level), value)
+                }));
+            Ok(Box::new(iter))
+        },
+    }
+}
+
 fn biggest_value_sizes(index: &Index, rtxn: &heed::RoTxn, limit: usize) -> anyhow::Result<()> {
     use std::cmp::Reverse;
     use std::collections::BinaryHeap;
     use heed::types::{Str, ByteSlice};
-    use crate::facet::FacetType;
-    use crate::heed_codec::facet::{FacetValueStringCodec, FacetValueF64Codec, FacetValueI64Codec};
 
     let Index {
         env: _env,
@@ -285,25 +325,9 @@ fn biggest_value_sizes(index: &Index, rtxn: &heed::RoTxn, limit: usize) -> anyho
         let fields_ids_map = index.fields_ids_map(rtxn)?;
         for (field_id, field_type) in faceted_fields {
             let facet_name = fields_ids_map.name(field_id).unwrap();
-            let iter = facet_field_id_value_docids.prefix_iter(&rtxn, &[field_id])?;
-            let iter = match field_type {
-                FacetType::String => {
-                    let iter = iter.remap_types::<FacetValueStringCodec, ByteSlice>()
-                        .map(|r| r.map(|((_, k), v)| (k.to_string(), v)));
-                    Box::new(iter) as Box<dyn Iterator<Item=_>>
-                },
-                FacetType::Float => {
-                    let iter = iter.remap_types::<FacetValueF64Codec, ByteSlice>()
-                        .map(|r| r.map(|((_, k), v)| (k.to_string(), v)));
-                    Box::new(iter)
-                },
-                FacetType::Integer => {
-                    let iter = iter.remap_types::<FacetValueI64Codec, ByteSlice>()
-                        .map(|r| r.map(|((_, k), v)| (k.to_string(), v)));
-                    Box::new(iter)
-                },
-            };
-            for result in iter {
+
+            let db = facet_field_id_value_docids.remap_data_type::<ByteSlice>();
+            for result in facet_values_iter(rtxn, db, field_id, field_type)? {
                 let (fvalue, value) = result?;
                 let key = format!("{} {}", facet_name, fvalue);
                 heap.push(Reverse((value.len(), key, facet_field_id_value_docids_name)));
@@ -349,10 +373,6 @@ fn words_docids(index: &Index, rtxn: &heed::RoTxn, debug: bool, words: Vec<Strin
 }
 
 fn facet_values_docids(index: &Index, rtxn: &heed::RoTxn, debug: bool, field_name: String) -> anyhow::Result<()> {
-    use crate::facet::FacetType;
-    use crate::heed_codec::facet::{FacetValueStringCodec, FacetValueF64Codec, FacetValueI64Codec};
-    use heed::{BytesDecode, Error::Decoding};
-
     let fields_ids_map = index.fields_ids_map(&rtxn)?;
     let faceted_fields = index.faceted_fields(&rtxn)?;
 
@@ -361,39 +381,12 @@ fn facet_values_docids(index: &Index, rtxn: &heed::RoTxn, debug: bool, field_nam
     let field_type = faceted_fields.get(&field_id)
         .with_context(|| format!("field {} is not faceted", field_name))?;
 
-    let iter = index.facet_field_id_value_docids.prefix_iter(&rtxn, &[field_id])?;
-    let iter = match field_type {
-        FacetType::String => {
-            let iter = iter
-                .map(|result| result.and_then(|(key, value)| {
-                    let (_, key) = FacetValueStringCodec::bytes_decode(key).ok_or(Decoding)?;
-                    Ok((key.to_string(), value))
-                }));
-            Box::new(iter) as Box<dyn Iterator<Item=_>>
-        },
-        FacetType::Float => {
-            let iter = iter
-                .map(|result| result.and_then(|(key, value)| {
-                    let (_, key) = FacetValueF64Codec::bytes_decode(key).ok_or(Decoding)?;
-                    Ok((key.to_string(), value))
-                }));
-            Box::new(iter)
-        },
-        FacetType::Integer => {
-            let iter = iter
-                .map(|result| result.and_then(|(key, value)| {
-                    let (_, key) = FacetValueI64Codec::bytes_decode(key).ok_or(Decoding)?;
-                    Ok((key.to_string(), value))
-                }));
-            Box::new(iter)
-        },
-    };
-
     let stdout = io::stdout();
     let mut wtr = csv::Writer::from_writer(stdout.lock());
     wtr.write_record(&["facet_value", "documents_ids"])?;
 
-    for result in iter {
+    let db = index.facet_field_id_value_docids;
+    for result in facet_values_iter(rtxn, db, field_id, *field_type)? {
         let (value, docids) = result?;
         let docids = if debug {
             format!("{:?}", docids)

src/update/index_documents/facet_level.rs

@@ -0,0 +1,125 @@
+use std::fs::File;
+
+use grenad::{CompressionType, Reader, Writer, FileFuse};
+use heed::types::{ByteSlice, DecodeIgnore};
+use heed::{BytesEncode, Error};
+use roaring::RoaringBitmap;
+
+use crate::facet::FacetType;
+use crate::heed_codec::{facet::FacetLevelValueI64Codec, CboRoaringBitmapCodec};
+use crate::update::index_documents::{create_writer, writer_into_reader};
+
+pub fn clear_field_levels(
+    wtxn: &mut heed::RwTxn,
+    db: heed::Database<ByteSlice, CboRoaringBitmapCodec>,
+    field_id: u8,
+) -> heed::Result<()>
+{
+    let range = (field_id, 1, i64::MIN, i64::MIN)..=(field_id, u8::MAX, i64::MAX, i64::MAX);
+    db.remap_key_type::<FacetLevelValueI64Codec>()
+        .delete_range(wtxn, &range)
+        .map(drop)
+}
+
+pub fn compute_facet_levels(
+    rtxn: &heed::RoTxn,
+    db: heed::Database<ByteSlice, CboRoaringBitmapCodec>,
+    compression_type: CompressionType,
+    compression_level: Option<u32>,
+    shrink_size: Option<u64>,
+    field_id: u8,
+    facet_type: FacetType,
+) -> anyhow::Result<Reader<FileFuse>>
+{
+    let last_level_size = 5;
+    let number_of_levels = 5;
+    let first_level_size = db.prefix_iter(rtxn, &[field_id])?
+        .remap_types::<DecodeIgnore, DecodeIgnore>()
+        .fold(Ok(0u64), |count, result| result.and(count).map(|c| c + 1))?;
+
+    // It is forbidden to keep a cursor and write in a database at the same time with LMDB
+    // therefore we write the facet levels entries into a grenad file before transfering them.
+    let mut writer = tempfile::tempfile().and_then(|file| {
+        create_writer(compression_type, compression_level, file)
+    })?;
+
+    let level_0_range = (field_id, 0, i64::MIN, i64::MIN)..=(field_id, 0, i64::MAX, i64::MAX);
+    let level_sizes_iter = levels_iterator(first_level_size, last_level_size, number_of_levels)
+        .enumerate()
+        .skip(1);
+
+    // TODO we must not create levels with identical group sizes.
+    for (level, size) in level_sizes_iter {
+        let level_entry_sizes = (first_level_size as f64 / size as f64).ceil() as usize;
+        let mut left = 0;
+        let mut right = 0;
+        let mut group_docids = RoaringBitmap::new();
+
+        let db = db.remap_key_type::<FacetLevelValueI64Codec>();
+        for (i, result) in db.range(rtxn, &level_0_range)?.enumerate() {
+            let ((_field_id, _level, value, _right), docids) = result?;
+
+            if i == 0 {
+                left = value;
+            } else if i % level_entry_sizes == 0 {
+                // we found the first bound of the next group, we must store the left
+                // and right bounds associated with the docids.
+                write_entry(&mut writer, field_id, level as u8, left, right, &group_docids)?;
+
+                // We save the left bound for the new group and also reset the docids.
+                group_docids = RoaringBitmap::new();
+                left = value;
+            }
+
+            // The right bound is always the bound we run through.
+            group_docids.union_with(&docids);
+            right = value;
+        }
+
+        if !group_docids.is_empty() {
+            write_entry(&mut writer, field_id, level as u8, left, right, &group_docids)?;
+        }
+    }
+
+    writer_into_reader(writer, shrink_size)
+}
+
+fn write_entry(
+    writer: &mut Writer<File>,
+    field_id: u8,
+    level: u8,
+    left: i64,
+    right: i64,
+    ids: &RoaringBitmap,
+) -> anyhow::Result<()>
+{
+    let key = (field_id, level, left, right);
+    let key = FacetLevelValueI64Codec::bytes_encode(&key).ok_or(Error::Encoding)?;
+    let data = CboRoaringBitmapCodec::bytes_encode(&ids).ok_or(Error::Encoding)?;
+    writer.insert(&key, &data)?;
+    Ok(())
+}
+
+fn levels_iterator(
+    first_level_size: u64, // biggest level
+    last_level_size: u64, // smallest level
+    number_of_levels: u64,
+) -> impl Iterator<Item=u64>
+{
+    // Go look at the function definitions here:
+    // https://docs.rs/easer/0.2.1/easer/index.html
+    // https://easings.net/#easeOutExpo
+    fn ease_out_expo(t: f64, b: f64, c: f64, d: f64) -> f64 {
+        if t == d {
+            b + c
+        } else {
+            c * (-2.0_f64.powf(-10.0 * t / d) + 1.0) + b
+        }
+    }
+
+    let b = last_level_size as f64;
+    let end = first_level_size as f64;
+    let c = end - b;
+    let d = number_of_levels;
+    (0..=d).map(move |t| ((end + b) - ease_out_expo(t as f64, b, c, d as f64)) as u64)
+}

src/update/index_documents/mod.rs

@@ -14,6 +14,7 @@ use memmap::Mmap;
 use rayon::prelude::*;
 use rayon::ThreadPool;
 
+use crate::facet::FacetType;
 use crate::index::Index;
 use crate::update::UpdateIndexingStep;
 use self::store::{Store, Readers};
@@ -22,10 +23,12 @@ use self::merge_function::{
     docid_word_positions_merge, documents_merge, facet_field_value_docids_merge,
 };
 pub use self::transform::{Transform, TransformOutput};
+pub use self::facet_level::{clear_field_levels, compute_facet_levels};
 
 use crate::MergeFn;
 use super::UpdateBuilder;
 
+mod facet_level;
 mod merge_function;
 mod store;
 mod transform;
@@ -327,7 +330,7 @@ impl<'t, 'u, 'i, 'a> IndexDocuments<'t, 'u, 'i, 'a> {
         enum DatabaseType {
             Main,
             WordDocids,
-            FacetValuesDocids,
+            FacetLevel0ValuesDocids,
         }
 
         let faceted_fields = self.index.faceted_fields(self.wtxn)?;
@@ -427,7 +430,7 @@ impl<'t, 'u, 'i, 'a> IndexDocuments<'t, 'u, 'i, 'a> {
                     (DatabaseType::Main, main_readers, main_merge as MergeFn),
                     (DatabaseType::WordDocids, word_docids_readers, word_docids_merge),
                     (
-                        DatabaseType::FacetValuesDocids,
+                        DatabaseType::FacetLevel0ValuesDocids,
                         facet_field_value_docids_readers,
                         facet_field_value_docids_merge,
                     ),
@@ -475,6 +478,9 @@ impl<'t, 'u, 'i, 'a> IndexDocuments<'t, 'u, 'i, 'a> {
         // We write the external documents ids into the main database.
         self.index.put_external_documents_ids(self.wtxn, &external_documents_ids)?;
 
+        // We get the faceted fields to be able to create the facet levels.
+        let faceted_fields = self.index.faceted_fields(self.wtxn)?;
+
         // We merge the new documents ids with the existing ones.
         documents_ids.union_with(&new_documents_ids);
         documents_ids.union_with(&replaced_documents_ids);
@@ -557,7 +563,7 @@ impl<'t, 'u, 'i, 'a> IndexDocuments<'t, 'u, 'i, 'a> {
                         write_method,
                     )?;
                 },
-                DatabaseType::FacetValuesDocids => {
+                DatabaseType::FacetLevel0ValuesDocids => {
                     debug!("Writing the facet values docids into LMDB on disk...");
                     let db = *self.index.facet_field_id_value_docids.as_polymorph();
                     write_into_lmdb_database(
@@ -577,6 +583,35 @@ impl<'t, 'u, 'i, 'a> IndexDocuments<'t, 'u, 'i, 'a> {
             });
         }
 
+        debug!("Computing and writing the facet values levels docids into LMDB on disk...");
+        for (field_id, facet_type) in faceted_fields {
+            if facet_type == FacetType::String { continue }
+
+            clear_field_levels(
+                self.wtxn,
+                self.index.facet_field_id_value_docids,
+                field_id,
+            )?;
+
+            let content = compute_facet_levels(
+                self.wtxn,
+                self.index.facet_field_id_value_docids,
+                chunk_compression_type,
+                chunk_compression_level,
+                chunk_fusing_shrink_size,
+                field_id,
+                facet_type,
+            )?;
+
+            write_into_lmdb_database(
+                self.wtxn,
+                *self.index.facet_field_id_value_docids.as_polymorph(),
+                content,
+                |_, _| anyhow::bail!("invalid facet level merging"),
+                WriteMethod::GetMergePut,
+            )?;
+        }
+
         debug_assert_eq!(database_count, total_databases);
 
         info!("Transform output indexed in {:.02?}", before_indexing.elapsed());

src/update/index_documents/store.rs

@@ -19,7 +19,7 @@ use tempfile::tempfile;
 
 use crate::facet::FacetType;
 use crate::heed_codec::{BoRoaringBitmapCodec, CboRoaringBitmapCodec};
-use crate::heed_codec::facet::{FacetValueStringCodec, FacetValueF64Codec, FacetValueI64Codec};
+use crate::heed_codec::facet::{FacetValueStringCodec, FacetLevelValueF64Codec, FacetLevelValueI64Codec};
 use crate::tokenizer::{simple_tokenizer, only_token};
 use crate::update::UpdateIndexingStep;
 use crate::{json_to_string, SmallVec8, SmallVec32, SmallString32, Position, DocumentId};
@@ -337,8 +337,8 @@ impl Store {
         for ((field_id, value), docids) in iter {
             let result = match value {
                 String(s) => FacetValueStringCodec::bytes_encode(&(field_id, &s)).map(Cow::into_owned),
-                Float(f) => FacetValueF64Codec::bytes_encode(&(field_id, *f)).map(Cow::into_owned),
-                Integer(i) => FacetValueI64Codec::bytes_encode(&(field_id, i)).map(Cow::into_owned),
+                Float(f) => FacetLevelValueF64Codec::bytes_encode(&(field_id, 0, *f, *f)).map(Cow::into_owned),
+                Integer(i) => FacetLevelValueI64Codec::bytes_encode(&(field_id, 0, i, i)).map(Cow::into_owned),
             };
             let key = result.context("could not serialize facet key")?;
             let bytes = CboRoaringBitmapCodec::bytes_encode(&docids)

src/update/settings.rs

@@ -412,7 +412,8 @@ mod tests {
         let rtxn = index.read_txn().unwrap();
         let fields_ids = index.faceted_fields(&rtxn).unwrap();
         assert_eq!(fields_ids, hashmap!{ 1 => FacetType::Integer });
-        let count = index.facet_field_id_value_docids.len(&rtxn).unwrap();
+        // Only count the field_id 0 and level 0 facet values.
+        let count = index.facet_field_id_value_docids.prefix_iter(&rtxn, &[1, 0]).unwrap().count();
         assert_eq!(count, 3);
         drop(rtxn);
 
@@ -425,7 +426,8 @@ mod tests {
         wtxn.commit().unwrap();
 
         let rtxn = index.read_txn().unwrap();
-        let count = index.facet_field_id_value_docids.len(&rtxn).unwrap();
+        // Only count the field_id 0 and level 0 facet values.
+        let count = index.facet_field_id_value_docids.prefix_iter(&rtxn, &[1, 0]).unwrap().count();
         assert_eq!(count, 4);
         drop(rtxn);
     }