use std::borrow::Cow;
use std::collections::{HashMap, HashSet};
use std::fmt;
use std::time::Instant;

use anyhow::{bail, Context};
use fst::{IntoStreamer, Streamer, Set};
use levenshtein_automata::DFA;
use levenshtein_automata::LevenshteinAutomatonBuilder as LevBuilder;
use log::debug;
use meilisearch_tokenizer::{AnalyzerConfig, Analyzer};
use once_cell::sync::Lazy;
use ordered_float::OrderedFloat;
use roaring::bitmap::RoaringBitmap;

use crate::facet::FacetType;
use crate::heed_codec::facet::{FacetLevelValueF64Codec, FacetLevelValueI64Codec};
use crate::heed_codec::facet::{FieldDocIdFacetF64Codec, FieldDocIdFacetI64Codec};
use crate::mdfs::Mdfs;
use crate::query_tokens::{query_tokens, QueryToken};
use crate::search::criteria::Criterion;
use crate::search::criteria::typo::Typo;
use crate::{Index, FieldId, DocumentId};

pub use self::facet::{FacetCondition, FacetDistribution, FacetNumberOperator, FacetStringOperator};
pub use self::facet::{FacetIter};
use self::query_tree::QueryTreeBuilder;

// Building these factories is not free.
static LEVDIST0: Lazy<LevBuilder> = Lazy::new(|| LevBuilder::new(0, true));
static LEVDIST1: Lazy<LevBuilder> = Lazy::new(|| LevBuilder::new(1, true));
static LEVDIST2: Lazy<LevBuilder> = Lazy::new(|| LevBuilder::new(2, true));

mod facet;
mod query_tree;
mod criteria;

pub struct Search<'a> {
    query: Option<String>,
    facet_condition: Option<FacetCondition>,
    offset: usize,
    limit: usize,
    rtxn: &'a heed::RoTxn<'a>,
    index: &'a Index,
}

impl<'a> Search<'a> {
    pub fn new(rtxn: &'a heed::RoTxn, index: &'a Index) -> Search<'a> {
        Search { query: None, facet_condition: None, offset: 0, limit: 20, rtxn, index }
    }

    pub fn query(&mut self, query: impl Into<String>) -> &mut Search<'a> {
        self.query = Some(query.into());
        self
    }

    pub fn offset(&mut self, offset: usize) -> &mut Search<'a> {
        self.offset = offset;
        self
    }

    pub fn limit(&mut self, limit: usize) -> &mut Search<'a> {
        self.limit = limit;
        self
    }

    pub fn facet_condition(&mut self, condition: FacetCondition) -> &mut Search<'a> {
        self.facet_condition = Some(condition);
        self
    }

    /// Extracts the query words from the query string and returns the DFAs accordingly.
    /// TODO introduce settings for the number of typos regarding the words lengths.
    fn generate_query_dfas(query: &str) -> Vec<(String, bool, DFA)> {
        let (lev0, lev1, lev2) = (&LEVDIST0, &LEVDIST1, &LEVDIST2);

        let stop_words = Set::default();
        let analyzer = Analyzer::new(AnalyzerConfig::default_with_stopwords(&stop_words));
        let analyzed = analyzer.analyze(query);
        let tokens = analyzed.tokens();
        let words: Vec<_> = query_tokens(tokens).collect();

        let ends_with_whitespace = query.chars().last().map_or(false, char::is_whitespace);
        let number_of_words = words.len();

        words.into_iter().enumerate().map(|(i, word)| {
            let (word, quoted) = match word {
                QueryToken::Free(token) => (token.text().to_string(), token.text().len() <= 3),
                QueryToken::Quoted(token) => (token.text().to_string(), true),
            };
            let is_last = i + 1 == number_of_words;
            let is_prefix = is_last && !ends_with_whitespace && !quoted;
            let lev = match word.len() {
                0..=4 => if quoted { lev0 } else { lev0 },
                5..=8 => if quoted { lev0 } else { lev1 },
                _     => if quoted { lev0 } else { lev2 },
            };

            let dfa = if is_prefix {
                lev.build_prefix_dfa(&word)
            } else {
                lev.build_dfa(&word)
            };

            (word, is_prefix, dfa)
        })
        .collect()
    }

    /// Fetch the words from the given FST related to the given DFAs along with
    /// the associated documents ids.
    fn fetch_words_docids(
        &self,
        fst: &fst::Set<Cow<[u8]>>,
        dfas: Vec<(String, bool, DFA)>,
    ) -> anyhow::Result<Vec<(HashMap<String, (u8, RoaringBitmap)>, RoaringBitmap)>>
    {
        // A Vec storing all the derived words from the original query words, associated
        // with the distance from the original word and the docids where the words appears.
        let mut derived_words = Vec::<(HashMap::<String, (u8, RoaringBitmap)>, RoaringBitmap)>::with_capacity(dfas.len());

        for (_word, _is_prefix, dfa) in dfas {

            let mut acc_derived_words = HashMap::new();
            let mut unions_docids = RoaringBitmap::new();
            let mut stream = fst.search_with_state(&dfa).into_stream();
            while let Some((word, state)) = stream.next() {

                let word = std::str::from_utf8(word)?;
                let docids = self.index.word_docids.get(self.rtxn, word)?.unwrap();
                let distance = dfa.distance(state);
                unions_docids.union_with(&docids);
                acc_derived_words.insert(word.to_string(), (distance.to_u8(), docids));
            }
            derived_words.push((acc_derived_words, unions_docids));
        }

        Ok(derived_words)
    }

    /// Returns the set of docids that contains all of the query words.
    fn compute_candidates(
        derived_words: &[(HashMap<String, (u8, RoaringBitmap)>, RoaringBitmap)],
    ) -> RoaringBitmap
    {
        // We sort the derived words by inverse popularity, this way intersections are faster.
        let mut derived_words: Vec<_> = derived_words.iter().collect();
        derived_words.sort_unstable_by_key(|(_, docids)| docids.len());

        // we do a union between all the docids of each of the derived words,
        // we got N unions (the number of original query words), we then intersect them.
        let mut candidates = RoaringBitmap::new();

        for (i, (_, union_docids)) in derived_words.iter().enumerate() {
            if i == 0 {
                candidates = union_docids.clone();
            } else {
                candidates.intersect_with(&union_docids);
            }
        }

        candidates
    }

    fn facet_ordered(
        &self,
        field_id: FieldId,
        facet_type: FacetType,
        ascending: bool,
        mut documents_ids: RoaringBitmap,
        limit: usize,
    ) -> anyhow::Result<Vec<DocumentId>>
    {
        let mut output: Vec<_> = match facet_type {
            FacetType::Float => {
                if documents_ids.len() <= 1000 {
                    let db = self.index.field_id_docid_facet_values.remap_key_type::<FieldDocIdFacetF64Codec>();
                    let mut docids_values = Vec::with_capacity(documents_ids.len() as usize);
                    for docid in documents_ids.iter() {
                        let left = (field_id, docid, f64::MIN);
                        let right = (field_id, docid, f64::MAX);
                        let mut iter = db.range(self.rtxn, &(left..=right))?;
                        let entry = if ascending { iter.next() } else { iter.last() };
                        if let Some(((_, _, value), ())) = entry.transpose()? {
                            docids_values.push((docid, OrderedFloat(value)));
                        }
                    }
                    docids_values.sort_unstable_by_key(|(_, value)| *value);
                    let iter = docids_values.into_iter().map(|(id, _)| id);
                    if ascending {
                        iter.take(limit).collect()
                    } else {
                        iter.rev().take(limit).collect()
                    }
                } else {
                    let facet_fn = if ascending {
                        FacetIter::<f64, FacetLevelValueF64Codec>::new_reducing
                    } else {
                        FacetIter::<f64, FacetLevelValueF64Codec>::new_reverse_reducing
                    };
                    let mut limit_tmp = limit;
                    let mut output = Vec::new();
                    for result in facet_fn(self.rtxn, self.index, field_id, documents_ids.clone())? {
                        let (_val, docids) = result?;
                        limit_tmp = limit_tmp.saturating_sub(docids.len() as usize);
                        output.push(docids);
                        if limit_tmp == 0 { break }
                    }
                    output.into_iter().flatten().take(limit).collect()
                }
            },
            FacetType::Integer => {
                if documents_ids.len() <= 1000 {
                    let db = self.index.field_id_docid_facet_values.remap_key_type::<FieldDocIdFacetI64Codec>();
                    let mut docids_values = Vec::with_capacity(documents_ids.len() as usize);
                    for docid in documents_ids.iter() {
                        let left = (field_id, docid, i64::MIN);
                        let right = (field_id, docid, i64::MAX);
                        let mut iter = db.range(self.rtxn, &(left..=right))?;
                        let entry = if ascending { iter.next() } else { iter.last() };
                        if let Some(((_, _, value), ())) = entry.transpose()? {
                            docids_values.push((docid, value));
                        }
                    }
                    docids_values.sort_unstable_by_key(|(_, value)| *value);
                    let iter = docids_values.into_iter().map(|(id, _)| id);
                    if ascending {
                        iter.take(limit).collect()
                    } else {
                        iter.rev().take(limit).collect()
                    }
                } else {
                    let facet_fn = if ascending {
                        FacetIter::<i64, FacetLevelValueI64Codec>::new_reducing
                    } else {
                        FacetIter::<i64, FacetLevelValueI64Codec>::new_reverse_reducing
                    };
                    let mut limit_tmp = limit;
                    let mut output = Vec::new();
                    for result in facet_fn(self.rtxn, self.index, field_id, documents_ids.clone())? {
                        let (_val, docids) = result?;
                        limit_tmp = limit_tmp.saturating_sub(docids.len() as usize);
                        output.push(docids);
                        if limit_tmp == 0 { break }
                    }
                    output.into_iter().flatten().take(limit).collect()
                }
            },
            FacetType::String => bail!("criteria facet type must be a number"),
        };

        // if there isn't enough documents to return we try to complete that list
        // with documents that are maybe not faceted under this field and therefore
        // not returned by the previous facet iteration.
        if output.len() < limit {
            output.iter().for_each(|n| { documents_ids.remove(*n); });
            let remaining = documents_ids.iter().take(limit - output.len());
            output.extend(remaining);
        }

        Ok(output)
    }

    pub fn execute(&self) -> anyhow::Result<SearchResult> {
        // We create the query tree by spliting the query into tokens.
        let before = Instant::now();
        let query_tree = match self.query.as_ref() {
            Some(query) => {
                let builder = QueryTreeBuilder::new(self.rtxn, self.index);
                let stop_words = &Set::default();
                let analyzer = Analyzer::new(AnalyzerConfig::default_with_stopwords(stop_words));
                let result = analyzer.analyze(query);
                let tokens = result.tokens();
                builder.build(false, true, tokens)
            },
            None => None,
        };

        debug!("query tree: {:?} took {:.02?}", query_tree, before.elapsed());

        // We create the original candidates with the facet conditions results.
        let before = Instant::now();
        let facet_candidates = match &self.facet_condition {
            Some(condition) => Some(condition.evaluate(self.rtxn, self.index)?),
            None => None,
        };

        debug!("facet candidates: {:?} took {:.02?}", facet_candidates, before.elapsed());

        // We aretesting the typo criteria but there will be more of them soon.
        let mut criteria = Typo::initial(self.index, self.rtxn, query_tree, facet_candidates)?;

        let mut offset = self.offset;
        let mut limit = self.limit;
        let mut documents_ids = Vec::new();
        while let Some((_qt, docids)) = criteria.next()? {

            let mut len = docids.len() as usize;
            let mut docids = docids.into_iter();

            if offset != 0 {
                docids.by_ref().skip(offset).for_each(drop);
                offset = offset.saturating_sub(len.min(offset));
                len = len.saturating_sub(len.min(offset));
            }

            if len != 0 {
                documents_ids.extend(docids.take(limit));
                limit = limit.saturating_sub(len.min(limit));
            }

            if limit == 0 { break }
        }

        let found_words = HashSet::new();
        let candidates = RoaringBitmap::new();
        Ok(SearchResult { found_words, candidates, documents_ids })

        // let order_by_facet = {
        //     let criteria = self.index.criteria(self.rtxn)?;
        //     let result = criteria.into_iter().flat_map(|criterion| {
        //         match criterion {
        //             Criterion::Asc(fid) => Some((fid, true)),
        //             Criterion::Desc(fid) => Some((fid, false)),
        //             _ => None
        //         }
        //     }).next();
        //     match result {
        //         Some((attr_name, is_ascending)) => {
        //             let field_id_map = self.index.fields_ids_map(self.rtxn)?;
        //             let fid = field_id_map.id(&attr_name).with_context(|| format!("unknown field: {:?}", attr_name))?;
        //             let faceted_fields = self.index.faceted_fields_ids(self.rtxn)?;
        //             let ftype = *faceted_fields.get(&fid)
        //                 .with_context(|| format!("{:?} not found in the faceted fields.", attr_name))
        //                 .expect("corrupted data: ");
        //             Some((fid, ftype, is_ascending))
        //         },
        //         None => None,
        //     }
        // };

        // let before = Instant::now();
        // let (candidates, derived_words) = match (facet_candidates, derived_words) {
        //     (Some(mut facet_candidates), Some(derived_words)) => {
        //         let words_candidates = Self::compute_candidates(&derived_words);
        //         facet_candidates.intersect_with(&words_candidates);
        //         (facet_candidates, derived_words)
        //     },
        //     (None, Some(derived_words)) => {
        //         (Self::compute_candidates(&derived_words), derived_words)
        //     },
        //     (Some(facet_candidates), None) => {
        //         // If the query is not set or results in no DFAs but
        //         // there is some facet conditions we return a placeholder.
        //         let documents_ids = match order_by_facet {
        //             Some((fid, ftype, is_ascending)) => {
        //                 self.facet_ordered(fid, ftype, is_ascending, facet_candidates.clone(), limit)?
        //             },
        //             None => facet_candidates.iter().take(limit).collect(),
        //         };
        //         return Ok(SearchResult {
        //             documents_ids,
        //             candidates: facet_candidates,
        //             ..Default::default()
        //         })
        //     },
        //     (None, None) => {
        //         // If the query is not set or results in no DFAs we return a placeholder.
        //         let all_docids = self.index.documents_ids(self.rtxn)?;
        //         let documents_ids = match order_by_facet {
        //             Some((fid, ftype, is_ascending)) => {
        //                 self.facet_ordered(fid, ftype, is_ascending, all_docids.clone(), limit)?
        //             },
        //             None => all_docids.iter().take(limit).collect(),
        //         };
        //         return Ok(SearchResult { documents_ids, candidates: all_docids,..Default::default() })
        //     },
        // };

        // debug!("candidates: {:?} took {:.02?}", candidates, before.elapsed());

        // // The mana depth first search is a revised DFS that explore
        // // solutions in the order of their proximities.
        // let mut mdfs = Mdfs::new(self.index, self.rtxn, &derived_words, candidates.clone());
        // let mut documents = Vec::new();

        // // We execute the Mdfs iterator until we find enough documents.
        // while documents.iter().map(RoaringBitmap::len).sum::<u64>() < limit as u64 {
        //     match mdfs.next().transpose()? {
        //         Some((proximity, answer)) => {
        //             debug!("answer with a proximity of {}: {:?}", proximity, answer);
        //             documents.push(answer);
        //         },
        //         None => break,
        //     }
        // }

        // let found_words = derived_words.into_iter().flat_map(|(w, _)| w).map(|(w, _)| w).collect();
        // let documents_ids = match order_by_facet {
        //     Some((fid, ftype, order)) => {
        //         let mut ordered_documents = Vec::new();
        //         for documents_ids in documents {
        //             let docids = self.facet_ordered(fid, ftype, order, documents_ids, limit)?;
        //             ordered_documents.push(docids);
        //             if ordered_documents.iter().map(Vec::len).sum::<usize>() >= limit { break }
        //         }
        //         ordered_documents.into_iter().flatten().take(limit).collect()
        //     },
        //     None => documents.into_iter().flatten().take(limit).collect(),
        // };

        // Ok(SearchResult { found_words, candidates, documents_ids })
    }
}

impl fmt::Debug for Search<'_> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let Search { query, facet_condition, offset, limit, rtxn: _, index: _ } = self;
        f.debug_struct("Search")
            .field("query", query)
            .field("facet_condition", facet_condition)
            .field("offset", offset)
            .field("limit", limit)
            .finish()
    }
}

#[derive(Default)]
pub struct SearchResult {
    pub found_words: HashSet<String>,
    pub candidates: RoaringBitmap,
    // TODO those documents ids should be associated with their criteria scores.
    pub documents_ids: Vec<DocumentId>,
}

pub fn word_typos(word: &str, is_prefix: bool, max_typo: u8, fst: &fst::Set<Cow<[u8]>>) -> anyhow::Result<Vec<(String, u8)>> {
    let dfa = {
        let lev = match max_typo {
            0 => &LEVDIST0,
            1 => &LEVDIST1,
            _ => &LEVDIST2,
        };

        if is_prefix {
            lev.build_prefix_dfa(&word)
        } else {
            lev.build_dfa(&word)
        }
    };

    let mut derived_words = Vec::new();
    let mut stream = fst.search_with_state(&dfa).into_stream();

    while let Some((word, state)) = stream.next() {
        let word = std::str::from_utf8(word)?;
        let distance = dfa.distance(state);
        derived_words.push((word.to_string(), distance.to_u8()));
    }

    Ok(derived_words)
}