use std::borrow::Cow;
use std::collections::hash_map::Entry;
use std::hash::Hash;

use fxhash::FxHashMap;
use grenad::MergeFunction;
use heed::types::Bytes;
use heed::{BytesEncode, Database, RoTxn};
use roaring::RoaringBitmap;

use super::interner::Interned;
use super::Word;
use crate::heed_codec::{BytesDecodeOwned, StrBEU16Codec};
use crate::proximity::ProximityPrecision;
use crate::update::MergeCboRoaringBitmaps;
use crate::{
    CboRoaringBitmapCodec, CboRoaringBitmapLenCodec, Result, SearchContext, U8StrStrCodec,
};

/// A cache storing pointers to values in the LMDB databases.
///
/// Used for performance reasons only. By using this cache, we avoid performing a
/// database lookup and instead get a direct reference to the value using a fast
/// local HashMap lookup.
#[derive(Default)]
pub struct DatabaseCache<'ctx> {
    pub word_pair_proximity_docids:
        FxHashMap<(u8, Interned<String>, Interned<String>), Option<Cow<'ctx, [u8]>>>,
    pub word_prefix_pair_proximity_docids:
        FxHashMap<(u8, Interned<String>, Interned<String>), Option<RoaringBitmap>>,
    pub prefix_word_pair_proximity_docids:
        FxHashMap<(u8, Interned<String>, Interned<String>), Option<Cow<'ctx, [u8]>>>,
    pub word_docids: FxHashMap<Interned<String>, Option<Cow<'ctx, [u8]>>>,
    pub exact_word_docids: FxHashMap<Interned<String>, Option<Cow<'ctx, [u8]>>>,
    pub word_prefix_docids: FxHashMap<Interned<String>, Option<Cow<'ctx, [u8]>>>,
    pub exact_word_prefix_docids: FxHashMap<Interned<String>, Option<Cow<'ctx, [u8]>>>,

    pub words_fst: Option<fst::Set<Cow<'ctx, [u8]>>>,
    pub word_position_docids: FxHashMap<(Interned<String>, u16), Option<Cow<'ctx, [u8]>>>,
    pub word_prefix_position_docids: FxHashMap<(Interned<String>, u16), Option<Cow<'ctx, [u8]>>>,
    pub word_positions: FxHashMap<Interned<String>, Vec<u16>>,
    pub word_prefix_positions: FxHashMap<Interned<String>, Vec<u16>>,

    pub word_fid_docids: FxHashMap<(Interned<String>, u16), Option<Cow<'ctx, [u8]>>>,
    pub word_prefix_fid_docids: FxHashMap<(Interned<String>, u16), Option<Cow<'ctx, [u8]>>>,
    pub word_fids: FxHashMap<Interned<String>, Vec<u16>>,
    pub word_prefix_fids: FxHashMap<Interned<String>, Vec<u16>>,
}
impl<'ctx> DatabaseCache<'ctx> {
    fn get_value<'v, K1, KC>(
        txn: &'ctx RoTxn<'_>,
        cache_key: K1,
        db_key: &'v KC::EItem,
        cache: &mut FxHashMap<K1, Option<Cow<'ctx, [u8]>>>,
        universe: Option<&RoaringBitmap>,
        db: Database<KC, Bytes>,
    ) -> Result<Option<RoaringBitmap>>
    where
        K1: Copy + Eq + Hash,
        KC: BytesEncode<'v>,
    {
        if let Entry::Vacant(entry) = cache.entry(cache_key) {
            let bitmap_ptr = db.get(txn, db_key)?.map(Cow::Borrowed);
            entry.insert(bitmap_ptr);
        }

        let bitmap_bytes = match cache.get(&cache_key).unwrap() {
            Some(Cow::Borrowed(bytes)) => bytes,
            Some(Cow::Owned(bytes)) => bytes.as_slice(),
            None => return Ok(None),
        };

        match (bitmap_bytes, universe) {
            (bytes, Some(universe)) => {
                CboRoaringBitmapCodec::intersection_with_serialized(bytes, universe)
                    .map(Some)
                    .map_err(Into::into)
            }
            (bytes, None) => CboRoaringBitmapCodec::bytes_decode_owned(bytes)
                .map(Some)
                .map_err(heed::Error::Decoding)
                .map_err(Into::into),
        }
    }

    fn get_value_length<'v, K1, KC>(
        txn: &'ctx RoTxn<'_>,
        cache_key: K1,
        db_key: &'v KC::EItem,
        cache: &mut FxHashMap<K1, Option<Cow<'ctx, [u8]>>>,
        db: Database<KC, Bytes>,
    ) -> Result<Option<u64>>
    where
        K1: Copy + Eq + Hash,
        KC: BytesEncode<'v>,
    {
        if let Entry::Vacant(entry) = cache.entry(cache_key) {
            let bitmap_ptr = db.get(txn, db_key)?.map(Cow::Borrowed);
            entry.insert(bitmap_ptr);
        }

        let bitmap_bytes = match cache.get(&cache_key).unwrap() {
            Some(Cow::Borrowed(bytes)) => bytes,
            Some(Cow::Owned(bytes)) => bytes.as_slice(),
            None => return Ok(None),
        };

        CboRoaringBitmapLenCodec::bytes_decode_owned(bitmap_bytes)
            .map(Some)
            .map_err(heed::Error::Decoding)
            .map_err(Into::into)
    }

    fn get_value_from_keys<'v, K1, KC, MF>(
        txn: &'ctx RoTxn<'_>,
        cache_key: K1,
        db_keys: &'v [KC::EItem],
        cache: &mut FxHashMap<K1, Option<Cow<'ctx, [u8]>>>,
        db: Database<KC, Bytes>,
        universe: Option<&RoaringBitmap>,
        merger: MF,
    ) -> Result<Option<RoaringBitmap>>
    where
        K1: Copy + Eq + Hash,
        KC: BytesEncode<'v>,
        KC::EItem: Sized,
        MF: MergeFunction,
        crate::Error: From<MF::Error>,
    {
        if let Entry::Vacant(entry) = cache.entry(cache_key) {
            let bitmap_ptr: Option<Cow<'ctx, [u8]>> = match db_keys {
                [] => None,
                [key] => db.get(txn, key)?.map(Cow::Borrowed),
                keys => {
                    let bitmaps = keys
                        .iter()
                        .filter_map(|key| db.get(txn, key).transpose())
                        .map(|v| v.map(Cow::Borrowed))
                        .collect::<std::result::Result<Vec<Cow<'_, [u8]>>, _>>()?;

                    if bitmaps.is_empty() {
                        None
                    } else {
                        Some(merger.merge(&[], &bitmaps[..])?)
                    }
                }
            };

            entry.insert(bitmap_ptr);
        }

        let bitmap_bytes = match cache.get(&cache_key).unwrap() {
            Some(Cow::Borrowed(bytes)) => bytes,
            Some(Cow::Owned(bytes)) => bytes.as_slice(),
            None => return Ok(None),
        };

        match (bitmap_bytes, universe) {
            (bytes, Some(universe)) => {
                CboRoaringBitmapCodec::intersection_with_serialized(bytes, universe)
                    .map(Some)
                    .map_err(Into::into)
            }
            (bytes, None) => CboRoaringBitmapCodec::bytes_decode_owned(bytes)
                .map(Some)
                .map_err(heed::Error::Decoding)
                .map_err(Into::into),
        }
    }
}

impl<'ctx> SearchContext<'ctx> {
    pub fn get_words_fst(&mut self) -> Result<fst::Set<Cow<'ctx, [u8]>>> {
        if let Some(fst) = self.db_cache.words_fst.clone() {
            Ok(fst)
        } else {
            let fst = self.index.words_fst(self.txn)?;
            self.db_cache.words_fst = Some(fst.clone());
            Ok(fst)
        }
    }

    pub fn word_docids(
        &mut self,
        universe: Option<&RoaringBitmap>,
        word: Word,
    ) -> Result<Option<RoaringBitmap>> {
        match word {
            Word::Original(word) => {
                let exact = self.get_db_exact_word_docids(universe, word)?;
                let tolerant = self.get_db_word_docids(universe, word)?;
                Ok(match (exact, tolerant) {
                    (None, None) => None,
                    (None, Some(tolerant)) => Some(tolerant),
                    (Some(exact), None) => Some(exact),
                    (Some(exact), Some(tolerant)) => {
                        let mut both = exact;
                        both |= tolerant;
                        Some(both)
                    }
                })
            }
            Word::Derived(word) => self.get_db_word_docids(universe, word),
        }
    }

    /// Retrieve or insert the given value in the `word_docids` database.
    fn get_db_word_docids(
        &mut self,
        universe: Option<&RoaringBitmap>,
        word: Interned<String>,
    ) -> Result<Option<RoaringBitmap>> {
        match &self.restricted_fids {
            Some(restricted_fids) => {
                let interned = self.word_interner.get(word).as_str();
                let keys: Vec<_> =
                    restricted_fids.tolerant.iter().map(|(fid, _)| (interned, *fid)).collect();

                DatabaseCache::get_value_from_keys(
                    self.txn,
                    word,
                    &keys[..],
                    &mut self.db_cache.word_docids,
                    self.index.word_fid_docids.remap_data_type::<Bytes>(),
                    universe,
                    MergeCboRoaringBitmaps,
                )
            }
            None => DatabaseCache::get_value(
                self.txn,
                word,
                self.word_interner.get(word).as_str(),
                &mut self.db_cache.word_docids,
                universe,
                self.index.word_docids.remap_data_type::<Bytes>(),
            ),
        }
    }

    fn get_db_exact_word_docids(
        &mut self,
        universe: Option<&RoaringBitmap>,
        word: Interned<String>,
    ) -> Result<Option<RoaringBitmap>> {
        match &self.restricted_fids {
            Some(restricted_fids) => {
                let interned = self.word_interner.get(word).as_str();
                let keys: Vec<_> =
                    restricted_fids.exact.iter().map(|(fid, _)| (interned, *fid)).collect();

                DatabaseCache::get_value_from_keys(
                    self.txn,
                    word,
                    &keys[..],
                    &mut self.db_cache.exact_word_docids,
                    self.index.word_fid_docids.remap_data_type::<Bytes>(),
                    universe,
                    MergeCboRoaringBitmaps,
                )
            }
            None => DatabaseCache::get_value(
                self.txn,
                word,
                self.word_interner.get(word).as_str(),
                &mut self.db_cache.exact_word_docids,
                universe,
                self.index.exact_word_docids.remap_data_type::<Bytes>(),
            ),
        }
    }

    pub fn word_prefix_docids(
        &mut self,
        universe: Option<&RoaringBitmap>,
        prefix: Word,
    ) -> Result<Option<RoaringBitmap>> {
        match prefix {
            Word::Original(prefix) => {
                let exact = self.get_db_exact_word_prefix_docids(universe, prefix)?;
                let tolerant = self.get_db_word_prefix_docids(universe, prefix)?;
                Ok(match (exact, tolerant) {
                    (None, None) => None,
                    (None, Some(tolerant)) => Some(tolerant),
                    (Some(exact), None) => Some(exact),
                    (Some(exact), Some(tolerant)) => {
                        let mut both = exact;
                        both |= tolerant;
                        Some(both)
                    }
                })
            }
            Word::Derived(prefix) => self.get_db_word_prefix_docids(universe, prefix),
        }
    }

    /// Retrieve or insert the given value in the `word_prefix_docids` database.
    fn get_db_word_prefix_docids(
        &mut self,
        universe: Option<&RoaringBitmap>,
        prefix: Interned<String>,
    ) -> Result<Option<RoaringBitmap>> {
        match &self.restricted_fids {
            Some(restricted_fids) => {
                let interned = self.word_interner.get(prefix).as_str();
                let keys: Vec<_> =
                    restricted_fids.tolerant.iter().map(|(fid, _)| (interned, *fid)).collect();

                DatabaseCache::get_value_from_keys(
                    self.txn,
                    prefix,
                    &keys[..],
                    &mut self.db_cache.word_prefix_docids,
                    self.index.word_prefix_fid_docids.remap_data_type::<Bytes>(),
                    universe,
                    MergeCboRoaringBitmaps,
                )
            }
            None => DatabaseCache::get_value(
                self.txn,
                prefix,
                self.word_interner.get(prefix).as_str(),
                &mut self.db_cache.word_prefix_docids,
                universe,
                self.index.word_prefix_docids.remap_data_type::<Bytes>(),
            ),
        }
    }

    fn get_db_exact_word_prefix_docids(
        &mut self,
        universe: Option<&RoaringBitmap>,
        prefix: Interned<String>,
    ) -> Result<Option<RoaringBitmap>> {
        match &self.restricted_fids {
            Some(restricted_fids) => {
                let interned = self.word_interner.get(prefix).as_str();
                let keys: Vec<_> =
                    restricted_fids.exact.iter().map(|(fid, _)| (interned, *fid)).collect();

                DatabaseCache::get_value_from_keys(
                    self.txn,
                    prefix,
                    &keys[..],
                    &mut self.db_cache.exact_word_prefix_docids,
                    self.index.word_prefix_fid_docids.remap_data_type::<Bytes>(),
                    universe,
                    MergeCboRoaringBitmaps,
                )
            }
            None => DatabaseCache::get_value(
                self.txn,
                prefix,
                self.word_interner.get(prefix).as_str(),
                &mut self.db_cache.exact_word_prefix_docids,
                universe,
                self.index.exact_word_prefix_docids.remap_data_type::<Bytes>(),
            ),
        }
    }

    pub fn get_db_word_pair_proximity_docids(
        &mut self,
        universe: Option<&RoaringBitmap>,
        word1: Interned<String>,
        word2: Interned<String>,
        proximity: u8,
    ) -> Result<Option<RoaringBitmap>> {
        match self.index.proximity_precision(self.txn)?.unwrap_or_default() {
            ProximityPrecision::ByAttribute => {
                // Force proximity to 0 because:
                // in ByAttribute, there are only 2 possible distances:
                // 1. words in same attribute: in that the DB contains (0, word1, word2)
                // 2. words in different attributes: no DB entry for these two words.
                let proximity = 0;
                let docids = if let Some(docids) =
                    self.db_cache.word_pair_proximity_docids.get(&(proximity, word1, word2))
                {
                    docids
                        .as_ref()
                        .map(|d| CboRoaringBitmapCodec::bytes_decode_owned(d))
                        .transpose()
                        .map_err(heed::Error::Decoding)?
                } else {
                    // Compute the distance at the attribute level and store it in the cache.
                    let fids = self.index.searchable_fields_ids(self.txn)?;
                    let mut docids = RoaringBitmap::new();
                    for fid in fids {
                        // for each field, intersect left word bitmap and right word bitmap,
                        // then merge the result in a global bitmap before storing it in the cache.
                        let word1_docids = self.get_db_word_fid_docids(universe, word1, fid)?;
                        let word2_docids = self.get_db_word_fid_docids(universe, word2, fid)?;
                        if let (Some(word1_docids), Some(word2_docids)) =
                            (word1_docids, word2_docids)
                        {
                            docids |= word1_docids & word2_docids;
                        }
                    }
                    let encoded = CboRoaringBitmapCodec::bytes_encode(&docids)
                        .map(Cow::into_owned)
                        .map(Cow::Owned)
                        .map(Some)
                        .map_err(heed::Error::Decoding)?;
                    self.db_cache
                        .word_pair_proximity_docids
                        .insert((proximity, word1, word2), encoded);
                    Some(docids)
                };

                Ok(docids)
            }
            ProximityPrecision::ByWord => DatabaseCache::get_value(
                self.txn,
                (proximity, word1, word2),
                &(
                    proximity,
                    self.word_interner.get(word1).as_str(),
                    self.word_interner.get(word2).as_str(),
                ),
                &mut self.db_cache.word_pair_proximity_docids,
                universe,
                self.index.word_pair_proximity_docids.remap_data_type::<Bytes>(),
            ),
        }
    }

    pub fn get_db_word_pair_proximity_docids_len(
        &mut self,
        universe: Option<&RoaringBitmap>,
        word1: Interned<String>,
        word2: Interned<String>,
        proximity: u8,
    ) -> Result<Option<u64>> {
        match self.index.proximity_precision(self.txn)?.unwrap_or_default() {
            ProximityPrecision::ByAttribute => Ok(self
                .get_db_word_pair_proximity_docids(universe, word1, word2, proximity)?
                .map(|d| d.len())),
            ProximityPrecision::ByWord => DatabaseCache::get_value_length::<_, _>(
                self.txn,
                (proximity, word1, word2),
                &(
                    proximity,
                    self.word_interner.get(word1).as_str(),
                    self.word_interner.get(word2).as_str(),
                ),
                &mut self.db_cache.word_pair_proximity_docids,
                self.index.word_pair_proximity_docids.remap_data_type::<Bytes>(),
            ),
        }
    }

    pub fn get_db_word_prefix_pair_proximity_docids(
        &mut self,
        universe: Option<&RoaringBitmap>,
        word1: Interned<String>,
        prefix2: Interned<String>,
        mut proximity: u8,
    ) -> Result<Option<RoaringBitmap>> {
        let proximity_precision = self.index.proximity_precision(self.txn)?.unwrap_or_default();
        if proximity_precision == ProximityPrecision::ByAttribute {
            // Force proximity to 0 because:
            // in ByAttribute, there are only 2 possible distances:
            // 1. words in same attribute: in that the DB contains (0, word1, word2)
            // 2. words in different attributes: no DB entry for these two words.
            proximity = 0;
        }

        let docids = if let Some(docids) =
            self.db_cache.word_prefix_pair_proximity_docids.get(&(proximity, word1, prefix2))
        {
            docids.clone()
        } else {
            let prefix_docids = match proximity_precision {
                ProximityPrecision::ByAttribute => {
                    // Compute the distance at the attribute level and store it in the cache.
                    let fids = self.index.searchable_fields_ids(self.txn)?;
                    let mut prefix_docids = RoaringBitmap::new();
                    // for each field, intersect left word bitmap and right word bitmap,
                    // then merge the result in a global bitmap before storing it in the cache.
                    for fid in fids {
                        let word1_docids = self.get_db_word_fid_docids(universe, word1, fid)?;
                        let prefix2_docids =
                            self.get_db_word_prefix_fid_docids(universe, prefix2, fid)?;
                        if let (Some(word1_docids), Some(prefix2_docids)) =
                            (word1_docids, prefix2_docids)
                        {
                            prefix_docids |= word1_docids & prefix2_docids;
                        }
                    }
                    prefix_docids
                }
                ProximityPrecision::ByWord => {
                    // compute docids using prefix iter and store the result in the cache.
                    let key = U8StrStrCodec::bytes_encode(&(
                        proximity,
                        self.word_interner.get(word1).as_str(),
                        self.word_interner.get(prefix2).as_str(),
                    ))
                    .unwrap()
                    .into_owned();
                    let mut prefix_docids = RoaringBitmap::new();
                    let remap_key_type = self
                        .index
                        .word_pair_proximity_docids
                        .remap_key_type::<Bytes>()
                        .prefix_iter(self.txn, &key)?;
                    for result in remap_key_type {
                        let (_, docids) = result?;

                        prefix_docids |= docids;
                    }
                    prefix_docids
                }
            };
            self.db_cache
                .word_prefix_pair_proximity_docids
                .insert((proximity, word1, prefix2), Some(prefix_docids.clone()));
            Some(prefix_docids)
        };
        Ok(docids)
    }

    pub fn get_db_prefix_word_pair_proximity_docids(
        &mut self,
        universe: Option<&RoaringBitmap>,
        left_prefix: Interned<String>,
        right: Interned<String>,
        proximity: u8,
    ) -> Result<Option<RoaringBitmap>> {
        // only accept exact matches on reverted positions
        self.get_db_word_pair_proximity_docids(universe, left_prefix, right, proximity)
    }

    pub fn get_db_word_fid_docids(
        &mut self,
        universe: Option<&RoaringBitmap>,
        word: Interned<String>,
        fid: u16,
    ) -> Result<Option<RoaringBitmap>> {
        // if the requested fid isn't in the restricted list, return None.
        if self.restricted_fids.as_ref().is_some_and(|fids| !fids.contains(&fid)) {
            return Ok(None);
        }

        DatabaseCache::get_value(
            self.txn,
            (word, fid),
            &(self.word_interner.get(word).as_str(), fid),
            &mut self.db_cache.word_fid_docids,
            universe,
            self.index.word_fid_docids.remap_data_type::<Bytes>(),
        )
    }

    pub fn get_db_word_prefix_fid_docids(
        &mut self,
        universe: Option<&RoaringBitmap>,
        word_prefix: Interned<String>,
        fid: u16,
    ) -> Result<Option<RoaringBitmap>> {
        // if the requested fid isn't in the restricted list, return None.
        if self.restricted_fids.as_ref().is_some_and(|fids| !fids.contains(&fid)) {
            return Ok(None);
        }

        DatabaseCache::get_value(
            self.txn,
            (word_prefix, fid),
            &(self.word_interner.get(word_prefix).as_str(), fid),
            &mut self.db_cache.word_prefix_fid_docids,
            universe,
            self.index.word_prefix_fid_docids.remap_data_type::<Bytes>(),
        )
    }

    pub fn get_db_word_fids(&mut self, word: Interned<String>) -> Result<Vec<u16>> {
        let fids = match self.db_cache.word_fids.entry(word) {
            Entry::Occupied(fids) => fids.get().clone(),
            Entry::Vacant(entry) => {
                let mut key = self.word_interner.get(word).as_bytes().to_owned();
                key.push(0);
                let mut fids = vec![];
                let remap_key_type = self
                    .index
                    .word_fid_docids
                    .remap_types::<Bytes, Bytes>()
                    .prefix_iter(self.txn, &key)?
                    .remap_key_type::<StrBEU16Codec>();
                for result in remap_key_type {
                    let ((_, fid), value) = result?;
                    // filling other caches to avoid searching for them again
                    self.db_cache.word_fid_docids.insert((word, fid), Some(Cow::Borrowed(value)));
                    fids.push(fid);
                }
                entry.insert(fids.clone());
                fids
            }
        };
        Ok(fids)
    }

    pub fn get_db_word_prefix_fids(&mut self, word_prefix: Interned<String>) -> Result<Vec<u16>> {
        let fids = match self.db_cache.word_prefix_fids.entry(word_prefix) {
            Entry::Occupied(fids) => fids.get().clone(),
            Entry::Vacant(entry) => {
                let mut key = self.word_interner.get(word_prefix).as_bytes().to_owned();
                key.push(0);
                let mut fids = vec![];
                let remap_key_type = self
                    .index
                    .word_prefix_fid_docids
                    .remap_types::<Bytes, Bytes>()
                    .prefix_iter(self.txn, &key)?
                    .remap_key_type::<StrBEU16Codec>();
                for result in remap_key_type {
                    let ((_, fid), value) = result?;
                    // filling other caches to avoid searching for them again
                    self.db_cache
                        .word_prefix_fid_docids
                        .insert((word_prefix, fid), Some(Cow::Borrowed(value)));
                    fids.push(fid);
                }
                entry.insert(fids.clone());
                fids
            }
        };
        Ok(fids)
    }

    pub fn get_db_word_position_docids(
        &mut self,
        universe: Option<&RoaringBitmap>,
        word: Interned<String>,
        position: u16,
    ) -> Result<Option<RoaringBitmap>> {
        DatabaseCache::get_value(
            self.txn,
            (word, position),
            &(self.word_interner.get(word).as_str(), position),
            &mut self.db_cache.word_position_docids,
            universe,
            self.index.word_position_docids.remap_data_type::<Bytes>(),
        )
    }

    pub fn get_db_word_prefix_position_docids(
        &mut self,
        universe: Option<&RoaringBitmap>,
        word_prefix: Interned<String>,
        position: u16,
    ) -> Result<Option<RoaringBitmap>> {
        DatabaseCache::get_value(
            self.txn,
            (word_prefix, position),
            &(self.word_interner.get(word_prefix).as_str(), position),
            &mut self.db_cache.word_prefix_position_docids,
            universe,
            self.index.word_prefix_position_docids.remap_data_type::<Bytes>(),
        )
    }

    pub fn get_db_word_positions(&mut self, word: Interned<String>) -> Result<Vec<u16>> {
        let positions = match self.db_cache.word_positions.entry(word) {
            Entry::Occupied(positions) => positions.get().clone(),
            Entry::Vacant(entry) => {
                let mut key = self.word_interner.get(word).as_bytes().to_owned();
                key.push(0);
                let mut positions = vec![];
                let remap_key_type = self
                    .index
                    .word_position_docids
                    .remap_types::<Bytes, Bytes>()
                    .prefix_iter(self.txn, &key)?
                    .remap_key_type::<StrBEU16Codec>();
                for result in remap_key_type {
                    let ((_, position), value) = result?;
                    // filling other caches to avoid searching for them again
                    self.db_cache
                        .word_position_docids
                        .insert((word, position), Some(Cow::Borrowed(value)));
                    positions.push(position);
                }
                entry.insert(positions.clone());
                positions
            }
        };
        Ok(positions)
    }

    pub fn get_db_word_prefix_positions(
        &mut self,
        word_prefix: Interned<String>,
    ) -> Result<Vec<u16>> {
        let positions = match self.db_cache.word_prefix_positions.entry(word_prefix) {
            Entry::Occupied(positions) => positions.get().clone(),
            Entry::Vacant(entry) => {
                let mut key = self.word_interner.get(word_prefix).as_bytes().to_owned();
                key.push(0);
                let mut positions = vec![];
                let remap_key_type = self
                    .index
                    .word_prefix_position_docids
                    .remap_types::<Bytes, Bytes>()
                    .prefix_iter(self.txn, &key)?
                    .remap_key_type::<StrBEU16Codec>();
                for result in remap_key_type {
                    let ((_, position), value) = result?;
                    // filling other caches to avoid searching for them again
                    self.db_cache
                        .word_prefix_position_docids
                        .insert((word_prefix, position), Some(Cow::Borrowed(value)));
                    positions.push(position);
                }
                entry.insert(positions.clone());
                positions
            }
        };
        Ok(positions)
    }
}