Implement facet search exctraction

milli/src/update/new/channel.rs

@@ -144,6 +144,8 @@ pub enum Database {
     FacetIdExistsDocids,
     FacetIdF64NumberDocids,
     FacetIdStringDocids,
+    FacetIdNormalizedStringStrings,
+    FacetIdStringFst,
 }
 
 impl Database {
@@ -163,6 +165,10 @@ impl Database {
             Database::FacetIdExistsDocids => index.facet_id_exists_docids.remap_types(),
             Database::FacetIdF64NumberDocids => index.facet_id_f64_docids.remap_types(),
             Database::FacetIdStringDocids => index.facet_id_string_docids.remap_types(),
+            Database::FacetIdNormalizedStringStrings => {
+                index.facet_id_normalized_string_strings.remap_types()
+            }
+            Database::FacetIdStringFst => index.facet_id_string_fst.remap_types(),
         }
     }
 }
@@ -240,6 +246,10 @@ impl MergerSender {
         DocumentsSender(self)
     }
 
+    pub fn facet_searchable(&self) -> FacetSearchableSender<'_> {
+        FacetSearchableSender { sender: self }
+    }
+
     pub fn send_documents_ids(&self, documents_ids: RoaringBitmap) -> StdResult<(), SendError<()>> {
         let entry = EntryOperation::Write(KeyValueEntry::from_small_key_bitmap(
             DOCUMENTS_IDS_KEY.as_bytes(),
@@ -445,6 +455,50 @@ impl DocidsSender for FacetDocidsSender<'_> {
     }
 }
 
+pub struct FacetSearchableSender<'a> {
+    sender: &'a MergerSender,
+}
+
+impl FacetSearchableSender<'_> {
+    pub fn write_facet(&self, key: &[u8], value: &[u8]) -> StdResult<(), SendError<()>> {
+        let entry = EntryOperation::Write(KeyValueEntry::from_small_key_value(key, value));
+        match self
+            .sender
+            .send(WriterOperation { database: Database::FacetIdNormalizedStringStrings, entry })
+        {
+            Ok(()) => Ok(()),
+            Err(SendError(_)) => Err(SendError(())),
+        }
+    }
+
+    pub fn delete_facet(&self, key: &[u8]) -> StdResult<(), SendError<()>> {
+        let entry = EntryOperation::Delete(KeyEntry::from_key(key));
+        match self
+            .sender
+            .send(WriterOperation { database: Database::FacetIdNormalizedStringStrings, entry })
+        {
+            Ok(()) => Ok(()),
+            Err(SendError(_)) => Err(SendError(())),
+        }
+    }
+
+    pub fn write_fst(&self, key: &[u8], value: Mmap) -> StdResult<(), SendError<()>> {
+        let entry = EntryOperation::Write(KeyValueEntry::from_large_key_value(key, value));
+        match self.sender.send(WriterOperation { database: Database::FacetIdStringFst, entry }) {
+            Ok(()) => Ok(()),
+            Err(SendError(_)) => Err(SendError(())),
+        }
+    }
+
+    pub fn delete_fst(&self, key: &[u8]) -> StdResult<(), SendError<()>> {
+        let entry = EntryOperation::Delete(KeyEntry::from_key(key));
+        match self.sender.send(WriterOperation { database: Database::FacetIdStringFst, entry }) {
+            Ok(()) => Ok(()),
+            Err(SendError(_)) => Err(SendError(())),
+        }
+    }
+}
+
 pub struct DocumentsSender<'a>(&'a MergerSender);
 
 impl DocumentsSender<'_> {

milli/src/update/new/facet_search_builder.rs

@@ -0,0 +1,275 @@
+use std::collections::{BTreeSet, HashMap};
+
+use charabia::{normalizer::NormalizerOption, Language, Normalize, StrDetection, Token};
+use grenad::Sorter;
+use heed::{
+    types::{Bytes, SerdeJson},
+    BytesDecode, BytesEncode, RoTxn,
+};
+
+use crate::{
+    heed_codec::{
+        facet::{FacetGroupKey, FacetGroupKeyCodec},
+        StrRefCodec,
+    },
+    update::{
+        create_sorter,
+        del_add::{DelAdd, KvWriterDelAdd},
+        MergeDeladdBtreesetString,
+    },
+    BEU16StrCodec, FieldId, GlobalFieldsIdsMap, Index, LocalizedAttributesRule, Result,
+    MAX_FACET_VALUE_LENGTH,
+};
+
+use super::{
+    channel::FacetSearchableSender, extract::FacetKind, fst_merger_builder::FstMergerBuilder,
+    KvReaderDelAdd,
+};
+
+pub struct FacetSearchBuilder<'indexer> {
+    registered_facets: HashMap<FieldId, usize>,
+    normalized_facet_string_docids_sorter: Sorter<MergeDeladdBtreesetString>,
+    global_fields_ids_map: GlobalFieldsIdsMap<'indexer>,
+    localized_attributes_rules: Vec<LocalizedAttributesRule>,
+    // Buffered data below
+    buffer: Vec<u8>,
+    localized_field_ids: HashMap<FieldId, Option<Vec<Language>>>,
+}
+
+impl<'indexer> FacetSearchBuilder<'indexer> {
+    pub fn new(
+        global_fields_ids_map: GlobalFieldsIdsMap<'indexer>,
+        localized_attributes_rules: Vec<LocalizedAttributesRule>,
+    ) -> Self {
+        let registered_facets = HashMap::new();
+        let normalized_facet_string_docids_sorter = create_sorter(
+            grenad::SortAlgorithm::Stable,
+            MergeDeladdBtreesetString,
+            grenad::CompressionType::None,
+            None,
+            None,
+            Some(0),
+        );
+
+        Self {
+            registered_facets,
+            normalized_facet_string_docids_sorter,
+            buffer: Vec::new(),
+            global_fields_ids_map,
+            localized_attributes_rules,
+            localized_field_ids: HashMap::new(),
+        }
+    }
+
+    fn extract_key_data<'k>(&self, key: &'k [u8]) -> Result<Option<FacetGroupKey<&'k str>>> {
+        match FacetKind::from(key[0]) {
+            // Only strings are searchable
+            FacetKind::String => Ok(Some(
+                FacetGroupKeyCodec::<StrRefCodec>::bytes_decode(&key[1..])
+                    .map_err(heed::Error::Encoding)?,
+            )),
+            _ => Ok(None),
+        }
+    }
+
+    pub fn register_from_key(&mut self, deladd: DelAdd, facet_key: &[u8]) -> Result<()> {
+        let Some(FacetGroupKey { field_id, level: _level, left_bound }) =
+            self.extract_key_data(facet_key)?
+        else {
+            return Ok(());
+        };
+
+        if deladd == DelAdd::Addition {
+            self.registered_facets.entry(field_id).and_modify(|count| *count += 1).or_insert(1);
+        }
+
+        let locales = self.locales(field_id);
+        let hyper_normalized_value = normalize_facet_string(left_bound, locales.as_deref());
+
+        let set = BTreeSet::from_iter(std::iter::once(left_bound));
+
+        // as the facet string is the same, we can put the deletion and addition in the same obkv.
+        self.buffer.clear();
+        let mut obkv = KvWriterDelAdd::new(&mut self.buffer);
+        let val = SerdeJson::bytes_encode(&set).map_err(heed::Error::Encoding)?;
+        obkv.insert(deladd, val)?;
+        obkv.finish()?;
+
+        let key: (u16, &str) = (field_id, hyper_normalized_value.as_ref());
+        let key_bytes = BEU16StrCodec::bytes_encode(&key).map_err(heed::Error::Encoding)?;
+        self.normalized_facet_string_docids_sorter.insert(key_bytes, &self.buffer)?;
+
+        Ok(())
+    }
+
+    fn locales(&mut self, field_id: FieldId) -> Option<&[Language]> {
+        if self.localized_field_ids.get(&field_id).is_none() {
+            let Some(field_name) = self.global_fields_ids_map.name(field_id) else {
+                unreachable!("Field id {} not found in the global fields ids map", field_id);
+            };
+
+            let locales = self
+                .localized_attributes_rules
+                .iter()
+                .find(|rule| rule.match_str(field_name))
+                .map(|rule| rule.locales.clone());
+
+            self.localized_field_ids.insert(field_id, locales);
+        }
+
+        self.localized_field_ids.get(&field_id).unwrap().as_deref()
+    }
+
+    #[tracing::instrument(level = "trace", skip_all, target = "indexing::facet_fst")]
+    pub fn merge_and_send(
+        self,
+        index: &Index,
+        rtxn: &RoTxn<'_>,
+        sender: FacetSearchableSender,
+    ) -> Result<()> {
+        let reader = self.normalized_facet_string_docids_sorter.into_reader_cursors()?;
+        let mut builder = grenad::MergerBuilder::new(MergeDeladdBtreesetString);
+        builder.extend(reader);
+
+        let database = index.facet_id_normalized_string_strings.remap_types::<Bytes, Bytes>();
+
+        let mut merger_iter = builder.build().into_stream_merger_iter()?;
+        let mut current_field_id = None;
+        let mut fst;
+        let mut fst_merger_builder: Option<FstMergerBuilder> = None;
+        while let Some((key, deladd)) = merger_iter.next()? {
+            let (field_id, normalized_facet_string) =
+                BEU16StrCodec::bytes_decode(&key).map_err(heed::Error::Encoding)?;
+
+            if current_field_id != Some(field_id) {
+                if let Some(fst_merger_builder) = fst_merger_builder {
+                    // send the previous fst to the channel
+                    let mmap = fst_merger_builder.build(&mut callback)?;
+                    sender.write_fst(&field_id.to_be_bytes(), mmap).unwrap();
+                }
+
+                println!("getting fst for field_id: {}", field_id);
+                fst = index.facet_id_string_fst.get(rtxn, &field_id)?;
+                fst_merger_builder = Some(FstMergerBuilder::new(fst.as_ref())?);
+                current_field_id = Some(field_id);
+            }
+
+            let current = database.get(rtxn, key)?;
+            let deladd: &KvReaderDelAdd = deladd.into();
+            let del = deladd.get(DelAdd::Deletion);
+            let add = deladd.get(DelAdd::Addition);
+
+            match merge_btreesets(current, del, add)? {
+                Operation::Write(value) => {
+                    match fst_merger_builder.as_mut() {
+                        Some(fst_merger_builder) => {
+                            fst_merger_builder.register(
+                                DelAdd::Addition,
+                                normalized_facet_string.as_bytes(),
+                                &mut callback,
+                            )?;
+                        }
+                        None => unreachable!(),
+                    }
+                    let key = (field_id, normalized_facet_string);
+                    let key_bytes =
+                        BEU16StrCodec::bytes_encode(&key).map_err(heed::Error::Encoding)?;
+                    sender.write_facet(&key_bytes, &value).unwrap();
+                }
+                Operation::Delete => {
+                    match fst_merger_builder.as_mut() {
+                        Some(fst_merger_builder) => {
+                            fst_merger_builder.register(
+                                DelAdd::Deletion,
+                                normalized_facet_string.as_bytes(),
+                                &mut callback,
+                            )?;
+                        }
+                        None => unreachable!(),
+                    }
+                    let key = (field_id, normalized_facet_string);
+                    let key_bytes =
+                        BEU16StrCodec::bytes_encode(&key).map_err(heed::Error::Encoding)?;
+                    sender.delete_facet(&key_bytes).unwrap();
+                }
+                Operation::Ignore => (),
+            }
+        }
+
+        if let (Some(field_id), Some(fst_merger_builder)) = (current_field_id, fst_merger_builder) {
+            let mmap = fst_merger_builder.build(&mut callback)?;
+            sender.write_fst(&field_id.to_be_bytes(), mmap).unwrap();
+        }
+
+        Ok(())
+    }
+}
+
+fn callback(_bytes: &[u8], _deladd: DelAdd, _is_modified: bool) -> Result<()> {
+    Ok(())
+}
+
+fn merge_btreesets<'a>(
+    current: Option<&[u8]>,
+    del: Option<&[u8]>,
+    add: Option<&[u8]>,
+) -> Result<Operation> {
+    let mut result: BTreeSet<String> = match current {
+        Some(current) => SerdeJson::bytes_decode(current).map_err(heed::Error::Encoding)?,
+        None => BTreeSet::new(),
+    };
+    if let Some(del) = del {
+        let del: BTreeSet<String> = SerdeJson::bytes_decode(del).map_err(heed::Error::Encoding)?;
+        result = result.difference(&del).cloned().collect();
+    }
+    if let Some(add) = add {
+        let add: BTreeSet<String> = SerdeJson::bytes_decode(add).map_err(heed::Error::Encoding)?;
+        result.extend(add);
+    }
+
+    /// TODO remove allocation
+    let result = SerdeJson::bytes_encode(&result).map_err(heed::Error::Encoding)?.into_owned();
+    if Some(result.as_ref()) == current {
+        Ok(Operation::Ignore)
+    } else if result.is_empty() {
+        Ok(Operation::Delete)
+    } else {
+        Ok(Operation::Write(result))
+    }
+}
+
+/// Normalizes the facet string and truncates it to the max length.
+fn normalize_facet_string(facet_string: &str, locales: Option<&[Language]>) -> String {
+    let options: NormalizerOption = NormalizerOption { lossy: true, ..Default::default() };
+    let mut detection = StrDetection::new(facet_string, locales);
+
+    let script = detection.script();
+    // Detect the language of the facet string only if several locales are explicitly provided.
+    let language = match locales {
+        Some(&[language]) => Some(language),
+        Some(multiple_locales) if multiple_locales.len() > 1 => detection.language(),
+        _ => None,
+    };
+
+    let token = Token {
+        lemma: std::borrow::Cow::Borrowed(facet_string),
+        script,
+        language,
+        ..Default::default()
+    };
+
+    // truncate the facet string to the max length
+    token
+        .normalize(&options)
+        .lemma
+        .char_indices()
+        .take_while(|(idx, _)| *idx < MAX_FACET_VALUE_LENGTH)
+        .map(|(_, c)| c)
+        .collect()
+}
+
+enum Operation {
+    Write(Vec<u8>),
+    Delete,
+    Ignore,
+}

milli/src/update/new/fst_merger_builder.rs

@@ -0,0 +1,155 @@
+use std::{fs::File, io::BufWriter};
+
+use fst::{Set, SetBuilder, Streamer};
+use memmap2::Mmap;
+use tempfile::tempfile;
+
+use crate::{update::del_add::DelAdd, InternalError, Result};
+
+pub struct FstMergerBuilder<'a> {
+    stream: Option<fst::set::Stream<'a>>,
+    fst_builder: SetBuilder<BufWriter<File>>,
+    last: Option<Vec<u8>>,
+    inserted_words: usize,
+}
+
+impl<'a> FstMergerBuilder<'a> {
+    pub fn new<D: AsRef<[u8]>>(fst: Option<&'a Set<D>>) -> Result<Self> {
+        Ok(Self {
+            stream: fst.map(|fst| fst.stream()),
+            fst_builder: SetBuilder::new(BufWriter::new(tempfile()?))?,
+            last: None,
+            inserted_words: 0,
+        })
+    }
+
+    pub fn register(
+        &mut self,
+        deladd: DelAdd,
+        right: &[u8],
+        insertion_callback: &mut impl FnMut(&[u8], DelAdd, bool) -> Result<()>,
+    ) -> Result<()> {
+        if let Some(left) = self.last.take() {
+            let (left_inserted, right_inserted) =
+                self.compare_and_insert(deladd, left.as_slice(), right, insertion_callback)?;
+
+            // left was not inserted, so we keep it for the next iteration
+            if !left_inserted {
+                self.last = Some(left);
+            }
+
+            // right was inserted, so we can stop
+            if right_inserted {
+                return Ok(());
+            }
+        }
+
+        if let Some(mut stream) = self.stream.take() {
+            while let Some(left) = stream.next() {
+                let (left_inserted, right_inserted) =
+                    self.compare_and_insert(deladd, left, right, insertion_callback)?;
+
+                // left was not inserted, so we keep it for the next iteration
+                if !left_inserted {
+                    self.last = Some(left.to_vec());
+                }
+
+                // right was inserted, so we can stop
+                if right_inserted {
+                    self.stream = Some(stream);
+                    return Ok(());
+                }
+            }
+        }
+
+        // If we reach this point, it means that the stream is empty
+        // and we need to insert the incoming word
+        self.insert(right, deladd, true, insertion_callback)?;
+
+        Ok(())
+    }
+
+    fn compare_and_insert(
+        &mut self,
+        deladd: DelAdd,
+        left: &[u8],
+        right: &[u8],
+        insertion_callback: &mut impl FnMut(&[u8], DelAdd, bool) -> Result<()>,
+    ) -> Result<(bool, bool)> {
+        let mut left_inserted = false;
+        let mut right_inserted = false;
+        match left.cmp(right) {
+            std::cmp::Ordering::Less => {
+                // We need to insert the last word from the current fst
+                self.insert(left, DelAdd::Addition, false, insertion_callback)?;
+
+                left_inserted = true;
+            }
+            std::cmp::Ordering::Equal => {
+                self.insert(right, deladd, true, insertion_callback)?;
+
+                left_inserted = true;
+                right_inserted = true;
+            }
+            std::cmp::Ordering::Greater => {
+                self.insert(right, deladd, true, insertion_callback)?;
+
+                right_inserted = true;
+            }
+        }
+
+        Ok((left_inserted, right_inserted))
+    }
+
+    fn insert(
+        &mut self,
+        bytes: &[u8],
+        deladd: DelAdd,
+        is_modified: bool,
+        insertion_callback: &mut impl FnMut(&[u8], DelAdd, bool) -> Result<()>,
+    ) -> Result<()> {
+        // Addition: We insert the word
+        // Deletion: We delete the word by not inserting it
+        if deladd == DelAdd::Addition {
+            self.inserted_words += 1;
+            self.fst_builder.insert(bytes)?;
+        }
+
+        insertion_callback(bytes, deladd, is_modified)?;
+
+        Ok(())
+    }
+
+    fn drain_stream(
+        &mut self,
+        insertion_callback: &mut impl FnMut(&[u8], DelAdd, bool) -> Result<()>,
+    ) -> Result<()> {
+        if let Some(last) = self.last.take() {
+            self.insert(last.as_slice(), DelAdd::Addition, false, insertion_callback)?;
+        }
+
+        if let Some(mut stream) = self.stream.take() {
+            while let Some(current) = stream.next() {
+                self.insert(current, DelAdd::Addition, false, insertion_callback)?;
+            }
+        }
+
+        Ok(())
+    }
+
+    pub fn build(
+        mut self,
+        insertion_callback: &mut impl FnMut(&[u8], DelAdd, bool) -> Result<()>,
+    ) -> Result<Mmap> {
+        self.drain_stream(insertion_callback)?;
+
+        let fst_file = self
+            .fst_builder
+            .into_inner()?
+            .into_inner()
+            .map_err(|_| InternalError::IndexingMergingKeys { process: "building-fst" })?;
+        let fst_mmap = unsafe { Mmap::map(&fst_file)? };
+
+        Ok(fst_mmap)
+    }
+}

milli/src/update/new/merger.rs

@@ -10,13 +10,17 @@ use roaring::RoaringBitmap;
 
 use super::channel::*;
 use super::extract::FacetKind;
+use super::facet_search_builder::FacetSearchBuilder;
 use super::word_fst_builder::{PrefixData, PrefixDelta};
 use super::{Deletion, DocumentChange, KvReaderDelAdd, KvReaderFieldId};
 use crate::update::del_add::DelAdd;
 use crate::update::new::channel::MergerOperation;
 use crate::update::new::word_fst_builder::WordFstBuilder;
 use crate::update::MergeDeladdCboRoaringBitmaps;
-use crate::{CboRoaringBitmapCodec, Error, FieldId, GeoPoint, GlobalFieldsIdsMap, Index, Result};
+use crate::{
+    localized_attributes_rules, CboRoaringBitmapCodec, Error, FieldId, GeoPoint,
+    GlobalFieldsIdsMap, Index, Result,
+};
 
 /// TODO We must return some infos/stats
 #[tracing::instrument(level = "trace", skip_all, target = "indexing::documents", name = "merge")]
@@ -170,6 +174,12 @@ pub fn merge_grenad_entries(
                     tracing::trace_span!(target: "indexing::documents::merge", "facet_docids");
                 let _entered = span.enter();
                 let mut facet_field_ids_delta = FacetFieldIdsDelta::new();
+                let localized_attributes_rules =
+                    index.localized_attributes_rules(rtxn)?.unwrap_or_default();
+                let mut facet_search_builder = FacetSearchBuilder::new(
+                    global_fields_ids_map.clone(),
+                    localized_attributes_rules,
+                );
                 merge_and_send_facet_docids(
                     merger,
                     FacetDatabases::new(index),
@@ -177,9 +187,12 @@ pub fn merge_grenad_entries(
                     &mut buffer,
                     sender.facet_docids(),
                     &mut facet_field_ids_delta,
+                    &mut facet_search_builder,
                 )?;
 
                 merger_result.facet_field_ids_delta = Some(facet_field_ids_delta);
+                // merge and send the facet fst and the searchable facet values
+                facet_search_builder.merge_and_send(index, rtxn, sender.facet_searchable())?;
             }
         }
     }
@@ -294,6 +307,7 @@ fn merge_and_send_facet_docids(
     buffer: &mut Vec<u8>,
     docids_sender: impl DocidsSender,
     facet_field_ids_delta: &mut FacetFieldIdsDelta,
+    facet_search_builder: &mut FacetSearchBuilder,
 ) -> Result<()> {
     let mut merger_iter = merger.into_stream_merger_iter().unwrap();
     while let Some((key, deladd)) = merger_iter.next().unwrap() {
@@ -305,11 +319,13 @@ fn merge_and_send_facet_docids(
         match merge_cbo_bitmaps(current, del, add)? {
             Operation::Write(bitmap) => {
                 facet_field_ids_delta.register_from_key(key);
+                facet_search_builder.register_from_key(DelAdd::Addition, key)?;
                 let value = cbo_bitmap_serialize_into_vec(&bitmap, buffer);
                 docids_sender.write(key, value).unwrap();
             }
             Operation::Delete => {
                 facet_field_ids_delta.register_from_key(key);
+                facet_search_builder.register_from_key(DelAdd::Deletion, key)?;
                 docids_sender.delete(key).unwrap();
             }
             Operation::Ignore => (),

milli/src/update/new/mod.rs

@@ -8,6 +8,8 @@ mod channel;
 pub mod document;
 mod document_change;
 mod extract;
+mod facet_search_builder;
+mod fst_merger_builder;
 pub mod indexer;
 mod merger;
 mod parallel_iterator_ext;

milli/src/update/new/word_fst_builder.rs

@@ -1,4 +1,4 @@
-use std::{fs::File, io::BufWriter};
+use std::io::BufWriter;
 
 use fst::{Set, SetBuilder, Streamer};
 use memmap2::Mmap;
@@ -7,23 +7,19 @@ use tempfile::tempfile;
 
 use crate::{index::PrefixSettings, update::del_add::DelAdd, InternalError, Prefix, Result};
 
+use super::fst_merger_builder::FstMergerBuilder;
+
 pub struct WordFstBuilder<'a> {
-    stream: Option<fst::set::Stream<'a>>,
-    word_fst_builder: SetBuilder<BufWriter<File>>,
-    last_word: Option<Vec<u8>>,
+    word_fst_builder: FstMergerBuilder<'a>,
     prefix_fst_builder: Option<PrefixFstBuilder>,
-    inserted_words: usize,
     registered_words: usize,
 }
 
 impl<'a> WordFstBuilder<'a> {
     pub fn new(words_fst: &'a Set<std::borrow::Cow<'a, [u8]>>) -> Result<Self> {
         Ok(Self {
-            stream: Some(words_fst.stream()),
-            word_fst_builder: SetBuilder::new(BufWriter::new(tempfile()?))?,
+            word_fst_builder: FstMergerBuilder::new(Some(words_fst))?,
             prefix_fst_builder: None,
-            last_word: None,
-            inserted_words: 0,
             registered_words: 0,
         })
     }
@@ -38,100 +34,13 @@ impl<'a> WordFstBuilder<'a> {
             self.registered_words += 1;
         }
 
-        if let Some(left) = self.last_word.take() {
-            let (left_inserted, right_inserted) =
-                self.compare_and_insert(deladd, left.as_slice(), right)?;
-
-            // left was not inserted, so we keep it for the next iteration
-            if !left_inserted {
-                self.last_word = Some(left);
+        self.word_fst_builder.register(deladd, right, &mut |bytes, deladd, is_modified| {
+            if let Some(prefix_fst_builder) = &mut self.prefix_fst_builder {
+                prefix_fst_builder.insert_word(bytes, deladd, is_modified)
+            } else {
+                Ok(())
             }
-
-            // right was inserted, so we can stop
-            if right_inserted {
-                return Ok(());
-            }
-        }
-
-        if let Some(mut stream) = self.stream.take() {
-            while let Some(left) = stream.next() {
-                let (left_inserted, right_inserted) =
-                    self.compare_and_insert(deladd, left, right)?;
-
-                // left was not inserted, so we keep it for the next iteration
-                if !left_inserted {
-                    self.last_word = Some(left.to_vec());
-                }
-
-                // right was inserted, so we can stop
-                if right_inserted {
-                    self.stream = Some(stream);
-                    return Ok(());
-                }
-            }
-
-            // If we reach this point, it means that the stream is empty
-            // and we need to insert the incoming word
-            self.insert_word(right, deladd, true)?;
-
-            self.stream = Some(stream);
-        }
-
-        Ok(())
-    }
-
-    pub fn compare_and_insert(
-        &mut self,
-        deladd: DelAdd,
-        left: &[u8],
-        right: &[u8],
-    ) -> Result<(bool, bool)> {
-        let mut left_inserted = false;
-        let mut right_inserted = false;
-        match left.cmp(right) {
-            std::cmp::Ordering::Less => {
-                // We need to insert the last word from the current fst
-                self.insert_word(left, DelAdd::Addition, false)?;
-
-                left_inserted = true;
-            }
-            std::cmp::Ordering::Equal => {
-                self.insert_word(right, deladd, true)?;
-
-                left_inserted = true;
-                right_inserted = true;
-            }
-            std::cmp::Ordering::Greater => {
-                self.insert_word(right, deladd, true)?;
-
-                right_inserted = true;
-            }
-        }
-
-        Ok((left_inserted, right_inserted))
-    }
-
-    fn insert_word(&mut self, bytes: &[u8], deladd: DelAdd, is_modified: bool) -> Result<()> {
-        // Addition: We insert the word
-        // Deletion: We delete the word by not inserting it
-        if deladd == DelAdd::Addition {
-            self.inserted_words += 1;
-            self.word_fst_builder.insert(bytes)?;
-        }
-
-        if let Some(prefix_fst_builder) = self.prefix_fst_builder.as_mut() {
-            prefix_fst_builder.insert_word(bytes, deladd, is_modified)?;
-        }
-
-        Ok(())
-    }
-
-    fn drain_stream(&mut self) -> Result<()> {
-        if let Some(mut stream) = self.stream.take() {
-            while let Some(current) = stream.next() {
-                self.insert_word(current, DelAdd::Addition, false)?;
-            }
-        }
+        })?;
 
         Ok(())
     }
@@ -141,13 +50,13 @@ impl<'a> WordFstBuilder<'a> {
         index: &crate::Index,
         rtxn: &heed::RoTxn,
     ) -> Result<(Mmap, Option<PrefixData>)> {
-        self.drain_stream()?;
-
-        let words_fst_file =
-            self.word_fst_builder.into_inner()?.into_inner().map_err(|_| {
-                InternalError::IndexingMergingKeys { process: "building-words-fst" }
-            })?;
-        let words_fst_mmap = unsafe { Mmap::map(&words_fst_file)? };
+        let words_fst_mmap = self.word_fst_builder.build(&mut |bytes, deladd, is_modified| {
+            if let Some(prefix_fst_builder) = &mut self.prefix_fst_builder {
+                prefix_fst_builder.insert_word(bytes, deladd, is_modified)
+            } else {
+                Ok(())
+            }
+        })?;
 
         let prefix_data = self
             .prefix_fst_builder