Introduce an heed codec that reduce the size of small amount of serialized integers

src/bin/indexer.rs

@@ -21,7 +21,7 @@ use rayon::prelude::*;
 use roaring::RoaringBitmap;
 use structopt::StructOpt;
 
-use milli::heed_codec::CsvStringRecordCodec;
+use milli::heed_codec::{CsvStringRecordCodec, ByteorderXRoaringBitmapCodec};
 use milli::tokenizer::{simple_tokenizer, only_words};
 use milli::{SmallVec32, Index, DocumentId, BEU32};
 
@@ -197,7 +197,6 @@ impl Store {
     {
         // postings positions ids keys are all prefixed
         let mut key = vec![WORD_DOCID_POSITIONS_BYTE];
-        let mut buffer = Vec::new();
 
         // We prefix the words by the document id.
         key.extend_from_slice(&id.to_be_bytes());
@@ -207,12 +206,11 @@ impl Store {
             key.truncate(base_size);
             key.extend_from_slice(word.as_bytes());
             // We serialize the positions into a buffer.
-            buffer.clear();
-            buffer.reserve(positions.serialized_size());
-            positions.serialize_into(&mut buffer)?;
+            let bytes = ByteorderXRoaringBitmapCodec::bytes_encode(&positions)
+                .with_context(|| format!("could not serialize positions"))?;
             // that we write under the generated key into MTBL
             if lmdb_key_valid_size(&key) {
-                sorter.insert(&key, &buffer)?;
+                sorter.insert(&key, &bytes)?;
             }
         }
 
@@ -309,7 +307,11 @@ fn merge(key: &[u8], values: &[Vec<u8>]) -> Result<Vec<u8>, ()> {
             Ok(values[0].to_vec())
         },
         key => match key[0] {
-            DOCUMENTS_IDS_BYTE | WORD_DOCIDS_BYTE | WORD_DOCID_POSITIONS_BYTE => {
+            WORD_DOCID_POSITIONS_BYTE => {
+                assert!(values.windows(2).all(|vs| vs[0] == vs[1]));
+                Ok(values[0].to_vec())
+            },
+            DOCUMENTS_IDS_BYTE | WORD_DOCIDS_BYTE => {
                 let (head, tail) = values.split_first().unwrap();
 
                 let mut head = RoaringBitmap::deserialize_from(head.as_slice()).unwrap();

src/bin/infos.rs

@@ -257,13 +257,13 @@ fn average_number_of_words_by_doc(index: &Index, rtxn: &heed::RoTxn) -> anyhow::
 
 fn average_number_of_positions(index: &Index, rtxn: &heed::RoTxn) -> anyhow::Result<()> {
     use heed::types::DecodeIgnore;
-    use milli::RoaringBitmapCodec;
+    use milli::ByteorderXRoaringBitmapCodec;
 
     let mut values_length = Vec::new();
     let mut count = 0;
 
-    let iter = index.docid_word_positions.as_polymorph().iter::<_, DecodeIgnore, RoaringBitmapCodec>(rtxn)?;
-    for result in iter {
+    let db = index.docid_word_positions.as_polymorph();
+    for result in db.iter::<_, DecodeIgnore, ByteorderXRoaringBitmapCodec>(rtxn)? {
         let ((), val) = result?;
         values_length.push(val.len() as u32);
         count += 1;

src/heed_codec/byteorder_x_roaring_bitmap_codec.rs

@@ -0,0 +1,29 @@
+use std::borrow::Cow;
+use byteorder::{NativeEndian, ReadBytesExt, WriteBytesExt};
+use roaring::RoaringBitmap;
+
+pub struct ByteorderXRoaringBitmapCodec;
+
+impl heed::BytesDecode<'_> for ByteorderXRoaringBitmapCodec {
+    type DItem = RoaringBitmap;
+
+    fn bytes_decode(mut bytes: &[u8]) -> Option<Self::DItem> {
+        let mut bitmap = RoaringBitmap::new();
+        while let Ok(integer) = bytes.read_u32::<NativeEndian>() {
+            bitmap.insert(integer);
+        }
+        Some(bitmap)
+    }
+}
+
+impl heed::BytesEncode<'_> for ByteorderXRoaringBitmapCodec {
+    type EItem = RoaringBitmap;
+
+    fn bytes_encode(item: &Self::EItem) -> Option<Cow<[u8]>> {
+        let mut bytes = Vec::with_capacity(item.len() as usize * 4);
+        for integer in item.iter() {
+            bytes.write_u32::<NativeEndian>(integer).ok()?;
+        }
+        Some(Cow::Owned(bytes))
+    }
+}

src/heed_codec/mod.rs

@@ -1,7 +1,9 @@
+mod beu32_str_codec;
+mod byteorder_x_roaring_bitmap_codec;
 mod csv_string_record_codec;
 mod roaring_bitmap_codec;
-mod beu32_str_codec;
 
+pub use self::beu32_str_codec::BEU32StrCodec;
+pub use self::byteorder_x_roaring_bitmap_codec::ByteorderXRoaringBitmapCodec;
 pub use self::csv_string_record_codec::CsvStringRecordCodec;
 pub use self::roaring_bitmap_codec::RoaringBitmapCodec;
-pub use self::beu32_str_codec::BEU32StrCodec;

src/lib.rs

@@ -15,7 +15,10 @@ use heed::{PolyDatabase, Database};
 
 pub use self::search::{Search, SearchResult};
 pub use self::criterion::{Criterion, default_criteria};
-pub use self::heed_codec::{RoaringBitmapCodec, BEU32StrCodec, CsvStringRecordCodec};
+pub use self::heed_codec::{
+    RoaringBitmapCodec, BEU32StrCodec, CsvStringRecordCodec,
+    ByteorderXRoaringBitmapCodec,
+};
 
 pub type FastMap4<K, V> = HashMap<K, V, BuildHasherDefault<FxHasher32>>;
 pub type FastMap8<K, V> = HashMap<K, V, BuildHasherDefault<FxHasher64>>;
@@ -38,7 +41,7 @@ pub struct Index {
     /// A word and all the documents ids containing the word.
     pub word_docids: Database<Str, RoaringBitmapCodec>,
     /// Maps a word and a document id (u32) to all the positions where the given word appears.
-    pub docid_word_positions: Database<BEU32StrCodec, RoaringBitmapCodec>,
+    pub docid_word_positions: Database<BEU32StrCodec, ByteorderXRoaringBitmapCodec>,
     /// Maps the document id to the document as a CSV line.
     pub documents: Database<OwnedType<BEU32>, ByteSlice>,
 }