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

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;