Introduce the CboRoaringBitmapCodec

src/heed_codec/cbo_roaring_bitmap_codec.rs

@@ -0,0 +1,40 @@
+use std::borrow::Cow;
+use std::mem::size_of;
+use roaring::RoaringBitmap;
+use super::{BoRoaringBitmapCodec, RoaringBitmapCodec};
+
+/// A conditionnal codec that either use the RoaringBitmap
+/// or a lighter ByteOrder en/decoding method.
+pub struct CboRoaringBitmapCodec;
+
+impl heed::BytesDecode<'_> for CboRoaringBitmapCodec {
+    type DItem = RoaringBitmap;
+
+    fn bytes_decode(bytes: &[u8]) -> Option<Self::DItem> {
+        if bytes.len() <= 4 * size_of::<u32>() {
+            // If there is 4 or less than 4 integers that can fit into this array
+            // of bytes it means that we used the ByteOrder codec serializer.
+            BoRoaringBitmapCodec::bytes_decode(bytes)
+        } else {
+            // Otherwise, it means we used the classic RoaringBitmapCodec and
+            // that the header takes 4 integers.
+            RoaringBitmapCodec::bytes_decode(bytes)
+        }
+    }
+}
+
+impl heed::BytesEncode<'_> for CboRoaringBitmapCodec {
+    type EItem = RoaringBitmap;
+
+    fn bytes_encode(item: &Self::EItem) -> Option<Cow<[u8]>> {
+        if item.len() <= 4 {
+            // If the number of items (u32s) to encode is less than or equal to 4
+            // it means that it would weigh the same or less than the RoaringBitmap
+            // header, so we directly encode them using ByteOrder instead.
+            BoRoaringBitmapCodec::bytes_encode(item)
+        } else {
+            // Otherwise, we use the classic RoaringBitmapCodec that writes a header.
+            RoaringBitmapCodec::bytes_encode(item)
+        }
+    }
+}

src/heed_codec/mod.rs

@@ -1,11 +1,13 @@
 mod beu32_str_codec;
 mod bo_roaring_bitmap_codec;
+mod cbo_roaring_bitmap_codec;
 mod csv_string_record_codec;
 mod roaring_bitmap_codec;
 mod str_str_u8_codec;
 
 pub use self::beu32_str_codec::BEU32StrCodec;
 pub use self::bo_roaring_bitmap_codec::BoRoaringBitmapCodec;
+pub use self::cbo_roaring_bitmap_codec::CboRoaringBitmapCodec;
 pub use self::csv_string_record_codec::CsvStringRecordCodec;
 pub use self::roaring_bitmap_codec::RoaringBitmapCodec;
 pub use self::str_str_u8_codec::StrStrU8Codec;