Introduce the compressed obkv readers and writers

milli/src/heed_codec/compressed_obkv_codec.rs

@@ -0,0 +1,50 @@
+use std::borrow::Cow;
+
+use heed::BoxedError;
+use obkv::KvReaderU16;
+
+pub struct ObkvCompressedCodec;
+
+impl<'a> heed::BytesDecode<'a> for ObkvCompressedCodec {
+    type DItem = CompressedKvReaderU16<'a>;
+
+    fn bytes_decode(bytes: &'a [u8]) -> Result<Self::DItem, BoxedError> {
+        Ok(CompressedKvReaderU16(bytes))
+    }
+}
+
+impl heed::BytesEncode<'_> for ObkvCompressedCodec {
+    type EItem = CompressedKvWriterU16;
+
+    fn bytes_encode(item: &Self::EItem) -> Result<Cow<[u8]>, BoxedError> {
+        Ok(Cow::Borrowed(&item.0))
+    }
+}
+
+pub struct CompressedKvReaderU16<'a>(&'a [u8]);
+
+impl<'a> CompressedKvReaderU16<'a> {
+    pub fn decompress_with<'b>(
+        &self,
+        buffer: &'b mut Vec<u8>,
+        dictionnary: &[u8],
+    ) -> Result<KvReaderU16<'b>, lz4_flex::block::DecompressError> {
+        let max_size = lz4_flex::block::get_maximum_output_size(self.0.len());
+        buffer.resize(max_size, 0);
+        let size = lz4_flex::block::decompress_into_with_dict(
+            self.0,
+            &mut buffer[..max_size],
+            dictionnary,
+        )?;
+        Ok(KvReaderU16::new(&buffer[..size]))
+    }
+}
+
+pub struct CompressedKvWriterU16(Vec<u8>);
+
+impl CompressedKvWriterU16 {
+    // TODO ask for a KvReaderU16 here
+    pub fn new_with_dictionnary(writer: &[u8], dictionnary: &[u8]) -> Self {
+        CompressedKvWriterU16(lz4_flex::block::compress_with_dict(writer, dictionnary))
+    }
+}

milli/src/heed_codec/mod.rs

@@ -1,6 +1,7 @@
 mod beu16_str_codec;
 mod beu32_str_codec;
 mod byte_slice_ref;
+mod compressed_obkv_codec;
 pub mod facet;
 mod field_id_word_count_codec;
 mod fst_set_codec;