Improve the FacetLevelF64 serialization

src/heed_codec/facet/facet_level_value_f64_codec.rs

@@ -13,8 +13,14 @@ impl<'a> heed::BytesDecode<'a> for FacetLevelValueF64Codec {
         let (field_id, bytes) = bytes.split_first()?;
         let (level, bytes) = bytes.split_first()?;
 
+        let (left, right) = if *level != 0 {
             let left = bytes[16..24].try_into().ok().map(f64::from_be_bytes)?;
             let right = bytes[24..].try_into().ok().map(f64::from_be_bytes)?;
+            (left, right)
+        } else {
+            let left = bytes[8..].try_into().ok().map(f64::from_be_bytes)?;
+            (left, left)
+        };
 
         Some((*field_id, *level, left, right))
     }
@@ -26,6 +32,7 @@ impl heed::BytesEncode<'_> for FacetLevelValueF64Codec {
     fn bytes_encode((field_id, level, left, right): &Self::EItem) -> Option<Cow<[u8]>> {
         let mut buffer = [0u8; 32];
 
+        let len = if *level != 0 {
             // Write the globally ordered floats.
             let bytes = f64_into_bytes(*left)?;
             buffer[..8].copy_from_slice(&bytes[..]);
@@ -40,10 +47,23 @@ impl heed::BytesEncode<'_> for FacetLevelValueF64Codec {
             let bytes = right.to_be_bytes();
             buffer[24..].copy_from_slice(&bytes[..]);
 
-        let mut bytes = Vec::with_capacity(buffer.len() + 2);
+            32 // length
+        } else {
+            // Write the globally ordered floats.
+            let bytes = f64_into_bytes(*left)?;
+            buffer[..8].copy_from_slice(&bytes[..]);
+
+            // Then the f64 values just to be able to read them back.
+            let bytes = left.to_be_bytes();
+            buffer[8..16].copy_from_slice(&bytes[..]);
+
+            16 // length
+        };
+
+        let mut bytes = Vec::with_capacity(len + 2);
         bytes.push(*field_id);
         bytes.push(*level);
-        bytes.extend_from_slice(&buffer[..]);
+        bytes.extend_from_slice(&buffer[..len]);
         Some(Cow::Owned(bytes))
     }
 }
@@ -55,8 +75,12 @@ mod tests {
 
     #[test]
     fn globally_ordered_f64() {
-        let bytes = FacetLevelValueF64Codec::bytes_encode(&(3, 0, -32.0, 32.0)).unwrap();
+        let bytes = FacetLevelValueF64Codec::bytes_encode(&(3, 0, 32.0, 0.0)).unwrap();
         let (name, level, left, right) = FacetLevelValueF64Codec::bytes_decode(&bytes).unwrap();
-        assert_eq!((name, level, left, right), (3, 0, -32.0, 32.0));
+        assert_eq!((name, level, left, right), (3, 0, 32.0, 32.0));
+
+        let bytes = FacetLevelValueF64Codec::bytes_encode(&(3, 1, -32.0, 32.0)).unwrap();
+        let (name, level, left, right) = FacetLevelValueF64Codec::bytes_decode(&bytes).unwrap();
+        assert_eq!((name, level, left, right), (3, 1, -32.0, 32.0));
     }
 }