Refactor facet-related codecs

milli/src/heed_codec/facet/ordered_f64_codec.rs

@@ -0,0 +1,37 @@
+use std::borrow::Cow;
+use std::convert::TryInto;
+
+use heed::BytesDecode;
+
+use crate::facet::value_encoding::f64_into_bytes;
+
+pub struct OrderedF64Codec;
+
+impl<'a> BytesDecode<'a> for OrderedF64Codec {
+    type DItem = f64;
+
+    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
+        if bytes.len() < 16 {
+            return None;
+        }
+        let f = bytes[8..].try_into().ok().map(f64::from_be_bytes)?;
+        Some(f)
+    }
+}
+
+impl heed::BytesEncode<'_> for OrderedF64Codec {
+    type EItem = f64;
+
+    fn bytes_encode(f: &Self::EItem) -> Option<Cow<[u8]>> {
+        let mut buffer = [0u8; 16];
+
+        // write the globally ordered float
+        let bytes = f64_into_bytes(*f)?;
+        buffer[..8].copy_from_slice(&bytes[..]);
+        // Then the f64 value just to be able to read it back
+        let bytes = f.to_be_bytes();
+        buffer[8..16].copy_from_slice(&bytes[..]);
+
+        Some(Cow::Owned(buffer.to_vec()))
+    }
+}