Store the facet string values in multiple FSTs

milli/src/heed_codec/fst_set_codec.rs

@@ -0,0 +1,23 @@
+use fst::Set;
+use std::borrow::Cow;
+
+use heed::{BytesDecode, BytesEncode};
+
+/// A codec for values of type `Set<&[u8]>`.
+pub struct FstSetCodec;
+
+impl<'a> BytesEncode<'a> for FstSetCodec {
+    type EItem = Set<Vec<u8>>;
+
+    fn bytes_encode(item: &'a Self::EItem) -> Option<Cow<'a, [u8]>> {
+        Some(Cow::Borrowed(item.as_fst().as_bytes()))
+    }
+}
+
+impl<'a> BytesDecode<'a> for FstSetCodec {
+    type DItem = Set<&'a [u8]>;
+
+    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
+        Some(Set::new(bytes).ok()?)
+    }
+}