Add a database containing the docids where each field exists

milli/src/heed_codec/facet/mod.rs

@@ -25,3 +25,43 @@ pub fn try_split_at(slice: &[u8], mid: usize) -> Option<(&[u8], &[u8])> {
         None
     }
 }
+
+use crate::{try_split_array_at, DocumentId, FieldId};
+use std::borrow::Cow;
+use std::convert::TryInto;
+
+pub struct FieldIdCodec;
+
+impl<'a> heed::BytesDecode<'a> for FieldIdCodec {
+    type DItem = FieldId;
+
+    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
+        let (field_id_bytes, _) = try_split_array_at(bytes)?;
+        let field_id = u16::from_be_bytes(field_id_bytes);
+        Some(field_id)
+    }
+}
+
+impl<'a> heed::BytesEncode<'a> for FieldIdCodec {
+    type EItem = FieldId;
+
+    fn bytes_encode(field_id: &Self::EItem) -> Option<Cow<[u8]>> {
+        Some(Cow::Owned(field_id.to_be_bytes().to_vec()))
+    }
+}
+
+pub struct FieldIdDocIdCodec;
+
+impl<'a> heed::BytesDecode<'a> for FieldIdDocIdCodec {
+    type DItem = (FieldId, DocumentId);
+
+    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
+        let (field_id_bytes, bytes) = try_split_array_at(bytes)?;
+        let field_id = u16::from_be_bytes(field_id_bytes);
+
+        let document_id_bytes = bytes[..4].try_into().ok()?;
+        let document_id = u32::from_be_bytes(document_id_bytes);
+
+        Some((field_id, document_id))
+    }
+}