index: use local position in segment-local operations

I'm going to change the index format to store local positions in the lookup
table. That's not super important, but I think it makes sense because the
lookup table should never contain inter-segment links.

The mutable segment now stores local positions in its lookup map. The readonly
segment will be updated later.

lib/src/default_index/composite.rs

@@ -44,7 +44,7 @@ pub(super) trait IndexSegment: Send + Sync {
 
     fn name(&self) -> Option<String>;
 
-    fn commit_id_to_pos(&self, commit_id: &CommitId) -> Option<IndexPosition>;
+    fn commit_id_to_pos(&self, commit_id: &CommitId) -> Option<LocalPosition>;
 
     /// Suppose the given `commit_id` exists, returns the previous and next
     /// commit ids in lexicographical order.
@@ -158,8 +158,10 @@ impl<'a> CompositeIndex<'a> {
     }
 
     pub fn commit_id_to_pos(&self, commit_id: &CommitId) -> Option<IndexPosition> {
-        self.ancestor_index_segments()
-            .find_map(|segment| segment.commit_id_to_pos(commit_id))
+        self.ancestor_index_segments().find_map(|segment| {
+            let LocalPosition(local_pos) = segment.commit_id_to_pos(commit_id)?;
+            Some(IndexPosition(local_pos + segment.num_parent_commits()))
+        })
     }
 
     /// Suppose the given `commit_id` exists, returns the previous and next

lib/src/default_index/mutable.rs

@@ -56,7 +56,7 @@ pub(super) struct MutableIndexSegment {
     commit_id_length: usize,
     change_id_length: usize,
     graph: Vec<MutableGraphEntry>,
-    commit_lookup: BTreeMap<CommitId, IndexPosition>,
+    commit_lookup: BTreeMap<CommitId, LocalPosition>,
 }
 
 impl MutableIndexSegment {
@@ -125,7 +125,7 @@ impl MutableIndexSegment {
         }
         self.commit_lookup.insert(
             entry.commit_id.clone(),
-            IndexPosition(u32::try_from(self.graph.len()).unwrap() + self.num_parent_commits),
+            LocalPosition(u32::try_from(self.graph.len()).unwrap()),
         );
         self.graph.push(entry);
     }
@@ -222,9 +222,9 @@ impl MutableIndexSegment {
             buf.extend_from_slice(entry.commit_id.as_bytes());
         }
 
-        for (commit_id, pos) in &self.commit_lookup {
+        for (commit_id, LocalPosition(pos)) in &self.commit_lookup {
             buf.extend_from_slice(commit_id.as_bytes());
-            buf.extend(pos.0.to_le_bytes());
+            buf.extend((pos + self.num_parent_commits).to_le_bytes());
         }
 
         buf[parent_overflow_offset..][..4]
@@ -316,8 +316,8 @@ impl IndexSegment for MutableIndexSegment {
         None
     }
 
-    fn commit_id_to_pos(&self, commit_id: &CommitId) -> Option<IndexPosition> {
-        self.commit_lookup.get(commit_id).cloned()
+    fn commit_id_to_pos(&self, commit_id: &CommitId) -> Option<LocalPosition> {
+        self.commit_lookup.get(commit_id).copied()
     }
 
     fn resolve_neighbor_commit_ids(

lib/src/default_index/readonly.rs

@@ -122,6 +122,7 @@ impl CommitGraphEntry<'_> {
 struct CommitLookupEntry<'a> {
     data: &'a [u8],
     commit_id_length: usize,
+    num_parent_commits: u32,
 }
 
 impl CommitLookupEntry<'_> {
@@ -142,6 +143,13 @@ impl CommitLookupEntry<'_> {
         let pos = u32::from_le_bytes(self.data[self.commit_id_length..][..4].try_into().unwrap());
         IndexPosition(pos)
     }
+
+    // TODO: better to store local pos in lookup table since there should be no
+    // inter-segment entry.
+    fn local_pos(&self) -> LocalPosition {
+        let IndexPosition(pos) = self.pos();
+        LocalPosition(pos - self.num_parent_commits)
+    }
 }
 
 /// Commit index segment backed by immutable file.
@@ -336,6 +344,7 @@ impl ReadonlyIndexSegment {
         CommitLookupEntry {
             data: &self.data[offset..][..self.commit_lookup_entry_size],
             commit_id_length: self.commit_id_length,
+            num_parent_commits: self.num_parent_commits,
         }
     }
 
@@ -388,10 +397,10 @@ impl IndexSegment for ReadonlyIndexSegment {
         Some(self.name.clone())
     }
 
-    fn commit_id_to_pos(&self, commit_id: &CommitId) -> Option<IndexPosition> {
+    fn commit_id_to_pos(&self, commit_id: &CommitId) -> Option<LocalPosition> {
         let lookup_pos = self.commit_id_byte_prefix_to_lookup_pos(commit_id)?;
         let entry = self.commit_lookup_entry(lookup_pos);
-        (&entry.commit_id() == commit_id).then(|| entry.pos())
+        (&entry.commit_id() == commit_id).then(|| entry.local_pos())
     }
 
     fn resolve_neighbor_commit_ids(