index: extract helper functions for id lookup in mutable table

Similar to the previous commit, these functions will be reused by the change id
lookup methods. The return value isn't cloned because resolve_id_prefix() will
return (key, value) pair, and the current caller doesn't need a cloned value.

lib/src/default_index/mutable.rs

@@ -334,32 +334,13 @@ impl IndexSegment for MutableIndexSegment {
         &self,
         commit_id: &CommitId,
     ) -> (Option<CommitId>, Option<CommitId>) {
-        let prev_id = self
-            .commit_lookup
-            .range((Bound::Unbounded, Bound::Excluded(commit_id)))
-            .next_back()
-            .map(|(id, _)| id.clone());
-        let next_id = self
-            .commit_lookup
-            .range((Bound::Excluded(commit_id), Bound::Unbounded))
-            .next()
-            .map(|(id, _)| id.clone());
-        (prev_id, next_id)
+        let (prev_id, next_id) = resolve_neighbor_ids(&self.commit_lookup, commit_id);
+        (prev_id.cloned(), next_id.cloned())
     }
 
     fn resolve_commit_id_prefix(&self, prefix: &HexPrefix) -> PrefixResolution<CommitId> {
         let min_bytes_prefix = CommitId::from_bytes(prefix.min_prefix_bytes());
-        let mut matches = self
-            .commit_lookup
-            .range((Bound::Included(&min_bytes_prefix), Bound::Unbounded))
-            .map(|(id, _pos)| id)
-            .take_while(|&id| prefix.matches(id))
-            .fuse();
-        match (matches.next(), matches.next()) {
-            (Some(id), None) => PrefixResolution::SingleMatch(id.clone()),
-            (Some(_), Some(_)) => PrefixResolution::AmbiguousMatch,
-            (None, _) => PrefixResolution::NoMatch,
-        }
+        resolve_id_prefix(&self.commit_lookup, prefix, &min_bytes_prefix).map(|id| id.clone())
     }
 
     fn generation_number(&self, local_pos: LocalPosition) -> u32 {
@@ -499,3 +480,35 @@ impl MutableIndex for DefaultMutableIndex {
         self.0.merge_in(other.as_segment().clone());
     }
 }
+
+fn resolve_neighbor_ids<'a, K: Ord, V>(
+    lookup_table: &'a BTreeMap<K, V>,
+    id: &K,
+) -> (Option<&'a K>, Option<&'a K>) {
+    let prev_id = lookup_table
+        .range((Bound::Unbounded, Bound::Excluded(id)))
+        .next_back()
+        .map(|(id, _)| id);
+    let next_id = lookup_table
+        .range((Bound::Excluded(id), Bound::Unbounded))
+        .next()
+        .map(|(id, _)| id);
+    (prev_id, next_id)
+}
+
+fn resolve_id_prefix<'a, K: ObjectId + Ord, V>(
+    lookup_table: &'a BTreeMap<K, V>,
+    prefix: &HexPrefix,
+    min_bytes_prefix: &K,
+) -> PrefixResolution<&'a K> {
+    let mut matches = lookup_table
+        .range((Bound::Included(min_bytes_prefix), Bound::Unbounded))
+        .map(|(id, _pos)| id)
+        .take_while(|&id| prefix.matches(id))
+        .fuse();
+    match (matches.next(), matches.next()) {
+        (Some(id), None) => PrefixResolution::SingleMatch(id),
+        (Some(_), Some(_)) => PrefixResolution::AmbiguousMatch,
+        (None, _) => PrefixResolution::NoMatch,
+    }
+}