perf: speed up impostor-commit's fast path (#158)

src/audit/impostor_commit.rs

@@ -58,14 +58,28 @@ impl ImpostorCommit {
     /// i.e. resolves due to presence in GitHub's fork network but is not actually
     /// present in any of the specified `owner/repo`'s tags or branches.
     fn impostor(&self, uses: Uses<'_>) -> Result<bool> {
-        let (branches, tags) = self.named_refs(uses)?;
-
         // If there's no ref or the ref is not a commit, there's nothing to impersonate.
         let Some(head_ref) = uses.commit_ref() else {
             return Ok(false);
         };
 
-        for branch in branches {
+        let (branches, tags) = self.named_refs(uses)?;
+
+        // Fast path: almost all commit refs will be at the tip of
+        // the branch or tag's history, so check those first.
+        for branch in &branches {
+            if branch.commit.sha == head_ref {
+                return Ok(false);
+            }
+        }
+
+        for tag in &tags {
+            if tag.commit.sha == head_ref {
+                return Ok(false);
+            }
+        }
+
+        for branch in &branches {
             if self.named_ref_contains_commit(
                 &uses,
                 &format!("refs/heads/{}", &branch.name),
@@ -75,7 +89,7 @@ impl ImpostorCommit {
             }
         }
 
-        for tag in tags {
+        for tag in &tags {
             if self.named_ref_contains_commit(
                 &uses,
                 &format!("refs/tags/{}", &tag.name),

src/github_api.rs

@@ -206,6 +206,7 @@ impl Client {
 #[derive(Deserialize, Clone)]
 pub(crate) struct Branch {
     pub(crate) name: String,
+    pub(crate) commit: Object,
 }
 
 /// A single tag, as returned by GitHub's tags endpoints.
@@ -214,23 +215,18 @@ pub(crate) struct Branch {
 #[derive(Deserialize, Clone)]
 pub(crate) struct Tag {
     pub(crate) name: String,
-    pub(crate) commit: TagCommit,
+    pub(crate) commit: Object,
 }
 
-/// Represents the SHA ref bound to a tag.
+/// Represents a git object.
 #[derive(Deserialize, Clone)]
-pub(crate) struct TagCommit {
+pub(crate) struct Object {
     pub(crate) sha: String,
 }
 
 #[derive(Deserialize)]
 pub(crate) struct GitRef {
-    pub(crate) object: GitObj,
-}
-
-#[derive(Deserialize)]
-pub(crate) struct GitObj {
-    pub(crate) sha: String,
+    pub(crate) object: Object,
 }
 
 #[derive(Clone, Deserialize)]