feat: describe() aborts search early if there is no input name in t…

…he name map.

git-revision/src/describe.rs

@@ -177,18 +177,8 @@ pub(crate) mod function {
         Find: for<'b> FnMut(&oid, &'b mut Vec<u8>) -> Result<Option<CommitRefIter<'b>>, E>,
         E: std::error::Error + Send + Sync + 'static,
     {
-        if let Some(name) = name_by_oid.get(commit) {
-            return Ok(Some(Outcome {
-                name: name.clone().into(),
-                id: commit.to_owned(),
-                depth: 0,
-                name_by_oid,
-                commits_seen: 0,
-            }));
-        }
-
         max_candidates = max_candidates.min(MAX_CANDIDATES);
-        if max_candidates == 0 {
+        if max_candidates == 0 || name_by_oid.is_empty() {
             return if fallback_to_oid {
                 Ok(Some(Outcome {
                     id: commit.to_owned(),
@@ -202,6 +192,16 @@ pub(crate) mod function {
             };
         }
 
+        if let Some(name) = name_by_oid.get(commit) {
+            return Ok(Some(Outcome {
+                name: name.clone().into(),
+                id: commit.to_owned(),
+                depth: 0,
+                name_by_oid,
+                commits_seen: 0,
+            }));
+        }
+
         let mut buf = Vec::new();
         let mut parent_buf = Vec::new();
 

git-revision/tests/describe/mod.rs

@@ -24,7 +24,7 @@ fn option_none_if_no_tag_found() -> crate::Result {
 }
 
 #[test]
-fn fallback_if_configured_in_options_but_no_candidate() -> crate::Result {
+fn fallback_if_configured_in_options_but_no_candidate_or_names() -> crate::Result {
     let repo = repo();
     let commit = repo.head_commit()?;
     let res = git_revision::describe(
@@ -38,7 +38,10 @@ fn fallback_if_configured_in_options_but_no_candidate() -> crate::Result {
     .expect("fallback activated");
     assert!(res.name.is_none(), "no name can be found");
     assert_eq!(res.depth, 0, "just a default, not relevant as there is no name");
-    assert_eq!(res.commits_seen, 8, "a traversal is performed");
+    assert_eq!(
+        res.commits_seen, 0,
+        "a traversal is isn't performed as name map is empty, and that's the whole point"
+    );
     assert_eq!(res.into_format(7).to_string(), "01ec18a");
     Ok(())
 }
@@ -91,6 +94,7 @@ fn not_enough_candidates() -> crate::Result {
 
     assert_eq!(res.name, Some(name), "it finds the youngest/most-recent name");
     assert_eq!(res.id, commit.id);
+    assert_eq!(res.commits_seen, 6, "it has to traverse commits");
     assert_eq!(
         res.depth, 3,
         "it calculates the final number of commits even though it aborted early"
@@ -146,6 +150,7 @@ fn typical_usecases() {
     );
     assert_eq!(res.id, commit.id);
     assert_eq!(res.depth, 3);
+    assert_eq!(res.commits_seen, 6);
 
     let res = git_revision::describe(
         &commit.id,