support for ancestor traversal (#427)

git-repository/src/revision/spec/parse/delegate.rs

@@ -324,7 +324,34 @@ impl<'repo> delegate::Navigate for Delegate<'repo> {
                         Err(err) => errors.push((*obj, err)),
                     }
                 }
-                Traversal::NthAncestor(_num) => todo!("ancestor"),
+                Traversal::NthAncestor(num) => {
+                    let id = obj.attach(repo);
+                    match id
+                        .ancestors()
+                        .first_parent_only()
+                        .all()
+                        .expect("cannot fail without sorting")
+                        .skip(num)
+                        .filter_map(Result::ok)
+                        .next()
+                    {
+                        Some(id) => replacements.push((*obj, id.detach())),
+                        None => errors.push((
+                            *obj,
+                            Error::AncestorOutOfRange {
+                                oid: id.shorten_or_id(),
+                                desired: num,
+                                available: id
+                                    .ancestors()
+                                    .first_parent_only()
+                                    .all()
+                                    .expect("cannot fail without sorting")
+                                    .skip(1)
+                                    .count(),
+                            },
+                        )),
+                    }
+                }
             }
         }
 

git-repository/src/revision/spec/parse/types.rs

@@ -61,6 +61,14 @@ pub struct Options {
 #[derive(Debug, thiserror::Error)]
 #[allow(missing_docs)]
 pub enum Error {
+    #[error(
+        "Commit {oid} has {available} ancestors along the first parent and ancestor number {desired} is out of range"
+    )]
+    AncestorOutOfRange {
+        oid: git_hash::Prefix,
+        desired: usize,
+        available: usize,
+    },
     #[error("Commit {oid} has {available} parents and parent number {desired} is out of range")]
     ParentOutOfRange {
         oid: git_hash::Prefix,

git-repository/tests/fixtures/make_rev_spec_parse_repos.sh

@@ -350,6 +350,10 @@ git init complex_graph
   # parents
   baseline "a"
   baseline "a^1"
+  baseline "a~1"
   baseline "a^0"
+  baseline "a~0"
   baseline "a^42"
+  baseline "a~42"
+  baseline "a~3"
 )

git-repository/tests/revision/spec/from_bytes/mod.rs

@@ -27,6 +27,24 @@ mod traverse {
             "Commit 55e825e has 2 parents and parent number 42 is out of range"
         );
     }
+
+    #[test]
+    fn ancestors() {
+        let repo = repo("complex_graph").unwrap();
+        assert_eq!(
+            parse_spec("a~1", &repo).unwrap(),
+            Spec::from_id(hex_to_id("5b3f9e24965d0b28780b7ce5daf2b5b7f7e0459f").attach(&repo))
+        );
+        assert_eq!(parse_spec("a", &repo).unwrap(), parse_spec("a~0", &repo).unwrap(),);
+        assert_eq!(
+            parse_spec("a~3", &repo).unwrap(),
+            Spec::from_id(hex_to_id("9f9eac6bd1cd4b4cc6a494f044b28c985a22972b").attach(&repo))
+        );
+        assert_eq!(
+            parse_spec("a~42", &repo).unwrap_err().to_string(),
+            "Commit 55e825e has 3 ancestors along the first parent and ancestor number 42 is out of range"
+        );
+    }
 }
 
 mod index {