From ac2105f2e3dfdf2b1775e6479594daf0503b27cc Mon Sep 17 00:00:00 2001
From: Sebastian Thiel <sebastian.thiel@icloud.com>
Date: Tue, 2 Aug 2022 13:59:13 +0800
Subject: [PATCH] support for ancestor traversal (#427)

---
 .../src/revision/spec/parse/delegate.rs       | 29 ++++++++++++++++++-
 .../src/revision/spec/parse/types.rs          |  8 +++++
 .../make_rev_spec_parse_repos.tar.xz          |  4 +--
 .../fixtures/make_rev_spec_parse_repos.sh     |  4 +++
 .../tests/revision/spec/from_bytes/mod.rs     | 18 ++++++++++++
 5 files changed, 60 insertions(+), 3 deletions(-)

diff --git a/git-repository/src/revision/spec/parse/delegate.rs b/git-repository/src/revision/spec/parse/delegate.rs
index 7eb1dba99f..5094bac5f8 100644
--- a/git-repository/src/revision/spec/parse/delegate.rs
+++ b/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(),
+                            },
+                        )),
+                    }
+                }
             }
         }
 
diff --git a/git-repository/src/revision/spec/parse/types.rs b/git-repository/src/revision/spec/parse/types.rs
index 70b4871b85..60f3e2ba6c 100644
--- a/git-repository/src/revision/spec/parse/types.rs
+++ b/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,
diff --git a/git-repository/tests/fixtures/generated-archives/make_rev_spec_parse_repos.tar.xz b/git-repository/tests/fixtures/generated-archives/make_rev_spec_parse_repos.tar.xz
index c1310f8e22..cec8beee10 100644
--- a/git-repository/tests/fixtures/generated-archives/make_rev_spec_parse_repos.tar.xz
+++ b/git-repository/tests/fixtures/generated-archives/make_rev_spec_parse_repos.tar.xz
@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:a6db0e8ed85ec2e7a3457d411a65a3508683de5156f45baf551c8d019b26e2e3
-size 28012
+oid sha256:15daf80cdca30bfed1844c71a4a2e88becb4e2e08972f4a6ea42eefc23f43000
+size 27980
diff --git a/git-repository/tests/fixtures/make_rev_spec_parse_repos.sh b/git-repository/tests/fixtures/make_rev_spec_parse_repos.sh
index 46949e4574..9cdcf3fdbf 100644
--- a/git-repository/tests/fixtures/make_rev_spec_parse_repos.sh
+++ b/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"
 )
diff --git a/git-repository/tests/revision/spec/from_bytes/mod.rs b/git-repository/tests/revision/spec/from_bytes/mod.rs
index 390617cee9..d00249365f 100644
--- a/git-repository/tests/revision/spec/from_bytes/mod.rs
+++ b/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 {