1 files changed, 49 insertions, 0 deletions
diff --git a/commitquery/query_is_ancestor.go b/commitquery/query_is_ancestor.go
new file mode 100644
index 00000000..c21892c8
--- /dev/null
+++ b/commitquery/query_is_ancestor.go
@@ -0,0 +1,49 @@
+package commitquery
+
+import objectid "codeberg.org/lindenii/furgit/object/id"
+
+// IsAncestor reports whether ancestor is reachable from descendant through
+// commit parent edges.
+//
+// Both inputs are peeled through annotated tags before commit traversal.
+func (query *query) IsAncestor(ancestor, descendant objectid.ObjectID) (bool, error) {
+	ancestorIdx, err := query.resolveCommitish(ancestor)
+	if err != nil {
+		return false, err
+	}
+
+	descendantIdx, err := query.resolveCommitish(descendant)
+	if err != nil {
+		return false, err
+	}
+
+	return query.isAncestor(ancestorIdx, descendantIdx)
+}
+
+// isAncestor answers one ancestry query between two resolved internal nodes.
+func (query *query) isAncestor(ancestor, descendant nodeIndex) (bool, error) {
+	if ancestor == descendant {
+		return true, nil
+	}
+
+	ancestorGeneration := query.effectiveGeneration(ancestor)
+	descendantGeneration := query.effectiveGeneration(descendant)
+
+	if ancestorGeneration != generationInfinity &&
+		descendantGeneration != generationInfinity &&
+		ancestorGeneration > descendantGeneration {
+		return false, nil
+	}
+
+	minGeneration := uint64(0)
+	if ancestorGeneration != generationInfinity {
+		minGeneration = ancestorGeneration
+	}
+
+	err := query.paintDownToCommon(ancestor, []nodeIndex{descendant}, minGeneration)
+	if err != nil {
+		return false, err
+	}
+
+	return query.hasAnyMarks(ancestor, markRight), nil
+}