format/commitgraph: Add initial commit-graph support

1 files changed, 65 insertions, 0 deletions

diff --git a/format/commitgraph/parents.go b/format/commitgraph/parents.go
new file mode 100644
index 00000000..df6d33ff
--- /dev/null
+++ b/format/commitgraph/parents.go
@@ -0,0 +1,65 @@
+package commitgraph
+
+// ParentRef references one parent position.
+type ParentRef struct {
+	Valid bool
+	Pos   Position
+}
+
+func (reader *Reader) decodeParents(layer *layer, p1, p2 uint32) (ParentRef, ParentRef, []Position, error) {
+	parent1, err := reader.decodeSingleParent(p1)
+	if err != nil {
+		return ParentRef{}, ParentRef{}, nil, err
+	}
+
+	if p2 == parentNone {
+		return parent1, ParentRef{}, nil, nil
+	}
+
+	if p2&parentExtraMask == 0 {
+		parent2, err := reader.decodeSingleParent(p2)
+		if err != nil {
+			return ParentRef{}, ParentRef{}, nil, err
+		}
+
+		return parent1, parent2, nil, nil
+	}
+
+	edgeStart := p2 & parentLastMask
+
+	parents, err := reader.decodeExtraEdgeList(layer, edgeStart)
+	if err != nil {
+		return ParentRef{}, ParentRef{}, nil, err
+	}
+
+	if len(parents) == 0 {
+		return ParentRef{}, ParentRef{}, nil, &ErrMalformed{Path: layer.path, Reason: "empty EDGE list"}
+	}
+
+	parent2 := ParentRef{Valid: true, Pos: parents[0]}
+	if len(parents) == 1 {
+		return parent1, parent2, nil, nil
+	}
+
+	return parent1, parent2, parents[1:], nil
+}
+
+func (reader *Reader) decodeSingleParent(raw uint32) (ParentRef, error) {
+	if raw == parentNone {
+		return ParentRef{}, nil
+	}
+
+	if raw&parentExtraMask != 0 {
+		return ParentRef{}, &ErrMalformed{
+			Path:   "commit-graph",
+			Reason: "unexpected EDGE marker in single-parent slot",
+		}
+	}
+
+	pos, err := reader.globalToPosition(raw)
+	if err != nil {
+		return ParentRef{}, err
+	}
+
+	return ParentRef{Valid: true, Pos: pos}, nil
+}