format/commitgraph: Add initial commit-graph support

1 files changed, 36 insertions, 0 deletions

diff --git a/format/commitgraph/oidat.go b/format/commitgraph/oidat.go
new file mode 100644
index 00000000..e277125b
--- /dev/null
+++ b/format/commitgraph/oidat.go
@@ -0,0 +1,36 @@
+package commitgraph
+
+import (
+	"codeberg.org/lindenii/furgit/internal/intconv"
+	"codeberg.org/lindenii/furgit/objectid"
+)
+
+// OIDAt returns object ID at one position.
+func (reader *Reader) OIDAt(pos Position) (objectid.ObjectID, error) {
+	layer, err := reader.layerByPosition(pos)
+	if err != nil {
+		return objectid.ObjectID{}, err
+	}
+
+	hashSize := reader.algo.Size()
+
+	hashSizeU64, err := intconv.IntToUint64(hashSize)
+	if err != nil {
+		return objectid.ObjectID{}, err
+	}
+
+	start64 := uint64(pos.Index) * hashSizeU64
+	end64 := start64 + hashSizeU64
+
+	start, err := intconv.Uint64ToInt(start64)
+	if err != nil {
+		return objectid.ObjectID{}, err
+	}
+
+	end, err := intconv.Uint64ToInt(end64)
+	if err != nil {
+		return objectid.ObjectID{}, err
+	}
+
+	return objectid.FromBytes(reader.algo, layer.chunkOIDLookup[start:end])
+}