1 files changed, 53 insertions, 0 deletions
diff --git a/commitgraph/read/layer_lookup.go b/commitgraph/read/layer_lookup.go
new file mode 100644
index 00000000..9a03505a
--- /dev/null
+++ b/commitgraph/read/layer_lookup.go
@@ -0,0 +1,53 @@
+package read
+
+import (
+	"bytes"
+	"encoding/binary"
+
+	"codeberg.org/lindenii/furgit/internal/intconv"
+	"codeberg.org/lindenii/furgit/objectid"
+)
+
+func layerLookup(layer *layer, oid objectid.ObjectID) (uint32, bool) {
+	hashSize := oid.Size()
+	first := int(oid.RawBytes()[0])
+
+	var lo uint32
+	if first > 0 {
+		lo = binary.BigEndian.Uint32(layer.chunkOIDFanout[(first-1)*4 : first*4])
+	}
+
+	hi := binary.BigEndian.Uint32(layer.chunkOIDFanout[first*4 : (first+1)*4])
+	if hi == 0 || lo >= hi {
+		return 0, false
+	}
+
+	target := oid.RawBytes()
+	left := int(lo)
+
+	right := int(hi) - 1
+	for left <= right {
+		mid := left + (right-left)/2
+		start := mid * hashSize
+		end := start + hashSize
+
+		current := layer.chunkOIDLookup[start:end]
+
+		cmp := bytes.Compare(current, target)
+		switch {
+		case cmp == 0:
+			pos, err := intconv.IntToUint32(mid)
+			if err != nil {
+				return 0, false
+			}
+
+			return pos, true
+		case cmp < 0:
+			left = mid + 1
+		default:
+			right = mid - 1
+		}
+	}
+
+	return 0, false
+}