1 files changed, 34 insertions, 0 deletions
diff --git a/object/store/packed/pack_idx_checksum.go b/object/store/packed/pack_idx_checksum.go
new file mode 100644
index 00000000..28d4c3db
--- /dev/null
+++ b/object/store/packed/pack_idx_checksum.go
@@ -0,0 +1,34 @@
+package packed
+
+import (
+	"bytes"
+	"fmt"
+
+	objectid "codeberg.org/lindenii/furgit/object/id"
+)
+
+// verifyMappedPackMatchesMappedIdx compares one mapped pack trailer hash with
+// the pack hash recorded in one mapped idx trailer.
+func verifyMappedPackMatchesMappedIdx(packData, idxData []byte, algo objectid.Algorithm) error {
+	hashSize := algo.Size()
+	if hashSize <= 0 {
+		return objectid.ErrInvalidAlgorithm
+	}
+
+	if len(packData) < hashSize {
+		return fmt.Errorf("objectstore/packed: pack too short for trailer hash")
+	}
+
+	if len(idxData) < hashSize*2 {
+		return fmt.Errorf("objectstore/packed: idx too short for trailer hashes")
+	}
+
+	packTrailerHash := packData[len(packData)-hashSize:]
+
+	idxPackHash := idxData[len(idxData)-hashSize*2 : len(idxData)-hashSize]
+	if !bytes.Equal(packTrailerHash, idxPackHash) {
+		return fmt.Errorf("objectstore/packed: pack hash does not match idx")
+	}
+
+	return nil
+}