1 files changed, 108 insertions, 0 deletions
diff --git a/format/pack/checksum/checksum.go b/format/pack/checksum/checksum.go
new file mode 100644
index 00000000..7fc1c954
--- /dev/null
+++ b/format/pack/checksum/checksum.go
@@ -0,0 +1,108 @@
+// Package checksum provides Git pack/index checksum primitives.
+package checksum
+
+import (
+	"bytes"
+	"fmt"
+
+	"codeberg.org/lindenii/furgit/objectid"
+)
+
+// VerifyPackTrailer verifies one pack trailer hash against the pack payload.
+//
+// This computes the object hash over all bytes except the trailing hash, so it
+// is O(pack size).
+func VerifyPackTrailer(data []byte, algo objectid.Algorithm) error {
+	hashSize := algo.Size()
+	if hashSize <= 0 {
+		return objectid.ErrInvalidAlgorithm
+	}
+	if len(data) < hashSize {
+		return fmt.Errorf("format/pack/checksum: pack too short for trailer hash")
+	}
+
+	hash, err := algo.New()
+	if err != nil {
+		return err
+	}
+	if _, err := hash.Write(data[:len(data)-hashSize]); err != nil {
+		return err
+	}
+	computed := hash.Sum(nil)
+	trailer := data[len(data)-hashSize:]
+	if !bytes.Equal(computed, trailer) {
+		return fmt.Errorf("format/pack/checksum: pack trailer hash mismatch")
+	}
+	return nil
+}
+
+// PackTrailerHash returns the trailer hash bytes from one pack file.
+func PackTrailerHash(data []byte, algo objectid.Algorithm) ([]byte, error) {
+	hashSize := algo.Size()
+	if hashSize <= 0 {
+		return nil, objectid.ErrInvalidAlgorithm
+	}
+	if len(data) < hashSize {
+		return nil, fmt.Errorf("format/pack/checksum: pack too short for trailer hash")
+	}
+	return data[len(data)-hashSize:], nil
+}
+
+// ParseIdxTrailer parses one idx v2 trailer and returns (packHash, idxHash).
+func ParseIdxTrailer(data []byte, algo objectid.Algorithm) ([]byte, []byte, error) {
+	hashSize := algo.Size()
+	if hashSize <= 0 {
+		return nil, nil, objectid.ErrInvalidAlgorithm
+	}
+	if len(data) < hashSize*2 {
+		return nil, nil, fmt.Errorf("format/pack/checksum: idx too short for trailer hashes")
+	}
+	packHashOff := len(data) - hashSize*2
+	idxHashOff := len(data) - hashSize
+	return data[packHashOff:idxHashOff], data[idxHashOff:], nil
+}
+
+// VerifyIdxTrailer verifies one idx trailer checksum against preceding bytes.
+func VerifyIdxTrailer(data []byte, algo objectid.Algorithm) error {
+	hashSize := algo.Size()
+	if hashSize <= 0 {
+		return objectid.ErrInvalidAlgorithm
+	}
+	if len(data) < hashSize*2 {
+		return fmt.Errorf("format/pack/checksum: idx too short for trailer hashes")
+	}
+
+	_, idxHash, err := ParseIdxTrailer(data, algo)
+	if err != nil {
+		return err
+	}
+	hash, err := algo.New()
+	if err != nil {
+		return err
+	}
+	if _, err := hash.Write(data[:len(data)-hashSize]); err != nil {
+		return err
+	}
+	computed := hash.Sum(nil)
+	if !bytes.Equal(computed, idxHash) {
+		return fmt.Errorf("format/pack/checksum: idx trailer hash mismatch")
+	}
+	return nil
+}
+
+// VerifyPackMatchesIdx compares a pack trailer hash with one idx-recorded pack
+// hash.
+func VerifyPackMatchesIdx(packData, idxData []byte, algo objectid.Algorithm) error {
+	packTrailerHash, err := PackTrailerHash(packData, algo)
+	if err != nil {
+		return err
+	}
+	idxPackHash, _, err := ParseIdxTrailer(idxData, algo)
+	if err != nil {
+		return err
+	}
+	if !bytes.Equal(packTrailerHash, idxPackHash) {
+		return fmt.Errorf("format/pack/checksum: pack hash does not match idx")
+	}
+	return nil
+}