1 files changed, 52 insertions, 0 deletions
diff --git a/packfile/pack.go b/packfile/pack.go
new file mode 100644
index 00000000..88d8fd9a
--- /dev/null
+++ b/packfile/pack.go
@@ -0,0 +1,52 @@
+// Package pack provides Git packfile format parsing primitives.
+package pack
+
+import (
+	"fmt"
+
+	"codeberg.org/lindenii/furgit/objecttype"
+)
+
+// Signature is the 4-byte "PACK" magic at the start of pack files.
+const Signature = 0x5041434b
+
+// VersionSupported reports whether one pack version is supported.
+func VersionSupported(version uint32) bool {
+	return version == 2 || version == 3
+}
+
+// IsBaseObjectType reports whether ty is one of the four canonical object
+// types encoded directly in pack entries.
+func IsBaseObjectType(ty objecttype.Type) bool {
+	switch ty {
+	case objecttype.TypeCommit, objecttype.TypeTree, objecttype.TypeBlob, objecttype.TypeTag:
+		return true
+	case objecttype.TypeInvalid, objecttype.TypeFuture, objecttype.TypeOfsDelta, objecttype.TypeRefDelta:
+		return false
+	default:
+		return false
+	}
+}
+
+// ParseOfsDeltaDistance parses one ofs-delta backward distance.
+func ParseOfsDeltaDistance(buf []byte) (uint64, int, error) {
+	if len(buf) == 0 {
+		return 0, 0, fmt.Errorf("packfile: malformed ofs-delta distance")
+	}
+
+	b := buf[0]
+	dist := uint64(b & 0x7f)
+
+	consumed := 1
+	for b&0x80 != 0 {
+		if consumed >= len(buf) {
+			return 0, 0, fmt.Errorf("packfile: malformed ofs-delta distance")
+		}
+
+		b = buf[consumed]
+		consumed++
+		dist = ((dist + 1) << 7) + uint64(b&0x7f)
+	}
+
+	return dist, consumed, nil
+}