2 files changed, 96 insertions, 0 deletions
diff --git a/format/pack/entry.go b/format/pack/entry.go
new file mode 100644
index 00000000..512b45f8
--- /dev/null
+++ b/format/pack/entry.go
@@ -0,0 +1,48 @@
+package pack
+
+import (
+	"fmt"
+
+	"codeberg.org/lindenii/furgit/objecttype"
+)
+
+// EntryHeader is one parsed pack entry header prefix.
+type EntryHeader struct {
+	// Type is the entry type tag from the first header byte.
+	Type objecttype.Type
+	// Size is the declared resulting object size.
+	Size int64
+	// HeaderSize is the number of bytes consumed by the type/size header.
+	HeaderSize int
+}
+
+// ParseEntryHeader parses one pack entry type/size header from data.
+func ParseEntryHeader(data []byte) (EntryHeader, error) {
+	var zero EntryHeader
+	if len(data) == 0 {
+		return zero, fmt.Errorf("format/pack: truncated entry header")
+	}
+
+	first := data[0]
+	header := EntryHeader{
+		Type:       objecttype.Type((first >> 4) & 0x07),
+		Size:       int64(first & 0x0f),
+		HeaderSize: 1,
+	}
+
+	shift := uint(4)
+	b := first
+	for b&0x80 != 0 {
+		if header.HeaderSize >= len(data) {
+			return zero, fmt.Errorf("format/pack: truncated entry header")
+		}
+		b = data[header.HeaderSize]
+		header.HeaderSize++
+		header.Size |= int64(b&0x7f) << shift
+		shift += 7
+	}
+	if header.Size < 0 {
+		return zero, fmt.Errorf("format/pack: negative entry size")
+	}
+	return header, nil
+}
diff --git a/format/pack/pack.go b/format/pack/pack.go
new file mode 100644
index 00000000..45fe6a1c
--- /dev/null
+++ b/format/pack/pack.go
@@ -0,0 +1,48 @@
+// 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("format/pack: 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("format/pack: malformed ofs-delta distance")
+		}
+		b = buf[consumed]
+		consumed++
+		dist = ((dist + 1) << 7) + uint64(b&0x7f)
+	}
+	return dist, consumed, nil
+}