1 files changed, 59 insertions, 5 deletions
diff --git a/internal/format/packfile/header.go b/internal/format/packfile/header.go
index 1a156f36..64c7d140 100644
--- a/internal/format/packfile/header.go
+++ b/internal/format/packfile/header.go
@@ -1,9 +1,63 @@
 package packfile
 
-// Signature is the 4-byte "PACK" magic at the start of pack files.
-const Signature = 0x5041434b
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+)
 
-// SupportedVersion reports whether a pack version is supported.
-func SupportedVersion(version uint32) bool {
-	return version == 2
+const (
+	// signature is the 4-byte "PACK" magic at the start of pack files.
+	signature = 0x5041434b
+
+	// version is the only pack version this package reads or writes.
+	version = 2
+)
+
+// HeaderLen is the size of a pack header:
+// signature, version, and object count.
+const HeaderLen = 12
+
+// ErrMalformedHeader reports that
+// a pack header is truncated,
+// or has a bad signature or unsupported version.
+var ErrMalformedHeader = errors.New("internal/format/packfile: malformed pack header")
+
+// Header is one parsed pack header.
+type Header struct {
+	// ObjectCount is the number of object entries
+	// declared to follow the header.
+	ObjectCount uint32
+}
+
+// ParseHeader parses and validates the pack header
+// at the start of data.
+func ParseHeader(data []byte) (Header, error) {
+	var zero Header
+
+	if len(data) < HeaderLen {
+		return zero, fmt.Errorf("%w: truncated", ErrMalformedHeader)
+	}
+
+	if binary.BigEndian.Uint32(data[0:4]) != signature {
+		return zero, fmt.Errorf("%w: bad signature", ErrMalformedHeader)
+	}
+
+	parsedVersion := binary.BigEndian.Uint32(data[4:8])
+	if parsedVersion != version {
+		return zero, fmt.Errorf("%w: unsupported version %d", ErrMalformedHeader, parsedVersion)
+	}
+
+	return Header{
+		ObjectCount: binary.BigEndian.Uint32(data[8:12]),
+	}, nil
+}
+
+// AppendHeader appends an encoded pack header for objectCount to dst.
+func AppendHeader(dst []byte, objectCount uint32) []byte {
+	dst = binary.BigEndian.AppendUint32(dst, signature)
+	dst = binary.BigEndian.AppendUint32(dst, version)
+	dst = binary.BigEndian.AppendUint32(dst, objectCount)
+
+	return dst
 }