format/pack: Extract general constants and such from objectstore/packed

1 files changed, 48 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
+}