1 files changed, 121 insertions, 0 deletions
diff --git a/packfile/entry.go b/packfile/entry.go
new file mode 100644
index 00000000..c17fe619
--- /dev/null
+++ b/packfile/entry.go
@@ -0,0 +1,121 @@
+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("packfile: 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("packfile: 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("packfile: negative entry size")
+	}
+
+	return header, nil
+}
+
+// Entry is one parsed pack entry prefix, including any delta base reference
+// data that appears before the compressed payload.
+type Entry struct {
+	// Type is the pack entry type.
+	Type objecttype.Type
+	// Size is the declared resulting object size.
+	Size int64
+	// DataOffset is the byte offset from the start of the entry to the zlib
+	// payload bytes.
+	DataOffset int
+	// RefBaseID is the referenced base object ID bytes for ref-delta entries.
+	RefBaseID []byte
+	// OfsBaseDistance is the backward distance for ofs-delta entries.
+	OfsBaseDistance uint64
+}
+
+// ParseEntry parses one full pack entry prefix from data.
+//
+// hashSize must match the hash algorithm size used by the pack/index.
+func ParseEntry(data []byte, hashSize int) (Entry, error) {
+	var zero Entry
+
+	header, err := ParseEntryHeader(data)
+	if err != nil {
+		return zero, err
+	}
+
+	entry := Entry{
+		Type:       header.Type,
+		Size:       header.Size,
+		DataOffset: header.HeaderSize,
+	}
+
+	switch entry.Type {
+	case objecttype.TypeCommit, objecttype.TypeTree, objecttype.TypeBlob, objecttype.TypeTag:
+		// Base object entries have no extra prefix fields.
+	case objecttype.TypeRefDelta:
+		if hashSize <= 0 {
+			return zero, fmt.Errorf("packfile: invalid hash size %d", hashSize)
+		}
+
+		end := entry.DataOffset + hashSize
+		if end > len(data) {
+			return zero, fmt.Errorf("packfile: truncated ref-delta base id")
+		}
+
+		entry.RefBaseID = data[entry.DataOffset:end]
+		entry.DataOffset = end
+	case objecttype.TypeOfsDelta:
+		dist, consumed, err := ParseOfsDeltaDistance(data[entry.DataOffset:])
+		if err != nil {
+			return zero, err
+		}
+
+		entry.OfsBaseDistance = dist
+		entry.DataOffset += consumed
+	case objecttype.TypeInvalid, objecttype.TypeFuture:
+		return zero, fmt.Errorf("packfile: unsupported object type %d", entry.Type)
+	default:
+		return zero, fmt.Errorf("packfile: unsupported object type %d", entry.Type)
+	}
+
+	if entry.DataOffset > len(data) {
+		return zero, fmt.Errorf("packfile: entry data offset out of bounds")
+	}
+
+	return entry, nil
+}