1 files changed, 117 insertions, 0 deletions
diff --git a/objectstore/packed/entry_parse.go b/objectstore/packed/entry_parse.go
new file mode 100644
index 00000000..e3cbeac3
--- /dev/null
+++ b/objectstore/packed/entry_parse.go
@@ -0,0 +1,117 @@
+package packed
+
+import (
+	"fmt"
+
+	"codeberg.org/lindenii/furgit/objectid"
+	"codeberg.org/lindenii/furgit/objecttype"
+)
+
+// entryMeta describes one parsed pack entry header.
+type entryMeta struct {
+	// ty is the pack entry type tag.
+	ty objecttype.Type
+	// size is the declared resulting content size.
+	size int64
+	// dataOffset points to the zlib payload start.
+	dataOffset int
+	// baseRefID is set for ref-delta entries.
+	baseRefID objectid.ObjectID
+	// baseOfs is set for ofs-delta entries.
+	baseOfs uint64
+}
+
+// parseEntryMeta parses one pack entry header at offset.
+func parseEntryMeta(pack *packFile, algo objectid.Algorithm, offset uint64) (entryMeta, error) {
+	var zero entryMeta
+	if offset >= uint64(len(pack.data)) {
+		return zero, fmt.Errorf("objectstore/packed: pack %q offset %d out of bounds", pack.name, offset)
+	}
+
+	pos := int(offset)
+	first := pack.data[pos]
+	pos++
+
+	meta := entryMeta{
+		ty:   objecttype.Type((first >> 4) & 0x07),
+		size: int64(first & 0x0f),
+	}
+
+	shift := uint(4)
+	b := first
+	for b&0x80 != 0 {
+		if pos >= len(pack.data) {
+			return zero, fmt.Errorf("objectstore/packed: pack %q truncated entry header", pack.name)
+		}
+		b = pack.data[pos]
+		pos++
+		meta.size |= int64(b&0x7f) << shift
+		shift += 7
+	}
+	if meta.size < 0 {
+		return zero, fmt.Errorf("objectstore/packed: pack %q entry has negative size", pack.name)
+	}
+
+	switch meta.ty {
+	case objecttype.TypeCommit, objecttype.TypeTree, objecttype.TypeBlob, objecttype.TypeTag:
+		// Base object entries have no extra header fields.
+	case objecttype.TypeRefDelta:
+		hashSize := algo.Size()
+		if pos+hashSize > len(pack.data) {
+			return zero, fmt.Errorf("objectstore/packed: pack %q truncated ref-delta base id", pack.name)
+		}
+		baseID, err := objectid.FromBytes(algo, pack.data[pos:pos+hashSize])
+		if err != nil {
+			return zero, err
+		}
+		meta.baseRefID = baseID
+		pos += hashSize
+	case objecttype.TypeOfsDelta:
+		dist, consumed, err := parseOfsDeltaDistance(pack.data[pos:])
+		if err != nil {
+			return zero, err
+		}
+		pos += consumed
+		if offset <= dist {
+			return zero, fmt.Errorf("objectstore/packed: pack %q has invalid ofs-delta base", pack.name)
+		}
+		meta.baseOfs = offset - dist
+	default:
+		return zero, fmt.Errorf("objectstore/packed: pack %q has unsupported object type %d", pack.name, meta.ty)
+	}
+
+	meta.dataOffset = pos
+	if meta.dataOffset > len(pack.data) {
+		return zero, fmt.Errorf("objectstore/packed: pack %q entry data offset out of bounds", pack.name)
+	}
+	return meta, nil
+}
+
+// parseOfsDeltaDistance parses one ofs-delta backward distance.
+func parseOfsDeltaDistance(buf []byte) (uint64, int, error) {
+	if len(buf) == 0 {
+		return 0, 0, fmt.Errorf("objectstore/packed: 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("objectstore/packed: malformed ofs-delta distance")
+		}
+		b = buf[consumed]
+		consumed++
+		dist = ((dist + 1) << 7) + uint64(b&0x7f)
+	}
+	return dist, consumed, nil
+}
+
+// isBaseObjectType reports whether ty is one of the four canonical object types.
+func isBaseObjectType(ty objecttype.Type) bool {
+	switch ty {
+	case objecttype.TypeCommit, objecttype.TypeTree, objecttype.TypeBlob, objecttype.TypeTag:
+		return true
+	default:
+		return false
+	}
+}