1 files changed, 95 insertions, 0 deletions
diff --git a/format/packfile/ingest/entry_prefix.go b/format/packfile/ingest/entry_prefix.go
new file mode 100644
index 00000000..a107b4e8
--- /dev/null
+++ b/format/packfile/ingest/entry_prefix.go
@@ -0,0 +1,95 @@
+package ingest
+
+import (
+	"fmt"
+
+	"codeberg.org/lindenii/furgit/internal/intconv"
+	objectid "codeberg.org/lindenii/furgit/object/id"
+	objecttype "codeberg.org/lindenii/furgit/object/type"
+)
+
+// parseEntryPrefix parses one entry prefix from stream.
+func parseEntryPrefix(state *ingestState, startOffset uint64) (objectRecord, error) {
+	var record objectRecord
+
+	record.offset = startOffset
+
+	first, err := state.stream.ReadByte()
+	if err != nil {
+		return record, &MalformedPackEntryError{Offset: startOffset, Reason: fmt.Sprintf("read first header byte: %v", err)}
+	}
+
+	record.packedType = objecttype.Type((first >> 4) & 0x07)
+	size := int64(first & 0x0f)
+	headerLen := uint32(1)
+	shift := uint(4)
+	b := first
+
+	for b&0x80 != 0 {
+		b, err = state.stream.ReadByte()
+		if err != nil {
+			return record, &MalformedPackEntryError{Offset: startOffset, Reason: fmt.Sprintf("read size continuation: %v", err)}
+		}
+
+		headerLen++
+		size |= int64(b&0x7f) << shift
+		shift += 7
+	}
+
+	if size < 0 {
+		return record, &MalformedPackEntryError{Offset: startOffset, Reason: "negative declared size"}
+	}
+
+	record.declaredSize = size
+
+	switch record.packedType {
+	case objecttype.TypeCommit, objecttype.TypeTree, objecttype.TypeBlob, objecttype.TypeTag:
+	case objecttype.TypeRefDelta:
+		baseRaw := make([]byte, state.algo.Size())
+
+		err := state.stream.readFull(baseRaw)
+		if err != nil {
+			return record, &MalformedPackEntryError{Offset: startOffset, Reason: fmt.Sprintf("read ref base: %v", err)}
+		}
+
+		baseID, err := objectid.FromBytes(state.algo, baseRaw)
+		if err != nil {
+			return record, &MalformedPackEntryError{Offset: startOffset, Reason: fmt.Sprintf("parse ref base: %v", err)}
+		}
+
+		record.baseObject = baseID
+
+		baseRawLen, err := intconv.IntToUint32(len(baseRaw))
+		if err != nil {
+			return record, err
+		}
+
+		headerLen += baseRawLen
+	case objecttype.TypeOfsDelta:
+		dist, consumed, err := readOfsDistanceFromStream(state.stream)
+		if err != nil {
+			return record, &MalformedPackEntryError{Offset: startOffset, Reason: err.Error()}
+		}
+
+		if startOffset <= dist {
+			return record, &MalformedPackEntryError{Offset: startOffset, Reason: "ofs base offset out of bounds"}
+		}
+
+		record.baseOffset = startOffset - dist
+
+		consumedUint32, err := intconv.IntToUint32(consumed)
+		if err != nil {
+			return record, err
+		}
+
+		headerLen += consumedUint32
+	case objecttype.TypeInvalid, objecttype.TypeFuture:
+		return record, &MalformedPackEntryError{Offset: startOffset, Reason: fmt.Sprintf("unsupported object type %d", record.packedType)}
+	default:
+		return record, &MalformedPackEntryError{Offset: startOffset, Reason: fmt.Sprintf("unsupported object type %d", record.packedType)}
+	}
+
+	record.headerLen = headerLen
+
+	return record, nil
+}