objectstore/packed: Add initial pack reading support

1 files changed, 166 insertions, 0 deletions

diff --git a/objectstore/packed/delta_apply.go b/objectstore/packed/delta_apply.go
new file mode 100644
index 00000000..9d34c245
--- /dev/null
+++ b/objectstore/packed/delta_apply.go
@@ -0,0 +1,166 @@
+package packed
+
+import (
+	"fmt"
+
+	"codeberg.org/lindenii/furgit/objecttype"
+)
+
+// deltaResolveContent resolves one object's content bytes from its pack location.
+func (store *Store) deltaResolveContent(start location) (objecttype.Type, []byte, error) {
+	plan, err := store.deltaPlanFor(start)
+	if err != nil {
+		return objecttype.TypeInvalid, nil, err
+	}
+
+	baseType, out, err := store.deltaResolveBase(plan)
+	if err != nil {
+		return objecttype.TypeInvalid, nil, err
+	}
+	for i := len(plan.frames) - 1; i >= 0; i-- {
+		frame := plan.frames[i]
+		pack, err := store.openPack(frame.packName)
+		if err != nil {
+			return objecttype.TypeInvalid, nil, err
+		}
+		delta, err := inflateAt(pack, frame.dataOffset, -1)
+		if err != nil {
+			return objecttype.TypeInvalid, nil, err
+		}
+		out, err = applyDelta(out, delta)
+		if err != nil {
+			return objecttype.TypeInvalid, nil, err
+		}
+	}
+	if int64(len(out)) != plan.declaredSize {
+		return objecttype.TypeInvalid, nil, fmt.Errorf(
+			"objectstore/packed: resolved content size mismatch: got %d want %d",
+			len(out),
+			plan.declaredSize,
+		)
+	}
+	return baseType, out, nil
+}
+
+// applyDelta applies one Git delta instruction stream to base.
+func applyDelta(base, delta []byte) ([]byte, error) {
+	pos := 0
+	srcSize, err := readDeltaVarint(delta, &pos)
+	if err != nil {
+		return nil, err
+	}
+	dstSize, err := readDeltaVarint(delta, &pos)
+	if err != nil {
+		return nil, err
+	}
+	if srcSize != len(base) {
+		return nil, fmt.Errorf("objectstore/packed: delta source size mismatch: got %d want %d", srcSize, len(base))
+	}
+
+	out := make([]byte, dstSize)
+	outPos := 0
+	for pos < len(delta) {
+		op := delta[pos]
+		pos++
+		if op&0x80 != 0 {
+			off := 0
+			if op&0x01 != 0 {
+				if pos >= len(delta) {
+					return nil, fmt.Errorf("objectstore/packed: malformed delta copy offset")
+				}
+				off |= int(delta[pos])
+				pos++
+			}
+			if op&0x02 != 0 {
+				if pos >= len(delta) {
+					return nil, fmt.Errorf("objectstore/packed: malformed delta copy offset")
+				}
+				off |= int(delta[pos]) << 8
+				pos++
+			}
+			if op&0x04 != 0 {
+				if pos >= len(delta) {
+					return nil, fmt.Errorf("objectstore/packed: malformed delta copy offset")
+				}
+				off |= int(delta[pos]) << 16
+				pos++
+			}
+			if op&0x08 != 0 {
+				if pos >= len(delta) {
+					return nil, fmt.Errorf("objectstore/packed: malformed delta copy offset")
+				}
+				off |= int(delta[pos]) << 24
+				pos++
+			}
+
+			n := 0
+			if op&0x10 != 0 {
+				if pos >= len(delta) {
+					return nil, fmt.Errorf("objectstore/packed: malformed delta copy size")
+				}
+				n |= int(delta[pos])
+				pos++
+			}
+			if op&0x20 != 0 {
+				if pos >= len(delta) {
+					return nil, fmt.Errorf("objectstore/packed: malformed delta copy size")
+				}
+				n |= int(delta[pos]) << 8
+				pos++
+			}
+			if op&0x40 != 0 {
+				if pos >= len(delta) {
+					return nil, fmt.Errorf("objectstore/packed: malformed delta copy size")
+				}
+				n |= int(delta[pos]) << 16
+				pos++
+			}
+			if n == 0 {
+				n = 0x10000
+			}
+			if off < 0 || n < 0 || off+n > len(base) || outPos+n > len(out) {
+				return nil, fmt.Errorf("objectstore/packed: delta copy out of bounds")
+			}
+			copy(out[outPos:outPos+n], base[off:off+n])
+			outPos += n
+			continue
+		}
+
+		if op == 0 {
+			return nil, fmt.Errorf("objectstore/packed: invalid delta opcode 0")
+		}
+		n := int(op)
+		if pos+n > len(delta) || outPos+n > len(out) {
+			return nil, fmt.Errorf("objectstore/packed: delta insert out of bounds")
+		}
+		copy(out[outPos:outPos+n], delta[pos:pos+n])
+		outPos += n
+		pos += n
+	}
+	if outPos != len(out) {
+		return nil, fmt.Errorf("objectstore/packed: delta output size mismatch: got %d want %d", outPos, len(out))
+	}
+	return out, nil
+}
+
+// readDeltaVarint parses one Git delta varint and advances pos.
+func readDeltaVarint(buf []byte, pos *int) (int, error) {
+	value := 0
+	shift := uint(0)
+	for {
+		if *pos >= len(buf) {
+			return 0, fmt.Errorf("objectstore/packed: malformed delta varint")
+		}
+		b := buf[*pos]
+		*pos++
+		value |= int(b&0x7f) << shift
+		if b&0x80 == 0 {
+			break
+		}
+		shift += 7
+		if shift > 63 {
+			return 0, fmt.Errorf("objectstore/packed: delta varint overflow")
+		}
+	}
+	return value, nil
+}