packed: Write packs with deltas

1 files changed, 159 insertions, 0 deletions

diff --git a/delta_read_apply.go b/delta_read_apply.go
new file mode 100644
index 00000000..48233292
--- /dev/null
+++ b/delta_read_apply.go
@@ -0,0 +1,159 @@
+package furgit
+
+import (
+	"io"
+
+	"codeberg.org/lindenii/furgit/internal/bufpool"
+)
+
+func packDeltaReadOfsDistance(data []byte) (uint64, int, error) {
+	if len(data) == 0 {
+		return 0, 0, io.ErrUnexpectedEOF
+	}
+	b := data[0]
+	dist := uint64(b & 0x7f)
+	consumed := 1
+	for (b & 0x80) != 0 {
+		if consumed >= len(data) {
+			return 0, 0, io.ErrUnexpectedEOF
+		}
+		b = data[consumed]
+		consumed++
+		dist += 1
+		dist <<= 7
+		dist |= uint64(b & 0x7f)
+	}
+	return dist, consumed, nil
+}
+
+func packDeltaApply(base, delta bufpool.Buffer) (bufpool.Buffer, error) {
+	pos := 0
+	baseBytes := base.Bytes()
+	deltaBytes := delta.Bytes()
+	srcSize, err := packVarintRead(deltaBytes, &pos)
+	if err != nil {
+		return bufpool.Buffer{}, err
+	}
+	dstSize, err := packVarintRead(deltaBytes, &pos)
+	if err != nil {
+		return bufpool.Buffer{}, err
+	}
+	if srcSize != len(baseBytes) {
+		return bufpool.Buffer{}, ErrInvalidObject
+	}
+	out := bufpool.Borrow(dstSize)
+	out.Resize(dstSize)
+	outBytes := out.Bytes()
+	outPos := 0
+
+	for pos < len(deltaBytes) {
+		op := deltaBytes[pos]
+		pos++
+		switch {
+		case op&0x80 != 0:
+			off := 0
+			n := 0
+			if op&0x01 != 0 {
+				if pos >= len(deltaBytes) {
+					out.Release()
+					return bufpool.Buffer{}, ErrInvalidObject
+				}
+				off |= int(deltaBytes[pos])
+				pos++
+			}
+			if op&0x02 != 0 {
+				if pos >= len(deltaBytes) {
+					out.Release()
+					return bufpool.Buffer{}, ErrInvalidObject
+				}
+				off |= int(deltaBytes[pos]) << 8
+				pos++
+			}
+			if op&0x04 != 0 {
+				if pos >= len(deltaBytes) {
+					out.Release()
+					return bufpool.Buffer{}, ErrInvalidObject
+				}
+				off |= int(deltaBytes[pos]) << 16
+				pos++
+			}
+			if op&0x08 != 0 {
+				if pos >= len(deltaBytes) {
+					out.Release()
+					return bufpool.Buffer{}, ErrInvalidObject
+				}
+				off |= int(deltaBytes[pos]) << 24
+				pos++
+			}
+			if op&0x10 != 0 {
+				if pos >= len(deltaBytes) {
+					out.Release()
+					return bufpool.Buffer{}, ErrInvalidObject
+				}
+				n |= int(deltaBytes[pos])
+				pos++
+			}
+			if op&0x20 != 0 {
+				if pos >= len(deltaBytes) {
+					out.Release()
+					return bufpool.Buffer{}, ErrInvalidObject
+				}
+				n |= int(deltaBytes[pos]) << 8
+				pos++
+			}
+			if op&0x40 != 0 {
+				if pos >= len(deltaBytes) {
+					out.Release()
+					return bufpool.Buffer{}, ErrInvalidObject
+				}
+				n |= int(deltaBytes[pos]) << 16
+				pos++
+			}
+			if n == 0 {
+				n = 0x10000
+			}
+			if off+n > len(baseBytes) || outPos+n > len(outBytes) {
+				out.Release()
+				return bufpool.Buffer{}, ErrInvalidObject
+			}
+			copy(outBytes[outPos:], baseBytes[off:off+n])
+			outPos += n
+		case op != 0:
+			n := int(op)
+			if pos+n > len(deltaBytes) || outPos+n > len(outBytes) {
+				out.Release()
+				return bufpool.Buffer{}, ErrInvalidObject
+			}
+			copy(outBytes[outPos:], deltaBytes[pos:pos+n])
+			pos += n
+			outPos += n
+		default:
+			out.Release()
+			return bufpool.Buffer{}, ErrInvalidObject
+		}
+	}
+
+	if outPos != len(outBytes) {
+		out.Release()
+		return bufpool.Buffer{}, ErrInvalidObject
+	}
+	return out, nil
+}
+
+func packVarintRead(buf []byte, pos *int) (int, error) {
+	res := 0
+	shift := 0
+	for {
+		if *pos >= len(buf) {
+			return 0, ErrInvalidObject
+		}
+		b := buf[*pos]
+		*pos++
+		res |= int(b&0x7f) << shift
+		if (b & 0x80) == 0 {
+			break
+		}
+		shift += 7
+	}
+	return res, nil
+}