format/delta/apply: Move core delta apply algorithm here

2 files changed, 4 insertions, 169 deletions

diff --git a/objectstore/packed/delta_apply.go b/objectstore/packed/delta_apply.go
index 9d34c245..a067525d 100644
--- a/objectstore/packed/delta_apply.go
+++ b/objectstore/packed/delta_apply.go
@@ -3,6 +3,7 @@ package packed
 import (
 	"fmt"
 
+	deltaapply "codeberg.org/lindenii/furgit/format/delta/apply"
 	"codeberg.org/lindenii/furgit/objecttype"
 )
 
@@ -27,7 +28,7 @@ func (store *Store) deltaResolveContent(start location) (objecttype.Type, []byte
 		if err != nil {
 			return objecttype.TypeInvalid, nil, err
 		}
-		out, err = applyDelta(out, delta)
+		out, err = deltaapply.Apply(out, delta)
 		if err != nil {
 			return objecttype.TypeInvalid, nil, err
 		}
@@ -41,126 +42,3 @@ func (store *Store) deltaResolveContent(start location) (objecttype.Type, []byte
 	}
 	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
-}
diff --git a/objectstore/packed/delta_plan.go b/objectstore/packed/delta_plan.go
index 2fca7c0e..58e0fc0c 100644
--- a/objectstore/packed/delta_plan.go
+++ b/objectstore/packed/delta_plan.go
@@ -2,8 +2,8 @@ package packed
 
 import (
 	"fmt"
-	"io"
 
+	deltaapply "codeberg.org/lindenii/furgit/format/delta/apply"
 	"codeberg.org/lindenii/furgit/objecttype"
 )
 
@@ -102,52 +102,9 @@ func deltaDeclaredSizeAt(pack *packFile, dataOffset int) (int64, error) {
 	}
 	defer func() { _ = reader.Close() }()
 
-	_, size, err := readDeltaHeaderSizes(reader)
+	_, size, err := deltaapply.ReadHeaderSizes(reader)
 	if err != nil {
 		return 0, err
 	}
 	return int64(size), nil
 }
-
-// readDeltaHeaderSizes reads the first two varints in one inflated delta stream.
-func readDeltaHeaderSizes(reader io.Reader) (int, int, error) {
-	// Two Git varints are read here. Each can take up to 10 bytes.
-	var buf [20]byte
-	n := 0
-
-	for {
-		if n >= len(buf) {
-			return 0, 0, fmt.Errorf("objectstore/packed: malformed delta varint")
-		}
-		if _, err := io.ReadFull(reader, buf[n:n+1]); err != nil {
-			return 0, 0, fmt.Errorf("objectstore/packed: malformed delta varint: %w", err)
-		}
-		n++
-		if buf[n-1]&0x80 == 0 {
-			break
-		}
-	}
-	pos := 0
-	srcSize, err := readDeltaVarint(buf[:n], &pos)
-	if err != nil {
-		return 0, 0, err
-	}
-
-	for {
-		if n >= len(buf) {
-			return 0, 0, fmt.Errorf("objectstore/packed: malformed delta varint")
-		}
-		if _, err := io.ReadFull(reader, buf[n:n+1]); err != nil {
-			return 0, 0, fmt.Errorf("objectstore/packed: malformed delta varint: %w", err)
-		}
-		n++
-		if buf[n-1]&0x80 == 0 {
-			break
-		}
-	}
-	dstSize, err := readDeltaVarint(buf[:n], &pos)
-	if err != nil {
-		return 0, 0, err
-	}
-	return srcSize, dstSize, nil
-}