Repository: bluekeyes/go-gitdiff (go)

Issue:
Split apply logic by fragment type
The single `Applier` type does some messy internal state tracking to avoid mixing `ApplyFile`, `ApplyTextFragment`, and `ApplyBinaryFragment`. I think the following would be better:

1. Create a `TextApplier` (in `apply_text.go`) and `BinaryApplier` (in `apply_binary.go`). Each of these has methods to apply single fragments (and multiple fragments, in the case of `TextApplier`.)
2. Remove the `Reset` method and rename `Flush` to `Close` to better indicate that apply types are single-use
3. Remove the `Applier` type and the `ApplyFile` method
4. Move the logic for `ApplyFile` to the global `Apply` function. This is the convenience function to select an applier based on the file type and execute it.

This should reduce confusion and provides an obvious place for the eventual text-only options for fuzzy apply.

Pull request:
Split apply logic by fragment type
Remove the Applier type and replace it with TextApplier and
BinaryApplier, both of which operate on fragments instead of on full
files. Move the logic that previously existed in Applier.ApplyFile to
the top-level Apply function.

Also restructure arguments and methods to make it clear that appliers
are one-time-use objects. The destination is now set when creating an
applier and the Reset() method was replaced by Close().

Closes #11.

Patch:
diff --git a/.golangci.yml b/.golangci.yml
index 12cdbd2..82dbad2 100644
--- a/.golangci.yml
+++ b/.golangci.yml
@@ -11,6 +11,7 @@ linters:
     - golint
     - govet
     - ineffassign
+    - misspell
     - typecheck
     - unconvert
     - varcheck
diff --git a/README.md b/README.md
index b7522e3..58bb675 100644
--- a/README.md
+++ b/README.md
@@ -29,7 +29,7 @@ if err != nil {
 
 // apply the changes in the patch to a source file
 var output bytes.Buffer
-if err := gitdiff.NewApplier(code).ApplyFile(&output, files[0]); err != nil {
+if err := gitdiff.Apply(&output, code, files[0]); err != nil {
     log.Fatal(err)
 }
 ```
diff --git a/gitdiff/apply.go b/gitdiff/apply.go
index 05a4526..cc4c3f0 100644
--- a/gitdiff/apply.go
+++ b/gitdiff/apply.go
@@ -89,85 +89,36 @@ func applyError(err error, args ...interface{}) error {
 
 var (
 	errApplyInProgress = errors.New("gitdiff: incompatible apply in progress")
+	errApplierClosed   = errors.New("gitdiff: applier is closed")
 )
 
-const (
-	applyInitial = iota
-	applyText
-	applyBinary
-	applyFile
-)
-
-// Apply is a convenience function that creates an Applier for src with default
-// settings and applies the changes in f, writing the result to dst.
-func Apply(dst io.Writer, src io.ReaderAt, f *File) error {
-	return NewApplier(src).ApplyFile(dst, f)
-}
-
-// Applier applies changes described in fragments to source data. If changes
-// are described in multiple fragments, those fragments must be applied in
-// order, usually by calling ApplyFile.
-//
-// By default, Applier operates in "strict" mode, where fragment content and
-// positions must exactly match those of the source.
-//
-// If an error occurs while applying, methods on Applier return instances of
-// *ApplyError that annotate the wrapped error with additional information
-// when available. If the error is because of a conflict between a fragment and
-// the source, the wrapped error will be a *Conflict.
+// Apply applies the changes in f to src, writing the result to dst. It can
+// apply both text and binary changes.
 //
-// While an Applier can apply both text and binary fragments, only one fragment
-// type can be used without resetting the Applier. The first fragment applied
-// sets the type for the Applier. Mixing fragment types or mixing
-// fragment-level and file-level applies results in an error.
-type Applier struct {
-	src       io.ReaderAt
-	lineSrc   LineReaderAt
-	nextLine  int64
-	applyType int
-}
-
-// NewApplier creates an Applier that reads data from src. If src is a
-// LineReaderAt, it is used directly to apply text fragments.
-func NewApplier(src io.ReaderAt) *Applier {
-	a := new(Applier)
-	a.Reset(src)
-	return a
-}
-
-// Reset resets the input and internal state of the Applier. If src is nil, the
-// existing source is reused.
-func (a *Applier) Reset(src io.ReaderAt) {
-	if src != nil {
-		a.src = src
-		if lineSrc, ok := src.(LineReaderAt); ok {
-			a.lineSrc = lineSrc
-		} else {
-			a.lineSrc = &lineReaderAt{r: src}
+// If an error occurs while applying, Apply returns an *ApplyError that
+// annotates the error with additional information. If the error is because of
+// a conflict with the source, the wrapped error will be a *Conflict.
+func Apply(dst io.Writer, src io.ReaderAt, f *File) error {
+	if f.IsBinary {
+		if len(f.TextFragments) > 0 {
+			return applyError(errors.New("binary file contains text fragments"))
+		}
+		if f.BinaryFragment == nil {
+			return applyError(errors.New("binary file does not contain a binary fragment"))
+		}
+	} else {
+		if f.BinaryFragment != nil {
+			return applyError(errors.New("text file contains a binary fragment"))
 		}
-	}
-	a.nextLine = 0
-	a.applyType = applyInitial
-}
-
-// ApplyFile applies the changes in all of the fragments of f and writes the
-// result to dst.
-func (a *Applier) ApplyFile(dst io.Writer, f *File) error {
-	if a.applyType != applyInitial {
-		return applyError(errApplyInProgress)
-	}
-	defer func() { a.applyType = applyFile }()
-
-	if f.IsBinary && len(f.TextFragments) > 0 {
-		return applyError(errors.New("binary file contains text fragments"))
-	}
-	if !f.IsBinary && f.BinaryFragment != nil {
-		return applyError(errors.New("text file contains binary fragment"))
 	}
 
 	switch {
 	case f.BinaryFragment != nil:
-		return a.ApplyBinaryFragment(dst, f.BinaryFragment)
+		applier := NewBinaryApplier(dst, src)
+		if err := applier.ApplyFragment(f.BinaryFragment); err != nil {
+			return err
+		}
+		return applier.Close()
 
 	case len(f.TextFragments) > 0:
 		frags := make([]*TextFragment, len(f.TextFragments))
@@ -181,271 +132,17 @@ func (a *Applier) ApplyFile(dst io.Writer, f *File) error {
 		// right now, the application fails if fragments overlap, but it should be
 		// possible to precompute the result of applying them in order
 
+		applier := NewTextApplier(dst, src)
 		for i, frag := range frags {
-			if err := a.ApplyTextFragment(dst, frag); err != nil {
+			if err := applier.ApplyFragment(frag); err != nil {
 				return applyError(err, fragNum(i))
 			}
 		}
-	}
-
-	return applyError(a.Flush(dst))
-}
+		return applier.Close()
 
-// ApplyTextFragment applies the changes in the fragment f and writes unwritten
-// data before the start of the fragment and the result to dst. If multiple
-// text fragments apply to the same source, ApplyTextFragment must be called in
-// order of increasing start position. As a result, each fragment can be
-// applied at most once before a call to Reset.
-func (a *Applier) ApplyTextFragment(dst io.Writer, f *TextFragment) error {
-	if a.applyType != applyInitial && a.applyType != applyText {
-		return applyError(errApplyInProgress)
-	}
-	defer func() { a.applyType = applyText }()
-
-	// application code assumes fragment fields are consistent
-	if err := f.Validate(); err != nil {
-		return applyError(err)
-	}
-
-	// lines are 0-indexed, positions are 1-indexed (but new files have position = 0)
-	fragStart := f.OldPosition - 1
-	if fragStart < 0 {
-		fragStart = 0
-	}
-	fragEnd := fragStart + f.OldLines
-
-	start := a.nextLine
-	if fragStart < start {
-		return applyError(&Conflict{"fragment overlaps with an applied fragment"})
-	}
-
-	if f.OldPosition == 0 {
-		ok, err := isLen(a.src, 0)
-		if err != nil {
-			return applyError(err)
-		}
-		if !ok {
-			return applyError(&Conflict{"cannot create new file from non-empty src"})
-		}
-	}
-
-	preimage := make([][]byte, fragEnd-start)
-	n, err := a.lineSrc.ReadLinesAt(preimage, start)
-	if err != nil {
-		return applyError(err, lineNum(start+int64(n)))
-	}
-
-	// copy leading data before the fragment starts
-	for i, line := range preimage[:fragStart-start] {
-		if _, err := dst.Write(line); err != nil {
-			a.nextLine = start + int64(i)
-			return applyError(err, lineNum(a.nextLine))
-		}
-	}
-	preimage = preimage[fragStart-start:]
-
-	// apply the changes in the fragment
-	used := int64(0)
-	for i, line := range f.Lines {
-		if err := applyTextLine(dst, line, preimage, used); err != nil {
-			a.nextLine = fragStart + used
-			return applyError(err, lineNum(a.nextLine), fragLineNum(i))
-		}
-		if line.Old() {
-			used++
-		}
-	}
-	a.nextLine = fragStart + used
-
-	// new position of +0,0 mean a full delete, so check for leftovers
-	if f.NewPosition == 0 && f.NewLines == 0 {
-		var b [1][]byte
-		n, err := a.lineSrc.ReadLinesAt(b[:], a.nextLine)
-		if err != nil && err != io.EOF {
-			return applyError(err, lineNum(a.nextLine))
-		}
-		if n > 0 {
-			return applyError(&Conflict{"src still has content after full delete"}, lineNum(a.nextLine))
-		}
-	}
-
-	return nil
-}
-
-func applyTextLine(dst io.Writer, line Line, preimage [][]byte, i int64) (err error) {
-	if line.Old() && string(preimage[i]) != line.Line {
-		return &Conflict{"fragment line does not match src line"}
-	}
-	if line.New() {
-		_, err = io.WriteString(dst, line.Line)
-	}
-	return err
-}
-
-// Flush writes any data following the last applied fragment to dst.
-func (a *Applier) Flush(dst io.Writer) (err error) {
-	switch a.applyType {
-	case applyInitial:
-		_, err = copyFrom(dst, a.src, 0)
-	case applyText:
-		_, err = copyLinesFrom(dst, a.lineSrc, a.nextLine)
-	case applyBinary:
-		// nothing to flush, binary apply "consumes" full source
-	}
-	return err
-}
-
-// ApplyBinaryFragment applies the changes in the fragment f and writes the
-// result to dst. At most one binary fragment can be applied before a call to
-// Reset.
-func (a *Applier) ApplyBinaryFragment(dst io.Writer, f *BinaryFragment) error {
-	if a.applyType != applyInitial {
-		return applyError(errApplyInProgress)
-	}
-	defer func() { a.applyType = applyBinary }()
-
-	if f == nil {
-		return applyError(errors.New("nil fragment"))
-	}
-
-	switch f.Method {
-	case BinaryPatchLiteral:
-		if _, err := dst.Write(f.Data); err != nil {
-			return applyError(err)
-		}
-	case BinaryPatchDelta:
-		if err := applyBinaryDeltaFragment(dst, a.src, f.Data); err != nil {
-			return applyError(err)
-		}
 	default:
-		return applyError(fmt.Errorf("unsupported binary patch method: %v", f.Method))
-	}
-	return nil
-}
-
-func applyBinaryDeltaFragment(dst io.Writer, src io.ReaderAt, frag []byte) error {
-	srcSize, delta := readBinaryDeltaSize(frag)
-	if err := checkBinarySrcSize(src, srcSize); err != nil {
-		return err
-	}
-
-	dstSize, delta := readBinaryDeltaSize(delta)
-
-	for len(delta) > 0 {
-		op := delta[0]
-		if op == 0 {
-			return errors.New("invalid delta opcode 0")
-		}
-
-		var n int64
-		var err error
-		switch op & 0x80 {
-		case 0x80:
-			n, delta, err = applyBinaryDeltaCopy(dst, op, delta[1:], src)
-		case 0x00:
-			n, delta, err = applyBinaryDeltaAdd(dst, op, delta[1:])
-		}
-		if err != nil {
-			return err
-		}
-		dstSize -= n
-	}
-
-	if dstSize != 0 {
-		return errors.New("corrupt binary delta: insufficient or extra data")
-	}
-	return nil
-}
-
-// readBinaryDeltaSize reads a variable length size from a delta-encoded binary
-// fragment, returing the size and the unused data. Data is encoded as:
-//
-//    [[1xxxxxxx]...] [0xxxxxxx]
-//
-// in little-endian order, with 7 bits of the value per byte.
-func readBinaryDeltaSize(d []byte) (size int64, rest []byte) {
-	shift := uint(0)
-	for i, b := range d {
-		size |= int64(b&0x7F) << shift
-		shift += 7
-		if b <= 0x7F {
-			return size, d[i+1:]
-		}
-	}
-	return size, nil
-}
-
-// applyBinaryDeltaAdd applies an add opcode in a delta-encoded binary
-// fragment, returning the amount of data written and the usused part of the
-// fragment. An add operation takes the form:
-//
-//     [0xxxxxx][[data1]...]
-//
-// where the lower seven bits of the opcode is the number of data bytes
-// following the opcode. See also pack-format.txt in the Git source.
-func applyBinaryDeltaAdd(w io.Writer, op byte, delta []byte) (n int64, rest []byte, err error) {
-	size := int(op)
-	if len(delta) < size {
-		return 0, delta, errors.New("corrupt binary delta: incomplete add")
-	}
-	_, err = w.Write(delta[:size])
-	return int64(size), delta[size:], err
-}
-
-// applyBinaryDeltaCopy applies a copy opcode in a delta-encoded binary
-// fragment, returing the amount of data written and the unused part of the
-// fragment. A copy operation takes the form:
-//
-//     [1xxxxxxx][offset1][offset2][offset3][offset4][size1][size2][size3]
-//
-// where the lower seven bits of the opcode determine which non-zero offset and
-// size bytes are present in little-endian order: if bit 0 is set, offset1 is
-// present, etc. If no offset or size bytes are present, offset is 0 and size
-// is 0x10000. See also pack-format.txt in the Git source.
-func applyBinaryDeltaCopy(w io.Writer, op byte, delta []byte, src io.ReaderAt) (n int64, rest []byte, err error) {
-	const defaultSize = 0x10000
-
-	unpack := func(start, bits uint) (v int64) {
-		for i := uint(0); i < bits; i++ {
-			mask := byte(1 << (i + start))
-			if op&mask > 0 {
-				if len(delta) == 0 {
-					err = errors.New("corrupt binary delta: incomplete copy")
-					return
-				}
-				v |= int64(delta[0]) << (8 * i)
-				delta = delta[1:]
-			}
-		}
-		return
-	}
-
-	offset := unpack(0, 4)
-	size := unpack(4, 3)
-	if err != nil {
-		return 0, delta, err
-	}
-	if size == 0 {
-		size = defaultSize
-	}
-
-	// TODO(bkeyes): consider pooling these buffers
-	b := make([]byte, size)
-	if _, err := src.ReadAt(b, offset); err != nil {
-		return 0, delta, err
-	}
-
-	_, err = w.Write(b)
-	return size, delta, err
-}
-
-func checkBinarySrcSize(r io.ReaderAt, size int64) error {
-	ok, err := isLen(r, size)
-	if err != nil {
+		// nothing to apply, just copy all the data
+		_, err := copyFrom(dst, src, 0)
 		return err
 	}
-	if !ok {
-		return &Conflict{"fragment src size does not match actual src size"}
-	}
-	return nil
 }
diff --git a/gitdiff/apply_binary.go b/gitdiff/apply_binary.go
new file mode 100644
index 0000000..b1ff4c1
--- /dev/null
+++ b/gitdiff/apply_binary.go
@@ -0,0 +1,206 @@
+package gitdiff
+
+import (
+	"errors"
+	"fmt"
+	"io"
+)
+
+// BinaryApplier applies binary changes described in a fragment to source data.
+// The applier must be closed after use.
+type BinaryApplier struct {
+	dst io.Writer
+	src io.ReaderAt
+
+	closed bool
+	dirty  bool
+}
+
+// NewBinaryApplier creates an BinaryApplier that reads data from src and
+// writes modified data to dst.
+func NewBinaryApplier(dst io.Writer, src io.ReaderAt) *BinaryApplier {
+	a := BinaryApplier{
+		dst: dst,
+		src: src,
+	}
+	return &a
+}
+
+// ApplyFragment applies the changes in the fragment f and writes the result to
+// dst. ApplyFragment can be called at most once.
+//
+// If an error occurs while applying, ApplyFragment returns an *ApplyError that
+// annotates the error with additional information. If the error is because of
+// a conflict between a fragment and the source, the wrapped error will be a
+// *Conflict.
+func (a *BinaryApplier) ApplyFragment(f *BinaryFragment) error {
+	if f == nil {
+		return applyError(errors.New("nil fragment"))
+	}
+	if a.closed {
+		return applyError(errApplierClosed)
+	}
+	if a.dirty {
+		return applyError(errApplyInProgress)
+	}
+
+	// mark an apply as in progress, even if it fails before making changes
+	a.dirty = true
+
+	switch f.Method {
+	case BinaryPatchLiteral:
+		if _, err := a.dst.Write(f.Data); err != nil {
+			return applyError(err)
+		}
+	case BinaryPatchDelta:
+		if err := applyBinaryDeltaFragment(a.dst, a.src, f.Data); err != nil {
+			return applyError(err)
+		}
+	default:
+		return applyError(fmt.Errorf("unsupported binary patch method: %v", f.Method))
+	}
+	return nil
+}
+
+// Close writes any data following the last applied fragment and prevents
+// future calls to ApplyFragment.
+func (a *BinaryApplier) Close() (err error) {
+	if a.closed {
+		return nil
+	}
+
+	a.closed = true
+	if !a.dirty {
+		_, err = copyFrom(a.dst, a.src, 0)
+	} else {
+		// do nothing, applying a binary fragment copies all data
+	}
+	return err
+}
+
+func applyBinaryDeltaFragment(dst io.Writer, src io.ReaderAt, frag []byte) error {
+	srcSize, delta := readBinaryDeltaSize(frag)
+	if err := checkBinarySrcSize(src, srcSize); err != nil {
+		return err
+	}
+
+	dstSize, delta := readBinaryDeltaSize(delta)
+
+	for len(delta) > 0 {
+		op := delta[0]
+		if op == 0 {
+			return errors.New("invalid delta opcode 0")
+		}
+
+		var n int64
+		var err error
+		switch op & 0x80 {
+		case 0x80:
+			n, delta, err = applyBinaryDeltaCopy(dst, op, delta[1:], src)
+		case 0x00:
+			n, delta, err = applyBinaryDeltaAdd(dst, op, delta[1:])
+		}
+		if err != nil {
+			return err
+		}
+		dstSize -= n
+	}
+
+	if dstSize != 0 {
+		return errors.New("corrupt binary delta: insufficient or extra data")
+	}
+	return nil
+}
+
+// readBinaryDeltaSize reads a variable length size from a delta-encoded binary
+// fragment, returing the size and the unused data. Data is encoded as:
+//
+//    [[1xxxxxxx]...] [0xxxxxxx]
+//
+// in little-endian order, with 7 bits of the value per byte.
+func readBinaryDeltaSize(d []byte) (size int64, rest []byte) {
+	shift := uint(0)
+	for i, b := range d {
+		size |= int64(b&0x7F) << shift
+		shift += 7
+		if b <= 0x7F {
+			return size, d[i+1:]
+		}
+	}
+	return size, nil
+}
+
+// applyBinaryDeltaAdd applies an add opcode in a delta-encoded binary
+// fragment, returning the amount of data written and the usused part of the
+// fragment. An add operation takes the form:
+//
+//     [0xxxxxx][[data1]...]
+//
+// where the lower seven bits of the opcode is the number of data bytes
+// following the opcode. See also pack-format.txt in the Git source.
+func applyBinaryDeltaAdd(w io.Writer, op byte, delta []byte) (n int64, rest []byte, err error) {
+	size := int(op)
+	if len(delta) < size {
+		return 0, delta, errors.New("corrupt binary delta: incomplete add")
+	}
+	_, err = w.Write(delta[:size])
+	return int64(size), delta[size:], err
+}
+
+// applyBinaryDeltaCopy applies a copy opcode in a delta-encoded binary
+// fragment, returing the amount of data written and the unused part of the
+// fragment. A copy operation takes the form:
+//
+//     [1xxxxxxx][offset1][offset2][offset3][offset4][size1][size2][size3]
+//
+// where the lower seven bits of the opcode determine which non-zero offset and
+// size bytes are present in little-endian order: if bit 0 is set, offset1 is
+// present, etc. If no offset or size bytes are present, offset is 0 and size
+// is 0x10000. See also pack-format.txt in the Git source.
+func applyBinaryDeltaCopy(w io.Writer, op byte, delta []byte, src io.ReaderAt) (n int64, rest []byte, err error) {
+	const defaultSize = 0x10000
+
+	unpack := func(start, bits uint) (v int64) {
+		for i := uint(0); i < bits; i++ {
+			mask := byte(1 << (i + start))
+			if op&mask > 0 {
+				if len(delta) == 0 {
+					err = errors.New("corrupt binary delta: incomplete copy")
+					return
+				}
+				v |= int64(delta[0]) << (8 * i)
+				delta = delta[1:]
+			}
+		}
+		return
+	}
+
+	offset := unpack(0, 4)
+	size := unpack(4, 3)
+	if err != nil {
+		return 0, delta, err
+	}
+	if size == 0 {
+		size = defaultSize
+	}
+
+	// TODO(bkeyes): consider pooling these buffers
+	b := make([]byte, size)
+	if _, err := src.ReadAt(b, offset); err != nil {
+		return 0, delta, err
+	}
+
+	_, err = w.Write(b)
+	return size, delta, err
+}
+
+func checkBinarySrcSize(r io.ReaderAt, size int64) error {
+	ok, err := isLen(r, size)
+	if err != nil {
+		return err
+	}
+	if !ok {
+		return &Conflict{"fragment src size does not match actual src size"}
+	}
+	return nil
+}
diff --git a/gitdiff/apply_text.go b/gitdiff/apply_text.go
new file mode 100644
index 0000000..a404552
--- /dev/null
+++ b/gitdiff/apply_text.go
@@ -0,0 +1,153 @@
+package gitdiff
+
+import (
+	"io"
+)
+
+// TextApplier applies changes described in text fragments to source data. If
+// changes are described in multiple fragments, those fragments must be applied
+// in order. The applier must be closed after use.
+//
+// By default, TextApplier operates in "strict" mode, where fragment content
+// and positions must exactly match those of the source.
+//
+type TextApplier struct {
+	dst      io.Writer
+	src      io.ReaderAt
+	lineSrc  LineReaderAt
+	nextLine int64
+
+	closed bool
+	dirty  bool
+}
+
+// NewTextApplier creates a TextApplier that reads data from src and writes
+// modified data to dst. If src implements LineReaderAt, it is used directly.
+func NewTextApplier(dst io.Writer, src io.ReaderAt) *TextApplier {
+	a := TextApplier{
+		dst: dst,
+		src: src,
+	}
+
+	if lineSrc, ok := src.(LineReaderAt); ok {
+		a.lineSrc = lineSrc
+	} else {
+		a.lineSrc = &lineReaderAt{r: src}
+	}
+
+	return &a
+}
+
+// ApplyFragment applies the changes in the fragment f, writing unwritten data
+// before the start of the fragment and any changes from the fragment. If
+// multiple text fragments apply to the same content, ApplyFragment must be
+// called in order of increasing start position. As a result, each fragment can
+// be applied at most once.
+//
+// If an error occurs while applying, ApplyFragment returns an *ApplyError that
+// annotates the error with additional information. If the error is because of
+// a conflict between the fragment and the source, the wrapped error will be a
+// *Conflict.
+func (a *TextApplier) ApplyFragment(f *TextFragment) error {
+	if a.closed {
+		return applyError(errApplierClosed)
+	}
+
+	// mark an apply as in progress, even if it fails before making changes
+	a.dirty = true
+
+	// application code assumes fragment fields are consistent
+	if err := f.Validate(); err != nil {
+		return applyError(err)
+	}
+
+	// lines are 0-indexed, positions are 1-indexed (but new files have position = 0)
+	fragStart := f.OldPosition - 1
+	if fragStart < 0 {
+		fragStart = 0
+	}
+	fragEnd := fragStart + f.OldLines
+
+	start := a.nextLine
+	if fragStart < start {
+		return applyError(&Conflict{"fragment overlaps with an applied fragment"})
+	}
+
+	if f.OldPosition == 0 {
+		ok, err := isLen(a.src, 0)
+		if err != nil {
+			return applyError(err)
+		}
+		if !ok {
+			return applyError(&Conflict{"cannot create new file from non-empty src"})
+		}
+	}
+
+	preimage := make([][]byte, fragEnd-start)
+	n, err := a.lineSrc.ReadLinesAt(preimage, start)
+	if err != nil {
+		return applyError(err, lineNum(start+int64(n)))
+	}
+
+	// copy leading data before the fragment starts
+	for i, line := range preimage[:fragStart-start] {
+		if _, err := a.dst.Write(line); err != nil {
+			a.nextLine = start + int64(i)
+			return applyError(err, lineNum(a.nextLine))
+		}
+	}
+	preimage = preimage[fragStart-start:]
+
+	// apply the changes in the fragment
+	used := int64(0)
+	for i, line := range f.Lines {
+		if err := applyTextLine(a.dst, line, preimage, used); err != nil {
+			a.nextLine = fragStart + used
+			return applyError(err, lineNum(a.nextLine), fragLineNum(i))
+		}
+		if line.Old() {
+			used++
+		}
+	}
+	a.nextLine = fragStart + used
+
+	// new position of +0,0 mean a full delete, so check for leftovers
+	if f.NewPosition == 0 && f.NewLines == 0 {
+		var b [1][]byte
+		n, err := a.lineSrc.ReadLinesAt(b[:], a.nextLine)
+		if err != nil && err != io.EOF {
+			return applyError(err, lineNum(a.nextLine))
+		}
+		if n > 0 {
+			return applyError(&Conflict{"src still has content after full delete"}, lineNum(a.nextLine))
+		}
+	}
+
+	return nil
+}
+
+func applyTextLine(dst io.Writer, line Line, preimage [][]byte, i int64) (err error) {
+	if line.Old() && string(preimage[i]) != line.Line {
+		return &Conflict{"fragment line does not match src line"}
+	}
+	if line.New() {
+		_, err = io.WriteString(dst, line.Line)
+	}
+	return err
+}
+
+// Close writes any data following the last applied fragment and prevents
+// future calls to ApplyFragment.
+func (a *TextApplier) Close() (err error) {
+	if a.closed {
+		return nil
+	}
+
+	a.closed = true
+	if !a.dirty {
+		_, err = copyFrom(a.dst, a.src, 0)
+	} else {
+		_, err = copyLinesFrom(a.dst, a.lineSrc, a.nextLine)
+	}
+	return err
+}


Test patch:
diff --git a/gitdiff/apply_test.go b/gitdiff/apply_test.go
index d981e96..dd076bb 100644
--- a/gitdiff/apply_test.go
+++ b/gitdiff/apply_test.go
@@ -9,69 +9,6 @@ import (
 	"testing"
 )
 
-func TestApplierInvariants(t *testing.T) {
-	binary := &BinaryFragment{
-		Method: BinaryPatchLiteral,
-		Size:   2,
-		Data:   []byte("\xbe\xef"),
-	}
-
-	text := &TextFragment{
-		NewPosition: 1,
-		NewLines:    1,
-		LinesAdded:  1,
-		Lines: []Line{
-			{Op: OpAdd, Line: "new line\n"},
-		},
-	}
-
-	file := &File{
-		TextFragments: []*TextFragment{text},
-	}
-
-	src := bytes.NewReader(nil)
-	dst := ioutil.Discard
-
-	assertInProgress := func(t *testing.T, kind string, err error) {
-		if !errors.Is(err, errApplyInProgress) {
-			t.Fatalf("expected in-progress error for %s apply, but got: %v", kind, err)
-		}
-	}
-
-	t.Run("binaryFirst", func(t *testing.T) {
-		a := NewApplier(src)
-		if err := a.ApplyBinaryFragment(dst, binary); err != nil {
-			t.Fatalf("unexpected error applying fragment: %v", err)
-		}
-		assertInProgress(t, "text", a.ApplyTextFragment(dst, text))
-		assertInProgress(t, "binary", a.ApplyBinaryFragment(dst, binary))
-		assertInProgress(t, "file", a.ApplyFile(dst, file))
-	})
-
-	t.Run("textFirst", func(t *testing.T) {
-		a := NewApplier(src)
-		if err := a.ApplyTextFragment(dst, text); err != nil {
-			t.Fatalf("unexpected error applying fragment: %v", err)
-		}
-		// additional text fragments are allowed
-		if err := a.ApplyTextFragment(dst, text); err != nil {
-			t.Fatalf("unexpected error applying second fragment: %v", err)
-		}
-		assertInProgress(t, "binary", a.ApplyBinaryFragment(dst, binary))
-		assertInProgress(t, "file", a.ApplyFile(dst, file))
-	})
-
-	t.Run("fileFirst", func(t *testing.T) {
-		a := NewApplier(src)
-		if err := a.ApplyFile(dst, file); err != nil {
-			t.Fatalf("unexpected error applying file: %v", err)
-		}
-		assertInProgress(t, "text", a.ApplyTextFragment(dst, text))
-		assertInProgress(t, "binary", a.ApplyBinaryFragment(dst, binary))
-		assertInProgress(t, "file", a.ApplyFile(dst, file))
-	})
-}
-
 func TestApplyTextFragment(t *testing.T) {
 	tests := map[string]applyTest{
 		"createFile": {Files: getApplyFiles("text_fragment_new")},
@@ -127,11 +64,12 @@ func TestApplyTextFragment(t *testing.T) {
 
 	for name, test := range tests {
 		t.Run(name, func(t *testing.T) {
-			test.run(t, func(w io.Writer, applier *Applier, file *File) error {
+			test.run(t, func(dst io.Writer, src io.ReaderAt, file *File) error {
 				if len(file.TextFragments) != 1 {
 					t.Fatalf("patch should contain exactly one fragment, but it has %d", len(file.TextFragments))
 				}
-				return applier.ApplyTextFragment(w, file.TextFragments[0])
+				applier := NewTextApplier(dst, src)
+				return applier.ApplyFragment(file.TextFragments[0])
 			})
 		})
 	}
@@ -176,8 +114,9 @@ func TestApplyBinaryFragment(t *testing.T) {
 
 	for name, test := range tests {
 		t.Run(name, func(t *testing.T) {
-			test.run(t, func(w io.Writer, applier *Applier, file *File) error {
-				return applier.ApplyBinaryFragment(w, file.BinaryFragment)
+			test.run(t, func(dst io.Writer, src io.ReaderAt, file *File) error {
+				applier := NewBinaryApplier(dst, src)
+				return applier.ApplyFragment(file.BinaryFragment)
 			})
 		})
 	}
@@ -216,8 +155,8 @@ func TestApplyFile(t *testing.T) {
 
 	for name, test := range tests {
 		t.Run(name, func(t *testing.T) {
-			test.run(t, func(w io.Writer, applier *Applier, file *File) error {
-				return applier.ApplyFile(w, file)
+			test.run(t, func(dst io.Writer, src io.ReaderAt, file *File) error {
+				return Apply(dst, src, file)
 			})
 		})
 	}
@@ -228,7 +167,7 @@ type applyTest struct {
 	Err   interface{}
 }
 
-func (at applyTest) run(t *testing.T, apply func(io.Writer, *Applier, *File) error) {
+func (at applyTest) run(t *testing.T, apply func(io.Writer, io.ReaderAt, *File) error) {
 	src, patch, out := at.Files.Load(t)
 
 	files, _, err := Parse(bytes.NewReader(patch))
@@ -239,10 +178,8 @@ func (at applyTest) run(t *testing.T, apply func(io.Writer, *Applier, *File) err
 		t.Fatalf("patch should contain exactly one file, but it has %d", len(files))
 	}
 
-	applier := NewApplier(bytes.NewReader(src))
-
 	var dst bytes.Buffer
-	err = apply(&dst, applier, files[0])
+	err = apply(&dst, bytes.NewReader(src), files[0])
 	if at.Err != nil {
 		assertError(t, at.Err, err, "applying fragment")
 		return