format/diff: unified diff encoder and public API

- Added Patch interface
- Added a Unified Diff encoder from Patches
- Added Change method to generate Patches
- Added Changes method to generate Patches
- Added Tree method to generate Patches
- Added Commit method to generate Patches

3 files changed, 1242 insertions, 0 deletions

diff --git a/plumbing/format/diff/patch.go b/plumbing/format/diff/patch.go
new file mode 100644
index 0000000..7c6cf4a
--- /dev/null
+++ b/plumbing/format/diff/patch.go
@@ -0,0 +1,58 @@
+package diff
+
+import (
+	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/plumbing/filemode"
+)
+
+// Operation defines the operation of a diff item.
+type Operation int
+
+const (
+	// Equal item represents a equals diff.
+	Equal Operation = iota
+	// Add item represents an insert diff.
+	Add
+	// Delete item represents a delete diff.
+	Delete
+)
+
+// Patch represents a collection of steps to transform several files.
+type Patch interface {
+	// FilePatches returns a slice of patches per file.
+	FilePatches() []FilePatch
+	// Message returns an optional message that can be at the top of the
+	// Patch representation.
+	Message() string
+}
+
+// FilePatch represents the necessary steps to transform one file to another.
+type FilePatch interface {
+	// IsBinary returns true if this patch is representing a binary file.
+	IsBinary() bool
+	// Files returns the from and to Files, with all the necessary metadata to
+	// about them. If the patch creates a new file, "from" will be nil.
+	// If the patch deletes a file, "to" will be nil.
+	Files() (from, to File)
+	// Chunks returns a slice of ordered changes to transform "from" File to
+	// "to" File. If the file is a binary one, Chunks will be empty.
+	Chunks() []Chunk
+}
+
+// File contains all the file metadata necessary to print some patch formats.
+type File interface {
+	// Hash returns the File Hash.
+	Hash() plumbing.Hash
+	// Mode returns the FileMode.
+	Mode() filemode.FileMode
+	// Path returns the complete Path to the file, including the filename.
+	Path() string
+}
+
+// Chunk represents a portion of a file transformation to another.
+type Chunk interface {
+	// Content contains the portion of the file.
+	Content() string
+	// Type contains the Operation to do with this Chunk.
+	Type() Operation
+}
diff --git a/plumbing/format/diff/unified_encoder.go b/plumbing/format/diff/unified_encoder.go
new file mode 100644
index 0000000..a4ff7ab
--- /dev/null
+++ b/plumbing/format/diff/unified_encoder.go
@@ -0,0 +1,355 @@
+package diff
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"strings"
+
+	"gopkg.in/src-d/go-git.v4/plumbing"
+)
+
+const (
+	diffInit = "diff --git a/%s b/%s\n"
+
+	chunkStart  = "@@ -"
+	chunkMiddle = " +"
+	chunkEnd    = " @@%s\n"
+	chunkCount  = "%d,%d"
+
+	noFilePath = "/dev/null"
+	aDir       = "a/"
+	bDir       = "b/"
+
+	fPath  = "--- %s\n"
+	tPath  = "+++ %s\n"
+	binary = "Binary files %s and %s differ\n"
+
+	addLine    = "+%s\n"
+	deleteLine = "-%s\n"
+	equalLine  = " %s\n"
+
+	oldMode         = "old mode %o\n"
+	newMode         = "new mode %o\n"
+	deletedFileMode = "deleted file mode %o\n"
+	newFileMode     = "new file mode %o\n"
+
+	renameFrom     = "from"
+	renameTo       = "to"
+	renameFileMode = "rename %s %s\n"
+
+	indexAndMode = "index %s..%s %o\n"
+	indexNoMode  = "index %s..%s\n"
+
+	DefaultContextLines = 3
+)
+
+var ErrBothFilesEmpty = errors.New("both files are empty")
+
+// UnifiedEncoder encodes an unified diff into the provided Writer.
+// There are some unsupported features:
+//     - Similarity index for renames
+//     - Sort hash representation
+type UnifiedEncoder struct {
+	io.Writer
+
+	// ctxLines is the count of unchanged lines that will appear
+	// surrounding a change.
+	ctxLines int
+
+	buf bytes.Buffer
+}
+
+func NewUnifiedEncoder(w io.Writer, ctxLines int) *UnifiedEncoder {
+	return &UnifiedEncoder{ctxLines: ctxLines, Writer: w}
+}
+
+func (e *UnifiedEncoder) Encode(patch Patch) error {
+	e.printMessage(patch.Message())
+
+	if err := e.encodeFilePatch(patch.FilePatches()); err != nil {
+		return err
+	}
+
+	_, err := e.buf.WriteTo(e)
+
+	return err
+}
+
+func (e *UnifiedEncoder) encodeFilePatch(filePatches []FilePatch) error {
+	for _, p := range filePatches {
+		f, t := p.Files()
+		if err := e.header(f, t, p.IsBinary()); err != nil {
+			return err
+		}
+
+		g := newHunksGenerator(p.Chunks(), e.ctxLines)
+		for _, c := range g.Generate() {
+			c.WriteTo(&e.buf)
+		}
+	}
+
+	return nil
+}
+
+func (e *UnifiedEncoder) printMessage(message string) {
+	isEmpty := message == ""
+	hasSuffix := strings.HasSuffix(message, "\n")
+	if !isEmpty && !hasSuffix {
+		message = message + "\n"
+	}
+
+	e.buf.WriteString(message)
+}
+
+func (e *UnifiedEncoder) header(from, to File, isBinary bool) error {
+	switch {
+	case from == nil && to == nil:
+		return ErrBothFilesEmpty
+	case from != nil && to != nil:
+		hashEquals := from.Hash() == to.Hash()
+
+		fmt.Fprintf(&e.buf, diffInit, from.Path(), to.Path())
+
+		if from.Mode() != to.Mode() {
+			fmt.Fprintf(&e.buf, oldMode+newMode, from.Mode(), to.Mode())
+		}
+
+		if from.Path() != to.Path() {
+			fmt.Fprintf(&e.buf,
+				renameFileMode+renameFileMode,
+				renameFrom, from.Path(), renameTo, to.Path())
+		}
+
+		if from.Mode() != to.Mode() && !hashEquals {
+			fmt.Fprintf(&e.buf, indexNoMode, from.Hash(), to.Hash())
+		} else if !hashEquals {
+			fmt.Fprintf(&e.buf, indexAndMode, from.Hash(), to.Hash(), from.Mode())
+		}
+
+		if !hashEquals {
+			e.pathLines(isBinary, aDir+from.Path(), bDir+to.Path())
+		}
+	case from == nil:
+		fmt.Fprintf(&e.buf, diffInit, to.Path(), to.Path())
+		fmt.Fprintf(&e.buf, newFileMode, to.Mode())
+		fmt.Fprintf(&e.buf, indexNoMode, plumbing.ZeroHash, to.Hash())
+		e.pathLines(isBinary, noFilePath, bDir+to.Path())
+	case to == nil:
+		fmt.Fprintf(&e.buf, diffInit, from.Path(), from.Path())
+		fmt.Fprintf(&e.buf, deletedFileMode, from.Mode())
+		fmt.Fprintf(&e.buf, indexNoMode, from.Hash(), plumbing.ZeroHash)
+		e.pathLines(isBinary, aDir+from.Path(), noFilePath)
+	}
+
+	return nil
+}
+
+func (e *UnifiedEncoder) pathLines(isBinary bool, fromPath, toPath string) {
+	format := fPath + tPath
+	if isBinary {
+		format = binary
+	}
+
+	fmt.Fprintf(&e.buf, format, fromPath, toPath)
+}
+
+type hunksGenerator struct {
+	fromLine, toLine            int
+	ctxLines                    int
+	chunks                      []Chunk
+	current                     *hunk
+	hunks                       []*hunk
+	beforeContext, afterContext []string
+}
+
+func newHunksGenerator(chunks []Chunk, ctxLines int) *hunksGenerator {
+	return &hunksGenerator{
+		chunks:   chunks,
+		ctxLines: ctxLines,
+	}
+}
+
+func (c *hunksGenerator) Generate() []*hunk {
+	for i, chunk := range c.chunks {
+		ls := splitLines(chunk.Content())
+		lsLen := len(ls)
+
+		switch chunk.Type() {
+		case Equal:
+			c.fromLine += lsLen
+			c.toLine += lsLen
+			c.processEqualsLines(ls, i)
+		case Delete:
+			if lsLen != 0 {
+				c.fromLine++
+			}
+
+			c.processHunk(i, chunk.Type())
+			c.fromLine += lsLen - 1
+			c.current.AddOp(chunk.Type(), ls...)
+		case Add:
+			if lsLen != 0 {
+				c.toLine++
+			}
+			c.processHunk(i, chunk.Type())
+			c.toLine += lsLen - 1
+			c.current.AddOp(chunk.Type(), ls...)
+		}
+
+		if i == len(c.chunks)-1 && c.current != nil {
+			c.hunks = append(c.hunks, c.current)
+		}
+	}
+
+	return c.hunks
+}
+
+func (c *hunksGenerator) processHunk(i int, op Operation) {
+	if c.current != nil {
+		return
+	}
+
+	var ctxPrefix string
+	linesBefore := len(c.beforeContext)
+	if linesBefore > c.ctxLines {
+		ctxPrefix = " " + c.beforeContext[linesBefore-c.ctxLines-1]
+		c.beforeContext = c.beforeContext[linesBefore-c.ctxLines:]
+		linesBefore = c.ctxLines
+	}
+
+	c.current = &hunk{ctxPrefix: ctxPrefix}
+	c.current.AddOp(Equal, c.beforeContext...)
+
+	switch op {
+	case Delete:
+		c.current.fromLine, c.current.toLine =
+			c.addLineNumbers(c.fromLine, c.toLine, linesBefore, i, Add)
+	case Add:
+		c.current.toLine, c.current.fromLine =
+			c.addLineNumbers(c.toLine, c.fromLine, linesBefore, i, Delete)
+	}
+
+	c.beforeContext = nil
+}
+
+// addLineNumbers obtains the line numbers in a new chunk
+func (c *hunksGenerator) addLineNumbers(la, lb int, linesBefore int, i int, op Operation) (cla, clb int) {
+	cla = la - linesBefore
+	// we need to search for a reference for the next diff
+	switch {
+	case linesBefore != 0 && c.ctxLines != 0:
+		clb = lb - c.ctxLines + 1
+	case c.ctxLines == 0:
+		clb = lb - c.ctxLines
+	case i != len(c.chunks)-1:
+		next := c.chunks[i+1]
+		if next.Type() == op || next.Type() == Equal {
+			// this diff will be into this chunk
+			clb = lb + 1
+		}
+	}
+
+	return
+}
+
+func (c *hunksGenerator) processEqualsLines(ls []string, i int) {
+	if c.current == nil {
+		c.beforeContext = append(c.beforeContext, ls...)
+		return
+	}
+
+	c.afterContext = append(c.afterContext, ls...)
+	if len(c.afterContext) <= c.ctxLines*2 && i != len(c.chunks)-1 {
+		c.current.AddOp(Equal, c.afterContext...)
+		c.afterContext = nil
+	} else {
+		c.current.AddOp(Equal, c.afterContext[:c.ctxLines]...)
+		c.hunks = append(c.hunks, c.current)
+
+		c.current = nil
+		c.beforeContext = c.afterContext[c.ctxLines:]
+		c.afterContext = nil
+	}
+}
+
+func splitLines(s string) []string {
+	out := strings.Split(s, "\n")
+	if out[len(out)-1] == "" {
+		out = out[:len(out)-1]
+	}
+
+	return out
+}
+
+type hunk struct {
+	fromLine int
+	toLine   int
+
+	fromCount int
+	toCount   int
+
+	ctxPrefix string
+	ops       []*op
+}
+
+func (c *hunk) WriteTo(buf *bytes.Buffer) {
+	buf.WriteString(chunkStart)
+
+	if c.fromCount == 1 {
+		fmt.Fprintf(buf, "%d", c.fromLine)
+	} else {
+		fmt.Fprintf(buf, chunkCount, c.fromLine, c.fromCount)
+	}
+
+	buf.WriteString(chunkMiddle)
+
+	if c.toCount == 1 {
+		fmt.Fprintf(buf, "%d", c.toLine)
+	} else {
+		fmt.Fprintf(buf, chunkCount, c.toLine, c.toCount)
+	}
+
+	fmt.Fprintf(buf, chunkEnd, c.ctxPrefix)
+
+	for _, d := range c.ops {
+		buf.WriteString(d.String())
+	}
+}
+
+func (c *hunk) AddOp(t Operation, s ...string) {
+	ls := len(s)
+	switch t {
+	case Add:
+		c.toCount += ls
+	case Delete:
+		c.fromCount += ls
+	case Equal:
+		c.toCount += ls
+		c.fromCount += ls
+	}
+
+	for _, l := range s {
+		c.ops = append(c.ops, &op{l, t})
+	}
+}
+
+type op struct {
+	text string
+	t    Operation
+}
+
+func (o *op) String() string {
+	var prefix string
+	switch o.t {
+	case Add:
+		prefix = addLine
+	case Delete:
+		prefix = deleteLine
+	case Equal:
+		prefix = equalLine
+	}
+
+	return fmt.Sprintf(prefix, o.text)
+}
diff --git a/plumbing/format/diff/unified_encoder_test.go b/plumbing/format/diff/unified_encoder_test.go
new file mode 100644
index 0000000..b832920
--- /dev/null
+++ b/plumbing/format/diff/unified_encoder_test.go
@@ -0,0 +1,829 @@
+package diff
+
+import (
+	"bytes"
+	"testing"
+
+	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/plumbing/filemode"
+
+	. "gopkg.in/check.v1"
+)
+
+func Test(t *testing.T) { TestingT(t) }
+
+type UnifiedEncoderTestSuite struct{}
+
+var _ = Suite(&UnifiedEncoderTestSuite{})
+
+func (s *UnifiedEncoderTestSuite) TestBothFilesEmpty(c *C) {
+	buffer := bytes.NewBuffer(nil)
+	e := NewUnifiedEncoder(buffer, 1)
+	err := e.Encode(testPatch{filePatches: []testFilePatch{{}}})
+	c.Assert(err, Equals, ErrBothFilesEmpty)
+}
+
+func (s *UnifiedEncoderTestSuite) TestBinaryFile(c *C) {
+	buffer := bytes.NewBuffer(nil)
+	e := NewUnifiedEncoder(buffer, 1)
+	p := testPatch{
+		message: "",
+		filePatches: []testFilePatch{{
+			from: &testFile{
+				mode: filemode.Regular,
+				path: "binary",
+				seed: "something",
+			},
+			to: &testFile{
+				mode: filemode.Regular,
+				path: "binary",
+				seed: "otherthing",
+			},
+		}},
+	}
+
+	err := e.Encode(p)
+	c.Assert(err, IsNil)
+
+	c.Assert(buffer.String(), Equals, `diff --git a/binary b/binary
+index a459bc245bdbc45e1bca99e7fe61731da5c48da4..6879395eacf3cc7e5634064ccb617ac7aa62be7d 100644
+Binary files a/binary and b/binary differ
+`)
+}
+
+func (s *UnifiedEncoderTestSuite) TestEncode(c *C) {
+	for _, f := range fixtures {
+		c.Log("executing: ", f.desc)
+
+		buffer := bytes.NewBuffer(nil)
+		e := NewUnifiedEncoder(buffer, f.context)
+
+		err := e.Encode(f.patch)
+		c.Assert(err, IsNil)
+
+		c.Assert(buffer.String(), Equals, f.diff)
+	}
+}
+
+var oneChunkPatch Patch = testPatch{
+	message: "",
+	filePatches: []testFilePatch{{
+		from: &testFile{
+			mode: filemode.Regular,
+			path: "onechunk.txt",
+			seed: "A\nB\nC\nD\nE\nF\nG\nH\nI\nJ\nK\nL\nM\nN\nÑ\nO\nP\nQ\nR\nS\nT\nU\nV\nW\nX\nY\nZ",
+		},
+		to: &testFile{
+			mode: filemode.Regular,
+			path: "onechunk.txt",
+			seed: "B\nC\nD\nE\nF\nG\nI\nJ\nK\nL\nM\nN\nO\nP\nQ\nR\nS\nT\nV\nW\nX\nY\nZ",
+		},
+
+		chunks: []testChunk{{
+			content: "A\n",
+			op:      Delete,
+		}, {
+			content: "B\nC\nD\nE\nF\nG",
+			op:      Equal,
+		}, {
+			content: "H\n",
+			op:      Delete,
+		}, {
+			content: "I\nJ\nK\nL\nM\nN\n",
+			op:      Equal,
+		}, {
+			content: "Ñ\n",
+			op:      Delete,
+		}, {
+			content: "O\nP\nQ\nR\nS\nT\n",
+			op:      Equal,
+		}, {
+			content: "U\n",
+			op:      Delete,
+		}, {
+			content: "V\nW\nX\nY\nZ",
+			op:      Equal,
+		}},
+	}},
+}
+
+var oneChunkPatchInverted Patch = testPatch{
+	message: "",
+	filePatches: []testFilePatch{{
+		to: &testFile{
+			mode: filemode.Regular,
+			path: "onechunk.txt",
+			seed: "A\nB\nC\nD\nE\nF\nG\nH\nI\nJ\nK\nL\nM\nN\nÑ\nO\nP\nQ\nR\nS\nT\nU\nV\nW\nX\nY\nZ",
+		},
+		from: &testFile{
+			mode: filemode.Regular,
+			path: "onechunk.txt",
+			seed: "B\nC\nD\nE\nF\nG\nI\nJ\nK\nL\nM\nN\nO\nP\nQ\nR\nS\nT\nV\nW\nX\nY\nZ",
+		},
+
+		chunks: []testChunk{{
+			content: "A\n",
+			op:      Add,
+		}, {
+			content: "B\nC\nD\nE\nF\nG",
+			op:      Equal,
+		}, {
+			content: "H\n",
+			op:      Add,
+		}, {
+			content: "I\nJ\nK\nL\nM\nN\n",
+			op:      Equal,
+		}, {
+			content: "Ñ\n",
+			op:      Add,
+		}, {
+			content: "O\nP\nQ\nR\nS\nT\n",
+			op:      Equal,
+		}, {
+			content: "U\n",
+			op:      Add,
+		}, {
+			content: "V\nW\nX\nY\nZ",
+			op:      Equal,
+		}},
+	}},
+}
+
+var fixtures []*fixture = []*fixture{{
+	patch: testPatch{
+		message: "",
+		filePatches: []testFilePatch{{
+			from: &testFile{
+				mode: filemode.Regular,
+				path: "test.txt",
+				seed: "test",
+			},
+			to: &testFile{
+				mode: filemode.Executable,
+				path: "test.txt",
+				seed: "test",
+			},
+			chunks: nil,
+		}},
+	},
+	desc:    "make executable",
+	context: 1,
+	diff: `diff --git a/test.txt b/test.txt
+old mode 100644
+new mode 100755
+`,
+}, {
+	patch: testPatch{
+		message: "",
+		filePatches: []testFilePatch{{
+			from: &testFile{
+				mode: filemode.Regular,
+				path: "test.txt",
+				seed: "test",
+			},
+			to: &testFile{
+				mode: filemode.Regular,
+				path: "test1.txt",
+				seed: "test",
+			},
+			chunks: nil,
+		}},
+	},
+	desc:    "rename file",
+	context: 1,
+	diff: `diff --git a/test.txt b/test1.txt
+rename from test.txt
+rename to test1.txt
+`,
+}, {
+	patch: testPatch{
+		message: "",
+		filePatches: []testFilePatch{{
+			from: &testFile{
+				mode: filemode.Regular,
+				path: "test.txt",
+				seed: "test",
+			},
+			to: &testFile{
+				mode: filemode.Regular,
+				path: "test1.txt",
+				seed: "test1",
+			},
+			chunks: []testChunk{{
+				content: "test",
+				op:      Delete,
+			}, {
+				content: "test1",
+				op:      Add,
+			}},
+		}},
+	},
+	desc:    "rename file with changes",
+	context: 1,
+	diff: `diff --git a/test.txt b/test1.txt
+rename from test.txt
+rename to test1.txt
+index 30d74d258442c7c65512eafab474568dd706c430..f079749c42ffdcc5f52ed2d3a6f15b09307e975e 100644
+--- a/test.txt
++++ b/test1.txt
+@@ -1 +1 @@
+-test
++test1
+`,
+}, {
+	patch: testPatch{
+		message: "",
+		filePatches: []testFilePatch{{
+			from: &testFile{
+				mode: filemode.Regular,
+				path: "test.txt",
+				seed: "test",
+			},
+			to: &testFile{
+				mode: filemode.Executable,
+				path: "test1.txt",
+				seed: "test",
+			},
+			chunks: nil,
+		}},
+	},
+	desc:    "rename with file mode change",
+	context: 1,
+	diff: `diff --git a/test.txt b/test1.txt
+old mode 100644
+new mode 100755
+rename from test.txt
+rename to test1.txt
+`,
+}, {
+	patch: testPatch{
+		message: "",
+		filePatches: []testFilePatch{{
+			from: &testFile{
+				mode: filemode.Regular,
+				path: "test.txt",
+				seed: "test",
+			},
+			to: &testFile{
+				mode: filemode.Regular,
+				path: "test.txt",
+				seed: "test2",
+			},
+
+			chunks: []testChunk{{
+				content: "test",
+				op:      Delete,
+			}, {
+				content: "test2",
+				op:      Add,
+			}},
+		}},
+	},
+
+	desc:    "one line change",
+	context: 1,
+	diff: `diff --git a/test.txt b/test.txt
+index 30d74d258442c7c65512eafab474568dd706c430..d606037cb232bfda7788a8322492312d55b2ae9d 100644
+--- a/test.txt
++++ b/test.txt
+@@ -1 +1 @@
+-test
++test2
+`,
+}, {
+	patch: testPatch{
+		message: "this is the message\n",
+		filePatches: []testFilePatch{{
+			from: &testFile{
+				mode: filemode.Regular,
+				path: "test.txt",
+				seed: "test",
+			},
+			to: &testFile{
+				mode: filemode.Regular,
+				path: "test.txt",
+				seed: "test2",
+			},
+
+			chunks: []testChunk{{
+				content: "test",
+				op:      Delete,
+			}, {
+				content: "test2",
+				op:      Add,
+			}},
+		}},
+	},
+
+	desc:    "one line change with message",
+	context: 1,
+	diff: `this is the message
+diff --git a/test.txt b/test.txt
+index 30d74d258442c7c65512eafab474568dd706c430..d606037cb232bfda7788a8322492312d55b2ae9d 100644
+--- a/test.txt
++++ b/test.txt
+@@ -1 +1 @@
+-test
++test2
+`,
+}, {
+	patch: testPatch{
+		message: "this is the message",
+		filePatches: []testFilePatch{{
+			from: &testFile{
+				mode: filemode.Regular,
+				path: "test.txt",
+				seed: "test",
+			},
+			to: &testFile{
+				mode: filemode.Regular,
+				path: "test.txt",
+				seed: "test2",
+			},
+
+			chunks: []testChunk{{
+				content: "test",
+				op:      Delete,
+			}, {
+				content: "test2",
+				op:      Add,
+			}},
+		}},
+	},
+
+	desc:    "one line change with message and no end of line",
+	context: 1,
+	diff: `this is the message
+diff --git a/test.txt b/test.txt
+index 30d74d258442c7c65512eafab474568dd706c430..d606037cb232bfda7788a8322492312d55b2ae9d 100644
+--- a/test.txt
++++ b/test.txt
+@@ -1 +1 @@
+-test
++test2
+`,
+}, {
+	patch: testPatch{
+		message: "",
+		filePatches: []testFilePatch{{
+			from: nil,
+			to: &testFile{
+				mode: filemode.Regular,
+				path: "new.txt",
+				seed: "test\ntest2\test3",
+			},
+
+			chunks: []testChunk{{
+				content: "test\ntest2\ntest3",
+				op:      Add,
+			}},
+		}},
+	},
+
+	desc:    "new file",
+	context: 1,
+	diff: `diff --git a/new.txt b/new.txt
+new file mode 100644
+index 0000000000000000000000000000000000000000..65c8dd02a42273038658a22b1cb29c8d9457ca12
+--- /dev/null
++++ b/new.txt
+@@ -0,0 +1,3 @@
++test
++test2
++test3
+`,
+}, {
+	patch: testPatch{
+		message: "",
+		filePatches: []testFilePatch{{
+			from: &testFile{
+				mode: filemode.Regular,
+				path: "old.txt",
+				seed: "test",
+			},
+			to: nil,
+
+			chunks: []testChunk{{
+				content: "test",
+				op:      Delete,
+			}},
+		}},
+	},
+
+	desc:    "delete file",
+	context: 1,
+	diff: `diff --git a/old.txt b/old.txt
+deleted file mode 100644
+index 30d74d258442c7c65512eafab474568dd706c430..0000000000000000000000000000000000000000
+--- a/old.txt
++++ /dev/null
+@@ -1 +0,0 @@
+-test
+`,
+}, {
+	patch:   oneChunkPatch,
+	desc:    "modified deleting lines file with context to 1",
+	context: 1,
+	diff: `diff --git a/onechunk.txt b/onechunk.txt
+index ab5eed5d4a2c33aeef67e0188ee79bed666bde6f..0adddcde4fd38042c354518351820eb06c417c82 100644
+--- a/onechunk.txt
++++ b/onechunk.txt
+@@ -1,2 +1 @@
+-A
+ B
+@@ -7,3 +6,2 @@ F
+ G
+-H
+ I
+@@ -14,3 +12,2 @@ M
+ N
+-Ñ
+ O
+@@ -21,3 +18,2 @@ S
+ T
+-U
+ V
+`,
+}, {
+	patch:   oneChunkPatch,
+	desc:    "modified deleting lines file with context to 2",
+	context: 2,
+	diff: `diff --git a/onechunk.txt b/onechunk.txt
+index ab5eed5d4a2c33aeef67e0188ee79bed666bde6f..0adddcde4fd38042c354518351820eb06c417c82 100644
+--- a/onechunk.txt
++++ b/onechunk.txt
+@@ -1,3 +1,2 @@
+-A
+ B
+ C
+@@ -6,5 +5,4 @@ E
+ F
+ G
+-H
+ I
+ J
+@@ -13,5 +11,4 @@ L
+ M
+ N
+-Ñ
+ O
+ P
+@@ -20,5 +17,4 @@ R
+ S
+ T
+-U
+ V
+ W
+`,
+}, {
+	patch: oneChunkPatch,
+
+	desc:    "modified deleting lines file with context to 3",
+	context: 3,
+	diff: `diff --git a/onechunk.txt b/onechunk.txt
+index ab5eed5d4a2c33aeef67e0188ee79bed666bde6f..0adddcde4fd38042c354518351820eb06c417c82 100644
+--- a/onechunk.txt
++++ b/onechunk.txt
+@@ -1,25 +1,21 @@
+-A
+ B
+ C
+ D
+ E
+ F
+ G
+-H
+ I
+ J
+ K
+ L
+ M
+ N
+-Ñ
+ O
+ P
+ Q
+ R
+ S
+ T
+-U
+ V
+ W
+ X
+`,
+}, {
+	patch:   oneChunkPatch,
+	desc:    "modified deleting lines file with context to 4",
+	context: 4,
+	diff: `diff --git a/onechunk.txt b/onechunk.txt
+index ab5eed5d4a2c33aeef67e0188ee79bed666bde6f..0adddcde4fd38042c354518351820eb06c417c82 100644
+--- a/onechunk.txt
++++ b/onechunk.txt
+@@ -1,26 +1,22 @@
+-A
+ B
+ C
+ D
+ E
+ F
+ G
+-H
+ I
+ J
+ K
+ L
+ M
+ N
+-Ñ
+ O
+ P
+ Q
+ R
+ S
+ T
+-U
+ V
+ W
+ X
+ Y
+`,
+}, {
+	patch:   oneChunkPatch,
+	desc:    "modified deleting lines file with context to 0",
+	context: 0,
+	diff: `diff --git a/onechunk.txt b/onechunk.txt
+index ab5eed5d4a2c33aeef67e0188ee79bed666bde6f..0adddcde4fd38042c354518351820eb06c417c82 100644
+--- a/onechunk.txt
++++ b/onechunk.txt
+@@ -1 +0,0 @@
+-A
+@@ -8 +6,0 @@ G
+-H
+@@ -15 +12,0 @@ N
+-Ñ
+@@ -22 +18,0 @@ T
+-U
+`,
+}, {
+	patch:   oneChunkPatchInverted,
+	desc:    "modified adding lines file with context to 1",
+	context: 1,
+	diff: `diff --git a/onechunk.txt b/onechunk.txt
+index 0adddcde4fd38042c354518351820eb06c417c82..ab5eed5d4a2c33aeef67e0188ee79bed666bde6f 100644
+--- a/onechunk.txt
++++ b/onechunk.txt
+@@ -1 +1,2 @@
++A
+ B
+@@ -6,2 +7,3 @@ F
+ G
++H
+ I
+@@ -12,2 +14,3 @@ M
+ N
++Ñ
+ O
+@@ -18,2 +21,3 @@ S
+ T
++U
+ V
+`,
+}, {
+	patch:   oneChunkPatchInverted,
+	desc:    "modified adding lines file with context to 2",
+	context: 2,
+	diff: `diff --git a/onechunk.txt b/onechunk.txt
+index 0adddcde4fd38042c354518351820eb06c417c82..ab5eed5d4a2c33aeef67e0188ee79bed666bde6f 100644
+--- a/onechunk.txt
++++ b/onechunk.txt
+@@ -1,2 +1,3 @@
++A
+ B
+ C
+@@ -5,4 +6,5 @@ E
+ F
+ G
++H
+ I
+ J
+@@ -11,4 +13,5 @@ L
+ M
+ N
++Ñ
+ O
+ P
+@@ -17,4 +20,5 @@ R
+ S
+ T
++U
+ V
+ W
+`,
+}, {
+	patch:   oneChunkPatchInverted,
+	desc:    "modified adding lines file with context to 3",
+	context: 3,
+	diff: `diff --git a/onechunk.txt b/onechunk.txt
+index 0adddcde4fd38042c354518351820eb06c417c82..ab5eed5d4a2c33aeef67e0188ee79bed666bde6f 100644
+--- a/onechunk.txt
++++ b/onechunk.txt
+@@ -1,21 +1,25 @@
++A
+ B
+ C
+ D
+ E
+ F
+ G
++H
+ I
+ J
+ K
+ L
+ M
+ N
++Ñ
+ O
+ P
+ Q
+ R
+ S
+ T
++U
+ V
+ W
+ X
+`,
+}, {
+	patch:   oneChunkPatchInverted,
+	desc:    "modified adding lines file with context to 4",
+	context: 4,
+	diff: `diff --git a/onechunk.txt b/onechunk.txt
+index 0adddcde4fd38042c354518351820eb06c417c82..ab5eed5d4a2c33aeef67e0188ee79bed666bde6f 100644
+--- a/onechunk.txt
++++ b/onechunk.txt
+@@ -1,22 +1,26 @@
++A
+ B
+ C
+ D
+ E
+ F
+ G
++H
+ I
+ J
+ K
+ L
+ M
+ N
++Ñ
+ O
+ P
+ Q
+ R
+ S
+ T
++U
+ V
+ W
+ X
+ Y
+`,
+}, {
+	patch:   oneChunkPatchInverted,
+	desc:    "modified adding lines file with context to 0",
+	context: 0,
+	diff: `diff --git a/onechunk.txt b/onechunk.txt
+index 0adddcde4fd38042c354518351820eb06c417c82..ab5eed5d4a2c33aeef67e0188ee79bed666bde6f 100644
+--- a/onechunk.txt
++++ b/onechunk.txt
+@@ -0,0 +1 @@
++A
+@@ -6,0 +8 @@ G
++H
+@@ -12,0 +15 @@ N
++Ñ
+@@ -18,0 +22 @@ T
++U
+`,
+}, {
+	patch: testPatch{
+		message: "",
+		filePatches: []testFilePatch{{
+			from: &testFile{
+				mode: filemode.Regular,
+				path: "onechunk.txt",
+				seed: "B\nC\nD\nE\nF\nG\nI\nJ\nK\nL\nM\nN\nO\nP\nQ\nR\nS\nT\nV\nW\nX\nY\nZ",
+			},
+			to: &testFile{
+				mode: filemode.Regular,
+				path: "onechunk.txt",
+				seed: "B\nC\nD\nE\nF\nG\nI\nJ\nK\nL\nM\nN\nO\nP\nQ\nR\nS\nT\nV\nW\nX\nY\n",
+			},
+
+			chunks: []testChunk{{
+				content: "B\nC\nD\nE\nF\nG\nI\nJ\nK\nL\nM\nN\nO\nP\nQ\nR\nS\nT\nV\nW\nX\nY\n",
+				op:      Equal,
+			}, {
+				content: "Z",
+				op:      Delete,
+			}},
+		}},
+	},
+	desc:    "remove last letter",
+	context: 0,
+	diff: `diff --git a/onechunk.txt b/onechunk.txt
+index 0adddcde4fd38042c354518351820eb06c417c82..553ae669c7a9303cf848fcc749a2569228ac5309 100644
+--- a/onechunk.txt
++++ b/onechunk.txt
+@@ -23 +22,0 @@ Y
+-Z
+`,
+}}
+
+type testPatch struct {
+	message     string
+	filePatches []testFilePatch
+}
+
+func (t testPatch) FilePatches() []FilePatch {
+	var result []FilePatch
+	for _, f := range t.filePatches {
+		result = append(result, f)
+	}
+
+	return result
+}
+
+func (t testPatch) Message() string {
+	return t.message
+}
+
+type testFilePatch struct {
+	from, to *testFile
+	chunks   []testChunk
+}
+
+func (t testFilePatch) IsBinary() bool {
+	return len(t.chunks) == 0
+}
+func (t testFilePatch) Files() (File, File) {
+	// Go is amazing
+	switch {
+	case t.from == nil && t.to == nil:
+		return nil, nil
+	case t.from == nil:
+		return nil, t.to
+	case t.to == nil:
+		return t.from, nil
+	}
+
+	return t.from, t.to
+}
+
+func (t testFilePatch) Chunks() []Chunk {
+	var result []Chunk
+	for _, c := range t.chunks {
+		result = append(result, c)
+	}
+	return result
+}
+
+type testFile struct {
+	path string
+	mode filemode.FileMode
+	seed string
+}
+
+func (t testFile) Hash() plumbing.Hash {
+	return plumbing.ComputeHash(plumbing.BlobObject, []byte(t.seed))
+}
+
+func (t testFile) Mode() filemode.FileMode {
+	return t.mode
+}
+
+func (t testFile) Path() string {
+	return t.path
+}
+
+type testChunk struct {
+	content string
+	op      Operation
+}
+
+func (t testChunk) Content() string {
+	return t.content
+}
+
+func (t testChunk) Type() Operation {
+	return t.op
+}
+
+type fixture struct {
+	desc    string
+	context int
+	diff    string
+	patch   Patch
+}