2 files changed, 53 insertions, 6 deletions

diff --git a/plumbing/format/diff/unified_encoder.go b/plumbing/format/diff/unified_encoder.go
index 413984a..fa605b1 100644
--- a/plumbing/format/diff/unified_encoder.go
+++ b/plumbing/format/diff/unified_encoder.go
@@ -38,6 +38,10 @@ type UnifiedEncoder struct {
 	// a change.
 	contextLines int
 
+	// srcPrefix and dstPrefix are prepended to file paths when encoding a diff.
+	srcPrefix string
+	dstPrefix string
+
 	// colorConfig is the color configuration. The default is no color.
 	color ColorConfig
 }
@@ -46,6 +50,8 @@ type UnifiedEncoder struct {
 func NewUnifiedEncoder(w io.Writer, contextLines int) *UnifiedEncoder {
 	return &UnifiedEncoder{
 		Writer:       w,
+		srcPrefix:    "a/",
+		dstPrefix:    "b/",
 		contextLines: contextLines,
 	}
 }
@@ -56,6 +62,18 @@ func (e *UnifiedEncoder) SetColor(colorConfig ColorConfig) *UnifiedEncoder {
 	return e
 }
 
+// SetSrcPrefix sets e's srcPrefix and returns e.
+func (e *UnifiedEncoder) SetSrcPrefix(prefix string) *UnifiedEncoder {
+	e.srcPrefix = prefix
+	return e
+}
+
+// SetDstPrefix sets e's dstPrefix and returns e.
+func (e *UnifiedEncoder) SetDstPrefix(prefix string) *UnifiedEncoder {
+	e.dstPrefix = prefix
+	return e
+}
+
 // Encode encodes patch.
 func (e *UnifiedEncoder) Encode(patch Patch) error {
 	sb := &strings.Builder{}
@@ -91,7 +109,8 @@ func (e *UnifiedEncoder) writeFilePatchHeader(sb *strings.Builder, filePatch Fil
 	case from != nil && to != nil:
 		hashEquals := from.Hash() == to.Hash()
 		lines = append(lines,
-			fmt.Sprintf("diff --git a/%s b/%s", from.Path(), to.Path()),
+			fmt.Sprintf("diff --git %s%s %s%s",
+				e.srcPrefix, from.Path(), e.dstPrefix, to.Path()),
 		)
 		if from.Mode() != to.Mode() {
 			lines = append(lines,
@@ -115,22 +134,22 @@ func (e *UnifiedEncoder) writeFilePatchHeader(sb *strings.Builder, filePatch Fil
 			)
 		}
 		if !hashEquals {
-			lines = e.appendPathLines(lines, "a/"+from.Path(), "b/"+to.Path(), isBinary)
+			lines = e.appendPathLines(lines, e.srcPrefix+from.Path(), e.dstPrefix+to.Path(), isBinary)
 		}
 	case from == nil:
 		lines = append(lines,
-			fmt.Sprintf("diff --git a/%s b/%s", to.Path(), to.Path()),
+			fmt.Sprintf("diff --git %s %s", e.srcPrefix+to.Path(), e.dstPrefix+to.Path()),
 			fmt.Sprintf("new file mode %o", to.Mode()),
 			fmt.Sprintf("index %s..%s", plumbing.ZeroHash, to.Hash()),
 		)
-		lines = e.appendPathLines(lines, "/dev/null", "b/"+to.Path(), isBinary)
+		lines = e.appendPathLines(lines, "/dev/null", e.dstPrefix+to.Path(), isBinary)
 	case to == nil:
 		lines = append(lines,
-			fmt.Sprintf("diff --git a/%s b/%s", from.Path(), from.Path()),
+			fmt.Sprintf("diff --git %s %s", e.srcPrefix+from.Path(), e.dstPrefix+from.Path()),
 			fmt.Sprintf("deleted file mode %o", from.Mode()),
 			fmt.Sprintf("index %s..%s", from.Hash(), plumbing.ZeroHash),
 		)
-		lines = e.appendPathLines(lines, "a/"+from.Path(), "/dev/null", isBinary)
+		lines = e.appendPathLines(lines, e.srcPrefix+from.Path(), "/dev/null", isBinary)
 	}
 
 	sb.WriteString(e.color[Meta])
diff --git a/plumbing/format/diff/unified_encoder_test.go b/plumbing/format/diff/unified_encoder_test.go
index 22dc4f1..3eee333 100644
--- a/plumbing/format/diff/unified_encoder_test.go
+++ b/plumbing/format/diff/unified_encoder_test.go
@@ -52,6 +52,34 @@ Binary files a/binary and b/binary differ
 `)
 }
 
+func (s *UnifiedEncoderTestSuite) TestCustomSrcDstPrefix(c *C) {
+	buffer := bytes.NewBuffer(nil)
+	e := NewUnifiedEncoder(buffer, 1).SetSrcPrefix("source/prefix/").SetDstPrefix("dest/prefix/")
+	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 source/prefix/binary dest/prefix/binary
+index a459bc245bdbc45e1bca99e7fe61731da5c48da4..6879395eacf3cc7e5634064ccb617ac7aa62be7d 100644
+Binary files source/prefix/binary and dest/prefix/binary differ
+`)
+}
+
 func (s *UnifiedEncoderTestSuite) TestEncode(c *C) {
 	for _, f := range fixtures {
 		c.Log("executing: ", f.desc)