Add patch option WillBeAppliedReverse

It's not used yet, but covered with tests already.

pkg/commands/patch/hunk.go

@@ -45,7 +45,7 @@ func headerInfo(header string) (int, int, string) {
 	return oldStart, newStart, heading
 }
 
-func (hunk *PatchHunk) updatedLines(lineIndices []int, reverse bool) []string {
+func (hunk *PatchHunk) updatedLines(lineIndices []int, reverse bool, willBeAppliedReverse bool) []string {
 	skippedNewlineMessageIndex := -1
 	newLines := []string{}
 
@@ -58,7 +58,7 @@ func (hunk *PatchHunk) updatedLines(lineIndices []int, reverse bool) []string {
 		isLineSelected := lo.Contains(lineIndices, lineIdx)
 
 		firstChar, content := line[:1], line[1:]
-		transformedFirstChar := transformedFirstChar(firstChar, reverse, isLineSelected)
+		transformedFirstChar := transformedFirstChar(firstChar, reverse, willBeAppliedReverse, isLineSelected)
 
 		if isLineSelected || (transformedFirstChar == "\\" && skippedNewlineMessageIndex != lineIdx) || transformedFirstChar == " " {
 			newLines = append(newLines, transformedFirstChar+content)
@@ -74,7 +74,7 @@ func (hunk *PatchHunk) updatedLines(lineIndices []int, reverse bool) []string {
 	return newLines
 }
 
-func transformedFirstChar(firstChar string, reverse bool, isLineSelected bool) string {
+func transformedFirstChar(firstChar string, reverse bool, willBeAppliedReverse bool, isLineSelected bool) string {
 	if reverse {
 		if !isLineSelected && firstChar == "+" {
 			return " "
@@ -87,7 +87,11 @@ func transformedFirstChar(firstChar string, reverse bool, isLineSelected bool) s
 		}
 	}
 
-	if !isLineSelected && firstChar == "-" {
+	linesToKeepInPatchContext := "-"
+	if willBeAppliedReverse {
+		linesToKeepInPatchContext = "+"
+	}
+	if !isLineSelected && firstChar == linesToKeepInPatchContext {
 		return " "
 	}
 
@@ -98,8 +102,8 @@ func (hunk *PatchHunk) formatHeader(oldStart int, oldLength int, newStart int, n
 	return fmt.Sprintf("@@ -%d,%d +%d,%d @@%s\n", oldStart, oldLength, newStart, newLength, heading)
 }
 
-func (hunk *PatchHunk) formatWithChanges(lineIndices []int, reverse bool, startOffset int) (int, string) {
-	bodyLines := hunk.updatedLines(lineIndices, reverse)
+func (hunk *PatchHunk) formatWithChanges(lineIndices []int, reverse bool, willBeAppliedReverse bool, startOffset int) (int, string) {
+	bodyLines := hunk.updatedLines(lineIndices, reverse, willBeAppliedReverse)
 	startOffset, header, ok := hunk.updatedHeader(bodyLines, startOffset, reverse)
 	if !ok {
 		return startOffset, ""

pkg/commands/patch/patch_modifier.go

@@ -18,6 +18,15 @@ type PatchOptions struct {
 	// generating the patch.
 	Reverse bool
 
+	// If true, we're building a patch that we are going to apply using
+	// "git apply --reverse". In other words, we are not flipping the '+' and
+	// '-' ourselves while creating the patch, but git is going to do that when
+	// applying. This has consequences for which lines we need to keep or
+	// discard when filtering lines from partial hunks.
+	//
+	// Currently incompatible with Reverse.
+	WillBeAppliedReverse bool
+
 	// Whether to keep or discard the original diff header including the
 	// "index deadbeef..fa1afe1 100644" line.
 	KeepOriginalHeader bool
@@ -105,7 +114,8 @@ outer:
 	formattedHunks := ""
 	var formattedHunk string
 	for _, hunk := range hunksInRange {
-		startOffset, formattedHunk = hunk.formatWithChanges(lineIndices, opts.Reverse, startOffset)
+		startOffset, formattedHunk = hunk.formatWithChanges(
+			lineIndices, opts.Reverse, opts.WillBeAppliedReverse, startOffset)
 		formattedHunks += formattedHunk
 	}
 

pkg/commands/patch/patch_modifier_test.go

@@ -69,6 +69,20 @@ index e48a11c..b2ab81b 100644
  ...
 `
 
+const twoChangesInOneHunk = `diff --git a/filename b/filename
+index 9320895..6d79956 100644
+--- a/filename
++++ b/filename
+@@ -1,5 +1,5 @@
+ apple
+-grape
++kiwi
+ orange
+-pear
++banana
+ lemon
+`
+
 const newFile = `diff --git a/newfile b/newfile
 new file mode 100644
 index 0000000..4e680cc
@@ -101,13 +115,14 @@ const exampleHunk = `@@ -1,5 +1,5 @@
 // TestModifyPatchForRange is a function.
 func TestModifyPatchForRange(t *testing.T) {
 	type scenario struct {
-		testName       string
-		filename       string
-		diffText       string
-		firstLineIndex int
-		lastLineIndex  int
-		reverse        bool
-		expected       string
+		testName             string
+		filename             string
+		diffText             string
+		firstLineIndex       int
+		lastLineIndex        int
+		reverse              bool
+		willBeAppliedReverse bool
+		expected             string
 	}
 
 	scenarios := []scenario{
@@ -506,6 +521,44 @@ func TestModifyPatchForRange(t *testing.T) {
 @@ -1,1 +0,0 @@
 -new line
 \ No newline at end of file
+`,
+		},
+		{
+			testName:             "adding part of a hunk",
+			filename:             "filename",
+			firstLineIndex:       6,
+			lastLineIndex:        7,
+			reverse:              false,
+			willBeAppliedReverse: false,
+			diffText:             twoChangesInOneHunk,
+			expected: `--- a/filename
++++ b/filename
+@@ -1,5 +1,5 @@
+ apple
+-grape
++kiwi
+ orange
+ pear
+ lemon
+`,
+		},
+		{
+			testName:             "adding part of a hunk, will-be-applied-reverse",
+			filename:             "filename",
+			firstLineIndex:       6,
+			lastLineIndex:        7,
+			reverse:              false,
+			willBeAppliedReverse: true,
+			diffText:             twoChangesInOneHunk,
+			expected: `--- a/filename
++++ b/filename
+@@ -1,5 +1,5 @@
+ apple
+-grape
++kiwi
+ orange
+ banana
+ lemon
 `,
 		},
 	}
@@ -514,7 +567,11 @@ func TestModifyPatchForRange(t *testing.T) {
 		s := s
 		t.Run(s.testName, func(t *testing.T) {
 			result := ModifiedPatchForRange(nil, s.filename, s.diffText, s.firstLineIndex, s.lastLineIndex,
-				PatchOptions{Reverse: s.reverse, KeepOriginalHeader: false})
+				PatchOptions{
+					Reverse:              s.reverse,
+					WillBeAppliedReverse: s.willBeAppliedReverse,
+					KeepOriginalHeader:   false,
+				})
 			if !assert.Equal(t, s.expected, result) {
 				fmt.Println(result)
 			}