move interdiff & combinediff into patchutil

appview/db/pulls.go

···
 	return false
 }
 
-func (s PullSubmission) AsNiceDiff(targetBranch string) types.NiceDiff {
+func (s PullSubmission) AsDiff(targetBranch string) ([]*gitdiff.File, error) {
 	patch := s.Patch
 
-	diffs, _, err := gitdiff.Parse(strings.NewReader(patch))
+	// if format-patch; then extract each patch
+	var diffs []*gitdiff.File
+	if patchutil.IsFormatPatch(patch) {
+		patches, err := patchutil.ExtractPatches(patch)
+		if err != nil {
+			return nil, err
+		}
+		var ps [][]*gitdiff.File
+		for _, p := range patches {
+			ps = append(ps, p.Files)
+		}
+
+		diffs = patchutil.CombineDiff(ps...)
+	} else {
+		d, _, err := gitdiff.Parse(strings.NewReader(patch))
+		if err != nil {
+			return nil, err
+		}
+		diffs = d
+	}
+
+	return diffs, nil
+}
+
+func (s PullSubmission) AsNiceDiff(targetBranch string) types.NiceDiff {
+	diffs, err := s.AsDiff(targetBranch)
 	if err != nil {
 		log.Println(err)
 	}

appview/pages/pages.go

···
 	"tangled.sh/tangled.sh/core/appview/db"
 	"tangled.sh/tangled.sh/core/appview/pages/markup"
 	"tangled.sh/tangled.sh/core/appview/state/userutil"
-	"tangled.sh/tangled.sh/core/interdiff"
+	"tangled.sh/tangled.sh/core/patchutil"
 	"tangled.sh/tangled.sh/core/types"
 
 	"github.com/alecthomas/chroma/v2"
···
 	RepoInfo     RepoInfo
 	Pull         *db.Pull
 	Round        int
-	Interdiff    *interdiff.InterdiffResult
+	Interdiff    *patchutil.InterdiffResult
 }
 
 // this name is a mouthful

appview/pages/templates/repo/pulls/pull.html

···
               </span>
             </div>
 
-            {{ if $.Pull.IsPatchBased }}
-            <!-- view patch -->
             <a class="btn flex items-center gap-2 no-underline hover:no-underline p-2"
                hx-boost="true"
                href="/{{ $.RepoInfo.FullName }}/pulls/{{ $.Pull.PullId }}/round/{{.RoundNumber}}">
               {{ i "file-diff" "w-4 h-4" }} <span class="hidden md:inline">view patch</span>
             </a>
-              {{ if not (eq .RoundNumber 0) }}
-              <a class="btn flex items-center gap-2 no-underline hover:no-underline p-2"
-                 hx-boost="true"
-                 href="/{{ $.RepoInfo.FullName }}/pulls/{{ $.Pull.PullId }}/round/{{.RoundNumber}}/interdiff">
-                {{ i "file-diff" "w-4 h-4" }} <span class="hidden md:inline">interdiff</span>
-              </a>
-              <span id="interdiff-error-{{.RoundNumber}}"></span>
-              {{ end }}
+            {{ if not (eq .RoundNumber 0) }}
+            <a class="btn flex items-center gap-2 no-underline hover:no-underline p-2"
+               hx-boost="true"
+               href="/{{ $.RepoInfo.FullName }}/pulls/{{ $.Pull.PullId }}/round/{{.RoundNumber}}/interdiff">
+              {{ i "file-diff" "w-4 h-4" }} <span class="hidden md:inline">interdiff</span>
+            </a>
+            <span id="interdiff-error-{{.RoundNumber}}"></span>
             {{ end }}
           </div>
         </summary>

appview/state/pull.go

···
 	"net/http"
 	"net/url"
 	"strconv"
-	"strings"
 	"time"
 
 	"tangled.sh/tangled.sh/core/api/tangled"
 	"tangled.sh/tangled.sh/core/appview/auth"
 	"tangled.sh/tangled.sh/core/appview/db"
 	"tangled.sh/tangled.sh/core/appview/pages"
-	"tangled.sh/tangled.sh/core/interdiff"
 	"tangled.sh/tangled.sh/core/patchutil"
 	"tangled.sh/tangled.sh/core/types"
 
-	"github.com/bluekeyes/go-gitdiff/gitdiff"
 	comatproto "github.com/bluesky-social/indigo/api/atproto"
 	"github.com/bluesky-social/indigo/atproto/syntax"
 	lexutil "github.com/bluesky-social/indigo/lex/util"
···
 		}
 	}
 
-	currentPatch, _, err := gitdiff.Parse(strings.NewReader(pull.Submissions[roundIdInt].Patch))
+	currentPatch, err := pull.Submissions[roundIdInt].AsDiff(pull.TargetBranch)
 	if err != nil {
 		log.Println("failed to interdiff; current patch malformed")
 		s.pages.Notice(w, fmt.Sprintf("interdiff-error-%d", roundIdInt), "Failed to calculate interdiff; current patch is invalid.")
 		return
 	}
 
-	previousPatch, _, err := gitdiff.Parse(strings.NewReader(pull.Submissions[roundIdInt-1].Patch))
+	previousPatch, err := pull.Submissions[roundIdInt-1].AsDiff(pull.TargetBranch)
 	if err != nil {
 		log.Println("failed to interdiff; previous patch malformed")
 		s.pages.Notice(w, fmt.Sprintf("interdiff-error-%d", roundIdInt), "Failed to calculate interdiff; previous patch is invalid.")
 		return
 	}
 
-	interdiff := interdiff.Interdiff(previousPatch, currentPatch)
+	interdiff := patchutil.Interdiff(previousPatch, currentPatch)
 
 	s.pages.RepoPullInterdiffPage(w, pages.RepoPullInterdiffParams{
 		LoggedInUser: s.auth.GetUser(r),

cmd/combinediff/main.go

···
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/bluekeyes/go-gitdiff/gitdiff"
+	"tangled.sh/tangled.sh/core/patchutil"
+)
+
+func main() {
+	if len(os.Args) != 3 {
+		fmt.Println("Usage: combinediff <patch1> <patch2>")
+		os.Exit(1)
+	}
+
+	patch1, err := os.Open(os.Args[1])
+	if err != nil {
+		fmt.Println(err)
+	}
+	patch2, err := os.Open(os.Args[2])
+	if err != nil {
+		fmt.Println(err)
+	}
+
+	files1, _, err := gitdiff.Parse(patch1)
+	if err != nil {
+		fmt.Println(err)
+	}
+
+	files2, _, err := gitdiff.Parse(patch2)
+	if err != nil {
+		fmt.Println(err)
+	}
+
+	combined := patchutil.CombineDiff(files1, files2)
+	fmt.Println(combined)
+}

cmd/interdiff/main.go

···
 	"os"
 
 	"github.com/bluekeyes/go-gitdiff/gitdiff"
-	"tangled.sh/tangled.sh/core/interdiff"
+	"tangled.sh/tangled.sh/core/patchutil"
 )
 
 func main() {
···
 		fmt.Println(err)
 	}
 
-	interDiffResult := interdiff.Interdiff(files1, files2)
+	interDiffResult := patchutil.Interdiff(files1, files2)
 	fmt.Println(interDiffResult)
 }

interdiff/interdiff.go

···
-package interdiff
-
-import (
-	"bytes"
-	"fmt"
-	"os"
-	"os/exec"
-	"strings"
-
-	"github.com/bluekeyes/go-gitdiff/gitdiff"
-)
-
-type ReconstructedLine struct {
-	LineNumber int64
-	Content    string
-	IsUnknown  bool
-}
-
-func NewLineAt(lineNumber int64, content string) ReconstructedLine {
-	return ReconstructedLine{
-		LineNumber: lineNumber,
-		Content:    content,
-		IsUnknown:  false,
-	}
-}
-
-type ReconstructedFile struct {
-	File string
-	Data []*ReconstructedLine
-}
-
-func (r *ReconstructedFile) String() string {
-	var i, j int64
-	var b strings.Builder
-	for {
-		i += 1
-
-		if int(j) >= (len(r.Data)) {
-			break
-		}
-
-		if r.Data[j].LineNumber == i {
-			// b.WriteString(fmt.Sprintf("%d:", r.Data[j].LineNumber))
-			b.WriteString(r.Data[j].Content)
-			j += 1
-		} else {
-			//b.WriteString(fmt.Sprintf("%d:\n", i))
-			b.WriteString("\n")
-		}
-	}
-
-	return b.String()
-}
-
-func (r *ReconstructedFile) AddLine(line *ReconstructedLine) {
-	r.Data = append(r.Data, line)
-}
-
-func bestName(file *gitdiff.File) string {
-	if file.IsDelete {
-		return file.OldName
-	} else {
-		return file.NewName
-	}
-}
-
-// in-place reverse of a diff
-func reverseDiff(file *gitdiff.File) {
-	file.OldName, file.NewName = file.NewName, file.OldName
-	file.OldMode, file.NewMode = file.NewMode, file.OldMode
-	file.BinaryFragment, file.ReverseBinaryFragment = file.ReverseBinaryFragment, file.BinaryFragment
-
-	for _, fragment := range file.TextFragments {
-		// swap postions
-		fragment.OldPosition, fragment.NewPosition = fragment.NewPosition, fragment.OldPosition
-		fragment.OldLines, fragment.NewLines = fragment.NewLines, fragment.OldLines
-		fragment.LinesAdded, fragment.LinesDeleted = fragment.LinesDeleted, fragment.LinesAdded
-
-		for i := range fragment.Lines {
-			switch fragment.Lines[i].Op {
-			case gitdiff.OpAdd:
-				fragment.Lines[i].Op = gitdiff.OpDelete
-			case gitdiff.OpDelete:
-				fragment.Lines[i].Op = gitdiff.OpAdd
-			default:
-				// do nothing
-			}
-		}
-	}
-}
-
-// rebuild the original file from a patch
-func CreateOriginal(file *gitdiff.File) ReconstructedFile {
-	rf := ReconstructedFile{
-		File: bestName(file),
-	}
-
-	for _, fragment := range file.TextFragments {
-		position := fragment.OldPosition
-		for _, line := range fragment.Lines {
-			switch line.Op {
-			case gitdiff.OpContext:
-				rl := NewLineAt(position, line.Line)
-				rf.Data = append(rf.Data, &rl)
-				position += 1
-			case gitdiff.OpDelete:
-				rl := NewLineAt(position, line.Line)
-				rf.Data = append(rf.Data, &rl)
-				position += 1
-			case gitdiff.OpAdd:
-				// do nothing here
-			}
-		}
-	}
-
-	return rf
-}
-
-type MergeError struct {
-	msg             string
-	mismatchingLine int64
-}
-
-func (m MergeError) Error() string {
-	return fmt.Sprintf("%s: %v", m.msg, m.mismatchingLine)
-}
-
-// best effort merging of two reconstructed files
-func (this *ReconstructedFile) Merge(other *ReconstructedFile) (*ReconstructedFile, error) {
-	mergedFile := ReconstructedFile{}
-
-	var i, j int64
-
-	for int(i) < len(this.Data) || int(j) < len(other.Data) {
-		if int(i) >= len(this.Data) {
-			// first file is done; the rest of the lines from file 2 can go in
-			mergedFile.AddLine(other.Data[j])
-			j++
-			continue
-		}
-
-		if int(j) >= len(other.Data) {
-			// first file is done; the rest of the lines from file 2 can go in
-			mergedFile.AddLine(this.Data[i])
-			i++
-			continue
-		}
-
-		line1 := this.Data[i]
-		line2 := other.Data[j]
-
-		if line1.LineNumber == line2.LineNumber {
-			if line1.Content != line2.Content {
-				return nil, MergeError{
-					msg:             "mismatching lines, this patch might have undergone rebase",
-					mismatchingLine: line1.LineNumber,
-				}
-			} else {
-				mergedFile.AddLine(line1)
-			}
-			i++
-			j++
-		} else if line1.LineNumber < line2.LineNumber {
-			mergedFile.AddLine(line1)
-			i++
-		} else {
-			mergedFile.AddLine(line2)
-			j++
-		}
-	}
-
-	return &mergedFile, nil
-}
-
-func (r *ReconstructedFile) Apply(patch *gitdiff.File) (string, error) {
-	original := r.String()
-	var buffer bytes.Buffer
-	reader := strings.NewReader(original)
-
-	err := gitdiff.Apply(&buffer, reader, patch)
-	if err != nil {
-		return "", err
-	}
-
-	return buffer.String(), nil
-}
-
-func Unified(oldText, oldFile, newText, newFile string) (string, error) {
-	oldTemp, err := os.CreateTemp("", "old_*")
-	if err != nil {
-		return "", fmt.Errorf("failed to create temp file for oldText: %w", err)
-	}
-	defer os.Remove(oldTemp.Name())
-	if _, err := oldTemp.WriteString(oldText); err != nil {
-		return "", fmt.Errorf("failed to write to old temp file: %w", err)
-	}
-	oldTemp.Close()
-
-	newTemp, err := os.CreateTemp("", "new_*")
-	if err != nil {
-		return "", fmt.Errorf("failed to create temp file for newText: %w", err)
-	}
-	defer os.Remove(newTemp.Name())
-	if _, err := newTemp.WriteString(newText); err != nil {
-		return "", fmt.Errorf("failed to write to new temp file: %w", err)
-	}
-	newTemp.Close()
-
-	cmd := exec.Command("diff", "-u", "--label", oldFile, "--label", newFile, oldTemp.Name(), newTemp.Name())
-	output, err := cmd.CombinedOutput()
-
-	if exitErr, ok := err.(*exec.ExitError); ok && exitErr.ExitCode() == 1 {
-		return string(output), nil
-	}
-	if err != nil {
-		return "", fmt.Errorf("diff command failed: %w", err)
-	}
-
-	return string(output), nil
-}
-
-type InterdiffResult struct {
-	Files []*InterdiffFile
-}
-
-func (i *InterdiffResult) String() string {
-	var b strings.Builder
-	for _, f := range i.Files {
-		b.WriteString(f.String())
-		b.WriteString("\n")
-	}
-
-	return b.String()
-}
-
-type InterdiffFile struct {
-	*gitdiff.File
-	Name   string
-	Status InterdiffFileStatus
-}
-
-func (s *InterdiffFile) String() string {
-	var b strings.Builder
-	b.WriteString(s.Status.String())
-	b.WriteString(" ")
-
-	if s.File != nil {
-		b.WriteString(bestName(s.File))
-		b.WriteString("\n")
-		b.WriteString(s.File.String())
-	}
-
-	return b.String()
-}
-
-type InterdiffFileStatus struct {
-	StatusKind StatusKind
-	Error      error
-}
-
-func (s *InterdiffFileStatus) String() string {
-	kind := s.StatusKind.String()
-	if s.Error != nil {
-		return fmt.Sprintf("%s [%s]", kind, s.Error.Error())
-	} else {
-		return kind
-	}
-}
-
-func (s *InterdiffFileStatus) IsOk() bool {
-	return s.StatusKind == StatusOk
-}
-
-func (s *InterdiffFileStatus) IsUnchanged() bool {
-	return s.StatusKind == StatusUnchanged
-}
-
-func (s *InterdiffFileStatus) IsOnlyInOne() bool {
-	return s.StatusKind == StatusOnlyInOne
-}
-
-func (s *InterdiffFileStatus) IsOnlyInTwo() bool {
-	return s.StatusKind == StatusOnlyInTwo
-}
-
-func (s *InterdiffFileStatus) IsRebased() bool {
-	return s.StatusKind == StatusRebased
-}
-
-func (s *InterdiffFileStatus) IsError() bool {
-	return s.StatusKind == StatusError
-}
-
-type StatusKind int
-
-func (k StatusKind) String() string {
-	switch k {
-	case StatusOnlyInOne:
-		return "only in one"
-	case StatusOnlyInTwo:
-		return "only in two"
-	case StatusUnchanged:
-		return "unchanged"
-	case StatusRebased:
-		return "rebased"
-	case StatusError:
-		return "error"
-	default:
-		return "changed"
-	}
-}
-
-const (
-	StatusOk StatusKind = iota
-	StatusOnlyInOne
-	StatusOnlyInTwo
-	StatusUnchanged
-	StatusRebased
-	StatusError
-)
-
-func interdiffFiles(f1, f2 *gitdiff.File) *InterdiffFile {
-	re1 := CreateOriginal(f1)
-	re2 := CreateOriginal(f2)
-
-	interdiffFile := InterdiffFile{
-		Name: bestName(f1),
-	}
-
-	merged, err := re1.Merge(&re2)
-	if err != nil {
-		interdiffFile.Status = InterdiffFileStatus{
-			StatusKind: StatusRebased,
-			Error:      err,
-		}
-		return &interdiffFile
-	}
-
-	rev1, err := merged.Apply(f1)
-	if err != nil {
-		interdiffFile.Status = InterdiffFileStatus{
-			StatusKind: StatusError,
-			Error:      err,
-		}
-		return &interdiffFile
-	}
-
-	rev2, err := merged.Apply(f2)
-	if err != nil {
-		interdiffFile.Status = InterdiffFileStatus{
-			StatusKind: StatusError,
-			Error:      err,
-		}
-		return &interdiffFile
-	}
-
-	diff, err := Unified(rev1, bestName(f1), rev2, bestName(f2))
-	if err != nil {
-		interdiffFile.Status = InterdiffFileStatus{
-			StatusKind: StatusError,
-			Error:      err,
-		}
-		return &interdiffFile
-	}
-
-	parsed, _, err := gitdiff.Parse(strings.NewReader(diff))
-	if err != nil {
-		interdiffFile.Status = InterdiffFileStatus{
-			StatusKind: StatusError,
-			Error:      err,
-		}
-		return &interdiffFile
-	}
-
-	if len(parsed) != 1 {
-		// files are identical?
-		interdiffFile.Status = InterdiffFileStatus{
-			StatusKind: StatusUnchanged,
-		}
-		return &interdiffFile
-	}
-
-	if interdiffFile.Status.StatusKind == StatusOk {
-		interdiffFile.File = parsed[0]
-	}
-
-	return &interdiffFile
-}
-
-func Interdiff(patch1, patch2 []*gitdiff.File) *InterdiffResult {
-	fileToIdx1 := make(map[string]int)
-	fileToIdx2 := make(map[string]int)
-	visited := make(map[string]struct{})
-	var result InterdiffResult
-
-	for idx, f := range patch1 {
-		fileToIdx1[bestName(f)] = idx
-	}
-
-	for idx, f := range patch2 {
-		fileToIdx2[bestName(f)] = idx
-	}
-
-	for _, f1 := range patch1 {
-		var interdiffFile *InterdiffFile
-
-		fileName := bestName(f1)
-		if idx, ok := fileToIdx2[fileName]; ok {
-			f2 := patch2[idx]
-
-			// we have f1 and f2, calculate interdiff
-			interdiffFile = interdiffFiles(f1, f2)
-		} else {
-			// only in patch 1, this change would have to be "inverted" to dissapear
-			// from patch 2, so we reverseDiff(f1)
-			reverseDiff(f1)
-
-			interdiffFile = &InterdiffFile{
-				File: f1,
-				Name: fileName,
-				Status: InterdiffFileStatus{
-					StatusKind: StatusOnlyInOne,
-				},
-			}
-		}
-
-		result.Files = append(result.Files, interdiffFile)
-		visited[fileName] = struct{}{}
-	}
-
-	// for all files in patch2 that remain unvisited; we can just add them into the output
-	for _, f2 := range patch2 {
-		fileName := bestName(f2)
-		if _, ok := visited[fileName]; ok {
-			continue
-		}
-
-		result.Files = append(result.Files, &InterdiffFile{
-			File: f2,
-			Name: fileName,
-			Status: InterdiffFileStatus{
-				StatusKind: StatusOnlyInTwo,
-			},
-		})
-	}
-
-	return &result
-}

patchutil/combinediff.go

···
+package patchutil
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/bluekeyes/go-gitdiff/gitdiff"
+)
+
+// original1 -> patch1 -> rev1
+// original2 -> patch2 -> rev2
+//
+// original2 must be equal to rev1, so we can merge them to get maximal context
+//
+// finally,
+// rev2' <- apply(patch2, merged)
+// combineddiff <- diff(rev2', original1)
+func combineFiles(file1, file2 *gitdiff.File) (*gitdiff.File, error) {
+	fileName := bestName(file1)
+
+	o1 := CreatePreImage(file1)
+	r1 := CreatePostImage(file1)
+	o2 := CreatePreImage(file2)
+
+	merged, err := r1.Merge(&o2)
+	if err != nil {
+		return nil, err
+	}
+
+	r2Prime, err := merged.Apply(file2)
+	if err != nil {
+		return nil, err
+	}
+
+	// produce combined diff
+	diff, err := Unified(o1.String(), fileName, r2Prime, fileName)
+	if err != nil {
+		return nil, err
+	}
+
+	parsed, _, err := gitdiff.Parse(strings.NewReader(diff))
+
+	if len(parsed) != 1 {
+		// no diff? the second commit reverted the changes from the first
+		return nil, nil
+	}
+
+	return parsed[0], nil
+}
+
+// use empty lines for lines we are unaware of
+//
+// this raises an error only if the two patches were invalid or non-contiguous
+func mergeLines(old, new string) (string, error) {
+	var i, j int
+
+	// TODO: use strings.Lines
+	linesOld := strings.Split(old, "\n")
+	linesNew := strings.Split(new, "\n")
+
+	result := []string{}
+
+	for i < len(linesOld) || j < len(linesNew) {
+		if i >= len(linesOld) {
+			// rest of the file is populated from `new`
+			result = append(result, linesNew[j])
+			j++
+			continue
+		}
+
+		if j >= len(linesNew) {
+			// rest of the file is populated from `old`
+			result = append(result, linesOld[i])
+			i++
+			continue
+		}
+
+		oldLine := linesOld[i]
+		newLine := linesNew[j]
+
+		if oldLine != newLine && (oldLine != "" && newLine != "") {
+			// context mismatch
+			return "", fmt.Errorf("failed to merge files, found context mismatch at %d; oldLine: `%s`, newline: `%s`", i+1, oldLine, newLine)
+		}
+
+		if oldLine == newLine {
+			result = append(result, oldLine)
+		} else if oldLine == "" {
+			result = append(result, newLine)
+		} else if newLine == "" {
+			result = append(result, oldLine)
+		}
+		i++
+		j++
+	}
+
+	return strings.Join(result, "\n"), nil
+}
+
+func combineTwo(patch1, patch2 []*gitdiff.File) []*gitdiff.File {
+	fileToIdx1 := make(map[string]int)
+	fileToIdx2 := make(map[string]int)
+	visited := make(map[string]struct{})
+	var result []*gitdiff.File
+
+	for idx, f := range patch1 {
+		fileToIdx1[bestName(f)] = idx
+	}
+
+	for idx, f := range patch2 {
+		fileToIdx2[bestName(f)] = idx
+	}
+
+	for _, f1 := range patch1 {
+		fileName := bestName(f1)
+		if idx, ok := fileToIdx2[fileName]; ok {
+			f2 := patch2[idx]
+
+			// we have f1 and f2, combine them
+			combined, err := combineFiles(f1, f2)
+			if err != nil {
+				fmt.Println(err)
+			}
+
+			result = append(result, combined)
+		} else {
+			// only in patch1; add as-is
+			result = append(result, f1)
+		}
+
+		visited[fileName] = struct{}{}
+	}
+
+	// for all files in patch2 that remain unvisited; we can just add them into the output
+	for _, f2 := range patch2 {
+		fileName := bestName(f2)
+		if _, ok := visited[fileName]; ok {
+			continue
+		}
+
+		result = append(result, f2)
+	}
+
+	return result
+}
+
+// pairwise combination from first to last patch
+func CombineDiff(patches ...[]*gitdiff.File) []*gitdiff.File {
+	if len(patches) == 0 {
+		return nil
+	}
+
+	if len(patches) == 1 {
+		return patches[0]
+	}
+
+	combined := combineTwo(patches[0], patches[1])
+
+	newPatches := [][]*gitdiff.File{}
+	newPatches = append(newPatches, combined)
+	for i, p := range patches {
+		if i >= 2 {
+			newPatches = append(newPatches, p)
+		}
+	}
+
+	return CombineDiff(newPatches...)
+}

patchutil/image.go

···
+package patchutil
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+
+	"github.com/bluekeyes/go-gitdiff/gitdiff"
+)
+
+type Line struct {
+	LineNumber int64
+	Content    string
+	IsUnknown  bool
+}
+
+func NewLineAt(lineNumber int64, content string) Line {
+	return Line{
+		LineNumber: lineNumber,
+		Content:    content,
+		IsUnknown:  false,
+	}
+}
+
+type Image struct {
+	File string
+	Data []*Line
+}
+
+func (r *Image) String() string {
+	var i, j int64
+	var b strings.Builder
+	for {
+		i += 1
+
+		if int(j) >= (len(r.Data)) {
+			break
+		}
+
+		if r.Data[j].LineNumber == i {
+			// b.WriteString(fmt.Sprintf("%d:", r.Data[j].LineNumber))
+			b.WriteString(r.Data[j].Content)
+			j += 1
+		} else {
+			//b.WriteString(fmt.Sprintf("%d:\n", i))
+			b.WriteString("\n")
+		}
+	}
+
+	return b.String()
+}
+
+func (r *Image) AddLine(line *Line) {
+	r.Data = append(r.Data, line)
+}
+
+// rebuild the original file from a patch
+func CreatePreImage(file *gitdiff.File) Image {
+	rf := Image{
+		File: bestName(file),
+	}
+
+	for _, fragment := range file.TextFragments {
+		position := fragment.OldPosition
+		for _, line := range fragment.Lines {
+			switch line.Op {
+			case gitdiff.OpContext:
+				rl := NewLineAt(position, line.Line)
+				rf.Data = append(rf.Data, &rl)
+				position += 1
+			case gitdiff.OpDelete:
+				rl := NewLineAt(position, line.Line)
+				rf.Data = append(rf.Data, &rl)
+				position += 1
+			case gitdiff.OpAdd:
+				// do nothing here
+			}
+		}
+	}
+
+	return rf
+}
+
+// rebuild the revised file from a patch
+func CreatePostImage(file *gitdiff.File) Image {
+	rf := Image{
+		File: bestName(file),
+	}
+
+	for _, fragment := range file.TextFragments {
+		position := fragment.NewPosition
+		for _, line := range fragment.Lines {
+			switch line.Op {
+			case gitdiff.OpContext:
+				rl := NewLineAt(position, line.Line)
+				rf.Data = append(rf.Data, &rl)
+				position += 1
+			case gitdiff.OpAdd:
+				rl := NewLineAt(position, line.Line)
+				rf.Data = append(rf.Data, &rl)
+				position += 1
+			case gitdiff.OpDelete:
+				// do nothing here
+			}
+		}
+	}
+
+	return rf
+}
+
+type MergeError struct {
+	msg             string
+	mismatchingLine int64
+}
+
+func (m MergeError) Error() string {
+	return fmt.Sprintf("%s: %v", m.msg, m.mismatchingLine)
+}
+
+// best effort merging of two reconstructed files
+func (this *Image) Merge(other *Image) (*Image, error) {
+	mergedFile := Image{}
+
+	var i, j int64
+
+	for int(i) < len(this.Data) || int(j) < len(other.Data) {
+		if int(i) >= len(this.Data) {
+			// first file is done; the rest of the lines from file 2 can go in
+			mergedFile.AddLine(other.Data[j])
+			j++
+			continue
+		}
+
+		if int(j) >= len(other.Data) {
+			// first file is done; the rest of the lines from file 2 can go in
+			mergedFile.AddLine(this.Data[i])
+			i++
+			continue
+		}
+
+		line1 := this.Data[i]
+		line2 := other.Data[j]
+
+		if line1.LineNumber == line2.LineNumber {
+			if line1.Content != line2.Content {
+				return nil, MergeError{
+					msg:             "mismatching lines, this patch might have undergone rebase",
+					mismatchingLine: line1.LineNumber,
+				}
+			} else {
+				mergedFile.AddLine(line1)
+			}
+			i++
+			j++
+		} else if line1.LineNumber < line2.LineNumber {
+			mergedFile.AddLine(line1)
+			i++
+		} else {
+			mergedFile.AddLine(line2)
+			j++
+		}
+	}
+
+	return &mergedFile, nil
+}
+
+func (r *Image) Apply(patch *gitdiff.File) (string, error) {
+	original := r.String()
+	var buffer bytes.Buffer
+	reader := strings.NewReader(original)
+
+	err := gitdiff.Apply(&buffer, reader, patch)
+	if err != nil {
+		return "", err
+	}
+
+	return buffer.String(), nil
+}

patchutil/interdiff.go

···
+package patchutil
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/bluekeyes/go-gitdiff/gitdiff"
+)
+
+type InterdiffResult struct {
+	Files []*InterdiffFile
+}
+
+func (i *InterdiffResult) String() string {
+	var b strings.Builder
+	for _, f := range i.Files {
+		b.WriteString(f.String())
+		b.WriteString("\n")
+	}
+
+	return b.String()
+}
+
+type InterdiffFile struct {
+	*gitdiff.File
+	Name   string
+	Status InterdiffFileStatus
+}
+
+func (s *InterdiffFile) String() string {
+	var b strings.Builder
+	b.WriteString(s.Status.String())
+	b.WriteString(" ")
+
+	if s.File != nil {
+		b.WriteString(bestName(s.File))
+		b.WriteString("\n")
+		b.WriteString(s.File.String())
+	}
+
+	return b.String()
+}
+
+type InterdiffFileStatus struct {
+	StatusKind StatusKind
+	Error      error
+}
+
+func (s *InterdiffFileStatus) String() string {
+	kind := s.StatusKind.String()
+	if s.Error != nil {
+		return fmt.Sprintf("%s [%s]", kind, s.Error.Error())
+	} else {
+		return kind
+	}
+}
+
+func (s *InterdiffFileStatus) IsOk() bool {
+	return s.StatusKind == StatusOk
+}
+
+func (s *InterdiffFileStatus) IsUnchanged() bool {
+	return s.StatusKind == StatusUnchanged
+}
+
+func (s *InterdiffFileStatus) IsOnlyInOne() bool {
+	return s.StatusKind == StatusOnlyInOne
+}
+
+func (s *InterdiffFileStatus) IsOnlyInTwo() bool {
+	return s.StatusKind == StatusOnlyInTwo
+}
+
+func (s *InterdiffFileStatus) IsRebased() bool {
+	return s.StatusKind == StatusRebased
+}
+
+func (s *InterdiffFileStatus) IsError() bool {
+	return s.StatusKind == StatusError
+}
+
+type StatusKind int
+
+func (k StatusKind) String() string {
+	switch k {
+	case StatusOnlyInOne:
+		return "only in one"
+	case StatusOnlyInTwo:
+		return "only in two"
+	case StatusUnchanged:
+		return "unchanged"
+	case StatusRebased:
+		return "rebased"
+	case StatusError:
+		return "error"
+	default:
+		return "changed"
+	}
+}
+
+const (
+	StatusOk StatusKind = iota
+	StatusOnlyInOne
+	StatusOnlyInTwo
+	StatusUnchanged
+	StatusRebased
+	StatusError
+)
+
+func interdiffFiles(f1, f2 *gitdiff.File) *InterdiffFile {
+	re1 := CreatePreImage(f1)
+	re2 := CreatePreImage(f2)
+
+	interdiffFile := InterdiffFile{
+		Name: bestName(f1),
+	}
+
+	merged, err := re1.Merge(&re2)
+	if err != nil {
+		interdiffFile.Status = InterdiffFileStatus{
+			StatusKind: StatusRebased,
+			Error:      err,
+		}
+		return &interdiffFile
+	}
+
+	rev1, err := merged.Apply(f1)
+	if err != nil {
+		interdiffFile.Status = InterdiffFileStatus{
+			StatusKind: StatusError,
+			Error:      err,
+		}
+		return &interdiffFile
+	}
+
+	rev2, err := merged.Apply(f2)
+	if err != nil {
+		interdiffFile.Status = InterdiffFileStatus{
+			StatusKind: StatusError,
+			Error:      err,
+		}
+		return &interdiffFile
+	}
+
+	diff, err := Unified(rev1, bestName(f1), rev2, bestName(f2))
+	if err != nil {
+		interdiffFile.Status = InterdiffFileStatus{
+			StatusKind: StatusError,
+			Error:      err,
+		}
+		return &interdiffFile
+	}
+
+	parsed, _, err := gitdiff.Parse(strings.NewReader(diff))
+	if err != nil {
+		interdiffFile.Status = InterdiffFileStatus{
+			StatusKind: StatusError,
+			Error:      err,
+		}
+		return &interdiffFile
+	}
+
+	if len(parsed) != 1 {
+		// files are identical?
+		interdiffFile.Status = InterdiffFileStatus{
+			StatusKind: StatusUnchanged,
+		}
+		return &interdiffFile
+	}
+
+	if interdiffFile.Status.StatusKind == StatusOk {
+		interdiffFile.File = parsed[0]
+	}
+
+	return &interdiffFile
+}
+
+func Interdiff(patch1, patch2 []*gitdiff.File) *InterdiffResult {
+	fileToIdx1 := make(map[string]int)
+	fileToIdx2 := make(map[string]int)
+	visited := make(map[string]struct{})
+	var result InterdiffResult
+
+	for idx, f := range patch1 {
+		fileToIdx1[bestName(f)] = idx
+	}
+
+	for idx, f := range patch2 {
+		fileToIdx2[bestName(f)] = idx
+	}
+
+	for _, f1 := range patch1 {
+		var interdiffFile *InterdiffFile
+
+		fileName := bestName(f1)
+		if idx, ok := fileToIdx2[fileName]; ok {
+			f2 := patch2[idx]
+
+			// we have f1 and f2, calculate interdiff
+			interdiffFile = interdiffFiles(f1, f2)
+		} else {
+			// only in patch 1, this change would have to be "inverted" to dissapear
+			// from patch 2, so we reverseDiff(f1)
+			reverseDiff(f1)
+
+			interdiffFile = &InterdiffFile{
+				File: f1,
+				Name: fileName,
+				Status: InterdiffFileStatus{
+					StatusKind: StatusOnlyInOne,
+				},
+			}
+		}
+
+		result.Files = append(result.Files, interdiffFile)
+		visited[fileName] = struct{}{}
+	}
+
+	// for all files in patch2 that remain unvisited; we can just add them into the output
+	for _, f2 := range patch2 {
+		fileName := bestName(f2)
+		if _, ok := visited[fileName]; ok {
+			continue
+		}
+
+		result.Files = append(result.Files, &InterdiffFile{
+			File: f2,
+			Name: fileName,
+			Status: InterdiffFileStatus{
+				StatusKind: StatusOnlyInTwo,
+			},
+		})
+	}
+
+	return &result
+}

patchutil/patchutil.go

···
 
 import (
 	"fmt"
+	"os"
+	"os/exec"
 	"regexp"
 	"strings"
 
···
 		patches[i] = strings.TrimSpace(patchText[startPos:endPos])
 	}
 	return patches
+}
+
+func bestName(file *gitdiff.File) string {
+	if file.IsDelete {
+		return file.OldName
+	} else {
+		return file.NewName
+	}
+}
+
+// in-place reverse of a diff
+func reverseDiff(file *gitdiff.File) {
+	file.OldName, file.NewName = file.NewName, file.OldName
+	file.OldMode, file.NewMode = file.NewMode, file.OldMode
+	file.BinaryFragment, file.ReverseBinaryFragment = file.ReverseBinaryFragment, file.BinaryFragment
+
+	for _, fragment := range file.TextFragments {
+		// swap postions
+		fragment.OldPosition, fragment.NewPosition = fragment.NewPosition, fragment.OldPosition
+		fragment.OldLines, fragment.NewLines = fragment.NewLines, fragment.OldLines
+		fragment.LinesAdded, fragment.LinesDeleted = fragment.LinesDeleted, fragment.LinesAdded
+
+		for i := range fragment.Lines {
+			switch fragment.Lines[i].Op {
+			case gitdiff.OpAdd:
+				fragment.Lines[i].Op = gitdiff.OpDelete
+			case gitdiff.OpDelete:
+				fragment.Lines[i].Op = gitdiff.OpAdd
+			default:
+				// do nothing
+			}
+		}
+	}
+}
+
+func Unified(oldText, oldFile, newText, newFile string) (string, error) {
+	oldTemp, err := os.CreateTemp("", "old_*")
+	if err != nil {
+		return "", fmt.Errorf("failed to create temp file for oldText: %w", err)
+	}
+	defer os.Remove(oldTemp.Name())
+	if _, err := oldTemp.WriteString(oldText); err != nil {
+		return "", fmt.Errorf("failed to write to old temp file: %w", err)
+	}
+	oldTemp.Close()
+
+	newTemp, err := os.CreateTemp("", "new_*")
+	if err != nil {
+		return "", fmt.Errorf("failed to create temp file for newText: %w", err)
+	}
+	defer os.Remove(newTemp.Name())
+	if _, err := newTemp.WriteString(newText); err != nil {
+		return "", fmt.Errorf("failed to write to new temp file: %w", err)
+	}
+	newTemp.Close()
+
+	cmd := exec.Command("diff", "-U", "9999", "--label", oldFile, "--label", newFile, oldTemp.Name(), newTemp.Name())
+	output, err := cmd.CombinedOutput()
+
+	if exitErr, ok := err.(*exec.ExitError); ok && exitErr.ExitCode() == 1 {
+		return string(output), nil
+	}
+	if err != nil {
+		return "", fmt.Errorf("diff command failed: %w", err)
+	}
+
+	return string(output), nil
 }