// Copyright 2014 The Gogs Authors. All rights reserved.
// Copyright 2019 The Gitea Authors. All rights reserved.
// Use of this source code is governed by a MIT-style
// license that can be found in the LICENSE file.

package gitdiff

import (
	"bufio"
	"bytes"
	"context"
	"fmt"
	"html"
	"html/template"
	"io"
	"net/url"
	"os"
	"regexp"
	"sort"
	"strings"
	"time"

	"code.gitea.io/gitea/models/db"
	git_model "code.gitea.io/gitea/models/git"
	issues_model "code.gitea.io/gitea/models/issues"
	pull_model "code.gitea.io/gitea/models/pull"
	user_model "code.gitea.io/gitea/models/user"
	"code.gitea.io/gitea/modules/analyze"
	"code.gitea.io/gitea/modules/base"
	"code.gitea.io/gitea/modules/charset"
	"code.gitea.io/gitea/modules/git"
	"code.gitea.io/gitea/modules/highlight"
	"code.gitea.io/gitea/modules/lfs"
	"code.gitea.io/gitea/modules/log"
	"code.gitea.io/gitea/modules/setting"

	"github.com/sergi/go-diff/diffmatchpatch"
	stdcharset "golang.org/x/net/html/charset"
	"golang.org/x/text/encoding"
	"golang.org/x/text/transform"
)

// DiffLineType represents the type of a DiffLine.
type DiffLineType uint8

// DiffLineType possible values.
const (
	DiffLinePlain DiffLineType = iota + 1
	DiffLineAdd
	DiffLineDel
	DiffLineSection
)

// DiffFileType represents the type of a DiffFile.
type DiffFileType uint8

// DiffFileType possible values.
const (
	DiffFileAdd DiffFileType = iota + 1
	DiffFileChange
	DiffFileDel
	DiffFileRename
	DiffFileCopy
)

// DiffLineExpandDirection represents the DiffLineSection expand direction
type DiffLineExpandDirection uint8

// DiffLineExpandDirection possible values.
const (
	DiffLineExpandNone DiffLineExpandDirection = iota + 1
	DiffLineExpandSingle
	DiffLineExpandUpDown
	DiffLineExpandUp
	DiffLineExpandDown
)

// DiffLine represents a line difference in a DiffSection.
type DiffLine struct {
	LeftIdx     int
	RightIdx    int
	Match       int
	Type        DiffLineType
	Content     string
	Comments    []*issues_model.Comment
	SectionInfo *DiffLineSectionInfo
}

// DiffLineSectionInfo represents diff line section meta data
type DiffLineSectionInfo struct {
	Path          string
	LastLeftIdx   int
	LastRightIdx  int
	LeftIdx       int
	RightIdx      int
	LeftHunkSize  int
	RightHunkSize int
}

// BlobExcerptChunkSize represent max lines of excerpt
const BlobExcerptChunkSize = 20

// GetType returns the type of a DiffLine.
func (d *DiffLine) GetType() int {
	return int(d.Type)
}

// CanComment returns whether or not a line can get commented
func (d *DiffLine) CanComment() bool {
	return len(d.Comments) == 0 && d.Type != DiffLineSection
}

// GetCommentSide returns the comment side of the first comment, if not set returns empty string
func (d *DiffLine) GetCommentSide() string {
	if len(d.Comments) == 0 {
		return ""
	}
	return d.Comments[0].DiffSide()
}

// GetLineTypeMarker returns the line type marker
func (d *DiffLine) GetLineTypeMarker() string {
	if strings.IndexByte(" +-", d.Content[0]) > -1 {
		return d.Content[0:1]
	}
	return ""
}

// GetBlobExcerptQuery builds query string to get blob excerpt
func (d *DiffLine) GetBlobExcerptQuery() string {
	query := fmt.Sprintf(
		"last_left=%d&last_right=%d&"+
			"left=%d&right=%d&"+
			"left_hunk_size=%d&right_hunk_size=%d&"+
			"path=%s",
		d.SectionInfo.LastLeftIdx, d.SectionInfo.LastRightIdx,
		d.SectionInfo.LeftIdx, d.SectionInfo.RightIdx,
		d.SectionInfo.LeftHunkSize, d.SectionInfo.RightHunkSize,
		url.QueryEscape(d.SectionInfo.Path))
	return query
}

// GetExpandDirection gets DiffLineExpandDirection
func (d *DiffLine) GetExpandDirection() DiffLineExpandDirection {
	if d.Type != DiffLineSection || d.SectionInfo == nil || d.SectionInfo.RightIdx-d.SectionInfo.LastRightIdx <= 1 {
		return DiffLineExpandNone
	}
	if d.SectionInfo.LastLeftIdx <= 0 && d.SectionInfo.LastRightIdx <= 0 {
		return DiffLineExpandUp
	} else if d.SectionInfo.RightIdx-d.SectionInfo.LastRightIdx > BlobExcerptChunkSize && d.SectionInfo.RightHunkSize > 0 {
		return DiffLineExpandUpDown
	} else if d.SectionInfo.LeftHunkSize <= 0 && d.SectionInfo.RightHunkSize <= 0 {
		return DiffLineExpandDown
	}
	return DiffLineExpandSingle
}

func getDiffLineSectionInfo(treePath, line string, lastLeftIdx, lastRightIdx int) *DiffLineSectionInfo {
	leftLine, leftHunk, rightLine, righHunk := git.ParseDiffHunkString(line)

	return &DiffLineSectionInfo{
		Path:          treePath,
		LastLeftIdx:   lastLeftIdx,
		LastRightIdx:  lastRightIdx,
		LeftIdx:       leftLine,
		RightIdx:      rightLine,
		LeftHunkSize:  leftHunk,
		RightHunkSize: righHunk,
	}
}

// escape a line's content or return <br> needed for copy/paste purposes
func getLineContent(content string) DiffInline {
	if len(content) > 0 {
		return DiffInlineWithUnicodeEscape(template.HTML(html.EscapeString(content)))
	}
	return DiffInline{Content: "<br>"}
}

// DiffSection represents a section of a DiffFile.
type DiffSection struct {
	file     *DiffFile
	FileName string
	Name     string
	Lines    []*DiffLine
}

var (
	addedCodePrefix   = []byte(`<span class="added-code">`)
	removedCodePrefix = []byte(`<span class="removed-code">`)
	codeTagSuffix     = []byte(`</span>`)
)

var (
	unfinishedtagRegex = regexp.MustCompile(`<[^>]*$`)
	trailingSpanRegex  = regexp.MustCompile(`<span\s*[[:alpha:]="]*?[>]?$`)
	entityRegex        = regexp.MustCompile(`&[#]*?[0-9[:alpha:]]*$`)
)

// shouldWriteInline represents combinations where we manually write inline changes
func shouldWriteInline(diff diffmatchpatch.Diff, lineType DiffLineType) bool {
	if true &&
		diff.Type == diffmatchpatch.DiffEqual ||
		diff.Type == diffmatchpatch.DiffInsert && lineType == DiffLineAdd ||
		diff.Type == diffmatchpatch.DiffDelete && lineType == DiffLineDel {
		return true
	}
	return false
}

func fixupBrokenSpans(diffs []diffmatchpatch.Diff) []diffmatchpatch.Diff {
	// Create a new array to store our fixed up blocks
	fixedup := make([]diffmatchpatch.Diff, 0, len(diffs))

	// semantically label some numbers
	const insert, delete, equal = 0, 1, 2

	// record the positions of the last type of each block in the fixedup blocks
	last := []int{-1, -1, -1}
	operation := []diffmatchpatch.Operation{diffmatchpatch.DiffInsert, diffmatchpatch.DiffDelete, diffmatchpatch.DiffEqual}

	// create a writer for insert and deletes
	toWrite := []strings.Builder{
		{},
		{},
	}

	// make some flags for insert and delete
	unfinishedTag := []bool{false, false}
	unfinishedEnt := []bool{false, false}

	// store stores the provided text in the writer for the typ
	store := func(text string, typ int) {
		(&(toWrite[typ])).WriteString(text)
	}

	// hasStored returns true if there is stored content
	hasStored := func(typ int) bool {
		return (&toWrite[typ]).Len() > 0
	}

	// stored will return that content
	stored := func(typ int) string {
		return (&toWrite[typ]).String()
	}

	// empty will empty the stored content
	empty := func(typ int) {
		(&toWrite[typ]).Reset()
	}

	// pop will remove the stored content appending to a diff block for that typ
	pop := func(typ int, fixedup []diffmatchpatch.Diff) []diffmatchpatch.Diff {
		if hasStored(typ) {
			if last[typ] > last[equal] {
				fixedup[last[typ]].Text += stored(typ)
			} else {
				fixedup = append(fixedup, diffmatchpatch.Diff{
					Type: operation[typ],
					Text: stored(typ),
				})
			}
			empty(typ)
		}
		return fixedup
	}

	// Now we walk the provided diffs and check the type of each block in turn
	for _, diff := range diffs {

		typ := delete // flag for handling insert or delete typs
		switch diff.Type {
		case diffmatchpatch.DiffEqual:
			// First check if there is anything stored
			if hasStored(insert) || hasStored(delete) {
				// There are two reasons for storing content:
				// 1. Unfinished Entity <- Could be more efficient here by not doing this if we're looking for a tag
				if unfinishedEnt[insert] || unfinishedEnt[delete] {
					// we look for a ';' to finish an entity
					idx := strings.IndexRune(diff.Text, ';')
					if idx >= 0 {
						// if we find a ';' store the preceding content to both insert and delete
						store(diff.Text[:idx+1], insert)
						store(diff.Text[:idx+1], delete)

						// and remove it from this block
						diff.Text = diff.Text[idx+1:]

						// reset the ent flags
						unfinishedEnt[insert] = false
						unfinishedEnt[delete] = false
					} else {
						// otherwise store it all on insert and delete
						store(diff.Text, insert)
						store(diff.Text, delete)
						// and empty this block
						diff.Text = ""
					}
				}
				// 2. Unfinished Tag
				if unfinishedTag[insert] || unfinishedTag[delete] {
					// we look for a '>' to finish a tag
					idx := strings.IndexRune(diff.Text, '>')
					if idx >= 0 {
						store(diff.Text[:idx+1], insert)
						store(diff.Text[:idx+1], delete)
						diff.Text = diff.Text[idx+1:]
						unfinishedTag[insert] = false
						unfinishedTag[delete] = false
					} else {
						store(diff.Text, insert)
						store(diff.Text, delete)
						diff.Text = ""
					}
				}

				// If we've completed the required tag/entities
				if !(unfinishedTag[insert] || unfinishedTag[delete] || unfinishedEnt[insert] || unfinishedEnt[delete]) {
					// pop off the stack
					fixedup = pop(insert, fixedup)
					fixedup = pop(delete, fixedup)
				}

				// If that has left this diff block empty then shortcut
				if len(diff.Text) == 0 {
					continue
				}
			}

			// check if this block ends in an unfinished tag?
			idx := unfinishedtagRegex.FindStringIndex(diff.Text)
			if idx != nil {
				unfinishedTag[insert] = true
				unfinishedTag[delete] = true
			} else {
				// otherwise does it end in an unfinished entity?
				idx = entityRegex.FindStringIndex(diff.Text)
				if idx != nil {
					unfinishedEnt[insert] = true
					unfinishedEnt[delete] = true
				}
			}

			// If there is an unfinished component
			if idx != nil {
				// Store the fragment
				store(diff.Text[idx[0]:], insert)
				store(diff.Text[idx[0]:], delete)
				// and remove it from this block
				diff.Text = diff.Text[:idx[0]]
			}

			// If that hasn't left the block empty
			if len(diff.Text) > 0 {
				// store the position of the last equal block and store it in our diffs
				last[equal] = len(fixedup)
				fixedup = append(fixedup, diff)
			}
			continue
		case diffmatchpatch.DiffInsert:
			typ = insert
			fallthrough
		case diffmatchpatch.DiffDelete:
			// First check if there is anything stored for this type
			if hasStored(typ) {
				// if there is prepend it to this block, empty the storage and reset our flags
				diff.Text = stored(typ) + diff.Text
				empty(typ)
				unfinishedEnt[typ] = false
				unfinishedTag[typ] = false
			}

			// check if this block ends in an unfinished tag
			idx := unfinishedtagRegex.FindStringIndex(diff.Text)
			if idx != nil {
				unfinishedTag[typ] = true
			} else {
				// otherwise does it end in an unfinished entity
				idx = entityRegex.FindStringIndex(diff.Text)
				if idx != nil {
					unfinishedEnt[typ] = true
				}
			}

			// If there is an unfinished component
			if idx != nil {
				// Store the fragment
				store(diff.Text[idx[0]:], typ)
				// and remove it from this block
				diff.Text = diff.Text[:idx[0]]
			}

			// If that hasn't left the block empty
			if len(diff.Text) > 0 {
				// if the last block of this type was after the last equal block
				if last[typ] > last[equal] {
					// store this blocks content on that block
					fixedup[last[typ]].Text += diff.Text
				} else {
					// otherwise store the position of the last block of this type and store the block
					last[typ] = len(fixedup)
					fixedup = append(fixedup, diff)
				}
			}
			continue
		}
	}

	// pop off any remaining stored content
	fixedup = pop(insert, fixedup)
	fixedup = pop(delete, fixedup)

	return fixedup
}

func diffToHTML(fileName string, diffs []diffmatchpatch.Diff, lineType DiffLineType) DiffInline {
	buf := bytes.NewBuffer(nil)
	match := ""

	diffs = fixupBrokenSpans(diffs)

	for _, diff := range diffs {
		if shouldWriteInline(diff, lineType) {
			if len(match) > 0 {
				diff.Text = match + diff.Text
				match = ""
			}
			// Chroma HTML syntax highlighting is done before diffing individual lines in order to maintain consistency.
			// Since inline changes might split in the middle of a chroma span tag or HTML entity, make we manually put it back together
			// before writing so we don't try insert added/removed code spans in the middle of one of those
			// and create broken HTML. This is done by moving incomplete HTML forward until it no longer matches our pattern of
			// a line ending with an incomplete HTML entity or partial/opening <span>.

			// EX:
			// diffs[{Type: dmp.DiffDelete, Text: "language</span><span "},
			// {Type: dmp.DiffEqual, Text: "c"},
			// {Type: dmp.DiffDelete, Text: "lass="p">}]

			// After first iteration
			// diffs[{Type: dmp.DiffDelete, Text: "language</span>"}, //write out
			// {Type: dmp.DiffEqual, Text: "<span c"},
			// {Type: dmp.DiffDelete, Text: "lass="p">,</span>}]

			// After second iteration
			// {Type: dmp.DiffEqual, Text: ""}, // write out
			// {Type: dmp.DiffDelete, Text: "<span class="p">,</span>}]

			// Final
			// {Type: dmp.DiffDelete, Text: "<span class="p">,</span>}]
			// end up writing <span class="removed-code"><span class="p">,</span></span>
			// Instead of <span class="removed-code">lass="p",</span></span>

			m := trailingSpanRegex.FindStringSubmatchIndex(diff.Text)
			if m != nil {
				match = diff.Text[m[0]:m[1]]
				diff.Text = strings.TrimSuffix(diff.Text, match)
			}
			m = entityRegex.FindStringSubmatchIndex(diff.Text)
			if m != nil {
				match = diff.Text[m[0]:m[1]]
				diff.Text = strings.TrimSuffix(diff.Text, match)
			}
			// Print an existing closing span first before opening added/remove-code span so it doesn't unintentionally close it
			if strings.HasPrefix(diff.Text, "</span>") {
				buf.WriteString("</span>")
				diff.Text = strings.TrimPrefix(diff.Text, "</span>")
			}
			// If we weren't able to fix it then this should avoid broken HTML by not inserting more spans below
			// The previous/next diff section will contain the rest of the tag that is missing here
			if strings.Count(diff.Text, "<") != strings.Count(diff.Text, ">") {
				buf.WriteString(diff.Text)
				continue
			}
		}
		switch {
		case diff.Type == diffmatchpatch.DiffEqual:
			buf.WriteString(diff.Text)
		case diff.Type == diffmatchpatch.DiffInsert && lineType == DiffLineAdd:
			buf.Write(addedCodePrefix)
			buf.WriteString(diff.Text)
			buf.Write(codeTagSuffix)
		case diff.Type == diffmatchpatch.DiffDelete && lineType == DiffLineDel:
			buf.Write(removedCodePrefix)
			buf.WriteString(diff.Text)
			buf.Write(codeTagSuffix)
		}
	}
	return DiffInlineWithUnicodeEscape(template.HTML(buf.String()))
}

// GetLine gets a specific line by type (add or del) and file line number
func (diffSection *DiffSection) GetLine(lineType DiffLineType, idx int) *DiffLine {
	var (
		difference    = 0
		addCount      = 0
		delCount      = 0
		matchDiffLine *DiffLine
	)

LOOP:
	for _, diffLine := range diffSection.Lines {
		switch diffLine.Type {
		case DiffLineAdd:
			addCount++
		case DiffLineDel:
			delCount++
		default:
			if matchDiffLine != nil {
				break LOOP
			}
			difference = diffLine.RightIdx - diffLine.LeftIdx
			addCount = 0
			delCount = 0
		}

		switch lineType {
		case DiffLineDel:
			if diffLine.RightIdx == 0 && diffLine.LeftIdx == idx-difference {
				matchDiffLine = diffLine
			}
		case DiffLineAdd:
			if diffLine.LeftIdx == 0 && diffLine.RightIdx == idx+difference {
				matchDiffLine = diffLine
			}
		}
	}

	if addCount == delCount {
		return matchDiffLine
	}
	return nil
}

var diffMatchPatch = diffmatchpatch.New()

func init() {
	diffMatchPatch.DiffEditCost = 100
}

// DiffInline is a struct that has a content and escape status
type DiffInline struct {
	EscapeStatus charset.EscapeStatus
	Content      template.HTML
}

// DiffInlineWithUnicodeEscape makes a DiffInline with hidden unicode characters escaped
func DiffInlineWithUnicodeEscape(s template.HTML) DiffInline {
	status, content := charset.EscapeControlString(string(s))
	return DiffInline{EscapeStatus: status, Content: template.HTML(content)}
}

// DiffInlineWithHighlightCode makes a DiffInline with code highlight and hidden unicode characters escaped
func DiffInlineWithHighlightCode(fileName, language, code string) DiffInline {
	status, content := charset.EscapeControlString(highlight.Code(fileName, language, code))
	return DiffInline{EscapeStatus: status, Content: template.HTML(content)}
}

// GetComputedInlineDiffFor computes inline diff for the given line.
func (diffSection *DiffSection) GetComputedInlineDiffFor(diffLine *DiffLine) DiffInline {
	if setting.Git.DisableDiffHighlight {
		return getLineContent(diffLine.Content[1:])
	}

	var (
		compareDiffLine *DiffLine
		diff1           string
		diff2           string
	)

	language := ""
	if diffSection.file != nil {
		language = diffSection.file.Language
	}

	// try to find equivalent diff line. ignore, otherwise
	switch diffLine.Type {
	case DiffLineSection:
		return getLineContent(diffLine.Content[1:])
	case DiffLineAdd:
		compareDiffLine = diffSection.GetLine(DiffLineDel, diffLine.RightIdx)
		if compareDiffLine == nil {
			return DiffInlineWithHighlightCode(diffSection.FileName, language, diffLine.Content[1:])
		}
		diff1 = compareDiffLine.Content
		diff2 = diffLine.Content
	case DiffLineDel:
		compareDiffLine = diffSection.GetLine(DiffLineAdd, diffLine.LeftIdx)
		if compareDiffLine == nil {
			return DiffInlineWithHighlightCode(diffSection.FileName, language, diffLine.Content[1:])
		}
		diff1 = diffLine.Content
		diff2 = compareDiffLine.Content
	default:
		if strings.IndexByte(" +-", diffLine.Content[0]) > -1 {
			return DiffInlineWithHighlightCode(diffSection.FileName, language, diffLine.Content[1:])
		}
		return DiffInlineWithHighlightCode(diffSection.FileName, language, diffLine.Content)
	}

	diffRecord := diffMatchPatch.DiffMain(highlight.Code(diffSection.FileName, language, diff1[1:]), highlight.Code(diffSection.FileName, language, diff2[1:]), true)
	diffRecord = diffMatchPatch.DiffCleanupEfficiency(diffRecord)

	return diffToHTML(diffSection.FileName, diffRecord, diffLine.Type)
}

// DiffFile represents a file diff.
type DiffFile struct {
	Name                      string
	NameHash                  string
	OldName                   string
	Index                     int
	Addition, Deletion        int
	Type                      DiffFileType
	IsCreated                 bool
	IsDeleted                 bool
	IsBin                     bool
	IsLFSFile                 bool
	IsRenamed                 bool
	IsAmbiguous               bool
	IsSubmodule               bool
	Sections                  []*DiffSection
	IsIncomplete              bool
	IsIncompleteLineTooLong   bool
	IsProtected               bool
	IsGenerated               bool
	IsVendored                bool
	IsViewed                  bool // User specific
	HasChangedSinceLastReview bool // User specific
	Language                  string
}

// GetType returns type of diff file.
func (diffFile *DiffFile) GetType() int {
	return int(diffFile.Type)
}

// GetTailSection creates a fake DiffLineSection if the last section is not the end of the file
func (diffFile *DiffFile) GetTailSection(gitRepo *git.Repository, leftCommitID, rightCommitID string) *DiffSection {
	if len(diffFile.Sections) == 0 || diffFile.Type != DiffFileChange || diffFile.IsBin || diffFile.IsLFSFile {
		return nil
	}
	leftCommit, err := gitRepo.GetCommit(leftCommitID)
	if err != nil {
		return nil
	}
	rightCommit, err := gitRepo.GetCommit(rightCommitID)
	if err != nil {
		return nil
	}
	lastSection := diffFile.Sections[len(diffFile.Sections)-1]
	lastLine := lastSection.Lines[len(lastSection.Lines)-1]
	leftLineCount := getCommitFileLineCount(leftCommit, diffFile.Name)
	rightLineCount := getCommitFileLineCount(rightCommit, diffFile.Name)
	if leftLineCount <= lastLine.LeftIdx || rightLineCount <= lastLine.RightIdx {
		return nil
	}
	tailDiffLine := &DiffLine{
		Type:    DiffLineSection,
		Content: " ",
		SectionInfo: &DiffLineSectionInfo{
			Path:         diffFile.Name,
			LastLeftIdx:  lastLine.LeftIdx,
			LastRightIdx: lastLine.RightIdx,
			LeftIdx:      leftLineCount,
			RightIdx:     rightLineCount,
		},
	}
	tailSection := &DiffSection{FileName: diffFile.Name, Lines: []*DiffLine{tailDiffLine}}
	return tailSection
}

// GetDiffFileName returns the name of the diff file, or its old name in case it was deleted
func (diffFile *DiffFile) GetDiffFileName() string {
	if diffFile.Name == "" {
		return diffFile.OldName
	}
	return diffFile.Name
}

func (diffFile *DiffFile) ShouldBeHidden() bool {
	return diffFile.IsGenerated || diffFile.IsViewed
}

func getCommitFileLineCount(commit *git.Commit, filePath string) int {
	blob, err := commit.GetBlobByPath(filePath)
	if err != nil {
		return 0
	}
	lineCount, err := blob.GetBlobLineCount()
	if err != nil {
		return 0
	}
	return lineCount
}

// Diff represents a difference between two git trees.
type Diff struct {
	Start, End                   string
	NumFiles                     int
	TotalAddition, TotalDeletion int
	Files                        []*DiffFile
	IsIncomplete                 bool
	NumViewedFiles               int // user-specific
}

// LoadComments loads comments into each line
func (diff *Diff) LoadComments(ctx context.Context, issue *issues_model.Issue, currentUser *user_model.User) error {
	allComments, err := issues_model.FetchCodeComments(ctx, issue, currentUser)
	if err != nil {
		return err
	}
	for _, file := range diff.Files {
		if lineCommits, ok := allComments[file.Name]; ok {
			for _, section := range file.Sections {
				for _, line := range section.Lines {
					if comments, ok := lineCommits[int64(line.LeftIdx*-1)]; ok {
						line.Comments = append(line.Comments, comments...)
					}
					if comments, ok := lineCommits[int64(line.RightIdx)]; ok {
						line.Comments = append(line.Comments, comments...)
					}
					sort.SliceStable(line.Comments, func(i, j int) bool {
						return line.Comments[i].CreatedUnix < line.Comments[j].CreatedUnix
					})
				}
			}
		}
	}
	return nil
}

const cmdDiffHead = "diff --git "

// ParsePatch builds a Diff object from a io.Reader and some parameters.
func ParsePatch(maxLines, maxLineCharacters, maxFiles int, reader io.Reader, skipToFile string) (*Diff, error) {
	log.Debug("ParsePatch(%d, %d, %d, ..., %s)", maxLines, maxLineCharacters, maxFiles, skipToFile)
	var curFile *DiffFile

	skipping := skipToFile != ""

	diff := &Diff{Files: make([]*DiffFile, 0)}

	sb := strings.Builder{}

	// OK let's set a reasonable buffer size.
	// This should be let's say at least the size of maxLineCharacters or 4096 whichever is larger.
	readerSize := maxLineCharacters
	if readerSize < 4096 {
		readerSize = 4096
	}

	input := bufio.NewReaderSize(reader, readerSize)
	line, err := input.ReadString('\n')
	if err != nil {
		if err == io.EOF {
			return diff, nil
		}
		return diff, err
	}
parsingLoop:
	for {
		// 1. A patch file always begins with `diff --git ` + `a/path b/path` (possibly quoted)
		// if it does not we have bad input!
		if !strings.HasPrefix(line, cmdDiffHead) {
			return diff, fmt.Errorf("invalid first file line: %s", line)
		}

		if maxFiles > -1 && len(diff.Files) >= maxFiles {
			lastFile := createDiffFile(diff, line)
			diff.End = lastFile.Name
			diff.IsIncomplete = true
			_, err := io.Copy(io.Discard, reader)
			if err != nil {
				// By the definition of io.Copy this never returns io.EOF
				return diff, fmt.Errorf("error during io.Copy: %w", err)
			}
			break parsingLoop
		}

		curFile = createDiffFile(diff, line)
		if skipping {
			if curFile.Name != skipToFile {
				line, err = skipToNextDiffHead(input)
				if err != nil {
					if err == io.EOF {
						return diff, nil
					}
					return diff, err
				}
				continue
			}
			skipping = false
		}

		diff.Files = append(diff.Files, curFile)

		// 2. It is followed by one or more extended header lines:
		//
		//     old mode <mode>
		//     new mode <mode>
		//     deleted file mode <mode>
		//     new file mode <mode>
		//     copy from <path>
		//     copy to <path>
		//     rename from <path>
		//     rename to <path>
		//     similarity index <number>
		//     dissimilarity index <number>
		//     index <hash>..<hash> <mode>
		//
		// * <mode> 6-digit octal numbers including the file type and file permission bits.
		// * <path> does not include the a/ and b/ prefixes
		// * <number> percentage of unchanged lines for similarity, percentage of changed
		//   lines dissimilarity as integer rounded down with terminal %. 100% => equal files.
		// * The index line includes the blob object names before and after the change.
		//   The <mode> is included if the file mode does not change; otherwise, separate
		//   lines indicate the old and the new mode.
		// 3. Following this header the "standard unified" diff format header may be encountered: (but not for every case...)
		//
		//     --- a/<path>
		//     +++ b/<path>
		//
		// With multiple hunks
		//
		//     @@ <hunk descriptor> @@
		//     +added line
		//     -removed line
		//      unchanged line
		//
		// 4. Binary files get:
		//
		//     Binary files a/<path> and b/<path> differ
		//
		// but one of a/<path> and b/<path> could be /dev/null.
	curFileLoop:
		for {
			line, err = input.ReadString('\n')
			if err != nil {
				if err != io.EOF {
					return diff, err
				}
				break parsingLoop
			}
			switch {
			case strings.HasPrefix(line, cmdDiffHead):
				break curFileLoop
			case strings.HasPrefix(line, "old mode ") ||
				strings.HasPrefix(line, "new mode "):
				if strings.HasSuffix(line, " 160000\n") {
					curFile.IsSubmodule = true
				}
			case strings.HasPrefix(line, "rename from "):
				curFile.IsRenamed = true
				curFile.Type = DiffFileRename
				if curFile.IsAmbiguous {
					curFile.OldName = line[len("rename from ") : len(line)-1]
				}
			case strings.HasPrefix(line, "rename to "):
				curFile.IsRenamed = true
				curFile.Type = DiffFileRename
				if curFile.IsAmbiguous {
					curFile.Name = line[len("rename to ") : len(line)-1]
					curFile.IsAmbiguous = false
				}
			case strings.HasPrefix(line, "copy from "):
				curFile.IsRenamed = true
				curFile.Type = DiffFileCopy
				if curFile.IsAmbiguous {
					curFile.OldName = line[len("copy from ") : len(line)-1]
				}
			case strings.HasPrefix(line, "copy to "):
				curFile.IsRenamed = true
				curFile.Type = DiffFileCopy
				if curFile.IsAmbiguous {
					curFile.Name = line[len("copy to ") : len(line)-1]
					curFile.IsAmbiguous = false
				}
			case strings.HasPrefix(line, "new file"):
				curFile.Type = DiffFileAdd
				curFile.IsCreated = true
				if strings.HasSuffix(line, " 160000\n") {
					curFile.IsSubmodule = true
				}
			case strings.HasPrefix(line, "deleted"):
				curFile.Type = DiffFileDel
				curFile.IsDeleted = true
				if strings.HasSuffix(line, " 160000\n") {
					curFile.IsSubmodule = true
				}
			case strings.HasPrefix(line, "index"):
				if strings.HasSuffix(line, " 160000\n") {
					curFile.IsSubmodule = true
				}
			case strings.HasPrefix(line, "similarity index 100%"):
				curFile.Type = DiffFileRename
			case strings.HasPrefix(line, "Binary"):
				curFile.IsBin = true
			case strings.HasPrefix(line, "--- "):
				// Handle ambiguous filenames
				if curFile.IsAmbiguous {
					// The shortest string that can end up here is:
					// "--- a\t\n" without the quotes.
					// This line has a len() of 7 but doesn't contain a oldName.
					// So the amount that the line need is at least 8 or more.
					// The code will otherwise panic for a out-of-bounds.
					if len(line) > 7 && line[4] == 'a' {
						curFile.OldName = line[6 : len(line)-1]
						if line[len(line)-2] == '\t' {
							curFile.OldName = curFile.OldName[:len(curFile.OldName)-1]
						}
					} else {
						curFile.OldName = ""
					}
				}
				// Otherwise do nothing with this line
			case strings.HasPrefix(line, "+++ "):
				// Handle ambiguous filenames
				if curFile.IsAmbiguous {
					if len(line) > 6 && line[4] == 'b' {
						curFile.Name = line[6 : len(line)-1]
						if line[len(line)-2] == '\t' {
							curFile.Name = curFile.Name[:len(curFile.Name)-1]
						}
						if curFile.OldName == "" {
							curFile.OldName = curFile.Name
						}
					} else {
						curFile.Name = curFile.OldName
					}
					curFile.IsAmbiguous = false
				}
				// Otherwise do nothing with this line, but now switch to parsing hunks
				lineBytes, isFragment, err := parseHunks(curFile, maxLines, maxLineCharacters, input)
				diff.TotalAddition += curFile.Addition
				diff.TotalDeletion += curFile.Deletion
				if err != nil {
					if err != io.EOF {
						return diff, err
					}
					break parsingLoop
				}
				sb.Reset()
				_, _ = sb.Write(lineBytes)
				for isFragment {
					lineBytes, isFragment, err = input.ReadLine()
					if err != nil {
						// Now by the definition of ReadLine this cannot be io.EOF
						return diff, fmt.Errorf("unable to ReadLine: %w", err)
					}
					_, _ = sb.Write(lineBytes)
				}
				line = sb.String()
				sb.Reset()

				break curFileLoop
			}
		}
	}

	// TODO: There are numerous issues with this:
	// - we might want to consider detecting encoding while parsing but...
	// - we're likely to fail to get the correct encoding here anyway as we won't have enough information
	diffLineTypeBuffers := make(map[DiffLineType]*bytes.Buffer, 3)
	diffLineTypeDecoders := make(map[DiffLineType]*encoding.Decoder, 3)
	diffLineTypeBuffers[DiffLinePlain] = new(bytes.Buffer)
	diffLineTypeBuffers[DiffLineAdd] = new(bytes.Buffer)
	diffLineTypeBuffers[DiffLineDel] = new(bytes.Buffer)
	for _, f := range diff.Files {
		f.NameHash = base.EncodeSha1(f.Name)

		for _, buffer := range diffLineTypeBuffers {
			buffer.Reset()
		}
		for _, sec := range f.Sections {
			for _, l := range sec.Lines {
				if l.Type == DiffLineSection {
					continue
				}
				diffLineTypeBuffers[l.Type].WriteString(l.Content[1:])
				diffLineTypeBuffers[l.Type].WriteString("\n")
			}
		}
		for lineType, buffer := range diffLineTypeBuffers {
			diffLineTypeDecoders[lineType] = nil
			if buffer.Len() == 0 {
				continue
			}
			charsetLabel, err := charset.DetectEncoding(buffer.Bytes())
			if charsetLabel != "UTF-8" && err == nil {
				encoding, _ := stdcharset.Lookup(charsetLabel)
				if encoding != nil {
					diffLineTypeDecoders[lineType] = encoding.NewDecoder()
				}
			}
		}
		for _, sec := range f.Sections {
			for _, l := range sec.Lines {
				decoder := diffLineTypeDecoders[l.Type]
				if decoder != nil {
					if c, _, err := transform.String(decoder, l.Content[1:]); err == nil {
						l.Content = l.Content[0:1] + c
					}
				}
			}
		}
	}

	diff.NumFiles = len(diff.Files)
	return diff, nil
}

func skipToNextDiffHead(input *bufio.Reader) (line string, err error) {
	// need to skip until the next cmdDiffHead
	isFragment, wasFragment := false, false
	var lineBytes []byte
	for {
		lineBytes, isFragment, err = input.ReadLine()
		if err != nil {
			return
		}
		if wasFragment {
			wasFragment = isFragment
			continue
		}
		if bytes.HasPrefix(lineBytes, []byte(cmdDiffHead)) {
			break
		}
		wasFragment = isFragment
	}
	line = string(lineBytes)
	if isFragment {
		var tail string
		tail, err = input.ReadString('\n')
		if err != nil {
			return
		}
		line += tail
	}
	return
}

func parseHunks(curFile *DiffFile, maxLines, maxLineCharacters int, input *bufio.Reader) (lineBytes []byte, isFragment bool, err error) {
	sb := strings.Builder{}

	var (
		curSection        *DiffSection
		curFileLinesCount int
		curFileLFSPrefix  bool
	)

	lastLeftIdx := -1
	leftLine, rightLine := 1, 1

	for {
		for isFragment {
			curFile.IsIncomplete = true
			curFile.IsIncompleteLineTooLong = true
			_, isFragment, err = input.ReadLine()
			if err != nil {
				// Now by the definition of ReadLine this cannot be io.EOF
				err = fmt.Errorf("unable to ReadLine: %w", err)
				return
			}
		}
		sb.Reset()
		lineBytes, isFragment, err = input.ReadLine()
		if err != nil {
			if err == io.EOF {
				return
			}
			err = fmt.Errorf("unable to ReadLine: %w", err)
			return
		}
		if lineBytes[0] == 'd' {
			// End of hunks
			return
		}

		switch lineBytes[0] {
		case '@':
			if maxLines > -1 && curFileLinesCount >= maxLines {
				curFile.IsIncomplete = true
				continue
			}

			_, _ = sb.Write(lineBytes)
			for isFragment {
				// This is very odd indeed - we're in a section header and the line is too long
				// This really shouldn't happen...
				lineBytes, isFragment, err = input.ReadLine()
				if err != nil {
					// Now by the definition of ReadLine this cannot be io.EOF
					err = fmt.Errorf("unable to ReadLine: %w", err)
					return
				}
				_, _ = sb.Write(lineBytes)
			}
			line := sb.String()

			// Create a new section to represent this hunk
			curSection = &DiffSection{file: curFile}
			lastLeftIdx = -1
			curFile.Sections = append(curFile.Sections, curSection)

			lineSectionInfo := getDiffLineSectionInfo(curFile.Name, line, leftLine-1, rightLine-1)
			diffLine := &DiffLine{
				Type:        DiffLineSection,
				Content:     line,
				SectionInfo: lineSectionInfo,
			}
			curSection.Lines = append(curSection.Lines, diffLine)
			curSection.FileName = curFile.Name
			// update line number.
			leftLine = lineSectionInfo.LeftIdx
			rightLine = lineSectionInfo.RightIdx
			continue
		case '\\':
			if maxLines > -1 && curFileLinesCount >= maxLines {
				curFile.IsIncomplete = true
				continue
			}
			// This is used only to indicate that the current file does not have a terminal newline
			if !bytes.Equal(lineBytes, []byte("\\ No newline at end of file")) {
				err = fmt.Errorf("unexpected line in hunk: %s", string(lineBytes))
				return
			}
			// Technically this should be the end the file!
			// FIXME: we should be putting a marker at the end of the file if there is no terminal new line
			continue
		case '+':
			curFileLinesCount++
			curFile.Addition++
			if maxLines > -1 && curFileLinesCount >= maxLines {
				curFile.IsIncomplete = true
				continue
			}
			diffLine := &DiffLine{Type: DiffLineAdd, RightIdx: rightLine, Match: -1}
			rightLine++
			if curSection == nil {
				// Create a new section to represent this hunk
				curSection = &DiffSection{file: curFile}
				curFile.Sections = append(curFile.Sections, curSection)
				lastLeftIdx = -1
			}
			if lastLeftIdx > -1 {
				diffLine.Match = lastLeftIdx
				curSection.Lines[lastLeftIdx].Match = len(curSection.Lines)
				lastLeftIdx++
				if lastLeftIdx >= len(curSection.Lines) || curSection.Lines[lastLeftIdx].Type != DiffLineDel {
					lastLeftIdx = -1
				}
			}
			curSection.Lines = append(curSection.Lines, diffLine)
		case '-':
			curFileLinesCount++
			curFile.Deletion++
			if maxLines > -1 && curFileLinesCount >= maxLines {
				curFile.IsIncomplete = true
				continue
			}
			diffLine := &DiffLine{Type: DiffLineDel, LeftIdx: leftLine, Match: -1}
			if leftLine > 0 {
				leftLine++
			}
			if curSection == nil {
				// Create a new section to represent this hunk
				curSection = &DiffSection{file: curFile}
				curFile.Sections = append(curFile.Sections, curSection)
				lastLeftIdx = -1
			}
			if len(curSection.Lines) == 0 || curSection.Lines[len(curSection.Lines)-1].Type != DiffLineDel {
				lastLeftIdx = len(curSection.Lines)
			}
			curSection.Lines = append(curSection.Lines, diffLine)
		case ' ':
			curFileLinesCount++
			if maxLines > -1 && curFileLinesCount >= maxLines {
				curFile.IsIncomplete = true
				continue
			}
			diffLine := &DiffLine{Type: DiffLinePlain, LeftIdx: leftLine, RightIdx: rightLine}
			leftLine++
			rightLine++
			lastLeftIdx = -1
			if curSection == nil {
				// Create a new section to represent this hunk
				curSection = &DiffSection{file: curFile}
				curFile.Sections = append(curFile.Sections, curSection)
			}
			curSection.Lines = append(curSection.Lines, diffLine)
		default:
			// This is unexpected
			err = fmt.Errorf("unexpected line in hunk: %s", string(lineBytes))
			return
		}

		line := string(lineBytes)
		if isFragment {
			curFile.IsIncomplete = true
			curFile.IsIncompleteLineTooLong = true
			for isFragment {
				lineBytes, isFragment, err = input.ReadLine()
				if err != nil {
					// Now by the definition of ReadLine this cannot be io.EOF
					err = fmt.Errorf("unable to ReadLine: %w", err)
					return
				}
			}
		}
		if len(line) > maxLineCharacters {
			curFile.IsIncomplete = true
			curFile.IsIncompleteLineTooLong = true
			line = line[:maxLineCharacters]
		}
		curSection.Lines[len(curSection.Lines)-1].Content = line

		// handle LFS
		if line[1:] == lfs.MetaFileIdentifier {
			curFileLFSPrefix = true
		} else if curFileLFSPrefix && strings.HasPrefix(line[1:], lfs.MetaFileOidPrefix) {
			oid := strings.TrimPrefix(line[1:], lfs.MetaFileOidPrefix)
			if len(oid) == 64 {
				m := &git_model.LFSMetaObject{Pointer: lfs.Pointer{Oid: oid}}
				count, err := db.CountByBean(db.DefaultContext, m)

				if err == nil && count > 0 {
					curFile.IsBin = true
					curFile.IsLFSFile = true
					curSection.Lines = nil
					lastLeftIdx = -1
				}
			}
		}
	}
}

func createDiffFile(diff *Diff, line string) *DiffFile {
	// The a/ and b/ filenames are the same unless rename/copy is involved.
	// Especially, even for a creation or a deletion, /dev/null is not used
	// in place of the a/ or b/ filenames.
	//
	// When rename/copy is involved, file1 and file2 show the name of the
	// source file of the rename/copy and the name of the file that rename/copy
	// produces, respectively.
	//
	// Path names are quoted if necessary.
	//
	// This means that you should always be able to determine the file name even when there
	// there is potential ambiguity...
	//
	// but we can be simpler with our heuristics by just forcing git to prefix things nicely
	curFile := &DiffFile{
		Index:    len(diff.Files) + 1,
		Type:     DiffFileChange,
		Sections: make([]*DiffSection, 0, 10),
	}

	rd := strings.NewReader(line[len(cmdDiffHead):] + " ")
	curFile.Type = DiffFileChange
	oldNameAmbiguity := false
	newNameAmbiguity := false

	curFile.OldName, oldNameAmbiguity = readFileName(rd)
	curFile.Name, newNameAmbiguity = readFileName(rd)
	if oldNameAmbiguity && newNameAmbiguity {
		curFile.IsAmbiguous = true
		// OK we should bet that the oldName and the newName are the same if they can be made to be same
		// So we need to start again ...
		if (len(line)-len(cmdDiffHead)-1)%2 == 0 {
			// diff --git a/b b/b b/b b/b b/b b/b
			//
			midpoint := (len(line) + len(cmdDiffHead) - 1) / 2
			new, old := line[len(cmdDiffHead):midpoint], line[midpoint+1:]
			if len(new) > 2 && len(old) > 2 && new[2:] == old[2:] {
				curFile.OldName = old[2:]
				curFile.Name = old[2:]
			}
		}
	}

	curFile.IsRenamed = curFile.Name != curFile.OldName
	return curFile
}

func readFileName(rd *strings.Reader) (string, bool) {
	ambiguity := false
	var name string
	char, _ := rd.ReadByte()
	_ = rd.UnreadByte()
	if char == '"' {
		fmt.Fscanf(rd, "%q ", &name)
		if len(name) == 0 {
			log.Error("Reader has no file name: %v", rd)
			return "", true
		}

		if name[0] == '\\' {
			name = name[1:]
		}
	} else {
		// This technique is potentially ambiguous it may not be possible to uniquely identify the filenames from the diff line alone
		ambiguity = true
		fmt.Fscanf(rd, "%s ", &name)
		char, _ := rd.ReadByte()
		_ = rd.UnreadByte()
		for !(char == 0 || char == '"' || char == 'b') {
			var suffix string
			fmt.Fscanf(rd, "%s ", &suffix)
			name += " " + suffix
			char, _ = rd.ReadByte()
			_ = rd.UnreadByte()
		}
	}
	if len(name) < 2 {
		log.Error("Unable to determine name from reader: %v", rd)
		return "", true
	}
	return name[2:], ambiguity
}

// DiffOptions represents the options for a DiffRange
type DiffOptions struct {
	BeforeCommitID     string
	AfterCommitID      string
	SkipTo             string
	MaxLines           int
	MaxLineCharacters  int
	MaxFiles           int
	WhitespaceBehavior string
	DirectComparison   bool
}

// GetDiff builds a Diff between two commits of a repository.
// Passing the empty string as beforeCommitID returns a diff from the parent commit.
// The whitespaceBehavior is either an empty string or a git flag
func GetDiff(gitRepo *git.Repository, opts *DiffOptions, files ...string) (*Diff, error) {
	repoPath := gitRepo.Path

	commit, err := gitRepo.GetCommit(opts.AfterCommitID)
	if err != nil {
		return nil, err
	}

	argsLength := 6
	if len(opts.WhitespaceBehavior) > 0 {
		argsLength++
	}
	if len(opts.SkipTo) > 0 {
		argsLength++
	}
	if len(files) > 0 {
		argsLength += len(files) + 1
	}

	diffArgs := make([]string, 0, argsLength)
	if (len(opts.BeforeCommitID) == 0 || opts.BeforeCommitID == git.EmptySHA) && commit.ParentCount() == 0 {
		diffArgs = append(diffArgs, "diff", "--src-prefix=\\a/", "--dst-prefix=\\b/", "-M")
		if len(opts.WhitespaceBehavior) != 0 {
			diffArgs = append(diffArgs, opts.WhitespaceBehavior)
		}
		// append empty tree ref
		diffArgs = append(diffArgs, "4b825dc642cb6eb9a060e54bf8d69288fbee4904")
		diffArgs = append(diffArgs, opts.AfterCommitID)
	} else {
		actualBeforeCommitID := opts.BeforeCommitID
		if len(actualBeforeCommitID) == 0 {
			parentCommit, _ := commit.Parent(0)
			actualBeforeCommitID = parentCommit.ID.String()
		}
		diffArgs = append(diffArgs, "diff", "--src-prefix=\\a/", "--dst-prefix=\\b/", "-M")
		if len(opts.WhitespaceBehavior) != 0 {
			diffArgs = append(diffArgs, opts.WhitespaceBehavior)
		}
		diffArgs = append(diffArgs, actualBeforeCommitID)
		diffArgs = append(diffArgs, opts.AfterCommitID)
		opts.BeforeCommitID = actualBeforeCommitID
	}

	// In git 2.31, git diff learned --skip-to which we can use to shortcut skip to file
	// so if we are using at least this version of git we don't have to tell ParsePatch to do
	// the skipping for us
	parsePatchSkipToFile := opts.SkipTo
	if opts.SkipTo != "" && git.CheckGitVersionAtLeast("2.31") == nil {
		diffArgs = append(diffArgs, "--skip-to="+opts.SkipTo)
		parsePatchSkipToFile = ""
	}

	if len(files) > 0 {
		diffArgs = append(diffArgs, "--")
		diffArgs = append(diffArgs, files...)
	}

	reader, writer := io.Pipe()
	defer func() {
		_ = reader.Close()
		_ = writer.Close()
	}()

	go func(ctx context.Context, diffArgs []string, repoPath string, writer *io.PipeWriter) {
		cmd := git.NewCommand(ctx, diffArgs...)
		cmd.SetDescription(fmt.Sprintf("GetDiffRange [repo_path: %s]", repoPath))
		if err := cmd.Run(&git.RunOpts{
			Timeout: time.Duration(setting.Git.Timeout.Default) * time.Second,
			Dir:     repoPath,
			Stderr:  os.Stderr,
			Stdout:  writer,
		}); err != nil {
			log.Error("error during RunWithContext: %w", err)
		}

		_ = writer.Close()
	}(gitRepo.Ctx, diffArgs, repoPath, writer)

	diff, err := ParsePatch(opts.MaxLines, opts.MaxLineCharacters, opts.MaxFiles, reader, parsePatchSkipToFile)
	if err != nil {
		return nil, fmt.Errorf("unable to ParsePatch: %w", err)
	}
	diff.Start = opts.SkipTo

	checker, deferable := gitRepo.CheckAttributeReader(opts.AfterCommitID)
	defer deferable()

	for _, diffFile := range diff.Files {

		gotVendor := false
		gotGenerated := false
		if checker != nil {
			attrs, err := checker.CheckPath(diffFile.Name)
			if err == nil {
				if vendored, has := attrs["linguist-vendored"]; has {
					if vendored == "set" || vendored == "true" {
						diffFile.IsVendored = true
						gotVendor = true
					} else {
						gotVendor = vendored == "false"
					}
				}
				if generated, has := attrs["linguist-generated"]; has {
					if generated == "set" || generated == "true" {
						diffFile.IsGenerated = true
						gotGenerated = true
					} else {
						gotGenerated = generated == "false"
					}
				}
				if language, has := attrs["linguist-language"]; has && language != "unspecified" && language != "" {
					diffFile.Language = language
				} else if language, has := attrs["gitlab-language"]; has && language != "unspecified" && language != "" {
					diffFile.Language = language
				}
			}
		}

		if !gotVendor {
			diffFile.IsVendored = analyze.IsVendor(diffFile.Name)
		}
		if !gotGenerated {
			diffFile.IsGenerated = analyze.IsGenerated(diffFile.Name)
		}

		tailSection := diffFile.GetTailSection(gitRepo, opts.BeforeCommitID, opts.AfterCommitID)
		if tailSection != nil {
			diffFile.Sections = append(diffFile.Sections, tailSection)
		}
	}

	separator := "..."
	if opts.DirectComparison {
		separator = ".."
	}

	shortstatArgs := []string{opts.BeforeCommitID + separator + opts.AfterCommitID}
	if len(opts.BeforeCommitID) == 0 || opts.BeforeCommitID == git.EmptySHA {
		shortstatArgs = []string{git.EmptyTreeSHA, opts.AfterCommitID}
	}
	diff.NumFiles, diff.TotalAddition, diff.TotalDeletion, err = git.GetDiffShortStat(gitRepo.Ctx, repoPath, shortstatArgs...)
	if err != nil && strings.Contains(err.Error(), "no merge base") {
		// git >= 2.28 now returns an error if base and head have become unrelated.
		// previously it would return the results of git diff --shortstat base head so let's try that...
		shortstatArgs = []string{opts.BeforeCommitID, opts.AfterCommitID}
		diff.NumFiles, diff.TotalAddition, diff.TotalDeletion, err = git.GetDiffShortStat(gitRepo.Ctx, repoPath, shortstatArgs...)
	}
	if err != nil {
		return nil, err
	}

	return diff, nil
}

// SyncAndGetUserSpecificDiff is like GetDiff, except that user specific data such as which files the given user has already viewed on the given PR will also be set
// Additionally, the database asynchronously is updated if files have changed since the last review
func SyncAndGetUserSpecificDiff(ctx context.Context, userID int64, pull *issues_model.PullRequest, gitRepo *git.Repository, opts *DiffOptions, files ...string) (*Diff, error) {
	diff, err := GetDiff(gitRepo, opts, files...)
	if err != nil {
		return nil, err
	}
	review, err := pull_model.GetNewestReviewState(ctx, userID, pull.ID)
	if err != nil || review == nil || review.UpdatedFiles == nil {
		return diff, err
	}

	latestCommit := opts.AfterCommitID
	if latestCommit == "" {
		latestCommit = pull.HeadBranch // opts.AfterCommitID is preferred because it handles PRs from forks correctly and the branch name doesn't
	}

	changedFiles, err := gitRepo.GetFilesChangedBetween(review.CommitSHA, latestCommit)
	// There are way too many possible errors.
	// Examples are various git errors such as the commit the review was based on was gc'ed and hence doesn't exist anymore as well as unrecoverable errors where we should serve a 500 response
	// Due to the current architecture and physical limitation of needing to compare explicit error messages, we can only choose one approach without the code getting ugly
	// For SOME of the errors such as the gc'ed commit, it would be best to mark all files as changed
	// But as that does not work for all potential errors, we simply mark all files as unchanged and drop the error which always works, even if not as good as possible
	if err != nil {
		log.Error("Could not get changed files between %s and %s for pull request %d in repo with path %s. Assuming no changes. Error: %w", review.CommitSHA, latestCommit, pull.Index, gitRepo.Path, err)
	}

	filesChangedSinceLastDiff := make(map[string]pull_model.ViewedState)
outer:
	for _, diffFile := range diff.Files {
		fileViewedState := review.UpdatedFiles[diffFile.GetDiffFileName()]

		// Check whether it was previously detected that the file has changed since the last review
		if fileViewedState == pull_model.HasChanged {
			diffFile.HasChangedSinceLastReview = true
			continue
		}

		filename := diffFile.GetDiffFileName()

		// Check explicitly whether the file has changed since the last review
		for _, changedFile := range changedFiles {
			diffFile.HasChangedSinceLastReview = filename == changedFile
			if diffFile.HasChangedSinceLastReview {
				filesChangedSinceLastDiff[filename] = pull_model.HasChanged
				continue outer // We don't want to check if the file is viewed here as that would fold the file, which is in this case unwanted
			}
		}
		// Check whether the file has already been viewed
		if fileViewedState == pull_model.Viewed {
			diffFile.IsViewed = true
			diff.NumViewedFiles++
		}
	}

	// Explicitly store files that have changed in the database, if any is present at all.
	// This has the benefit that the "Has Changed" attribute will be present as long as the user does not explicitly mark this file as viewed, so it will even survive a page reload after marking another file as viewed.
	// On the other hand, this means that even if a commit reverting an unseen change is committed, the file will still be seen as changed.
	if len(filesChangedSinceLastDiff) > 0 {
		err := pull_model.UpdateReviewState(ctx, review.UserID, review.PullID, review.CommitSHA, filesChangedSinceLastDiff)
		if err != nil {
			log.Warn("Could not update review for user %d, pull %d, commit %s and the changed files %v: %v", review.UserID, review.PullID, review.CommitSHA, filesChangedSinceLastDiff, err)
			return nil, err
		}
	}

	return diff, err
}

// CommentAsDiff returns c.Patch as *Diff
func CommentAsDiff(c *issues_model.Comment) (*Diff, error) {
	diff, err := ParsePatch(setting.Git.MaxGitDiffLines,
		setting.Git.MaxGitDiffLineCharacters, setting.Git.MaxGitDiffFiles, strings.NewReader(c.Patch), "")
	if err != nil {
		log.Error("Unable to parse patch: %v", err)
		return nil, err
	}
	if len(diff.Files) == 0 {
		return nil, fmt.Errorf("no file found for comment ID: %d", c.ID)
	}
	secs := diff.Files[0].Sections
	if len(secs) == 0 {
		return nil, fmt.Errorf("no sections found for comment ID: %d", c.ID)
	}
	return diff, nil
}

// CommentMustAsDiff executes AsDiff and logs the error instead of returning
func CommentMustAsDiff(c *issues_model.Comment) *Diff {
	if c == nil {
		return nil
	}
	defer func() {
		if err := recover(); err != nil {
			log.Error("PANIC whilst retrieving diff for comment[%d] Error: %v\nStack: %s", c.ID, err, log.Stack(2))
		}
	}()
	diff, err := CommentAsDiff(c)
	if err != nil {
		log.Warn("CommentMustAsDiff: %v", err)
	}
	return diff
}

// GetWhitespaceFlag returns git diff flag for treating whitespaces
func GetWhitespaceFlag(whitespaceBehavior string) string {
	whitespaceFlags := map[string]string{
		"ignore-all":    "-w",
		"ignore-change": "-b",
		"ignore-eol":    "--ignore-space-at-eol",
		"show-all":      "",
	}

	if flag, ok := whitespaceFlags[whitespaceBehavior]; ok {
		return flag
	}
	log.Warn("unknown whitespace behavior: %q, default to 'show-all'", whitespaceBehavior)
	return ""
}