diffmatchpatch/diff.go

// Copyright (c) 2012-2016 The go-diff authors. All rights reserved.
// https://github.com/sergi/go-diff
// See the included LICENSE file for license details.
//
// go-diff is a Go implementation of Google's Diff, Match, and Patch library
// Original library is Copyright (c) 2006 Google Inc.
// http://code.google.com/p/google-diff-match-patch/

package diffmatchpatch

import (
	"bytes"
	"errors"
	"fmt"
	"html"
	"math"
	"net/url"
	"regexp"
	"strconv"
	"strings"
	"time"
	"unicode/utf8"
)

// Operation defines the operation of a diff item.
type Operation int8

const (
	// DiffDelete item represents a delete diff.
	DiffDelete Operation = -1
	// DiffInsert item represents an insert diff.
	DiffInsert Operation = 1
	// DiffEqual item represents an equal diff.
	DiffEqual Operation = 0
)

// Diff represents one diff operation
type Diff struct {
	Type Operation
	Text string
}

func splice(slice []Diff, index int, amount int, elements ...Diff) []Diff {
	return append(slice[:index], append(elements, slice[index+amount:]...)...)
}

// DiffMain finds the differences between two texts.
func (dmp *DiffMatchPatch) DiffMain(text1, text2 string, checklines bool) []Diff {
	return dmp.DiffMainRunes([]rune(text1), []rune(text2), checklines)
}

// DiffMainRunes finds the differences between two rune sequences.
func (dmp *DiffMatchPatch) DiffMainRunes(text1, text2 []rune, checklines bool) []Diff {
	var deadline time.Time
	if dmp.DiffTimeout > 0 {
		deadline = time.Now().Add(dmp.DiffTimeout)
	}
	return dmp.diffMainRunes(text1, text2, checklines, deadline)
}

func (dmp *DiffMatchPatch) diffMainRunes(text1, text2 []rune, checklines bool, deadline time.Time) []Diff {
	if runesEqual(text1, text2) {
		var diffs []Diff
		if len(text1) > 0 {
			diffs = append(diffs, Diff{DiffEqual, string(text1)})
		}
		return diffs
	}
	// Trim off common prefix (speedup).
	commonlength := commonPrefixLength(text1, text2)
	commonprefix := text1[:commonlength]
	text1 = text1[commonlength:]
	text2 = text2[commonlength:]

	// Trim off common suffix (speedup).
	commonlength = commonSuffixLength(text1, text2)
	commonsuffix := text1[len(text1)-commonlength:]
	text1 = text1[:len(text1)-commonlength]
	text2 = text2[:len(text2)-commonlength]

	// Compute the diff on the middle block.
	diffs := dmp.diffCompute(text1, text2, checklines, deadline)

	// Restore the prefix and suffix.
	if len(commonprefix) != 0 {
		diffs = append([]Diff{Diff{DiffEqual, string(commonprefix)}}, diffs...)
	}
	if len(commonsuffix) != 0 {
		diffs = append(diffs, Diff{DiffEqual, string(commonsuffix)})
	}

	return dmp.DiffCleanupMerge(diffs)
}

// diffCompute finds the differences between two rune slices.  Assumes that the texts do not
// have any common prefix or suffix.
func (dmp *DiffMatchPatch) diffCompute(text1, text2 []rune, checklines bool, deadline time.Time) []Diff {
	diffs := []Diff{}
	if len(text1) == 0 {
		// Just add some text (speedup).
		return append(diffs, Diff{DiffInsert, string(text2)})
	} else if len(text2) == 0 {
		// Just delete some text (speedup).
		return append(diffs, Diff{DiffDelete, string(text1)})
	}

	var longtext, shorttext []rune
	if len(text1) > len(text2) {
		longtext = text1
		shorttext = text2
	} else {
		longtext = text2
		shorttext = text1
	}

	if i := runesIndex(longtext, shorttext); i != -1 {
		op := DiffInsert
		// Swap insertions for deletions if diff is reversed.
		if len(text1) > len(text2) {
			op = DiffDelete
		}
		// Shorter text is inside the longer text (speedup).
		return []Diff{
			Diff{op, string(longtext[:i])},
			Diff{DiffEqual, string(shorttext)},
			Diff{op, string(longtext[i+len(shorttext):])},
		}
	} else if len(shorttext) == 1 {
		// Single character string.
		// After the previous speedup, the character can't be an equality.
		return []Diff{
			Diff{DiffDelete, string(text1)},
			Diff{DiffInsert, string(text2)},
		}
		// Check to see if the problem can be split in two.
	} else if hm := dmp.diffHalfMatch(text1, text2); hm != nil {
		// A half-match was found, sort out the return data.
		text1A := hm[0]
		text1B := hm[1]
		text2A := hm[2]
		text2B := hm[3]
		midCommon := hm[4]
		// Send both pairs off for separate processing.
		diffsA := dmp.diffMainRunes(text1A, text2A, checklines, deadline)
		diffsB := dmp.diffMainRunes(text1B, text2B, checklines, deadline)
		// Merge the results.
		return append(diffsA, append([]Diff{Diff{DiffEqual, string(midCommon)}}, diffsB...)...)
	} else if checklines && len(text1) > 100 && len(text2) > 100 {
		return dmp.diffLineMode(text1, text2, deadline)
	}
	return dmp.diffBisect(text1, text2, deadline)
}

// diffLineMode does a quick line-level diff on both []runes, then rediff the parts for
// greater accuracy. This speedup can produce non-minimal diffs.
func (dmp *DiffMatchPatch) diffLineMode(text1, text2 []rune, deadline time.Time) []Diff {
	// Scan the text on a line-by-line basis first.
	text1, text2, linearray := dmp.diffLinesToRunes(text1, text2)

	diffs := dmp.diffMainRunes(text1, text2, false, deadline)

	// Convert the diff back to original text.
	diffs = dmp.DiffCharsToLines(diffs, linearray)
	// Eliminate freak matches (e.g. blank lines)
	diffs = dmp.DiffCleanupSemantic(diffs)

	// Rediff any replacement blocks, this time character-by-character.
	// Add a dummy entry at the end.
	diffs = append(diffs, Diff{DiffEqual, ""})

	pointer := 0
	countDelete := 0
	countInsert := 0

	// NOTE: Rune slices are slower than using strings in this case.
	textDelete := ""
	textInsert := ""

	for pointer < len(diffs) {
		switch diffs[pointer].Type {
		case DiffInsert:
			countInsert++
			textInsert += diffs[pointer].Text
		case DiffDelete:
			countDelete++
			textDelete += diffs[pointer].Text
		case DiffEqual:
			// Upon reaching an equality, check for prior redundancies.
			if countDelete >= 1 && countInsert >= 1 {
				// Delete the offending records and add the merged ones.
				diffs = splice(diffs, pointer-countDelete-countInsert,
					countDelete+countInsert)

				pointer = pointer - countDelete - countInsert
				a := dmp.diffMainRunes([]rune(textDelete), []rune(textInsert), false, deadline)
				for j := len(a) - 1; j >= 0; j-- {
					diffs = splice(diffs, pointer, 0, a[j])
				}
				pointer = pointer + len(a)
			}

			countInsert = 0
			countDelete = 0
			textDelete = ""
			textInsert = ""
		}
		pointer++
	}

	return diffs[:len(diffs)-1] // Remove the dummy entry at the end.
}

// DiffBisect finds the 'middle snake' of a diff, split the problem in two
// and return the recursively constructed diff.
// See Myers 1986 paper: An O(ND) Difference Algorithm and Its Variations.
func (dmp *DiffMatchPatch) DiffBisect(text1, text2 string, deadline time.Time) []Diff {
	// Unused in this code, but retained for interface compatibility.
	return dmp.diffBisect([]rune(text1), []rune(text2), deadline)
}

// diffBisect finds the 'middle snake' of a diff, splits the problem in two
// and returns the recursively constructed diff.
// See Myers's 1986 paper: An O(ND) Difference Algorithm and Its Variations.
func (dmp *DiffMatchPatch) diffBisect(runes1, runes2 []rune, deadline time.Time) []Diff {
	// Cache the text lengths to prevent multiple calls.
	runes1Len, runes2Len := len(runes1), len(runes2)

	maxD := (runes1Len + runes2Len + 1) / 2
	vOffset := maxD
	vLength := 2 * maxD

	v1 := make([]int, vLength)
	v2 := make([]int, vLength)
	for i := range v1 {
		v1[i] = -1
		v2[i] = -1
	}
	v1[vOffset+1] = 0
	v2[vOffset+1] = 0

	delta := runes1Len - runes2Len
	// If the total number of characters is odd, then the front path will collide
	// with the reverse path.
	front := (delta%2 != 0)
	// Offsets for start and end of k loop.
	// Prevents mapping of space beyond the grid.
	k1start := 0
	k1end := 0
	k2start := 0
	k2end := 0
	for d := 0; d < maxD; d++ {
		// Bail out if deadline is reached.
		if !deadline.IsZero() && time.Now().After(deadline) {
			break
		}

		// Walk the front path one step.
		for k1 := -d + k1start; k1 <= d-k1end; k1 += 2 {
			k1Offset := vOffset + k1
			var x1 int

			if k1 == -d || (k1 != d && v1[k1Offset-1] < v1[k1Offset+1]) {
				x1 = v1[k1Offset+1]
			} else {
				x1 = v1[k1Offset-1] + 1
			}

			y1 := x1 - k1
			for x1 < runes1Len && y1 < runes2Len {
				if runes1[x1] != runes2[y1] {
					break
				}
				x1++
				y1++
			}
			v1[k1Offset] = x1
			if x1 > runes1Len {
				// Ran off the right of the graph.
				k1end += 2
			} else if y1 > runes2Len {
				// Ran off the bottom of the graph.
				k1start += 2
			} else if front {
				k2Offset := vOffset + delta - k1
				if k2Offset >= 0 && k2Offset < vLength && v2[k2Offset] != -1 {
					// Mirror x2 onto top-left coordinate system.
					x2 := runes1Len - v2[k2Offset]
					if x1 >= x2 {
						// Overlap detected.
						return dmp.diffBisectSplit(runes1, runes2, x1, y1, deadline)
					}
				}
			}
		}
		// Walk the reverse path one step.
		for k2 := -d + k2start; k2 <= d-k2end; k2 += 2 {
			k2Offset := vOffset + k2
			var x2 int
			if k2 == -d || (k2 != d && v2[k2Offset-1] < v2[k2Offset+1]) {
				x2 = v2[k2Offset+1]
			} else {
				x2 = v2[k2Offset-1] + 1
			}
			var y2 = x2 - k2
			for x2 < runes1Len && y2 < runes2Len {
				if runes1[runes1Len-x2-1] != runes2[runes2Len-y2-1] {
					break
				}
				x2++
				y2++
			}
			v2[k2Offset] = x2
			if x2 > runes1Len {
				// Ran off the left of the graph.
				k2end += 2
			} else if y2 > runes2Len {
				// Ran off the top of the graph.
				k2start += 2
			} else if !front {
				k1Offset := vOffset + delta - k2
				if k1Offset >= 0 && k1Offset < vLength && v1[k1Offset] != -1 {
					x1 := v1[k1Offset]
					y1 := vOffset + x1 - k1Offset
					// Mirror x2 onto top-left coordinate system.
					x2 = runes1Len - x2
					if x1 >= x2 {
						// Overlap detected.
						return dmp.diffBisectSplit(runes1, runes2, x1, y1, deadline)
					}
				}
			}
		}
	}
	// Diff took too long and hit the deadline or
	// number of diffs equals number of characters, no commonality at all.
	return []Diff{
		Diff{DiffDelete, string(runes1)},
		Diff{DiffInsert, string(runes2)},
	}
}

func (dmp *DiffMatchPatch) diffBisectSplit(runes1, runes2 []rune, x, y int,
	deadline time.Time) []Diff {
	runes1a := runes1[:x]
	runes2a := runes2[:y]
	runes1b := runes1[x:]
	runes2b := runes2[y:]

	// Compute both diffs serially.
	diffs := dmp.diffMainRunes(runes1a, runes2a, false, deadline)
	diffsb := dmp.diffMainRunes(runes1b, runes2b, false, deadline)

	return append(diffs, diffsb...)
}

// DiffLinesToChars splits two texts into a list of strings.  Reduces the texts to a string of
// hashes where each Unicode character represents one line.
// It's slightly faster to call DiffLinesToRunes first, followed by DiffMainRunes.
func (dmp *DiffMatchPatch) DiffLinesToChars(text1, text2 string) (string, string, []string) {
	chars1, chars2, lineArray := dmp.DiffLinesToRunes(text1, text2)
	return string(chars1), string(chars2), lineArray
}

// DiffLinesToRunes splits two texts into a list of runes.  Each rune represents one line.
func (dmp *DiffMatchPatch) DiffLinesToRunes(text1, text2 string) ([]rune, []rune, []string) {
	// '\x00' is a valid character, but various debuggers don't like it.
	// So we'll insert a junk entry to avoid generating a null character.
	lineArray := []string{""}    // e.g. lineArray[4] == 'Hello\n'
	lineHash := map[string]int{} // e.g. lineHash['Hello\n'] == 4

	chars1 := dmp.diffLinesToRunesMunge(text1, &lineArray, lineHash)
	chars2 := dmp.diffLinesToRunesMunge(text2, &lineArray, lineHash)

	return chars1, chars2, lineArray
}

func (dmp *DiffMatchPatch) diffLinesToRunes(text1, text2 []rune) ([]rune, []rune, []string) {
	return dmp.DiffLinesToRunes(string(text1), string(text2))
}

// diffLinesToRunesMunge splits a text into an array of strings.  Reduces the
// texts to a []rune where each Unicode character represents one line.
// We use strings instead of []runes as input mainly because you can't use []rune as a map key.
func (dmp *DiffMatchPatch) diffLinesToRunesMunge(text string, lineArray *[]string, lineHash map[string]int) []rune {
	// Walk the text, pulling out a substring for each line.
	// text.split('\n') would would temporarily double our memory footprint.
	// Modifying text would create many large strings to garbage collect.
	lineStart := 0
	lineEnd := -1
	runes := []rune{}

	for lineEnd < len(text)-1 {
		lineEnd = indexOf(text, "\n", lineStart)

		if lineEnd == -1 {
			lineEnd = len(text) - 1
		}

		line := text[lineStart : lineEnd+1]
		lineStart = lineEnd + 1
		lineValue, ok := lineHash[line]

		if ok {
			runes = append(runes, rune(lineValue))
		} else {
			*lineArray = append(*lineArray, line)
			lineHash[line] = len(*lineArray) - 1
			runes = append(runes, rune(len(*lineArray)-1))
		}
	}

	return runes
}

// DiffCharsToLines rehydrates the text in a diff from a string of line hashes to real lines of
// text.
func (dmp *DiffMatchPatch) DiffCharsToLines(diffs []Diff, lineArray []string) []Diff {
	hydrated := make([]Diff, 0, len(diffs))
	for _, aDiff := range diffs {
		chars := aDiff.Text
		text := make([]string, len(chars))

		for i, r := range chars {
			text[i] = lineArray[r]
		}

		aDiff.Text = strings.Join(text, "")
		hydrated = append(hydrated, aDiff)
	}
	return hydrated
}

// DiffCommonPrefix determines the common prefix length of two strings.
func (dmp *DiffMatchPatch) DiffCommonPrefix(text1, text2 string) int {
	// Unused in this code, but retained for interface compatibility.
	return commonPrefixLength([]rune(text1), []rune(text2))
}

// DiffCommonSuffix determines the common suffix length of two strings.
func (dmp *DiffMatchPatch) DiffCommonSuffix(text1, text2 string) int {
	// Unused in this code, but retained for interface compatibility.
	return commonSuffixLength([]rune(text1), []rune(text2))
}

// commonPrefixLength returns the length of the common prefix of two rune slices.
func commonPrefixLength(text1, text2 []rune) int {
	short, long := text1, text2
	if len(short) > len(long) {
		short, long = long, short
	}
	for i, r := range short {
		if r != long[i] {
			return i
		}
	}
	return len(short)
}

// commonSuffixLength returns the length of the common suffix of two rune slices.
func commonSuffixLength(text1, text2 []rune) int {
	n := min(len(text1), len(text2))
	for i := 0; i < n; i++ {
		if text1[len(text1)-i-1] != text2[len(text2)-i-1] {
			return i
		}
	}
	return n

	// Binary search.
	// Performance analysis: http://neil.fraser.name/news/2007/10/09/
	/*
	   pointermin := 0
	   pointermax := math.Min(len(text1), len(text2))
	   pointermid := pointermax
	   pointerend := 0
	   for pointermin < pointermid {
	       if text1[len(text1)-pointermid:len(text1)-pointerend] ==
	           text2[len(text2)-pointermid:len(text2)-pointerend] {
	           pointermin = pointermid
	           pointerend = pointermin
	       } else {
	           pointermax = pointermid
	       }
	       pointermid = math.Floor((pointermax-pointermin)/2 + pointermin)
	   }
	   return pointermid
	*/
}

// DiffCommonOverlap determines if the suffix of one string is the prefix of another.
func (dmp *DiffMatchPatch) DiffCommonOverlap(text1 string, text2 string) int {
	// Cache the text lengths to prevent multiple calls.
	text1Length := len(text1)
	text2Length := len(text2)
	// Eliminate the null case.
	if text1Length == 0 || text2Length == 0 {
		return 0
	}
	// Truncate the longer string.
	if text1Length > text2Length {
		text1 = text1[text1Length-text2Length:]
	} else if text1Length < text2Length {
		text2 = text2[0:text1Length]
	}
	textLength := int(math.Min(float64(text1Length), float64(text2Length)))
	// Quick check for the worst case.
	if text1 == text2 {
		return textLength
	}

	// Start by looking for a single character match
	// and increase length until no match is found.
	// Performance analysis: http://neil.fraser.name/news/2010/11/04/
	best := 0
	length := 1
	for {
		pattern := text1[textLength-length:]
		found := strings.Index(text2, pattern)
		if found == -1 {
			break
		}
		length += found
		if found == 0 || text1[textLength-length:] == text2[0:length] {
			best = length
			length++
		}
	}

	return best
}

// DiffHalfMatch checks whether the two texts share a substring which is at
// least half the length of the longer text. This speedup can produce non-minimal diffs.
func (dmp *DiffMatchPatch) DiffHalfMatch(text1, text2 string) []string {
	// Unused in this code, but retained for interface compatibility.
	runeSlices := dmp.diffHalfMatch([]rune(text1), []rune(text2))
	if runeSlices == nil {
		return nil
	}

	result := make([]string, len(runeSlices))
	for i, r := range runeSlices {
		result[i] = string(r)
	}
	return result
}

func (dmp *DiffMatchPatch) diffHalfMatch(text1, text2 []rune) [][]rune {
	if dmp.DiffTimeout <= 0 {
		// Don't risk returning a non-optimal diff if we have unlimited time.
		return nil
	}

	var longtext, shorttext []rune
	if len(text1) > len(text2) {
		longtext = text1
		shorttext = text2
	} else {
		longtext = text2
		shorttext = text1
	}

	if len(longtext) < 4 || len(shorttext)*2 < len(longtext) {
		return nil // Pointless.
	}

	// First check if the second quarter is the seed for a half-match.
	hm1 := dmp.diffHalfMatchI(longtext, shorttext, int(float64(len(longtext)+3)/4))

	// Check again based on the third quarter.
	hm2 := dmp.diffHalfMatchI(longtext, shorttext, int(float64(len(longtext)+1)/2))

	hm := [][]rune{}
	if hm1 == nil && hm2 == nil {
		return nil
	} else if hm2 == nil {
		hm = hm1
	} else if hm1 == nil {
		hm = hm2
	} else {
		// Both matched.  Select the longest.
		if len(hm1[4]) > len(hm2[4]) {
			hm = hm1
		} else {
			hm = hm2
		}
	}

	// A half-match was found, sort out the return data.
	if len(text1) > len(text2) {
		return hm
	}

	return [][]rune{hm[2], hm[3], hm[0], hm[1], hm[4]}
}

// diffHalfMatchI checks if a substring of shorttext exist within longtext such that the substring is at least half the length of longtext?
// @param {string} longtext Longer string.
// @param {string} shorttext Shorter string.
// @param {number} i Start index of quarter length substring within longtext.
// @return {Array.<string>} Five element Array, containing the prefix of
//     longtext, the suffix of longtext, the prefix of shorttext, the suffix
//     of shorttext and the common middle.  Or null if there was no match.
func (dmp *DiffMatchPatch) diffHalfMatchI(l, s []rune, i int) [][]rune {
	var bestCommonA []rune
	var bestCommonB []rune
	var bestCommonLen int
	var bestLongtextA []rune
	var bestLongtextB []rune
	var bestShorttextA []rune
	var bestShorttextB []rune

	// Start with a 1/4 length substring at position i as a seed.
	seed := l[i : i+len(l)/4]

	for j := runesIndexOf(s, seed, 0); j != -1; j = runesIndexOf(s, seed, j+1) {
		prefixLength := commonPrefixLength(l[i:], s[j:])
		suffixLength := commonSuffixLength(l[:i], s[:j])

		if bestCommonLen < suffixLength+prefixLength {
			bestCommonA = s[j-suffixLength : j]
			bestCommonB = s[j : j+prefixLength]
			bestCommonLen = len(bestCommonA) + len(bestCommonB)
			bestLongtextA = l[:i-suffixLength]
			bestLongtextB = l[i+prefixLength:]
			bestShorttextA = s[:j-suffixLength]
			bestShorttextB = s[j+prefixLength:]
		}
	}

	if bestCommonLen*2 < len(l) {
		return nil
	}

	return [][]rune{
		bestLongtextA,
		bestLongtextB,
		bestShorttextA,
		bestShorttextB,
		append(bestCommonA, bestCommonB...),
	}
}

// DiffCleanupSemantic reduces the number of edits by eliminating
// semantically trivial equalities.
func (dmp *DiffMatchPatch) DiffCleanupSemantic(diffs []Diff) []Diff {
	changes := false
	// Stack of indices where equalities are found.
	type equality struct {
		data int
		next *equality
	}
	var equalities *equality

	var lastequality string
	// Always equal to diffs[equalities[equalitiesLength - 1]][1]
	var pointer int // Index of current position.
	// Number of characters that changed prior to the equality.
	var lengthInsertions1, lengthDeletions1 int
	// Number of characters that changed after the equality.
	var lengthInsertions2, lengthDeletions2 int

	for pointer < len(diffs) {
		if diffs[pointer].Type == DiffEqual { // Equality found.
			equalities = &equality{
				data: pointer,
				next: equalities,
			}
			lengthInsertions1 = lengthInsertions2
			lengthDeletions1 = lengthDeletions2
			lengthInsertions2 = 0
			lengthDeletions2 = 0
			lastequality = diffs[pointer].Text
		} else { // An insertion or deletion.
			if diffs[pointer].Type == DiffInsert {
				lengthInsertions2 += len(diffs[pointer].Text)
			} else {
				lengthDeletions2 += len(diffs[pointer].Text)
			}
			// Eliminate an equality that is smaller or equal to the edits on both
			// sides of it.
			difference1 := int(math.Max(float64(lengthInsertions1), float64(lengthDeletions1)))
			difference2 := int(math.Max(float64(lengthInsertions2), float64(lengthDeletions2)))
			if len(lastequality) > 0 &&
				(len(lastequality) <= difference1) &&
				(len(lastequality) <= difference2) {
				// Duplicate record.
				insPoint := equalities.data
				diffs = append(
					diffs[:insPoint],
					append([]Diff{Diff{DiffDelete, lastequality}}, diffs[insPoint:]...)...)

				// Change second copy to insert.
				diffs[insPoint+1].Type = DiffInsert
				// Throw away the equality we just deleted.
				equalities = equalities.next

				if equalities != nil {
					equalities = equalities.next
				}
				if equalities != nil {
					pointer = equalities.data
				} else {
					pointer = -1
				}

				lengthInsertions1 = 0 // Reset the counters.
				lengthDeletions1 = 0
				lengthInsertions2 = 0
				lengthDeletions2 = 0
				lastequality = ""
				changes = true
			}
		}
		pointer++
	}

	// Normalize the diff.
	if changes {
		diffs = dmp.DiffCleanupMerge(diffs)
	}
	diffs = dmp.DiffCleanupSemanticLossless(diffs)
	// Find any overlaps between deletions and insertions.
	// e.g: <del>abcxxx</del><ins>xxxdef</ins>
	//   -> <del>abc</del>xxx<ins>def</ins>
	// e.g: <del>xxxabc</del><ins>defxxx</ins>
	//   -> <ins>def</ins>xxx<del>abc</del>
	// Only extract an overlap if it is as big as the edit ahead or behind it.
	pointer = 1
	for pointer < len(diffs) {
		if diffs[pointer-1].Type == DiffDelete &&
			diffs[pointer].Type == DiffInsert {
			deletion := diffs[pointer-1].Text
			insertion := diffs[pointer].Text
			overlapLength1 := dmp.DiffCommonOverlap(deletion, insertion)
			overlapLength2 := dmp.DiffCommonOverlap(insertion, deletion)
			if overlapLength1 >= overlapLength2 {
				if float64(overlapLength1) >= float64(len(deletion))/2 ||
					float64(overlapLength1) >= float64(len(insertion))/2 {

					// Overlap found.  Insert an equality and trim the surrounding edits.
					diffs = append(
						diffs[:pointer],
						append([]Diff{Diff{DiffEqual, insertion[:overlapLength1]}}, diffs[pointer:]...)...)
					//diffs.splice(pointer, 0,
					//    [DiffEqual, insertion[0 : overlapLength1)]]
					diffs[pointer-1].Text =
						deletion[0 : len(deletion)-overlapLength1]
					diffs[pointer+1].Text = insertion[overlapLength1:]
					pointer++
				}
			} else {
				if float64(overlapLength2) >= float64(len(deletion))/2 ||
					float64(overlapLength2) >= float64(len(insertion))/2 {
					// Reverse overlap found.
					// Insert an equality and swap and trim the surrounding edits.
					overlap := Diff{DiffEqual, deletion[:overlapLength2]}
					diffs = append(
						diffs[:pointer],
						append([]Diff{overlap}, diffs[pointer:]...)...)
					// diffs.splice(pointer, 0,
					//     [DiffEqual, deletion[0 : overlapLength2)]]
					diffs[pointer-1].Type = DiffInsert
					diffs[pointer-1].Text = insertion[0 : len(insertion)-overlapLength2]
					diffs[pointer+1].Type = DiffDelete
					diffs[pointer+1].Text = deletion[overlapLength2:]
					pointer++
				}
			}
			pointer++
		}
		pointer++
	}

	return diffs
}

// Define some regex patterns for matching boundaries.
var (
	nonAlphaNumericRegex = regexp.MustCompile(`[^a-zA-Z0-9]`)
	whitespaceRegex      = regexp.MustCompile(`\s`)
	linebreakRegex       = regexp.MustCompile(`[\r\n]`)
	blanklineEndRegex    = regexp.MustCompile(`\n\r?\n$`)
	blanklineStartRegex  = regexp.MustCompile(`^\r?\n\r?\n`)
)

// DiffCleanupSemanticLossless looks for single edits surrounded on both sides by equalities
// which can be shifted sideways to align the edit to a word boundary.
// e.g: The c<ins>at c</ins>ame. -> The <ins>cat </ins>came.
func (dmp *DiffMatchPatch) DiffCleanupSemanticLossless(diffs []Diff) []Diff {

	/**
	 * Given two strings, compute a score representing whether the internal
	 * boundary falls on logical boundaries.
	 * Scores range from 6 (best) to 0 (worst).
	 * Closure, but does not reference any external variables.
	 * @param {string} one First string.
	 * @param {string} two Second string.
	 * @return {number} The score.
	 * @private
	 */
	diffCleanupSemanticScore := func(one, two string) int {
		if len(one) == 0 || len(two) == 0 {
			// Edges are the best.
			return 6
		}

		// Each port of this function behaves slightly differently due to
		// subtle differences in each language's definition of things like
		// 'whitespace'.  Since this function's purpose is largely cosmetic,
		// the choice has been made to use each language's native features
		// rather than force total conformity.
		rune1, _ := utf8.DecodeLastRuneInString(one)
		rune2, _ := utf8.DecodeRuneInString(two)
		char1 := string(rune1)
		char2 := string(rune2)

		nonAlphaNumeric1 := nonAlphaNumericRegex.MatchString(char1)
		nonAlphaNumeric2 := nonAlphaNumericRegex.MatchString(char2)
		whitespace1 := nonAlphaNumeric1 && whitespaceRegex.MatchString(char1)
		whitespace2 := nonAlphaNumeric2 && whitespaceRegex.MatchString(char2)
		lineBreak1 := whitespace1 && linebreakRegex.MatchString(char1)
		lineBreak2 := whitespace2 && linebreakRegex.MatchString(char2)
		blankLine1 := lineBreak1 && blanklineEndRegex.MatchString(one)
		blankLine2 := lineBreak2 && blanklineEndRegex.MatchString(two)

		if blankLine1 || blankLine2 {
			// Five points for blank lines.
			return 5
		} else if lineBreak1 || lineBreak2 {
			// Four points for line breaks.
			return 4
		} else if nonAlphaNumeric1 && !whitespace1 && whitespace2 {
			// Three points for end of sentences.
			return 3
		} else if whitespace1 || whitespace2 {
			// Two points for whitespace.
			return 2
		} else if nonAlphaNumeric1 || nonAlphaNumeric2 {
			// One point for non-alphanumeric.
			return 1
		}
		return 0
	}

	pointer := 1

	// Intentionally ignore the first and last element (don't need checking).
	for pointer < len(diffs)-1 {
		if diffs[pointer-1].Type == DiffEqual &&
			diffs[pointer+1].Type == DiffEqual {

			// This is a single edit surrounded by equalities.
			equality1 := diffs[pointer-1].Text
			edit := diffs[pointer].Text
			equality2 := diffs[pointer+1].Text

			// First, shift the edit as far left as possible.
			commonOffset := dmp.DiffCommonSuffix(equality1, edit)
			if commonOffset > 0 {
				commonString := edit[len(edit)-commonOffset:]
				equality1 = equality1[0 : len(equality1)-commonOffset]
				edit = commonString + edit[:len(edit)-commonOffset]
				equality2 = commonString + equality2
			}

			// Second, step character by character right, looking for the best fit.
			bestEquality1 := equality1
			bestEdit := edit
			bestEquality2 := equality2
			bestScore := diffCleanupSemanticScore(equality1, edit) +
				diffCleanupSemanticScore(edit, equality2)

			for len(edit) != 0 && len(equality2) != 0 {
				_, sz := utf8.DecodeRuneInString(edit)
				if len(equality2) < sz || edit[:sz] != equality2[:sz] {
					break
				}
				equality1 += edit[:sz]
				edit = edit[sz:] + equality2[:sz]
				equality2 = equality2[sz:]
				score := diffCleanupSemanticScore(equality1, edit) +
					diffCleanupSemanticScore(edit, equality2)
				// The >= encourages trailing rather than leading whitespace on
				// edits.
				if score >= bestScore {
					bestScore = score
					bestEquality1 = equality1
					bestEdit = edit
					bestEquality2 = equality2
				}
			}

			if diffs[pointer-1].Text != bestEquality1 {
				// We have an improvement, save it back to the diff.
				if len(bestEquality1) != 0 {
					diffs[pointer-1].Text = bestEquality1
				} else {
					diffs = splice(diffs, pointer-1, 1)
					pointer--
				}

				diffs[pointer].Text = bestEdit
				if len(bestEquality2) != 0 {
					diffs[pointer+1].Text = bestEquality2
				} else {
					//splice(diffs, pointer+1, 1)
					diffs = append(diffs[:pointer+1], diffs[pointer+2:]...)
					pointer--
				}
			}
		}
		pointer++
	}

	return diffs
}

// DiffCleanupEfficiency reduces the number of edits by eliminating
// operationally trivial equalities.
func (dmp *DiffMatchPatch) DiffCleanupEfficiency(diffs []Diff) []Diff {
	changes := false
	// Stack of indices where equalities are found.
	type equality struct {
		data int
		next *equality
	}
	var equalities *equality
	// Always equal to equalities[equalitiesLength-1][1]
	lastequality := ""
	pointer := 0 // Index of current position.
	// Is there an insertion operation before the last equality.
	preIns := false
	// Is there a deletion operation before the last equality.
	preDel := false
	// Is there an insertion operation after the last equality.
	postIns := false
	// Is there a deletion operation after the last equality.
	postDel := false
	for pointer < len(diffs) {
		if diffs[pointer].Type == DiffEqual { // Equality found.
			if len(diffs[pointer].Text) < dmp.DiffEditCost &&
				(postIns || postDel) {
				// Candidate found.
				equalities = &equality{
					data: pointer,
					next: equalities,
				}
				preIns = postIns
				preDel = postDel
				lastequality = diffs[pointer].Text
			} else {
				// Not a candidate, and can never become one.
				equalities = nil
				lastequality = ""
			}
			postIns = false
			postDel = false
		} else { // An insertion or deletion.
			if diffs[pointer].Type == DiffDelete {
				postDel = true
			} else {
				postIns = true
			}
			/*
			 * Five types to be split:
			 * <ins>A</ins><del>B</del>XY<ins>C</ins><del>D</del>
			 * <ins>A</ins>X<ins>C</ins><del>D</del>
			 * <ins>A</ins><del>B</del>X<ins>C</ins>
			 * <ins>A</del>X<ins>C</ins><del>D</del>
			 * <ins>A</ins><del>B</del>X<del>C</del>
			 */
			var sumPres int
			if preIns {
				sumPres++
			}
			if preDel {
				sumPres++
			}
			if postIns {
				sumPres++
			}
			if postDel {
				sumPres++
			}
			if len(lastequality) > 0 &&
				((preIns && preDel && postIns && postDel) ||
					((len(lastequality) < dmp.DiffEditCost/2) && sumPres == 3)) {

				insPoint := equalities.data

				// Duplicate record.
				diffs = append(diffs[:insPoint],
					append([]Diff{Diff{DiffDelete, lastequality}}, diffs[insPoint:]...)...)

				// Change second copy to insert.
				diffs[insPoint+1].Type = DiffInsert
				// Throw away the equality we just deleted.
				equalities = equalities.next
				lastequality = ""

				if preIns && preDel {
					// No changes made which could affect previous entry, keep going.
					postIns = true
					postDel = true
					equalities = nil
				} else {
					if equalities != nil {
						equalities = equalities.next
					}
					if equalities != nil {
						pointer = equalities.data
					} else {
						pointer = -1
					}
					postIns = false
					postDel = false
				}
				changes = true
			}
		}
		pointer++
	}

	if changes {
		diffs = dmp.DiffCleanupMerge(diffs)
	}

	return diffs
}

// DiffCleanupMerge reorders and merges like edit sections.  Merge equalities.
// Any edit section can move as long as it doesn't cross an equality.
func (dmp *DiffMatchPatch) DiffCleanupMerge(diffs []Diff) []Diff {
	// Add a dummy entry at the end.
	diffs = append(diffs, Diff{DiffEqual, ""})
	pointer := 0
	countDelete := 0
	countInsert := 0
	commonlength := 0
	textDelete := []rune(nil)
	textInsert := []rune(nil)

	for pointer < len(diffs) {
		switch diffs[pointer].Type {
		case DiffInsert:
			countInsert++
			textInsert = append(textInsert, []rune(diffs[pointer].Text)...)
			pointer++
			break
		case DiffDelete:
			countDelete++
			textDelete = append(textDelete, []rune(diffs[pointer].Text)...)
			pointer++
			break
		case DiffEqual:
			// Upon reaching an equality, check for prior redundancies.
			if countDelete+countInsert > 1 {
				if countDelete != 0 && countInsert != 0 {
					// Factor out any common prefixies.
					commonlength = commonPrefixLength(textInsert, textDelete)
					if commonlength != 0 {
						x := pointer - countDelete - countInsert
						if x > 0 && diffs[x-1].Type == DiffEqual {
							diffs[x-1].Text += string(textInsert[:commonlength])
						} else {
							diffs = append([]Diff{Diff{DiffEqual, string(textInsert[:commonlength])}}, diffs...)
							pointer++
						}
						textInsert = textInsert[commonlength:]
						textDelete = textDelete[commonlength:]
					}
					// Factor out any common suffixies.
					commonlength = commonSuffixLength(textInsert, textDelete)
					if commonlength != 0 {
						insertIndex := len(textInsert) - commonlength
						deleteIndex := len(textDelete) - commonlength
						diffs[pointer].Text = string(textInsert[insertIndex:]) + diffs[pointer].Text
						textInsert = textInsert[:insertIndex]
						textDelete = textDelete[:deleteIndex]
					}
				}
				// Delete the offending records and add the merged ones.
				if countDelete == 0 {
					diffs = splice(diffs, pointer-countInsert,
						countDelete+countInsert,
						Diff{DiffInsert, string(textInsert)})
				} else if countInsert == 0 {
					diffs = splice(diffs, pointer-countDelete,
						countDelete+countInsert,
						Diff{DiffDelete, string(textDelete)})
				} else {
					diffs = splice(diffs, pointer-countDelete-countInsert,
						countDelete+countInsert,
						Diff{DiffDelete, string(textDelete)},
						Diff{DiffInsert, string(textInsert)})
				}

				pointer = pointer - countDelete - countInsert + 1
				if countDelete != 0 {
					pointer++
				}
				if countInsert != 0 {
					pointer++
				}
			} else if pointer != 0 && diffs[pointer-1].Type == DiffEqual {
				// Merge this equality with the previous one.
				diffs[pointer-1].Text += diffs[pointer].Text
				diffs = append(diffs[:pointer], diffs[pointer+1:]...)
			} else {
				pointer++
			}
			countInsert = 0
			countDelete = 0
			textDelete = nil
			textInsert = nil
			break
		}
	}

	if len(diffs[len(diffs)-1].Text) == 0 {
		diffs = diffs[0 : len(diffs)-1] // Remove the dummy entry at the end.
	}

	// Second pass: look for single edits surrounded on both sides by
	// equalities which can be shifted sideways to eliminate an equality.
	// e.g: A<ins>BA</ins>C -> <ins>AB</ins>AC
	changes := false
	pointer = 1
	// Intentionally ignore the first and last element (don't need checking).
	for pointer < (len(diffs) - 1) {
		if diffs[pointer-1].Type == DiffEqual &&
			diffs[pointer+1].Type == DiffEqual {
			// This is a single edit surrounded by equalities.
			if strings.HasSuffix(diffs[pointer].Text, diffs[pointer-1].Text) {
				// Shift the edit over the previous equality.
				diffs[pointer].Text = diffs[pointer-1].Text +
					diffs[pointer].Text[:len(diffs[pointer].Text)-len(diffs[pointer-1].Text)]
				diffs[pointer+1].Text = diffs[pointer-1].Text + diffs[pointer+1].Text
				diffs = splice(diffs, pointer-1, 1)
				changes = true
			} else if strings.HasPrefix(diffs[pointer].Text, diffs[pointer+1].Text) {
				// Shift the edit over the next equality.
				diffs[pointer-1].Text += diffs[pointer+1].Text
				diffs[pointer].Text =
					diffs[pointer].Text[len(diffs[pointer+1].Text):] + diffs[pointer+1].Text
				diffs = splice(diffs, pointer+1, 1)
				changes = true
			}
		}
		pointer++
	}

	// If shifts were made, the diff needs reordering and another shift sweep.
	if changes {
		diffs = dmp.DiffCleanupMerge(diffs)
	}

	return diffs
}

// DiffXIndex returns the equivalent location in s2.
// loc is a location in text1, comAdde and return the equivalent location in
// text2.
// e.g. "The cat" vs "The big cat", 1->1, 5->8
func (dmp *DiffMatchPatch) DiffXIndex(diffs []Diff, loc int) int {
	chars1 := 0
	chars2 := 0
	lastChars1 := 0
	lastChars2 := 0
	lastDiff := Diff{}
	for i := 0; i < len(diffs); i++ {
		aDiff := diffs[i]
		if aDiff.Type != DiffInsert {
			// Equality or deletion.
			chars1 += len(aDiff.Text)
		}
		if aDiff.Type != DiffDelete {
			// Equality or insertion.
			chars2 += len(aDiff.Text)
		}
		if chars1 > loc {
			// Overshot the location.
			lastDiff = aDiff
			break
		}
		lastChars1 = chars1
		lastChars2 = chars2
	}
	if lastDiff.Type == DiffDelete {
		// The location was deleted.
		return lastChars2
	}
	// Add the remaining character length.
	return lastChars2 + (loc - lastChars1)
}

// DiffPrettyHtml converts a []Diff into a pretty HTML report.
// It is intended as an example from which to write one's own
// display functions.
func (dmp *DiffMatchPatch) DiffPrettyHtml(diffs []Diff) string {
	var buff bytes.Buffer
	for _, diff := range diffs {
		text := strings.Replace(html.EscapeString(diff.Text), "\n", "&para;<br>", -1)
		switch diff.Type {
		case DiffInsert:
			_, _ = buff.WriteString("<ins style=\"background:#e6ffe6;\">")
			_, _ = buff.WriteString(text)
			_, _ = buff.WriteString("</ins>")
		case DiffDelete:
			_, _ = buff.WriteString("<del style=\"background:#ffe6e6;\">")
			_, _ = buff.WriteString(text)
			_, _ = buff.WriteString("</del>")
		case DiffEqual:
			_, _ = buff.WriteString("<span>")
			_, _ = buff.WriteString(text)
			_, _ = buff.WriteString("</span>")
		}
	}
	return buff.String()
}

// DiffPrettyText converts a []Diff into a colored text report.
func (dmp *DiffMatchPatch) DiffPrettyText(diffs []Diff) string {
	var buff bytes.Buffer
	for _, diff := range diffs {
		text := diff.Text

		switch diff.Type {
		case DiffInsert:
			_, _ = buff.WriteString("\x1b[32m")
			_, _ = buff.WriteString(text)
			_, _ = buff.WriteString("\x1b[0m")
		case DiffDelete:
			_, _ = buff.WriteString("\x1b[31m")
			_, _ = buff.WriteString(text)
			_, _ = buff.WriteString("\x1b[0m")
		case DiffEqual:
			_, _ = buff.WriteString(text)
		}
	}

	return buff.String()
}

// DiffText1 computes and returns the source text (all equalities and deletions).
func (dmp *DiffMatchPatch) DiffText1(diffs []Diff) string {
	//StringBuilder text = new StringBuilder()
	var text bytes.Buffer

	for _, aDiff := range diffs {
		if aDiff.Type != DiffInsert {
			_, _ = text.WriteString(aDiff.Text)
		}
	}
	return text.String()
}

// DiffText2 computes and returns the destination text (all equalities and insertions).
func (dmp *DiffMatchPatch) DiffText2(diffs []Diff) string {
	var text bytes.Buffer

	for _, aDiff := range diffs {
		if aDiff.Type != DiffDelete {
			_, _ = text.WriteString(aDiff.Text)
		}
	}
	return text.String()
}

// DiffLevenshtein computes the Levenshtein distance; the number of inserted, deleted or
// substituted characters.
func (dmp *DiffMatchPatch) DiffLevenshtein(diffs []Diff) int {
	levenshtein := 0
	insertions := 0
	deletions := 0

	for _, aDiff := range diffs {
		switch aDiff.Type {
		case DiffInsert:
			insertions += len(aDiff.Text)
		case DiffDelete:
			deletions += len(aDiff.Text)
		case DiffEqual:
			// A deletion and an insertion is one substitution.
			levenshtein += max(insertions, deletions)
			insertions = 0
			deletions = 0
		}
	}

	levenshtein += max(insertions, deletions)
	return levenshtein
}

// DiffToDelta crushes the diff into an encoded string which describes the operations
// required to transform text1 into text2.
// E.g. =3\t-2\t+ing  -> Keep 3 chars, delete 2 chars, insert 'ing'.
// Operations are tab-separated.  Inserted text is escaped using %xx
// notation.
func (dmp *DiffMatchPatch) DiffToDelta(diffs []Diff) string {
	var text bytes.Buffer
	for _, aDiff := range diffs {
		switch aDiff.Type {
		case DiffInsert:
			_, _ = text.WriteString("+")
			_, _ = text.WriteString(strings.Replace(url.QueryEscape(aDiff.Text), "+", " ", -1))
			_, _ = text.WriteString("\t")
			break
		case DiffDelete:
			_, _ = text.WriteString("-")
			_, _ = text.WriteString(strconv.Itoa(utf8.RuneCountInString(aDiff.Text)))
			_, _ = text.WriteString("\t")
			break
		case DiffEqual:
			_, _ = text.WriteString("=")
			_, _ = text.WriteString(strconv.Itoa(utf8.RuneCountInString(aDiff.Text)))
			_, _ = text.WriteString("\t")
			break
		}
	}
	delta := text.String()
	if len(delta) != 0 {
		// Strip off trailing tab character.
		delta = delta[0 : utf8.RuneCountInString(delta)-1]
		delta = unescaper.Replace(delta)
	}
	return delta
}

// DiffFromDelta given the original text1, and an encoded string which describes the
// operations required to transform text1 into text2, comAdde the full diff.
func (dmp *DiffMatchPatch) DiffFromDelta(text1, delta string) (diffs []Diff, err error) {
	diffs = []Diff{}

	defer func() {
		if r := recover(); r != nil {
			err = r.(error)
		}
	}()

	pointer := 0 // Cursor in text1
	tokens := strings.Split(delta, "\t")

	for _, token := range tokens {
		if len(token) == 0 {
			// Blank tokens are ok (from a trailing \t).
			continue
		}

		// Each token begins with a one character parameter which specifies the
		// operation of this token (delete, insert, equality).
		param := token[1:]

		switch op := token[0]; op {
		case '+':
			// decode would Diff all "+" to " "
			param = strings.Replace(param, "+", "%2b", -1)
			param, err = url.QueryUnescape(param)
			if err != nil {
				return nil, err
			}
			if !utf8.ValidString(param) {
				return nil, fmt.Errorf("invalid UTF-8 token: %q", param)
			}
			diffs = append(diffs, Diff{DiffInsert, param})
		case '=', '-':
			n, err := strconv.ParseInt(param, 10, 0)
			if err != nil {
				return diffs, err
			} else if n < 0 {
				return diffs, errors.New("Negative number in DiffFromDelta: " + param)
			}

			// remember that string slicing is by byte - we want by rune here.
			text := string([]rune(text1)[pointer : pointer+int(n)])
			pointer += int(n)

			if op == '=' {
				diffs = append(diffs, Diff{DiffEqual, text})
			} else {
				diffs = append(diffs, Diff{DiffDelete, text})
			}
		default:
			// Anything else is an error.
			return diffs, errors.New("Invalid diff operation in DiffFromDelta: " + string(token[0]))
		}
	}

	if pointer != len([]rune(text1)) {
		return diffs, fmt.Errorf("Delta length (%v) smaller than source text length (%v)", pointer, len(text1))
	}
	return diffs, err
}