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// Copyright 2012 Google, Inc. All rights reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the LICENSE file in the root of the source
// tree.
// Package bytediff provides a simple diff utility for looking at differences in byte
// slices. It's slow, clunky, and not particularly good by any measure, but
// it does provide very useful visualizations for diffs between small byte
// slices.
//
// Our diff algorithm uses a dynamic programming implementation of longest common
// substring to find matching parts of slices, then recursively calls itself on
// the prefix/suffix of that matching part for each packet. This is a Bad Idea
// (tm) for normal (especially large) input, but for packets where large portions
// repeat frequently and we expect minor changes between results, it's actually
// quite useful.
package bytediff
import (
"bytes"
"fmt"
)
// OutputFormat tells a Differences.String call how to format the set of
// differences into a human-readable string. Its internals are currently
// unexported because we may want to change them drastically in the future. For
// the moment, please just use one of the provided OutputFormats that comes with
// this library.
type OutputFormat struct {
start, finish, add, remove, change, reset string
}
var (
// BashOutput uses bash escape sequences to color output.
BashOutput = &OutputFormat{
reset: "\033[0m",
remove: "\033[32m",
add: "\033[31m",
change: "\033[33m",
}
// HTMLOutput uses a <pre> to wrap output, and <span>s to color it.
// HTMLOutput is pretty experimental, so use at your own risk ;)
HTMLOutput = &OutputFormat{
start: "<pre>",
finish: "</pre>",
reset: "</span>",
remove: "<span style='color:red'>",
add: "<span style='color:green'>",
change: "<span style='color:yellow'>",
}
)
// longestCommonSubstring uses a O(MN) dynamic programming approach to find the
// longest common substring in a set of slices. It returns the index in each
// slice at which the substring begins, plus the length of the commonality.
func longestCommonSubstring(strA, strB []byte) (indexA, indexB, length int) {
lenA, lenB := len(strA), len(strB)
if lenA == 0 || lenB == 0 {
return 0, 0, 0
}
arr := make([][]int, lenA)
for i := 0; i < lenA; i++ {
arr[i] = make([]int, lenB)
}
var maxLength int
var maxA, maxB int
for a := 0; a < lenA; a++ {
for b := 0; b < lenB; b++ {
if strA[a] == strB[b] {
length := 1
if a > 0 && b > 0 {
length = arr[a-1][b-1] + 1
}
arr[a][b] = length
if length > maxLength {
maxLength = length
maxA = a
maxB = b
}
}
}
}
a, b := maxA, maxB
for a >= 0 && b >= 0 && strA[a] == strB[b] {
indexA = a
indexB = b
a--
b--
length++
}
return
}
func intMax(a, b int) int {
if a > b {
return a
}
return b
}
// Difference represents a single part of the data being diffed, containing
// information about both the original and new values.
// From and To are the sets of bytes in the original and the new byte slice.
// !Replace implies From == To (no change)
// len(To) == 0 implies From is being deleted
// len(From) == 0 implies To is being inserted
// else implies From is being replaced by To
type Difference struct {
Replace bool
From, To []byte
}
// color returns the bash color for a given difference.
func (c *OutputFormat) color(d Difference) string {
switch {
case !d.Replace:
return ""
case len(d.From) == 0:
return c.remove
case len(d.To) == 0:
return c.add
default:
return c.change
}
}
// Diff diffs strA and strB, returning a list of differences which
// can be used to construct either the original or new string.
//
// Diff is optimized for comparing VERY SHORT slices. It's meant for comparing
// things like packets off the wire, not large files or the like.
// As such, its runtime can be catastrophic if large inputs are passed in.
// You've been warned.
func Diff(strA, strB []byte) Differences {
if len(strA) == 0 && len(strB) == 0 {
return nil
}
ia, ib, l := longestCommonSubstring(strA, strB)
if l == 0 {
return Differences{
Difference{true, strA, strB},
}
}
beforeA, match, afterA := strA[:ia], strA[ia:ia+l], strA[ia+l:]
beforeB, afterB := strB[:ib], strB[ib+l:]
var diffs Differences
diffs = append(diffs, Diff(beforeA, beforeB)...)
diffs = append(diffs, Difference{false, match, match})
diffs = append(diffs, Diff(afterA, afterB)...)
return diffs
}
// Differences is a set of differences for a given diff'd pair of byte slices.
type Differences []Difference
// String outputs a previously diff'd set of strings, showing differences
// between them, highlighted by colors.
//
// The output format of this function is NOT guaranteed consistent, and may be
// changed at any time by the library authors. It's meant solely for human
// consumption.
func (c *OutputFormat) String(diffs Differences) string {
var buf bytes.Buffer
count := 0
fmt.Fprintf(&buf, "%s", c.start)
fmt.Fprintf(&buf, "00000000 ")
for i := 0; i < len(diffs); i++ {
diff := diffs[i]
color := c.color(diff)
reset := ""
if color != "" {
reset = c.reset
}
fmt.Fprint(&buf, color)
for _, b := range diff.From {
fmt.Fprintf(&buf, " %02x", b)
count++
switch count % 16 {
case 0:
fmt.Fprintf(&buf, "%v\n%08x%v ", reset, count, color)
case 8:
fmt.Fprintf(&buf, " ")
}
}
fmt.Fprint(&buf, reset)
}
fmt.Fprintf(&buf, "\n\n00000000 ")
count = 0
for i := 0; i < len(diffs); i++ {
diff := diffs[i]
str := diff.From
if diff.Replace {
str = diff.To
}
color := c.color(diff)
reset := ""
if color != "" {
reset = c.reset
}
fmt.Fprint(&buf, color)
for _, b := range str {
fmt.Fprintf(&buf, " %02x", b)
count++
switch count % 16 {
case 0:
fmt.Fprintf(&buf, "%v\n%08x%v ", reset, count, color)
case 8:
fmt.Fprintf(&buf, " ")
}
}
fmt.Fprint(&buf, reset)
}
fmt.Fprint(&buf, "\n")
fmt.Fprintf(&buf, "%s", c.finish)
return string(buf.Bytes())
}
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