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// Copyright 2013 Google Inc. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// Package diff implements a linewise diff algorithm.
package diff
import (
"bytes"
"fmt"
"strings"
)
// Chunk represents a piece of the diff. A chunk will not have both added and
// deleted lines. Equal lines are always after any added or deleted lines.
// A Chunk may or may not have any lines in it, especially for the first or last
// chunk in a computation.
type Chunk struct {
Added []string
Deleted []string
Equal []string
}
func (c *Chunk) empty() bool {
return len(c.Added) == 0 && len(c.Deleted) == 0 && len(c.Equal) == 0
}
// Diff returns a string containing a line-by-line unified diff of the linewise
// changes required to make A into B. Each line is prefixed with '+', '-', or
// ' ' to indicate if it should be added, removed, or is correct respectively.
func Diff(A, B string) string {
aLines := strings.Split(A, "\n")
bLines := strings.Split(B, "\n")
chunks := DiffChunks(aLines, bLines)
buf := new(bytes.Buffer)
for _, c := range chunks {
for _, line := range c.Added {
fmt.Fprintf(buf, "+%s\n", line)
}
for _, line := range c.Deleted {
fmt.Fprintf(buf, "-%s\n", line)
}
for _, line := range c.Equal {
fmt.Fprintf(buf, " %s\n", line)
}
}
return strings.TrimRight(buf.String(), "\n")
}
// DiffChunks uses an O(D(N+M)) shortest-edit-script algorithm
// to compute the edits required from A to B and returns the
// edit chunks.
func DiffChunks(a, b []string) []Chunk {
// algorithm: http://www.xmailserver.org/diff2.pdf
// We'll need these quantities a lot.
alen, blen := len(a), len(b) // M, N
// At most, it will require len(a) deletions and len(b) additions
// to transform a into b.
maxPath := alen + blen // MAX
if maxPath == 0 {
// degenerate case: two empty lists are the same
return nil
}
// Store the endpoint of the path for diagonals.
// We store only the a index, because the b index on any diagonal
// (which we know during the loop below) is aidx-diag.
// endpoint[maxPath] represents the 0 diagonal.
//
// Stated differently:
// endpoint[d] contains the aidx of a furthest reaching path in diagonal d
endpoint := make([]int, 2*maxPath+1) // V
saved := make([][]int, 0, 8) // Vs
save := func() {
dup := make([]int, len(endpoint))
copy(dup, endpoint)
saved = append(saved, dup)
}
var editDistance int // D
dLoop:
for editDistance = 0; editDistance <= maxPath; editDistance++ {
// The 0 diag(onal) represents equality of a and b. Each diagonal to
// the left is numbered one lower, to the right is one higher, from
// -alen to +blen. Negative diagonals favor differences from a,
// positive diagonals favor differences from b. The edit distance to a
// diagonal d cannot be shorter than d itself.
//
// The iterations of this loop cover either odds or evens, but not both,
// If odd indices are inputs, even indices are outputs and vice versa.
for diag := -editDistance; diag <= editDistance; diag += 2 { // k
var aidx int // x
switch {
case diag == -editDistance:
// This is a new diagonal; copy from previous iter
aidx = endpoint[maxPath-editDistance+1] + 0
case diag == editDistance:
// This is a new diagonal; copy from previous iter
aidx = endpoint[maxPath+editDistance-1] + 1
case endpoint[maxPath+diag+1] > endpoint[maxPath+diag-1]:
// diagonal d+1 was farther along, so use that
aidx = endpoint[maxPath+diag+1] + 0
default:
// diagonal d-1 was farther (or the same), so use that
aidx = endpoint[maxPath+diag-1] + 1
}
// On diagonal d, we can compute bidx from aidx.
bidx := aidx - diag // y
// See how far we can go on this diagonal before we find a difference.
for aidx < alen && bidx < blen && a[aidx] == b[bidx] {
aidx++
bidx++
}
// Store the end of the current edit chain.
endpoint[maxPath+diag] = aidx
// If we've found the end of both inputs, we're done!
if aidx >= alen && bidx >= blen {
save() // save the final path
break dLoop
}
}
save() // save the current path
}
if editDistance == 0 {
return nil
}
chunks := make([]Chunk, editDistance+1)
x, y := alen, blen
for d := editDistance; d > 0; d-- {
endpoint := saved[d]
diag := x - y
insert := diag == -d || (diag != d && endpoint[maxPath+diag-1] < endpoint[maxPath+diag+1])
x1 := endpoint[maxPath+diag]
var x0, xM, kk int
if insert {
kk = diag + 1
x0 = endpoint[maxPath+kk]
xM = x0
} else {
kk = diag - 1
x0 = endpoint[maxPath+kk]
xM = x0 + 1
}
y0 := x0 - kk
var c Chunk
if insert {
c.Added = b[y0:][:1]
} else {
c.Deleted = a[x0:][:1]
}
if xM < x1 {
c.Equal = a[xM:][:x1-xM]
}
x, y = x0, y0
chunks[d] = c
}
if x > 0 {
chunks[0].Equal = a[:x]
}
if chunks[0].empty() {
chunks = chunks[1:]
}
if len(chunks) == 0 {
return nil
}
return chunks
}
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