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// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package myers implements the Myers diff algorithm.
package myers
import (
"strings"
"golang.org/x/tools/internal/lsp/diff"
"golang.org/x/tools/internal/span"
)
// Sources:
// https://blog.jcoglan.com/2017/02/17/the-myers-diff-algorithm-part-3/
// https://www.codeproject.com/Articles/42279/%2FArticles%2F42279%2FInvestigating-Myers-diff-algorithm-Part-1-of-2
func ComputeEdits(uri span.URI, before, after string) []diff.TextEdit {
ops := operations(splitLines(before), splitLines(after))
edits := make([]diff.TextEdit, 0, len(ops))
for _, op := range ops {
s := span.New(uri, span.NewPoint(op.I1+1, 1, 0), span.NewPoint(op.I2+1, 1, 0))
switch op.Kind {
case diff.Delete:
// Delete: unformatted[i1:i2] is deleted.
edits = append(edits, diff.TextEdit{Span: s})
case diff.Insert:
// Insert: formatted[j1:j2] is inserted at unformatted[i1:i1].
if content := strings.Join(op.Content, ""); content != "" {
edits = append(edits, diff.TextEdit{Span: s, NewText: content})
}
}
}
return edits
}
type operation struct {
Kind diff.OpKind
Content []string // content from b
I1, I2 int // indices of the line in a
J1 int // indices of the line in b, J2 implied by len(Content)
}
// operations returns the list of operations to convert a into b, consolidating
// operations for multiple lines and not including equal lines.
func operations(a, b []string) []*operation {
if len(a) == 0 && len(b) == 0 {
return nil
}
trace, offset := shortestEditSequence(a, b)
snakes := backtrack(trace, len(a), len(b), offset)
M, N := len(a), len(b)
var i int
solution := make([]*operation, len(a)+len(b))
add := func(op *operation, i2, j2 int) {
if op == nil {
return
}
op.I2 = i2
if op.Kind == diff.Insert {
op.Content = b[op.J1:j2]
}
solution[i] = op
i++
}
x, y := 0, 0
for _, snake := range snakes {
if len(snake) < 2 {
continue
}
var op *operation
// delete (horizontal)
for snake[0]-snake[1] > x-y {
if op == nil {
op = &operation{
Kind: diff.Delete,
I1: x,
J1: y,
}
}
x++
if x == M {
break
}
}
add(op, x, y)
op = nil
// insert (vertical)
for snake[0]-snake[1] < x-y {
if op == nil {
op = &operation{
Kind: diff.Insert,
I1: x,
J1: y,
}
}
y++
}
add(op, x, y)
op = nil
// equal (diagonal)
for x < snake[0] {
x++
y++
}
if x >= M && y >= N {
break
}
}
return solution[:i]
}
// backtrack uses the trace for the edit sequence computation and returns the
// "snakes" that make up the solution. A "snake" is a single deletion or
// insertion followed by zero or diagonals.
func backtrack(trace [][]int, x, y, offset int) [][]int {
snakes := make([][]int, len(trace))
d := len(trace) - 1
for ; x > 0 && y > 0 && d > 0; d-- {
V := trace[d]
if len(V) == 0 {
continue
}
snakes[d] = []int{x, y}
k := x - y
var kPrev int
if k == -d || (k != d && V[k-1+offset] < V[k+1+offset]) {
kPrev = k + 1
} else {
kPrev = k - 1
}
x = V[kPrev+offset]
y = x - kPrev
}
if x < 0 || y < 0 {
return snakes
}
snakes[d] = []int{x, y}
return snakes
}
// shortestEditSequence returns the shortest edit sequence that converts a into b.
func shortestEditSequence(a, b []string) ([][]int, int) {
M, N := len(a), len(b)
V := make([]int, 2*(N+M)+1)
offset := N + M
trace := make([][]int, N+M+1)
// Iterate through the maximum possible length of the SES (N+M).
for d := 0; d <= N+M; d++ {
copyV := make([]int, len(V))
// k lines are represented by the equation y = x - k. We move in
// increments of 2 because end points for even d are on even k lines.
for k := -d; k <= d; k += 2 {
// At each point, we either go down or to the right. We go down if
// k == -d, and we go to the right if k == d. We also prioritize
// the maximum x value, because we prefer deletions to insertions.
var x int
if k == -d || (k != d && V[k-1+offset] < V[k+1+offset]) {
x = V[k+1+offset] // down
} else {
x = V[k-1+offset] + 1 // right
}
y := x - k
// Diagonal moves while we have equal contents.
for x < M && y < N && a[x] == b[y] {
x++
y++
}
V[k+offset] = x
// Return if we've exceeded the maximum values.
if x == M && y == N {
// Makes sure to save the state of the array before returning.
copy(copyV, V)
trace[d] = copyV
return trace, offset
}
}
// Save the state of the array.
copy(copyV, V)
trace[d] = copyV
}
return nil, 0
}
func splitLines(text string) []string {
lines := strings.SplitAfter(text, "\n")
if lines[len(lines)-1] == "" {
lines = lines[:len(lines)-1]
}
return lines
}
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