| // Copyright 2022 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 diff |
| |
| import ( |
| "bytes" |
| "fmt" |
| "sort" |
| "strings" |
| ) |
| |
| // A pair is a pair of values tracked for both the x and y side of a diff. |
| // It is typically a pair of line indexes. |
| type pair struct{ x, y int } |
| |
| // Diff returns an anchored diff of the two texts old and new |
| // in the “unified diff” format. If old and new are identical, |
| // Diff returns a nil slice (no output). |
| // |
| // Unix diff implementations typically look for a diff with |
| // the smallest number of lines inserted and removed, |
| // which can in the worst case take time quadratic in the |
| // number of lines in the texts. As a result, many implementations |
| // either can be made to run for a long time or cut off the search |
| // after a predetermined amount of work. |
| // |
| // In contrast, this implementation looks for a diff with the |
| // smallest number of “unique” lines inserted and removed, |
| // where unique means a line that appears just once in both old and new. |
| // We call this an “anchored diff” because the unique lines anchor |
| // the chosen matching regions. An anchored diff is usually clearer |
| // than a standard diff, because the algorithm does not try to |
| // reuse unrelated blank lines or closing braces. |
| // The algorithm also guarantees to run in O(n log n) time |
| // instead of the standard O(n²) time. |
| // |
| // Some systems call this approach a “patience diff,” named for |
| // the “patience sorting” algorithm, itself named for a solitaire card game. |
| // We avoid that name for two reasons. First, the name has been used |
| // for a few different variants of the algorithm, so it is imprecise. |
| // Second, the name is frequently interpreted as meaning that you have |
| // to wait longer (to be patient) for the diff, meaning that it is a slower algorithm, |
| // when in fact the algorithm is faster than the standard one. |
| func Diff(oldName string, old []byte, newName string, new []byte) []byte { |
| if bytes.Equal(old, new) { |
| return nil |
| } |
| x := lines(old) |
| y := lines(new) |
| |
| // Print diff header. |
| var out bytes.Buffer |
| fmt.Fprintf(&out, "diff %s %s\n", oldName, newName) |
| fmt.Fprintf(&out, "--- %s\n", oldName) |
| fmt.Fprintf(&out, "+++ %s\n", newName) |
| |
| // Loop over matches to consider, |
| // expanding each match to include surrounding lines, |
| // and then printing diff chunks. |
| // To avoid setup/teardown cases outside the loop, |
| // tgs returns a leading {0,0} and trailing {len(x), len(y)} pair |
| // in the sequence of matches. |
| var ( |
| done pair // printed up to x[:done.x] and y[:done.y] |
| chunk pair // start lines of current chunk |
| count pair // number of lines from each side in current chunk |
| ctext []string // lines for current chunk |
| ) |
| for _, m := range tgs(x, y) { |
| if m.x < done.x { |
| // Already handled scanning forward from earlier match. |
| continue |
| } |
| |
| // Expand matching lines as far as possible, |
| // establishing that x[start.x:end.x] == y[start.y:end.y]. |
| // Note that on the first (or last) iteration we may (or definitely do) |
| // have an empty match: start.x==end.x and start.y==end.y. |
| start := m |
| for start.x > done.x && start.y > done.y && x[start.x-1] == y[start.y-1] { |
| start.x-- |
| start.y-- |
| } |
| end := m |
| for end.x < len(x) && end.y < len(y) && x[end.x] == y[end.y] { |
| end.x++ |
| end.y++ |
| } |
| |
| // Emit the mismatched lines before start into this chunk. |
| // (No effect on first sentinel iteration, when start = {0,0}.) |
| for _, s := range x[done.x:start.x] { |
| ctext = append(ctext, "-"+s) |
| count.x++ |
| } |
| for _, s := range y[done.y:start.y] { |
| ctext = append(ctext, "+"+s) |
| count.y++ |
| } |
| |
| // If we're not at EOF and have too few common lines, |
| // the chunk includes all the common lines and continues. |
| const C = 3 // number of context lines |
| if (end.x < len(x) || end.y < len(y)) && |
| (end.x-start.x < C || (len(ctext) > 0 && end.x-start.x < 2*C)) { |
| for _, s := range x[start.x:end.x] { |
| ctext = append(ctext, " "+s) |
| count.x++ |
| count.y++ |
| } |
| done = end |
| continue |
| } |
| |
| // End chunk with common lines for context. |
| if len(ctext) > 0 { |
| n := end.x - start.x |
| if n > C { |
| n = C |
| } |
| for _, s := range x[start.x : start.x+n] { |
| ctext = append(ctext, " "+s) |
| count.x++ |
| count.y++ |
| } |
| done = pair{start.x + n, start.y + n} |
| |
| // Format and emit chunk. |
| // Convert line numbers to 1-indexed. |
| // Special case: empty file shows up as 0,0 not 1,0. |
| if count.x > 0 { |
| chunk.x++ |
| } |
| if count.y > 0 { |
| chunk.y++ |
| } |
| fmt.Fprintf(&out, "@@ -%d,%d +%d,%d @@\n", chunk.x, count.x, chunk.y, count.y) |
| for _, s := range ctext { |
| out.WriteString(s) |
| } |
| count.x = 0 |
| count.y = 0 |
| ctext = ctext[:0] |
| } |
| |
| // If we reached EOF, we're done. |
| if end.x >= len(x) && end.y >= len(y) { |
| break |
| } |
| |
| // Otherwise start a new chunk. |
| chunk = pair{end.x - C, end.y - C} |
| for _, s := range x[chunk.x:end.x] { |
| ctext = append(ctext, " "+s) |
| count.x++ |
| count.y++ |
| } |
| done = end |
| } |
| |
| return out.Bytes() |
| } |
| |
| // lines returns the lines in the file x, including newlines. |
| // If the file does not end in a newline, one is supplied |
| // along with a warning about the missing newline. |
| func lines(x []byte) []string { |
| l := strings.SplitAfter(string(x), "\n") |
| if l[len(l)-1] == "" { |
| l = l[:len(l)-1] |
| } else { |
| // Treat last line as having a message about the missing newline attached, |
| // using the same text as BSD/GNU diff (including the leading backslash). |
| l[len(l)-1] += "\n\\ No newline at end of file\n" |
| } |
| return l |
| } |
| |
| // tgs returns the pairs of indexes of the longest common subsequence |
| // of unique lines in x and y, where a unique line is one that appears |
| // once in x and once in y. |
| // |
| // The longest common subsequence algorithm is as described in |
| // Thomas G. Szymanski, “A Special Case of the Maximal Common |
| // Subsequence Problem,” Princeton TR #170 (January 1975), |
| // available at https://research.swtch.com/tgs170.pdf. |
| func tgs(x, y []string) []pair { |
| // Count the number of times each string appears in a and b. |
| // We only care about 0, 1, many, counted as 0, -1, -2 |
| // for the x side and 0, -4, -8 for the y side. |
| // Using negative numbers now lets us distinguish positive line numbers later. |
| m := make(map[string]int) |
| for _, s := range x { |
| if c := m[s]; c > -2 { |
| m[s] = c - 1 |
| } |
| } |
| for _, s := range y { |
| if c := m[s]; c > -8 { |
| m[s] = c - 4 |
| } |
| } |
| |
| // Now unique strings can be identified by m[s] = -1+-4. |
| // |
| // Gather the indexes of those strings in x and y, building: |
| // xi[i] = increasing indexes of unique strings in x. |
| // yi[i] = increasing indexes of unique strings in y. |
| // inv[i] = index j such that x[xi[i]] = y[yi[j]]. |
| var xi, yi, inv []int |
| for i, s := range y { |
| if m[s] == -1+-4 { |
| m[s] = len(yi) |
| yi = append(yi, i) |
| } |
| } |
| for i, s := range x { |
| if j, ok := m[s]; ok && j >= 0 { |
| xi = append(xi, i) |
| inv = append(inv, j) |
| } |
| } |
| |
| // Apply Algorithm A from Szymanski's paper. |
| // In those terms, A = J = inv and B = [0, n). |
| // We add sentinel pairs {0,0}, and {len(x),len(y)} |
| // to the returned sequence, to help the processing loop. |
| J := inv |
| n := len(xi) |
| T := make([]int, n) |
| L := make([]int, n) |
| for i := range T { |
| T[i] = n + 1 |
| } |
| for i := 0; i < n; i++ { |
| k := sort.Search(n, func(k int) bool { |
| return T[k] >= J[i] |
| }) |
| T[k] = J[i] |
| L[i] = k + 1 |
| } |
| k := 0 |
| for _, v := range L { |
| if k < v { |
| k = v |
| } |
| } |
| seq := make([]pair, 2+k) |
| seq[1+k] = pair{len(x), len(y)} // sentinel at end |
| lastj := n |
| for i := n - 1; i >= 0; i-- { |
| if L[i] == k && J[i] < lastj { |
| seq[k] = pair{xi[i], yi[J[i]]} |
| k-- |
| } |
| } |
| seq[0] = pair{0, 0} // sentinel at start |
| return seq |
| } |
