1 // Copyright 2022 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 package diff 6 7 import ( 8 "bytes" 9 "fmt" 10 "sort" 11 "strings" 12 ) 13 14 // A pair is a pair of values tracked for both the x and y side of a diff. 15 // It is typically a pair of line indexes. 16 type pair struct{ x, y int } 17 18 // Diff returns an anchored diff of the two texts old and new 19 // in the “unified diff” format. If old and new are identical, 20 // Diff returns a nil slice (no output). 21 // 22 // Unix diff implementations typically look for a diff with 23 // the smallest number of lines inserted and removed, 24 // which can in the worst case take time quadratic in the 25 // number of lines in the texts. As a result, many implementations 26 // either can be made to run for a long time or cut off the search 27 // after a predetermined amount of work. 28 // 29 // In contrast, this implementation looks for a diff with the 30 // smallest number of “unique” lines inserted and removed, 31 // where unique means a line that appears just once in both old and new. 32 // We call this an “anchored diff” because the unique lines anchor 33 // the chosen matching regions. An anchored diff is usually clearer 34 // than a standard diff, because the algorithm does not try to 35 // reuse unrelated blank lines or closing braces. 36 // The algorithm also guarantees to run in O(n log n) time 37 // instead of the standard O(n²) time. 38 // 39 // Some systems call this approach a “patience diff,” named for 40 // the “patience sorting” algorithm, itself named for a solitaire card game. 41 // We avoid that name for two reasons. First, the name has been used 42 // for a few different variants of the algorithm, so it is imprecise. 43 // Second, the name is frequently interpreted as meaning that you have 44 // to wait longer (to be patient) for the diff, meaning that it is a slower algorithm, 45 // when in fact the algorithm is faster than the standard one. 46 func Diff(oldName string, old []byte, newName string, new []byte) []byte { 47 if bytes.Equal(old, new) { 48 return nil 49 } 50 x := lines(old) 51 y := lines(new) 52 53 // Print diff header. 54 var out bytes.Buffer 55 fmt.Fprintf(&out, "diff %s %s\n", oldName, newName) 56 fmt.Fprintf(&out, "--- %s\n", oldName) 57 fmt.Fprintf(&out, "+++ %s\n", newName) 58 59 // Loop over matches to consider, 60 // expanding each match to include surrounding lines, 61 // and then printing diff chunks. 62 // To avoid setup/teardown cases outside the loop, 63 // tgs returns a leading {0,0} and trailing {len(x), len(y)} pair 64 // in the sequence of matches. 65 var ( 66 done pair // printed up to x[:done.x] and y[:done.y] 67 chunk pair // start lines of current chunk 68 count pair // number of lines from each side in current chunk 69 ctext []string // lines for current chunk 70 ) 71 for _, m := range tgs(x, y) { 72 if m.x < done.x { 73 // Already handled scanning forward from earlier match. 74 continue 75 } 76 77 // Expand matching lines as far as possible, 78 // establishing that x[start.x:end.x] == y[start.y:end.y]. 79 // Note that on the first (or last) iteration we may (or definitely do) 80 // have an empty match: start.x==end.x and start.y==end.y. 81 start := m 82 for start.x > done.x && start.y > done.y && x[start.x-1] == y[start.y-1] { 83 start.x-- 84 start.y-- 85 } 86 end := m 87 for end.x < len(x) && end.y < len(y) && x[end.x] == y[end.y] { 88 end.x++ 89 end.y++ 90 } 91 92 // Emit the mismatched lines before start into this chunk. 93 // (No effect on first sentinel iteration, when start = {0,0}.) 94 for _, s := range x[done.x:start.x] { 95 ctext = append(ctext, "-"+s) 96 count.x++ 97 } 98 for _, s := range y[done.y:start.y] { 99 ctext = append(ctext, "+"+s) 100 count.y++ 101 } 102 103 // If we're not at EOF and have too few common lines, 104 // the chunk includes all the common lines and continues. 105 const C = 3 // number of context lines 106 if (end.x < len(x) || end.y < len(y)) && 107 (end.x-start.x < C || (len(ctext) > 0 && end.x-start.x < 2*C)) { 108 for _, s := range x[start.x:end.x] { 109 ctext = append(ctext, " "+s) 110 count.x++ 111 count.y++ 112 } 113 done = end 114 continue 115 } 116 117 // End chunk with common lines for context. 118 if len(ctext) > 0 { 119 n := end.x - start.x 120 if n > C { 121 n = C 122 } 123 for _, s := range x[start.x : start.x+n] { 124 ctext = append(ctext, " "+s) 125 count.x++ 126 count.y++ 127 } 128 done = pair{start.x + n, start.y + n} 129 130 // Format and emit chunk. 131 // Convert line numbers to 1-indexed. 132 // Special case: empty file shows up as 0,0 not 1,0. 133 if count.x > 0 { 134 chunk.x++ 135 } 136 if count.y > 0 { 137 chunk.y++ 138 } 139 fmt.Fprintf(&out, "@@ -%d,%d +%d,%d @@\n", chunk.x, count.x, chunk.y, count.y) 140 for _, s := range ctext { 141 out.WriteString(s) 142 } 143 count.x = 0 144 count.y = 0 145 ctext = ctext[:0] 146 } 147 148 // If we reached EOF, we're done. 149 if end.x >= len(x) && end.y >= len(y) { 150 break 151 } 152 153 // Otherwise start a new chunk. 154 chunk = pair{end.x - C, end.y - C} 155 for _, s := range x[chunk.x:end.x] { 156 ctext = append(ctext, " "+s) 157 count.x++ 158 count.y++ 159 } 160 done = end 161 } 162 163 return out.Bytes() 164 } 165 166 // lines returns the lines in the file x, including newlines. 167 // If the file does not end in a newline, one is supplied 168 // along with a warning about the missing newline. 169 func lines(x []byte) []string { 170 l := strings.SplitAfter(string(x), "\n") 171 if l[len(l)-1] == "" { 172 l = l[:len(l)-1] 173 } else { 174 // Treat last line as having a message about the missing newline attached, 175 // using the same text as BSD/GNU diff (including the leading backslash). 176 l[len(l)-1] += "\n\\ No newline at end of file\n" 177 } 178 return l 179 } 180 181 // tgs returns the pairs of indexes of the longest common subsequence 182 // of unique lines in x and y, where a unique line is one that appears 183 // once in x and once in y. 184 // 185 // The longest common subsequence algorithm is as described in 186 // Thomas G. Szymanski, “A Special Case of the Maximal Common 187 // Subsequence Problem,” Princeton TR #170 (January 1975), 188 // available at https://research.swtch.com/tgs170.pdf. 189 func tgs(x, y []string) []pair { 190 // Count the number of times each string appears in a and b. 191 // We only care about 0, 1, many, counted as 0, -1, -2 192 // for the x side and 0, -4, -8 for the y side. 193 // Using negative numbers now lets us distinguish positive line numbers later. 194 m := make(map[string]int) 195 for _, s := range x { 196 if c := m[s]; c > -2 { 197 m[s] = c - 1 198 } 199 } 200 for _, s := range y { 201 if c := m[s]; c > -8 { 202 m[s] = c - 4 203 } 204 } 205 206 // Now unique strings can be identified by m[s] = -1+-4. 207 // 208 // Gather the indexes of those strings in x and y, building: 209 // xi[i] = increasing indexes of unique strings in x. 210 // yi[i] = increasing indexes of unique strings in y. 211 // inv[i] = index j such that x[xi[i]] = y[yi[j]]. 212 var xi, yi, inv []int 213 for i, s := range y { 214 if m[s] == -1+-4 { 215 m[s] = len(yi) 216 yi = append(yi, i) 217 } 218 } 219 for i, s := range x { 220 if j, ok := m[s]; ok && j >= 0 { 221 xi = append(xi, i) 222 inv = append(inv, j) 223 } 224 } 225 226 // Apply Algorithm A from Szymanski's paper. 227 // In those terms, A = J = inv and B = [0, n). 228 // We add sentinel pairs {0,0}, and {len(x),len(y)} 229 // to the returned sequence, to help the processing loop. 230 J := inv 231 n := len(xi) 232 T := make([]int, n) 233 L := make([]int, n) 234 for i := range T { 235 T[i] = n + 1 236 } 237 for i := 0; i < n; i++ { 238 k := sort.Search(n, func(k int) bool { 239 return T[k] >= J[i] 240 }) 241 T[k] = J[i] 242 L[i] = k + 1 243 } 244 k := 0 245 for _, v := range L { 246 if k < v { 247 k = v 248 } 249 } 250 seq := make([]pair, 2+k) 251 seq[1+k] = pair{len(x), len(y)} // sentinel at end 252 lastj := n 253 for i := n - 1; i >= 0; i-- { 254 if L[i] == k && J[i] < lastj { 255 seq[k] = pair{xi[i], yi[J[i]]} 256 k-- 257 } 258 } 259 seq[0] = pair{0, 0} // sentinel at start 260 return seq 261 } 262