includes/diff/DiffEngine.php

   1 <?php
   2 /**
   3  * New version of the difference engine
   4  *
   5  * Copyright © 2008 Guy Van den Broeck <guy@guyvdb.eu>
   6  *
   7  * This program is free software; you can redistribute it and/or modify
   8  * it under the terms of the GNU General Public License as published by
   9  * the Free Software Foundation; either version 2 of the License, or
  10  * (at your option) any later version.
  11  *
  12  * This program is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15  * GNU General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU General Public License
  18  * along with this program; if not, write to the Free Software
  19  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  20  * http://www.gnu.org/copyleft/gpl.html
  21  *
  22  * @file
  23  * @ingroup DifferenceEngine
  24  */
  25
  26 /**
  27  * This diff implementation is mainly lifted from the LCS algorithm of the Eclipse project which
  28  * in turn is based on Myers' "An O(ND) difference algorithm and its variations"
  29  * (http://citeseer.ist.psu.edu/myers86ond.html) with range compression (see Wu et al.'s
  30  * "An O(NP) Sequence Comparison Algorithm").
  31  *
  32  * This implementation supports an upper bound on the execution time.
  33  *
  34  * Some ideas (and a bit of code) are from analyze.c, from GNU
  35  * diffutils-2.7, which can be found at:
  36  *     ftp://gnudist.gnu.org/pub/gnu/diffutils/diffutils-2.7.tar.gz
  37  *
  38  * Complexity: O((M + N)D) worst case time, O(M + N + D^2) expected time, O(M + N) space
  39  *
  40  * @author Guy Van den Broeck, Geoffrey T. Dairiki, Tim Starling
  41  * @ingroup DifferenceEngine
  42  */
  43 class DiffEngine {
  44
  45         // Input variables
  46         private $from;
  47         private $to;
  48         private $m;
  49         private $n;
  50
  51         private $tooLong;
  52         private $powLimit;
  53
  54         // State variables
  55         private $maxDifferences;
  56         private $lcsLengthCorrectedForHeuristic = false;
  57
  58         // Output variables
  59         public $length;
  60         public $removed;
  61         public $added;
  62         public $heuristicUsed;
  63
  64         function __construct( $tooLong = 2000000, $powLimit = 1.45 ) {
  65                 $this->tooLong = $tooLong;
  66                 $this->powLimit = $powLimit;
  67         }
  68
  69         /**
  70          * @param string[] $from_lines
  71          * @param string[] $to_lines
  72          *
  73          * @return DiffOp[]
  74          */
  75         public function diff( $from_lines, $to_lines ) {
  76
  77                 // Diff and store locally
  78                 $this->diffInternal( $from_lines, $to_lines );
  79
  80                 // Merge edits when possible
  81                 $this->shiftBoundaries( $from_lines, $this->removed, $this->added );
  82                 $this->shiftBoundaries( $to_lines, $this->added, $this->removed );
  83
  84                 // Compute the edit operations.
  85                 $n_from = count( $from_lines );
  86                 $n_to = count( $to_lines );
  87
  88                 $edits = [];
  89                 $xi = $yi = 0;
  90                 while ( $xi < $n_from || $yi < $n_to ) {
  91                         assert( $yi < $n_to || $this->removed[$xi] );
  92                         assert( $xi < $n_from || $this->added[$yi] );
  93
  94                         // Skip matching "snake".
  95                         $copy = [];
  96                         while ( $xi < $n_from && $yi < $n_to
  97                                         && !$this->removed[$xi] && !$this->added[$yi]
  98                         ) {
  99                                 $copy[] = $from_lines[$xi++];
 100                                 ++$yi;
 101                         }
 102                         if ( $copy ) {
 103                                 $edits[] = new DiffOpCopy( $copy );
 104                         }
 105
 106                         // Find deletes & adds.
 107                         $delete = [];
 108                         while ( $xi < $n_from && $this->removed[$xi] ) {
 109                                 $delete[] = $from_lines[$xi++];
 110                         }
 111
 112                         $add = [];
 113                         while ( $yi < $n_to && $this->added[$yi] ) {
 114                                 $add[] = $to_lines[$yi++];
 115                         }
 116
 117                         if ( $delete && $add ) {
 118                                 $edits[] = new DiffOpChange( $delete, $add );
 119                         } elseif ( $delete ) {
 120                                 $edits[] = new DiffOpDelete( $delete );
 121                         } elseif ( $add ) {
 122                                 $edits[] = new DiffOpAdd( $add );
 123                         }
 124                 }
 125
 126                 return $edits;
 127         }
 128
 129         /**
 130          * Adjust inserts/deletes of identical lines to join changes
 131          * as much as possible.
 132          *
 133          * We do something when a run of changed lines include a
 134          * line at one end and has an excluded, identical line at the other.
 135          * We are free to choose which identical line is included.
 136          * `compareseq' usually chooses the one at the beginning,
 137          * but usually it is cleaner to consider the following identical line
 138          * to be the "change".
 139          *
 140          * This is extracted verbatim from analyze.c (GNU diffutils-2.7).
 141          */
 142         private function shiftBoundaries( $lines, &$changed, $other_changed ) {
 143                 $i = 0;
 144                 $j = 0;
 145
 146                 assert( count( $lines ) == count( $changed ) );
 147                 $len = count( $lines );
 148                 $other_len = count( $other_changed );
 149
 150                 while ( 1 ) {
 151                         /*
 152                          * Scan forwards to find beginning of another run of changes.
 153                          * Also keep track of the corresponding point in the other file.
 154                          *
 155                          * Throughout this code, $i and $j are adjusted together so that
 156                          * the first $i elements of $changed and the first $j elements
 157                          * of $other_changed both contain the same number of zeros
 158                          * (unchanged lines).
 159                          * Furthermore, $j is always kept so that $j == $other_len or
 160                          * $other_changed[$j] == false.
 161                          */
 162                         while ( $j < $other_len && $other_changed[$j] ) {
 163                                 $j++;
 164                         }
 165
 166                         while ( $i < $len && !$changed[$i] ) {
 167                                 assert( $j < $other_len && ! $other_changed[$j] );
 168                                 $i++;
 169                                 $j++;
 170                                 while ( $j < $other_len && $other_changed[$j] ) {
 171                                         $j++;
 172                                 }
 173                         }
 174
 175                         if ( $i == $len ) {
 176                                 break;
 177                         }
 178
 179                         $start = $i;
 180
 181                         // Find the end of this run of changes.
 182                         while ( ++$i < $len && $changed[$i] ) {
 183                                 continue;
 184                         }
 185
 186                         do {
 187                                 /*
 188                                  * Record the length of this run of changes, so that
 189                                  * we can later determine whether the run has grown.
 190                                  */
 191                                 $runlength = $i - $start;
 192
 193                                 /*
 194                                  * Move the changed region back, so long as the
 195                                  * previous unchanged line matches the last changed one.
 196                                  * This merges with previous changed regions.
 197                                  */
 198                                 while ( $start > 0 && $lines[$start - 1] == $lines[$i - 1] ) {
 199                                         $changed[--$start] = 1;
 200                                         $changed[--$i] = false;
 201                                         while ( $start > 0 && $changed[$start - 1] ) {
 202                                                 $start--;
 203                                         }
 204                                         assert( $j > 0 );
 205                                         while ( $other_changed[--$j] ) {
 206                                                 continue;
 207                                         }
 208                                         assert( $j >= 0 && !$other_changed[$j] );
 209                                 }
 210
 211                                 /*
 212                                  * Set CORRESPONDING to the end of the changed run, at the last
 213                                  * point where it corresponds to a changed run in the other file.
 214                                  * CORRESPONDING == LEN means no such point has been found.
 215                                  */
 216                                 $corresponding = $j < $other_len ? $i : $len;
 217
 218                                 /*
 219                                  * Move the changed region forward, so long as the
 220                                  * first changed line matches the following unchanged one.
 221                                  * This merges with following changed regions.
 222                                  * Do this second, so that if there are no merges,
 223                                  * the changed region is moved forward as far as possible.
 224                                  */
 225                                 while ( $i < $len && $lines[$start] == $lines[$i] ) {
 226                                         $changed[$start++] = false;
 227                                         $changed[$i++] = 1;
 228                                         while ( $i < $len && $changed[$i] ) {
 229                                                 $i++;
 230                                         }
 231
 232                                         assert( $j < $other_len && ! $other_changed[$j] );
 233                                         $j++;
 234                                         if ( $j < $other_len && $other_changed[$j] ) {
 235                                                 $corresponding = $i;
 236                                                 while ( $j < $other_len && $other_changed[$j] ) {
 237                                                         $j++;
 238                                                 }
 239                                         }
 240                                 }
 241                         } while ( $runlength != $i - $start );
 242
 243                         /*
 244                          * If possible, move the fully-merged run of changes
 245                          * back to a corresponding run in the other file.
 246                          */
 247                         while ( $corresponding < $i ) {
 248                                 $changed[--$start] = 1;
 249                                 $changed[--$i] = 0;
 250                                 assert( $j > 0 );
 251                                 while ( $other_changed[--$j] ) {
 252                                         continue;
 253                                 }
 254                                 assert( $j >= 0 && !$other_changed[$j] );
 255                         }
 256                 }
 257         }
 258
 259         protected function diffInternal( /*array*/ $from, /*array*/ $to ) {
 260                 // remember initial lengths
 261                 $m = count( $from );
 262                 $n = count( $to );
 263
 264                 $this->heuristicUsed = false;
 265
 266                 // output
 267                 $removed = $m > 0 ? array_fill( 0, $m, true ) : [];
 268                 $added = $n > 0 ? array_fill( 0, $n, true ) : [];
 269
 270                 // reduce the complexity for the next step (intentionally done twice)
 271                 // remove common tokens at the start
 272                 $i = 0;
 273                 while ( $i < $m && $i < $n && $from[$i] === $to[$i] ) {
 274                         $removed[$i] = $added[$i] = false;
 275                         unset( $from[$i], $to[$i] );
 276                         ++$i;
 277                 }
 278
 279                 // remove common tokens at the end
 280                 $j = 1;
 281                 while ( $i + $j <= $m && $i + $j <= $n && $from[$m - $j] === $to[$n - $j] ) {
 282                         $removed[$m - $j] = $added[$n - $j] = false;
 283                         unset( $from[$m - $j], $to[$n - $j] );
 284                         ++$j;
 285                 }
 286
 287                 $this->from = $newFromIndex = $this->to = $newToIndex = [];
 288
 289                 // remove tokens not in both sequences
 290                 $shared = [];
 291                 foreach ( $from as $key ) {
 292                         $shared[$key] = false;
 293                 }
 294
 295                 foreach ( $to as $index => &$el ) {
 296                         if ( array_key_exists( $el, $shared ) ) {
 297                                 // keep it
 298                                 $this->to[] = $el;
 299                                 $shared[$el] = true;
 300                                 $newToIndex[] = $index;
 301                         }
 302                 }
 303                 foreach ( $from as $index => &$el ) {
 304                         if ( $shared[$el] ) {
 305                                 // keep it
 306                                 $this->from[] = $el;
 307                                 $newFromIndex[] = $index;
 308                         }
 309                 }
 310
 311                 unset( $shared, $from, $to );
 312
 313                 $this->m = count( $this->from );
 314                 $this->n = count( $this->to );
 315
 316                 $this->removed = $this->m > 0 ? array_fill( 0, $this->m, true ) : [];
 317                 $this->added = $this->n > 0 ? array_fill( 0, $this->n, true ) : [];
 318
 319                 if ( $this->m == 0 || $this->n == 0 ) {
 320                         $this->length = 0;
 321                 } else {
 322                         $this->maxDifferences = ceil( ( $this->m + $this->n ) / 2.0 );
 323                         if ( $this->m * $this->n > $this->tooLong ) {
 324                                 // limit complexity to D^POW_LIMIT for long sequences
 325                                 $this->maxDifferences = floor( pow( $this->maxDifferences, $this->powLimit - 1.0 ) );
 326                                 wfDebug( "Limiting max number of differences to $this->maxDifferences\n" );
 327                         }
 328
 329                         /*
 330                          * The common prefixes and suffixes are always part of some LCS, include
 331                          * them now to reduce our search space
 332                          */
 333                         $max = min( $this->m, $this->n );
 334                         for ( $forwardBound = 0; $forwardBound < $max
 335                                 && $this->from[$forwardBound] === $this->to[$forwardBound];
 336                                 ++$forwardBound
 337                         ) {
 338                                 $this->removed[$forwardBound] = $this->added[$forwardBound] = false;
 339                         }
 340
 341                         $backBoundL1 = $this->m - 1;
 342                         $backBoundL2 = $this->n - 1;
 343
 344                         while ( $backBoundL1 >= $forwardBound && $backBoundL2 >= $forwardBound
 345                                 && $this->from[$backBoundL1] === $this->to[$backBoundL2]
 346                         ) {
 347                                 $this->removed[$backBoundL1--] = $this->added[$backBoundL2--] = false;
 348                         }
 349
 350                         $temp = array_fill( 0, $this->m + $this->n + 1, 0 );
 351                         $V = [ $temp, $temp ];
 352                         $snake = [ 0, 0, 0 ];
 353
 354                         $this->length = $forwardBound + $this->m - $backBoundL1 - 1
 355                                 + $this->lcs_rec(
 356                                         $forwardBound,
 357                                         $backBoundL1,
 358                                         $forwardBound,
 359                                         $backBoundL2,
 360                                         $V,
 361                                         $snake
 362                         );
 363                 }
 364
 365                 $this->m = $m;
 366                 $this->n = $n;
 367
 368                 $this->length += $i + $j - 1;
 369
 370                 foreach ( $this->removed as $key => &$removed_elem ) {
 371                         if ( !$removed_elem ) {
 372                                 $removed[$newFromIndex[$key]] = false;
 373                         }
 374                 }
 375                 foreach ( $this->added as $key => &$added_elem ) {
 376                         if ( !$added_elem ) {
 377                                 $added[$newToIndex[$key]] = false;
 378                         }
 379                 }
 380                 $this->removed = $removed;
 381                 $this->added = $added;
 382         }
 383
 384         function diff_range( $from_lines, $to_lines ) {
 385                 // Diff and store locally
 386                 $this->diff( $from_lines, $to_lines );
 387                 unset( $from_lines, $to_lines );
 388
 389                 $ranges = [];
 390                 $xi = $yi = 0;
 391                 while ( $xi < $this->m || $yi < $this->n ) {
 392                         // Matching "snake".
 393                         while ( $xi < $this->m && $yi < $this->n
 394                                 && !$this->removed[$xi]
 395                                 && !$this->added[$yi]
 396                         ) {
 397                                 ++$xi;
 398                                 ++$yi;
 399                         }
 400                         // Find deletes & adds.
 401                         $xstart = $xi;
 402                         while ( $xi < $this->m && $this->removed[$xi] ) {
 403                                 ++$xi;
 404                         }
 405
 406                         $ystart = $yi;
 407                         while ( $yi < $this->n && $this->added[$yi] ) {
 408                                 ++$yi;
 409                         }
 410
 411                         if ( $xi > $xstart || $yi > $ystart ) {
 412                                 $ranges[] = new RangeDifference( $xstart, $xi, $ystart, $yi );
 413                         }
 414                 }
 415
 416                 return $ranges;
 417         }
 418
 419         private function lcs_rec( $bottoml1, $topl1, $bottoml2, $topl2, &$V, &$snake ) {
 420                 // check that both sequences are non-empty
 421                 if ( $bottoml1 > $topl1 || $bottoml2 > $topl2 ) {
 422                         return 0;
 423                 }
 424
 425                 $d = $this->find_middle_snake( $bottoml1, $topl1, $bottoml2,
 426                         $topl2, $V, $snake );
 427
 428                 // need to store these so we don't lose them when they're
 429                 // overwritten by the recursion
 430                 $len = $snake[2];
 431                 $startx = $snake[0];
 432                 $starty = $snake[1];
 433
 434                 // the middle snake is part of the LCS, store it
 435                 for ( $i = 0; $i < $len; ++$i ) {
 436                         $this->removed[$startx + $i] = $this->added[$starty + $i] = false;
 437                 }
 438
 439                 if ( $d > 1 ) {
 440                         return $len
 441                         + $this->lcs_rec( $bottoml1, $startx - 1, $bottoml2,
 442                                 $starty - 1, $V, $snake )
 443                         + $this->lcs_rec( $startx + $len, $topl1, $starty + $len,
 444                                 $topl2, $V, $snake );
 445                 } elseif ( $d == 1 ) {
 446                         /*
 447                          * In this case the sequences differ by exactly 1 line. We have
 448                          * already saved all the lines after the difference in the for loop
 449                          * above, now we need to save all the lines before the difference.
 450                          */
 451                         $max = min( $startx - $bottoml1, $starty - $bottoml2 );
 452                         for ( $i = 0; $i < $max; ++$i ) {
 453                                 $this->removed[$bottoml1 + $i] =
 454                                         $this->added[$bottoml2 + $i] = false;
 455                         }
 456
 457                         return $max + $len;
 458                 }
 459
 460                 return $len;
 461         }
 462
 463         private function find_middle_snake( $bottoml1, $topl1, $bottoml2, $topl2, &$V, &$snake ) {
 464                 $from = &$this->from;
 465                 $to = &$this->to;
 466                 $V0 = &$V[0];
 467                 $V1 = &$V[1];
 468                 $snake0 = &$snake[0];
 469                 $snake1 = &$snake[1];
 470                 $snake2 = &$snake[2];
 471                 $bottoml1_min_1 = $bottoml1 - 1;
 472                 $bottoml2_min_1 = $bottoml2 - 1;
 473                 $N = $topl1 - $bottoml1_min_1;
 474                 $M = $topl2 - $bottoml2_min_1;
 475                 $delta = $N - $M;
 476                 $maxabsx = $N + $bottoml1;
 477                 $maxabsy = $M + $bottoml2;
 478                 $limit = min( $this->maxDifferences, ceil( ( $N + $M ) / 2 ) );
 479
 480                 // value_to_add_forward: a 0 or 1 that we add to the start
 481                 // offset to make it odd/even
 482                 if ( ( $M & 1 ) == 1 ) {
 483                         $value_to_add_forward = 1;
 484                 } else {
 485                         $value_to_add_forward = 0;
 486                 }
 487
 488                 if ( ( $N & 1 ) == 1 ) {
 489                         $value_to_add_backward = 1;
 490                 } else {
 491                         $value_to_add_backward = 0;
 492                 }
 493
 494                 $start_forward = -$M;
 495                 $end_forward = $N;
 496                 $start_backward = -$N;
 497                 $end_backward = $M;
 498
 499                 $limit_min_1 = $limit - 1;
 500                 $limit_plus_1 = $limit + 1;
 501
 502                 $V0[$limit_plus_1] = 0;
 503                 $V1[$limit_min_1] = $N;
 504                 $limit = min( $this->maxDifferences, ceil( ( $N + $M ) / 2 ) );
 505
 506                 if ( ( $delta & 1 ) == 1 ) {
 507                         for ( $d = 0; $d <= $limit; ++$d ) {
 508                                 $start_diag = max( $value_to_add_forward + $start_forward, -$d );
 509                                 $end_diag = min( $end_forward, $d );
 510                                 $value_to_add_forward = 1 - $value_to_add_forward;
 511
 512                                 // compute forward furthest reaching paths
 513                                 for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
 514                                         if ( $k == -$d || ( $k < $d
 515                                                         && $V0[$limit_min_1 + $k] < $V0[$limit_plus_1 + $k] )
 516                                         ) {
 517                                                 $x = $V0[$limit_plus_1 + $k];
 518                                         } else {
 519                                                 $x = $V0[$limit_min_1 + $k] + 1;
 520                                         }
 521
 522                                         $absx = $snake0 = $x + $bottoml1;
 523                                         $absy = $snake1 = $x - $k + $bottoml2;
 524
 525                                         while ( $absx < $maxabsx && $absy < $maxabsy && $from[$absx] === $to[$absy] ) {
 526                                                 ++$absx;
 527                                                 ++$absy;
 528                                         }
 529                                         $x = $absx - $bottoml1;
 530
 531                                         $snake2 = $absx - $snake0;
 532                                         $V0[$limit + $k] = $x;
 533                                         if ( $k >= $delta - $d + 1 && $k <= $delta + $d - 1
 534                                                 && $x >= $V1[$limit + $k - $delta]
 535                                         ) {
 536                                                 return 2 * $d - 1;
 537                                         }
 538
 539                                         // check to see if we can cut down the diagonal range
 540                                         if ( $x >= $N && $end_forward > $k - 1 ) {
 541                                                 $end_forward = $k - 1;
 542                                         } elseif ( $absy - $bottoml2 >= $M ) {
 543                                                 $start_forward = $k + 1;
 544                                                 $value_to_add_forward = 0;
 545                                         }
 546                                 }
 547
 548                                 $start_diag = max( $value_to_add_backward + $start_backward, -$d );
 549                                 $end_diag = min( $end_backward, $d );
 550                                 $value_to_add_backward = 1 - $value_to_add_backward;
 551
 552                                 // compute backward furthest reaching paths
 553                                 for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
 554                                         if ( $k == $d
 555                                                 || ( $k != -$d && $V1[$limit_min_1 + $k] < $V1[$limit_plus_1 + $k] )
 556                                         ) {
 557                                                 $x = $V1[$limit_min_1 + $k];
 558                                         } else {
 559                                                 $x = $V1[$limit_plus_1 + $k] - 1;
 560                                         }
 561
 562                                         $y = $x - $k - $delta;
 563
 564                                         $snake2 = 0;
 565                                         while ( $x > 0 && $y > 0
 566                                                 && $from[$x + $bottoml1_min_1] === $to[$y + $bottoml2_min_1]
 567                                         ) {
 568                                                 --$x;
 569                                                 --$y;
 570                                                 ++$snake2;
 571                                         }
 572                                         $V1[$limit + $k] = $x;
 573
 574                                         // check to see if we can cut down our diagonal range
 575                                         if ( $x <= 0 ) {
 576                                                 $start_backward = $k + 1;
 577                                                 $value_to_add_backward = 0;
 578                                         } elseif ( $y <= 0 && $end_backward > $k - 1 ) {
 579                                                 $end_backward = $k - 1;
 580                                         }
 581                                 }
 582                         }
 583                 } else {
 584                         for ( $d = 0; $d <= $limit; ++$d ) {
 585                                 $start_diag = max( $value_to_add_forward + $start_forward, -$d );
 586                                 $end_diag = min( $end_forward, $d );
 587                                 $value_to_add_forward = 1 - $value_to_add_forward;
 588
 589                                 // compute forward furthest reaching paths
 590                                 for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
 591                                         if ( $k == -$d
 592                                                 || ( $k < $d && $V0[$limit_min_1 + $k] < $V0[$limit_plus_1 + $k] )
 593                                         ) {
 594                                                 $x = $V0[$limit_plus_1 + $k];
 595                                         } else {
 596                                                 $x = $V0[$limit_min_1 + $k] + 1;
 597                                         }
 598
 599                                         $absx = $snake0 = $x + $bottoml1;
 600                                         $absy = $snake1 = $x - $k + $bottoml2;
 601
 602                                         while ( $absx < $maxabsx && $absy < $maxabsy && $from[$absx] === $to[$absy] ) {
 603                                                 ++$absx;
 604                                                 ++$absy;
 605                                         }
 606                                         $x = $absx - $bottoml1;
 607                                         $snake2 = $absx - $snake0;
 608                                         $V0[$limit + $k] = $x;
 609
 610                                         // check to see if we can cut down the diagonal range
 611                                         if ( $x >= $N && $end_forward > $k - 1 ) {
 612                                                 $end_forward = $k - 1;
 613                                         } elseif ( $absy - $bottoml2 >= $M ) {
 614                                                 $start_forward = $k + 1;
 615                                                 $value_to_add_forward = 0;
 616                                         }
 617                                 }
 618
 619                                 $start_diag = max( $value_to_add_backward + $start_backward, -$d );
 620                                 $end_diag = min( $end_backward, $d );
 621                                 $value_to_add_backward = 1 - $value_to_add_backward;
 622
 623                                 // compute backward furthest reaching paths
 624                                 for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
 625                                         if ( $k == $d
 626                                                 || ( $k != -$d && $V1[$limit_min_1 + $k] < $V1[$limit_plus_1 + $k] )
 627                                         ) {
 628                                                 $x = $V1[$limit_min_1 + $k];
 629                                         } else {
 630                                                 $x = $V1[$limit_plus_1 + $k] - 1;
 631                                         }
 632
 633                                         $y = $x - $k - $delta;
 634
 635                                         $snake2 = 0;
 636                                         while ( $x > 0 && $y > 0
 637                                                 && $from[$x + $bottoml1_min_1] === $to[$y + $bottoml2_min_1]
 638                                         ) {
 639                                                 --$x;
 640                                                 --$y;
 641                                                 ++$snake2;
 642                                         }
 643                                         $V1[$limit + $k] = $x;
 644
 645                                         if ( $k >= -$delta - $d && $k <= $d - $delta
 646                                                 && $x <= $V0[$limit + $k + $delta]
 647                                         ) {
 648                                                 $snake0 = $bottoml1 + $x;
 649                                                 $snake1 = $bottoml2 + $y;
 650
 651                                                 return 2 * $d;
 652                                         }
 653
 654                                         // check to see if we can cut down our diagonal range
 655                                         if ( $x <= 0 ) {
 656                                                 $start_backward = $k + 1;
 657                                                 $value_to_add_backward = 0;
 658                                         } elseif ( $y <= 0 && $end_backward > $k - 1 ) {
 659                                                 $end_backward = $k - 1;
 660                                         }
 661                                 }
 662                         }
 663                 }
 664                 /*
 665                  * computing the true LCS is too expensive, instead find the diagonal
 666                  * with the most progress and pretend a midle snake of length 0 occurs
 667                  * there.
 668                  */
 669
 670                 $most_progress = self::findMostProgress( $M, $N, $limit, $V );
 671
 672                 $snake0 = $bottoml1 + $most_progress[0];
 673                 $snake1 = $bottoml2 + $most_progress[1];
 674                 $snake2 = 0;
 675                 wfDebug( "Computing the LCS is too expensive. Using a heuristic.\n" );
 676                 $this->heuristicUsed = true;
 677
 678                 return 5; /*
 679                 * HACK: since we didn't really finish the LCS computation
 680                 * we don't really know the length of the SES. We don't do
 681                 * anything with the result anyway, unless it's <=1. We know
 682                 * for a fact SES > 1 so 5 is as good a number as any to
 683                 * return here
 684                 */
 685         }
 686
 687         private static function findMostProgress( $M, $N, $limit, $V ) {
 688                 $delta = $N - $M;
 689
 690                 if ( ( $M & 1 ) == ( $limit & 1 ) ) {
 691                         $forward_start_diag = max( -$M, -$limit );
 692                 } else {
 693                         $forward_start_diag = max( 1 - $M, -$limit );
 694                 }
 695
 696                 $forward_end_diag = min( $N, $limit );
 697
 698                 if ( ( $N & 1 ) == ( $limit & 1 ) ) {
 699                         $backward_start_diag = max( -$N, -$limit );
 700                 } else {
 701                         $backward_start_diag = max( 1 - $N, -$limit );
 702                 }
 703
 704                 $backward_end_diag = -min( $M, $limit );
 705
 706                 $temp = [ 0, 0, 0 ];
 707
 708                 $max_progress = array_fill( 0, ceil( max( $forward_end_diag - $forward_start_diag,
 709                                 $backward_end_diag - $backward_start_diag ) / 2 ), $temp );
 710                 $num_progress = 0; // the 1st entry is current, it is initialized
 711                 // with 0s
 712
 713                 // first search the forward diagonals
 714                 for ( $k = $forward_start_diag; $k <= $forward_end_diag; $k += 2 ) {
 715                         $x = $V[0][$limit + $k];
 716                         $y = $x - $k;
 717                         if ( $x > $N || $y > $M ) {
 718                                 continue;
 719                         }
 720
 721                         $progress = $x + $y;
 722                         if ( $progress > $max_progress[0][2] ) {
 723                                 $num_progress = 0;
 724                                 $max_progress[0][0] = $x;
 725                                 $max_progress[0][1] = $y;
 726                                 $max_progress[0][2] = $progress;
 727                         } elseif ( $progress == $max_progress[0][2] ) {
 728                                 ++$num_progress;
 729                                 $max_progress[$num_progress][0] = $x;
 730                                 $max_progress[$num_progress][1] = $y;
 731                                 $max_progress[$num_progress][2] = $progress;
 732                         }
 733                 }
 734
 735                 $max_progress_forward = true; // initially the maximum
 736                 // progress is in the forward
 737                 // direction
 738
 739                 // now search the backward diagonals
 740                 for ( $k = $backward_start_diag; $k <= $backward_end_diag; $k += 2 ) {
 741                         $x = $V[1][$limit + $k];
 742                         $y = $x - $k - $delta;
 743                         if ( $x < 0 || $y < 0 ) {
 744                                 continue;
 745                         }
 746
 747                         $progress = $N - $x + $M - $y;
 748                         if ( $progress > $max_progress[0][2] ) {
 749                                 $num_progress = 0;
 750                                 $max_progress_forward = false;
 751                                 $max_progress[0][0] = $x;
 752                                 $max_progress[0][1] = $y;
 753                                 $max_progress[0][2] = $progress;
 754                         } elseif ( $progress == $max_progress[0][2] && !$max_progress_forward ) {
 755                                 ++$num_progress;
 756                                 $max_progress[$num_progress][0] = $x;
 757                                 $max_progress[$num_progress][1] = $y;
 758                                 $max_progress[$num_progress][2] = $progress;
 759                         }
 760                 }
 761
 762                 // return the middle diagonal with maximal progress.
 763                 return $max_progress[(int)floor( $num_progress / 2 )];
 764         }
 765
 766         /**
 767          * @return mixed
 768          */
 769         public function getLcsLength() {
 770                 if ( $this->heuristicUsed && !$this->lcsLengthCorrectedForHeuristic ) {
 771                         $this->lcsLengthCorrectedForHeuristic = true;
 772                         $this->length = $this->m - array_sum( $this->added );
 773                 }
 774
 775                 return $this->length;
 776         }
 777
 778 }
 779
 780 /**
 781  * Alternative representation of a set of changes, by the index
 782  * ranges that are changed.
 783  *
 784  * @ingroup DifferenceEngine
 785  */
 786 class RangeDifference {
 787
 788         /** @var int */
 789         public $leftstart;
 790
 791         /** @var int */
 792         public $leftend;
 793
 794         /** @var int */
 795         public $leftlength;
 796
 797         /** @var int */
 798         public $rightstart;
 799
 800         /** @var int */
 801         public $rightend;
 802
 803         /** @var int */
 804         public $rightlength;
 805
 806         function __construct( $leftstart, $leftend, $rightstart, $rightend ) {
 807                 $this->leftstart = $leftstart;
 808                 $this->leftend = $leftend;
 809                 $this->leftlength = $leftend - $leftstart;
 810                 $this->rightstart = $rightstart;
 811                 $this->rightend = $rightend;
 812                 $this->rightlength = $rightend - $rightstart;
 813         }
 814
 815 }