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          * Performs diff
  71          *
  72          * @param string[] $from_lines
  73          * @param string[] $to_lines
  74          *
  75          * @return DiffOp[]
  76          */
  77         public function diff( $from_lines, $to_lines ) {
  78
  79                 // Diff and store locally
  80                 $this->diffInternal( $from_lines, $to_lines );
  81
  82                 // Merge edits when possible
  83                 $this->shiftBoundaries( $from_lines, $this->removed, $this->added );
  84                 $this->shiftBoundaries( $to_lines, $this->added, $this->removed );
  85
  86                 // Compute the edit operations.
  87                 $n_from = count( $from_lines );
  88                 $n_to = count( $to_lines );
  89
  90                 $edits = [];
  91                 $xi = $yi = 0;
  92                 while ( $xi < $n_from || $yi < $n_to ) {
  93                         assert( $yi < $n_to || $this->removed[$xi] );
  94                         assert( $xi < $n_from || $this->added[$yi] );
  95
  96                         // Skip matching "snake".
  97                         $copy = [];
  98                         while ( $xi < $n_from && $yi < $n_to
  99                                         && !$this->removed[$xi] && !$this->added[$yi]
 100                         ) {
 101                                 $copy[] = $from_lines[$xi++];
 102                                 ++$yi;
 103                         }
 104                         if ( $copy ) {
 105                                 $edits[] = new DiffOpCopy( $copy );
 106                         }
 107
 108                         // Find deletes & adds.
 109                         $delete = [];
 110                         while ( $xi < $n_from && $this->removed[$xi] ) {
 111                                 $delete[] = $from_lines[$xi++];
 112                         }
 113
 114                         $add = [];
 115                         while ( $yi < $n_to && $this->added[$yi] ) {
 116                                 $add[] = $to_lines[$yi++];
 117                         }
 118
 119                         if ( $delete && $add ) {
 120                                 $edits[] = new DiffOpChange( $delete, $add );
 121                         } elseif ( $delete ) {
 122                                 $edits[] = new DiffOpDelete( $delete );
 123                         } elseif ( $add ) {
 124                                 $edits[] = new DiffOpAdd( $add );
 125                         }
 126                 }
 127
 128                 return $edits;
 129         }
 130
 131         /**
 132          * Adjust inserts/deletes of identical lines to join changes
 133          * as much as possible.
 134          *
 135          * We do something when a run of changed lines include a
 136          * line at one end and has an excluded, identical line at the other.
 137          * We are free to choose which identical line is included.
 138          * `compareseq' usually chooses the one at the beginning,
 139          * but usually it is cleaner to consider the following identical line
 140          * to be the "change".
 141          *
 142          * This is extracted verbatim from analyze.c (GNU diffutils-2.7).
 143          *
 144          * @param string[] $lines
 145          * @param string[] $changed
 146          * @param string[] $other_changed
 147          */
 148         private function shiftBoundaries( array $lines, array &$changed, array $other_changed ) {
 149                 $i = 0;
 150                 $j = 0;
 151
 152                 assert( count( $lines ) == count( $changed ) );
 153                 $len = count( $lines );
 154                 $other_len = count( $other_changed );
 155
 156                 while ( 1 ) {
 157                         /*
 158                          * Scan forwards to find beginning of another run of changes.
 159                          * Also keep track of the corresponding point in the other file.
 160                          *
 161                          * Throughout this code, $i and $j are adjusted together so that
 162                          * the first $i elements of $changed and the first $j elements
 163                          * of $other_changed both contain the same number of zeros
 164                          * (unchanged lines).
 165                          * Furthermore, $j is always kept so that $j == $other_len or
 166                          * $other_changed[$j] == false.
 167                          */
 168                         while ( $j < $other_len && $other_changed[$j] ) {
 169                                 $j++;
 170                         }
 171
 172                         while ( $i < $len && !$changed[$i] ) {
 173                                 assert( $j < $other_len && ! $other_changed[$j] );
 174                                 $i++;
 175                                 $j++;
 176                                 while ( $j < $other_len && $other_changed[$j] ) {
 177                                         $j++;
 178                                 }
 179                         }
 180
 181                         if ( $i == $len ) {
 182                                 break;
 183                         }
 184
 185                         $start = $i;
 186
 187                         // Find the end of this run of changes.
 188                         while ( ++$i < $len && $changed[$i] ) {
 189                                 continue;
 190                         }
 191
 192                         do {
 193                                 /*
 194                                  * Record the length of this run of changes, so that
 195                                  * we can later determine whether the run has grown.
 196                                  */
 197                                 $runlength = $i - $start;
 198
 199                                 /*
 200                                  * Move the changed region back, so long as the
 201                                  * previous unchanged line matches the last changed one.
 202                                  * This merges with previous changed regions.
 203                                  */
 204                                 while ( $start > 0 && $lines[$start - 1] == $lines[$i - 1] ) {
 205                                         $changed[--$start] = 1;
 206                                         $changed[--$i] = false;
 207                                         while ( $start > 0 && $changed[$start - 1] ) {
 208                                                 $start--;
 209                                         }
 210                                         assert( $j > 0 );
 211                                         while ( $other_changed[--$j] ) {
 212                                                 continue;
 213                                         }
 214                                         assert( $j >= 0 && !$other_changed[$j] );
 215                                 }
 216
 217                                 /*
 218                                  * Set CORRESPONDING to the end of the changed run, at the last
 219                                  * point where it corresponds to a changed run in the other file.
 220                                  * CORRESPONDING == LEN means no such point has been found.
 221                                  */
 222                                 $corresponding = $j < $other_len ? $i : $len;
 223
 224                                 /*
 225                                  * Move the changed region forward, so long as the
 226                                  * first changed line matches the following unchanged one.
 227                                  * This merges with following changed regions.
 228                                  * Do this second, so that if there are no merges,
 229                                  * the changed region is moved forward as far as possible.
 230                                  */
 231                                 while ( $i < $len && $lines[$start] == $lines[$i] ) {
 232                                         $changed[$start++] = false;
 233                                         $changed[$i++] = 1;
 234                                         while ( $i < $len && $changed[$i] ) {
 235                                                 $i++;
 236                                         }
 237
 238                                         assert( $j < $other_len && ! $other_changed[$j] );
 239                                         $j++;
 240                                         if ( $j < $other_len && $other_changed[$j] ) {
 241                                                 $corresponding = $i;
 242                                                 while ( $j < $other_len && $other_changed[$j] ) {
 243                                                         $j++;
 244                                                 }
 245                                         }
 246                                 }
 247                         } while ( $runlength != $i - $start );
 248
 249                         /*
 250                          * If possible, move the fully-merged run of changes
 251                          * back to a corresponding run in the other file.
 252                          */
 253                         while ( $corresponding < $i ) {
 254                                 $changed[--$start] = 1;
 255                                 $changed[--$i] = 0;
 256                                 assert( $j > 0 );
 257                                 while ( $other_changed[--$j] ) {
 258                                         continue;
 259                                 }
 260                                 assert( $j >= 0 && !$other_changed[$j] );
 261                         }
 262                 }
 263         }
 264
 265         /**
 266          * @param string[] $from
 267          * @param string[] $to
 268          */
 269         protected function diffInternal( array $from, array $to ) {
 270                 // remember initial lengths
 271                 $m = count( $from );
 272                 $n = count( $to );
 273
 274                 $this->heuristicUsed = false;
 275
 276                 // output
 277                 $removed = $m > 0 ? array_fill( 0, $m, true ) : [];
 278                 $added = $n > 0 ? array_fill( 0, $n, true ) : [];
 279
 280                 // reduce the complexity for the next step (intentionally done twice)
 281                 // remove common tokens at the start
 282                 $i = 0;
 283                 while ( $i < $m && $i < $n && $from[$i] === $to[$i] ) {
 284                         $removed[$i] = $added[$i] = false;
 285                         unset( $from[$i], $to[$i] );
 286                         ++$i;
 287                 }
 288
 289                 // remove common tokens at the end
 290                 $j = 1;
 291                 while ( $i + $j <= $m && $i + $j <= $n && $from[$m - $j] === $to[$n - $j] ) {
 292                         $removed[$m - $j] = $added[$n - $j] = false;
 293                         unset( $from[$m - $j], $to[$n - $j] );
 294                         ++$j;
 295                 }
 296
 297                 $this->from = $newFromIndex = $this->to = $newToIndex = [];
 298
 299                 // remove tokens not in both sequences
 300                 $shared = [];
 301                 foreach ( $from as $key ) {
 302                         $shared[$key] = false;
 303                 }
 304
 305                 foreach ( $to as $index => &$el ) {
 306                         if ( array_key_exists( $el, $shared ) ) {
 307                                 // keep it
 308                                 $this->to[] = $el;
 309                                 $shared[$el] = true;
 310                                 $newToIndex[] = $index;
 311                         }
 312                 }
 313                 foreach ( $from as $index => &$el ) {
 314                         if ( $shared[$el] ) {
 315                                 // keep it
 316                                 $this->from[] = $el;
 317                                 $newFromIndex[] = $index;
 318                         }
 319                 }
 320
 321                 unset( $shared, $from, $to );
 322
 323                 $this->m = count( $this->from );
 324                 $this->n = count( $this->to );
 325
 326                 $this->removed = $this->m > 0 ? array_fill( 0, $this->m, true ) : [];
 327                 $this->added = $this->n > 0 ? array_fill( 0, $this->n, true ) : [];
 328
 329                 if ( $this->m == 0 || $this->n == 0 ) {
 330                         $this->length = 0;
 331                 } else {
 332                         $this->maxDifferences = ceil( ( $this->m + $this->n ) / 2.0 );
 333                         if ( $this->m * $this->n > $this->tooLong ) {
 334                                 // limit complexity to D^POW_LIMIT for long sequences
 335                                 $this->maxDifferences = floor( pow( $this->maxDifferences, $this->powLimit - 1.0 ) );
 336                                 wfDebug( "Limiting max number of differences to $this->maxDifferences\n" );
 337                         }
 338
 339                         /*
 340                          * The common prefixes and suffixes are always part of some LCS, include
 341                          * them now to reduce our search space
 342                          */
 343                         $max = min( $this->m, $this->n );
 344                         for ( $forwardBound = 0; $forwardBound < $max
 345                                 && $this->from[$forwardBound] === $this->to[$forwardBound];
 346                                 ++$forwardBound
 347                         ) {
 348                                 $this->removed[$forwardBound] = $this->added[$forwardBound] = false;
 349                         }
 350
 351                         $backBoundL1 = $this->m - 1;
 352                         $backBoundL2 = $this->n - 1;
 353
 354                         while ( $backBoundL1 >= $forwardBound && $backBoundL2 >= $forwardBound
 355                                 && $this->from[$backBoundL1] === $this->to[$backBoundL2]
 356                         ) {
 357                                 $this->removed[$backBoundL1--] = $this->added[$backBoundL2--] = false;
 358                         }
 359
 360                         $temp = array_fill( 0, $this->m + $this->n + 1, 0 );
 361                         $V = [ $temp, $temp ];
 362                         $snake = [ 0, 0, 0 ];
 363
 364                         $this->length = $forwardBound + $this->m - $backBoundL1 - 1
 365                                 + $this->lcs_rec(
 366                                         $forwardBound,
 367                                         $backBoundL1,
 368                                         $forwardBound,
 369                                         $backBoundL2,
 370                                         $V,
 371                                         $snake
 372                         );
 373                 }
 374
 375                 $this->m = $m;
 376                 $this->n = $n;
 377
 378                 $this->length += $i + $j - 1;
 379
 380                 foreach ( $this->removed as $key => &$removed_elem ) {
 381                         if ( !$removed_elem ) {
 382                                 $removed[$newFromIndex[$key]] = false;
 383                         }
 384                 }
 385                 foreach ( $this->added as $key => &$added_elem ) {
 386                         if ( !$added_elem ) {
 387                                 $added[$newToIndex[$key]] = false;
 388                         }
 389                 }
 390                 $this->removed = $removed;
 391                 $this->added = $added;
 392         }
 393
 394         function diff_range( $from_lines, $to_lines ) {
 395                 // Diff and store locally
 396                 $this->diff( $from_lines, $to_lines );
 397                 unset( $from_lines, $to_lines );
 398
 399                 $ranges = [];
 400                 $xi = $yi = 0;
 401                 while ( $xi < $this->m || $yi < $this->n ) {
 402                         // Matching "snake".
 403                         while ( $xi < $this->m && $yi < $this->n
 404                                 && !$this->removed[$xi]
 405                                 && !$this->added[$yi]
 406                         ) {
 407                                 ++$xi;
 408                                 ++$yi;
 409                         }
 410                         // Find deletes & adds.
 411                         $xstart = $xi;
 412                         while ( $xi < $this->m && $this->removed[$xi] ) {
 413                                 ++$xi;
 414                         }
 415
 416                         $ystart = $yi;
 417                         while ( $yi < $this->n && $this->added[$yi] ) {
 418                                 ++$yi;
 419                         }
 420
 421                         if ( $xi > $xstart || $yi > $ystart ) {
 422                                 $ranges[] = new RangeDifference( $xstart, $xi, $ystart, $yi );
 423                         }
 424                 }
 425
 426                 return $ranges;
 427         }
 428
 429         private function lcs_rec( $bottoml1, $topl1, $bottoml2, $topl2, &$V, &$snake ) {
 430                 // check that both sequences are non-empty
 431                 if ( $bottoml1 > $topl1 || $bottoml2 > $topl2 ) {
 432                         return 0;
 433                 }
 434
 435                 $d = $this->find_middle_snake( $bottoml1, $topl1, $bottoml2,
 436                         $topl2, $V, $snake );
 437
 438                 // need to store these so we don't lose them when they're
 439                 // overwritten by the recursion
 440                 $len = $snake[2];
 441                 $startx = $snake[0];
 442                 $starty = $snake[1];
 443
 444                 // the middle snake is part of the LCS, store it
 445                 for ( $i = 0; $i < $len; ++$i ) {
 446                         $this->removed[$startx + $i] = $this->added[$starty + $i] = false;
 447                 }
 448
 449                 if ( $d > 1 ) {
 450                         return $len
 451                         + $this->lcs_rec( $bottoml1, $startx - 1, $bottoml2,
 452                                 $starty - 1, $V, $snake )
 453                         + $this->lcs_rec( $startx + $len, $topl1, $starty + $len,
 454                                 $topl2, $V, $snake );
 455                 } elseif ( $d == 1 ) {
 456                         /*
 457                          * In this case the sequences differ by exactly 1 line. We have
 458                          * already saved all the lines after the difference in the for loop
 459                          * above, now we need to save all the lines before the difference.
 460                          */
 461                         $max = min( $startx - $bottoml1, $starty - $bottoml2 );
 462                         for ( $i = 0; $i < $max; ++$i ) {
 463                                 $this->removed[$bottoml1 + $i] =
 464                                         $this->added[$bottoml2 + $i] = false;
 465                         }
 466
 467                         return $max + $len;
 468                 }
 469
 470                 return $len;
 471         }
 472
 473         private function find_middle_snake( $bottoml1, $topl1, $bottoml2, $topl2, &$V, &$snake ) {
 474                 $from = &$this->from;
 475                 $to = &$this->to;
 476                 $V0 = &$V[0];
 477                 $V1 = &$V[1];
 478                 $snake0 = &$snake[0];
 479                 $snake1 = &$snake[1];
 480                 $snake2 = &$snake[2];
 481                 $bottoml1_min_1 = $bottoml1 - 1;
 482                 $bottoml2_min_1 = $bottoml2 - 1;
 483                 $N = $topl1 - $bottoml1_min_1;
 484                 $M = $topl2 - $bottoml2_min_1;
 485                 $delta = $N - $M;
 486                 $maxabsx = $N + $bottoml1;
 487                 $maxabsy = $M + $bottoml2;
 488                 $limit = min( $this->maxDifferences, ceil( ( $N + $M ) / 2 ) );
 489
 490                 // value_to_add_forward: a 0 or 1 that we add to the start
 491                 // offset to make it odd/even
 492                 if ( ( $M & 1 ) == 1 ) {
 493                         $value_to_add_forward = 1;
 494                 } else {
 495                         $value_to_add_forward = 0;
 496                 }
 497
 498                 if ( ( $N & 1 ) == 1 ) {
 499                         $value_to_add_backward = 1;
 500                 } else {
 501                         $value_to_add_backward = 0;
 502                 }
 503
 504                 $start_forward = -$M;
 505                 $end_forward = $N;
 506                 $start_backward = -$N;
 507                 $end_backward = $M;
 508
 509                 $limit_min_1 = $limit - 1;
 510                 $limit_plus_1 = $limit + 1;
 511
 512                 $V0[$limit_plus_1] = 0;
 513                 $V1[$limit_min_1] = $N;
 514                 $limit = min( $this->maxDifferences, ceil( ( $N + $M ) / 2 ) );
 515
 516                 if ( ( $delta & 1 ) == 1 ) {
 517                         for ( $d = 0; $d <= $limit; ++$d ) {
 518                                 $start_diag = max( $value_to_add_forward + $start_forward, -$d );
 519                                 $end_diag = min( $end_forward, $d );
 520                                 $value_to_add_forward = 1 - $value_to_add_forward;
 521
 522                                 // compute forward furthest reaching paths
 523                                 for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
 524                                         if ( $k == -$d || ( $k < $d
 525                                                         && $V0[$limit_min_1 + $k] < $V0[$limit_plus_1 + $k] )
 526                                         ) {
 527                                                 $x = $V0[$limit_plus_1 + $k];
 528                                         } else {
 529                                                 $x = $V0[$limit_min_1 + $k] + 1;
 530                                         }
 531
 532                                         $absx = $snake0 = $x + $bottoml1;
 533                                         $absy = $snake1 = $x - $k + $bottoml2;
 534
 535                                         while ( $absx < $maxabsx && $absy < $maxabsy && $from[$absx] === $to[$absy] ) {
 536                                                 ++$absx;
 537                                                 ++$absy;
 538                                         }
 539                                         $x = $absx - $bottoml1;
 540
 541                                         $snake2 = $absx - $snake0;
 542                                         $V0[$limit + $k] = $x;
 543                                         if ( $k >= $delta - $d + 1 && $k <= $delta + $d - 1
 544                                                 && $x >= $V1[$limit + $k - $delta]
 545                                         ) {
 546                                                 return 2 * $d - 1;
 547                                         }
 548
 549                                         // check to see if we can cut down the diagonal range
 550                                         if ( $x >= $N && $end_forward > $k - 1 ) {
 551                                                 $end_forward = $k - 1;
 552                                         } elseif ( $absy - $bottoml2 >= $M ) {
 553                                                 $start_forward = $k + 1;
 554                                                 $value_to_add_forward = 0;
 555                                         }
 556                                 }
 557
 558                                 $start_diag = max( $value_to_add_backward + $start_backward, -$d );
 559                                 $end_diag = min( $end_backward, $d );
 560                                 $value_to_add_backward = 1 - $value_to_add_backward;
 561
 562                                 // compute backward furthest reaching paths
 563                                 for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
 564                                         if ( $k == $d
 565                                                 || ( $k != -$d && $V1[$limit_min_1 + $k] < $V1[$limit_plus_1 + $k] )
 566                                         ) {
 567                                                 $x = $V1[$limit_min_1 + $k];
 568                                         } else {
 569                                                 $x = $V1[$limit_plus_1 + $k] - 1;
 570                                         }
 571
 572                                         $y = $x - $k - $delta;
 573
 574                                         $snake2 = 0;
 575                                         while ( $x > 0 && $y > 0
 576                                                 && $from[$x + $bottoml1_min_1] === $to[$y + $bottoml2_min_1]
 577                                         ) {
 578                                                 --$x;
 579                                                 --$y;
 580                                                 ++$snake2;
 581                                         }
 582                                         $V1[$limit + $k] = $x;
 583
 584                                         // check to see if we can cut down our diagonal range
 585                                         if ( $x <= 0 ) {
 586                                                 $start_backward = $k + 1;
 587                                                 $value_to_add_backward = 0;
 588                                         } elseif ( $y <= 0 && $end_backward > $k - 1 ) {
 589                                                 $end_backward = $k - 1;
 590                                         }
 591                                 }
 592                         }
 593                 } else {
 594                         for ( $d = 0; $d <= $limit; ++$d ) {
 595                                 $start_diag = max( $value_to_add_forward + $start_forward, -$d );
 596                                 $end_diag = min( $end_forward, $d );
 597                                 $value_to_add_forward = 1 - $value_to_add_forward;
 598
 599                                 // compute forward furthest reaching paths
 600                                 for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
 601                                         if ( $k == -$d
 602                                                 || ( $k < $d && $V0[$limit_min_1 + $k] < $V0[$limit_plus_1 + $k] )
 603                                         ) {
 604                                                 $x = $V0[$limit_plus_1 + $k];
 605                                         } else {
 606                                                 $x = $V0[$limit_min_1 + $k] + 1;
 607                                         }
 608
 609                                         $absx = $snake0 = $x + $bottoml1;
 610                                         $absy = $snake1 = $x - $k + $bottoml2;
 611
 612                                         while ( $absx < $maxabsx && $absy < $maxabsy && $from[$absx] === $to[$absy] ) {
 613                                                 ++$absx;
 614                                                 ++$absy;
 615                                         }
 616                                         $x = $absx - $bottoml1;
 617                                         $snake2 = $absx - $snake0;
 618                                         $V0[$limit + $k] = $x;
 619
 620                                         // check to see if we can cut down the diagonal range
 621                                         if ( $x >= $N && $end_forward > $k - 1 ) {
 622                                                 $end_forward = $k - 1;
 623                                         } elseif ( $absy - $bottoml2 >= $M ) {
 624                                                 $start_forward = $k + 1;
 625                                                 $value_to_add_forward = 0;
 626                                         }
 627                                 }
 628
 629                                 $start_diag = max( $value_to_add_backward + $start_backward, -$d );
 630                                 $end_diag = min( $end_backward, $d );
 631                                 $value_to_add_backward = 1 - $value_to_add_backward;
 632
 633                                 // compute backward furthest reaching paths
 634                                 for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
 635                                         if ( $k == $d
 636                                                 || ( $k != -$d && $V1[$limit_min_1 + $k] < $V1[$limit_plus_1 + $k] )
 637                                         ) {
 638                                                 $x = $V1[$limit_min_1 + $k];
 639                                         } else {
 640                                                 $x = $V1[$limit_plus_1 + $k] - 1;
 641                                         }
 642
 643                                         $y = $x - $k - $delta;
 644
 645                                         $snake2 = 0;
 646                                         while ( $x > 0 && $y > 0
 647                                                 && $from[$x + $bottoml1_min_1] === $to[$y + $bottoml2_min_1]
 648                                         ) {
 649                                                 --$x;
 650                                                 --$y;
 651                                                 ++$snake2;
 652                                         }
 653                                         $V1[$limit + $k] = $x;
 654
 655                                         if ( $k >= -$delta - $d && $k <= $d - $delta
 656                                                 && $x <= $V0[$limit + $k + $delta]
 657                                         ) {
 658                                                 $snake0 = $bottoml1 + $x;
 659                                                 $snake1 = $bottoml2 + $y;
 660
 661                                                 return 2 * $d;
 662                                         }
 663
 664                                         // check to see if we can cut down our diagonal range
 665                                         if ( $x <= 0 ) {
 666                                                 $start_backward = $k + 1;
 667                                                 $value_to_add_backward = 0;
 668                                         } elseif ( $y <= 0 && $end_backward > $k - 1 ) {
 669                                                 $end_backward = $k - 1;
 670                                         }
 671                                 }
 672                         }
 673                 }
 674                 /*
 675                  * computing the true LCS is too expensive, instead find the diagonal
 676                  * with the most progress and pretend a midle snake of length 0 occurs
 677                  * there.
 678                  */
 679
 680                 $most_progress = self::findMostProgress( $M, $N, $limit, $V );
 681
 682                 $snake0 = $bottoml1 + $most_progress[0];
 683                 $snake1 = $bottoml2 + $most_progress[1];
 684                 $snake2 = 0;
 685                 wfDebug( "Computing the LCS is too expensive. Using a heuristic.\n" );
 686                 $this->heuristicUsed = true;
 687
 688                 return 5; /*
 689                 * HACK: since we didn't really finish the LCS computation
 690                 * we don't really know the length of the SES. We don't do
 691                 * anything with the result anyway, unless it's <=1. We know
 692                 * for a fact SES > 1 so 5 is as good a number as any to
 693                 * return here
 694                 */
 695         }
 696
 697         private static function findMostProgress( $M, $N, $limit, $V ) {
 698                 $delta = $N - $M;
 699
 700                 if ( ( $M & 1 ) == ( $limit & 1 ) ) {
 701                         $forward_start_diag = max( -$M, -$limit );
 702                 } else {
 703                         $forward_start_diag = max( 1 - $M, -$limit );
 704                 }
 705
 706                 $forward_end_diag = min( $N, $limit );
 707
 708                 if ( ( $N & 1 ) == ( $limit & 1 ) ) {
 709                         $backward_start_diag = max( -$N, -$limit );
 710                 } else {
 711                         $backward_start_diag = max( 1 - $N, -$limit );
 712                 }
 713
 714                 $backward_end_diag = -min( $M, $limit );
 715
 716                 $temp = [ 0, 0, 0 ];
 717
 718                 $max_progress = array_fill( 0, ceil( max( $forward_end_diag - $forward_start_diag,
 719                                 $backward_end_diag - $backward_start_diag ) / 2 ), $temp );
 720                 $num_progress = 0; // the 1st entry is current, it is initialized
 721                 // with 0s
 722
 723                 // first search the forward diagonals
 724                 for ( $k = $forward_start_diag; $k <= $forward_end_diag; $k += 2 ) {
 725                         $x = $V[0][$limit + $k];
 726                         $y = $x - $k;
 727                         if ( $x > $N || $y > $M ) {
 728                                 continue;
 729                         }
 730
 731                         $progress = $x + $y;
 732                         if ( $progress > $max_progress[0][2] ) {
 733                                 $num_progress = 0;
 734                                 $max_progress[0][0] = $x;
 735                                 $max_progress[0][1] = $y;
 736                                 $max_progress[0][2] = $progress;
 737                         } elseif ( $progress == $max_progress[0][2] ) {
 738                                 ++$num_progress;
 739                                 $max_progress[$num_progress][0] = $x;
 740                                 $max_progress[$num_progress][1] = $y;
 741                                 $max_progress[$num_progress][2] = $progress;
 742                         }
 743                 }
 744
 745                 $max_progress_forward = true; // initially the maximum
 746                 // progress is in the forward
 747                 // direction
 748
 749                 // now search the backward diagonals
 750                 for ( $k = $backward_start_diag; $k <= $backward_end_diag; $k += 2 ) {
 751                         $x = $V[1][$limit + $k];
 752                         $y = $x - $k - $delta;
 753                         if ( $x < 0 || $y < 0 ) {
 754                                 continue;
 755                         }
 756
 757                         $progress = $N - $x + $M - $y;
 758                         if ( $progress > $max_progress[0][2] ) {
 759                                 $num_progress = 0;
 760                                 $max_progress_forward = false;
 761                                 $max_progress[0][0] = $x;
 762                                 $max_progress[0][1] = $y;
 763                                 $max_progress[0][2] = $progress;
 764                         } elseif ( $progress == $max_progress[0][2] && !$max_progress_forward ) {
 765                                 ++$num_progress;
 766                                 $max_progress[$num_progress][0] = $x;
 767                                 $max_progress[$num_progress][1] = $y;
 768                                 $max_progress[$num_progress][2] = $progress;
 769                         }
 770                 }
 771
 772                 // return the middle diagonal with maximal progress.
 773                 return $max_progress[(int)floor( $num_progress / 2 )];
 774         }
 775
 776         /**
 777          * @return mixed
 778          */
 779         public function getLcsLength() {
 780                 if ( $this->heuristicUsed && !$this->lcsLengthCorrectedForHeuristic ) {
 781                         $this->lcsLengthCorrectedForHeuristic = true;
 782                         $this->length = $this->m - array_sum( $this->added );
 783                 }
 784
 785                 return $this->length;
 786         }
 787
 788 }
 789
 790 /**
 791  * Alternative representation of a set of changes, by the index
 792  * ranges that are changed.
 793  *
 794  * @ingroup DifferenceEngine
 795  */
 796 class RangeDifference {
 797
 798         /** @var int */
 799         public $leftstart;
 800
 801         /** @var int */
 802         public $leftend;
 803
 804         /** @var int */
 805         public $leftlength;
 806
 807         /** @var int */
 808         public $rightstart;
 809
 810         /** @var int */
 811         public $rightend;
 812
 813         /** @var int */
 814         public $rightlength;
 815
 816         function __construct( $leftstart, $leftend, $rightstart, $rightend ) {
 817                 $this->leftstart = $leftstart;
 818                 $this->leftend = $leftend;
 819                 $this->leftlength = $leftend - $leftstart;
 820                 $this->rightstart = $rightstart;
 821                 $this->rightend = $rightend;
 822                 $this->rightlength = $rightend - $rightstart;
 823         }
 824
 825 }