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 use MediaWiki\Diff\ComplexityException;
  26
  27 /**
  28  * This diff implementation is mainly lifted from the LCS algorithm of the Eclipse project which
  29  * in turn is based on Myers' "An O(ND) difference algorithm and its variations"
  30  * (http://citeseer.ist.psu.edu/myers86ond.html) with range compression (see Wu et al.'s
  31  * "An O(NP) Sequence Comparison Algorithm").
  32  *
  33  * This implementation supports an upper bound on the execution time.
  34  *
  35  * Some ideas (and a bit of code) are from analyze.c, from GNU
  36  * diffutils-2.7, which can be found at:
  37  *     ftp://gnudist.gnu.org/pub/gnu/diffutils/diffutils-2.7.tar.gz
  38  *
  39  * Complexity: O((M + N)D) worst case time, O(M + N + D^2) expected time, O(M + N) space
  40  *
  41  * @author Guy Van den Broeck, Geoffrey T. Dairiki, Tim Starling
  42  * @ingroup DifferenceEngine
  43  */
  44 class DiffEngine {
  45
  46         // Input variables
  47         private $from;
  48         private $to;
  49         private $m;
  50         private $n;
  51
  52         private $tooLong;
  53         private $powLimit;
  54
  55         protected $bailoutComplexity = 0;
  56
  57         // State variables
  58         private $maxDifferences;
  59         private $lcsLengthCorrectedForHeuristic = false;
  60
  61         // Output variables
  62         public $length;
  63         public $removed;
  64         public $added;
  65         public $heuristicUsed;
  66
  67         function __construct( $tooLong = 2000000, $powLimit = 1.45 ) {
  68                 $this->tooLong = $tooLong;
  69                 $this->powLimit = $powLimit;
  70         }
  71
  72         /**
  73          * Performs diff
  74          *
  75          * @param string[] $from_lines
  76          * @param string[] $to_lines
  77          * @throws ComplexityException
  78          *
  79          * @return DiffOp[]
  80          */
  81         public function diff( $from_lines, $to_lines ) {
  82                 // Diff and store locally
  83                 $this->diffInternal( $from_lines, $to_lines );
  84
  85                 // Merge edits when possible
  86                 $this->shiftBoundaries( $from_lines, $this->removed, $this->added );
  87                 $this->shiftBoundaries( $to_lines, $this->added, $this->removed );
  88
  89                 // Compute the edit operations.
  90                 $n_from = count( $from_lines );
  91                 $n_to = count( $to_lines );
  92
  93                 $edits = [];
  94                 $xi = $yi = 0;
  95                 while ( $xi < $n_from || $yi < $n_to ) {
  96                         assert( $yi < $n_to || $this->removed[$xi] );
  97                         assert( $xi < $n_from || $this->added[$yi] );
  98
  99                         // Skip matching "snake".
 100                         $copy = [];
 101                         while ( $xi < $n_from && $yi < $n_to
 102                                         && !$this->removed[$xi] && !$this->added[$yi]
 103                         ) {
 104                                 $copy[] = $from_lines[$xi++];
 105                                 ++$yi;
 106                         }
 107                         if ( $copy ) {
 108                                 $edits[] = new DiffOpCopy( $copy );
 109                         }
 110
 111                         // Find deletes & adds.
 112                         $delete = [];
 113                         while ( $xi < $n_from && $this->removed[$xi] ) {
 114                                 $delete[] = $from_lines[$xi++];
 115                         }
 116
 117                         $add = [];
 118                         while ( $yi < $n_to && $this->added[$yi] ) {
 119                                 $add[] = $to_lines[$yi++];
 120                         }
 121
 122                         if ( $delete && $add ) {
 123                                 $edits[] = new DiffOpChange( $delete, $add );
 124                         } elseif ( $delete ) {
 125                                 $edits[] = new DiffOpDelete( $delete );
 126                         } elseif ( $add ) {
 127                                 $edits[] = new DiffOpAdd( $add );
 128                         }
 129                 }
 130
 131                 return $edits;
 132         }
 133
 134         /**
 135          * Sets the complexity (in comparison operations) that can't be exceeded
 136          * @param int $value
 137          */
 138         public function setBailoutComplexity( $value ) {
 139                 $this->bailoutComplexity = $value;
 140         }
 141
 142         /**
 143          * Adjust inserts/deletes of identical lines to join changes
 144          * as much as possible.
 145          *
 146          * We do something when a run of changed lines include a
 147          * line at one end and has an excluded, identical line at the other.
 148          * We are free to choose which identical line is included.
 149          * `compareseq' usually chooses the one at the beginning,
 150          * but usually it is cleaner to consider the following identical line
 151          * to be the "change".
 152          *
 153          * This is extracted verbatim from analyze.c (GNU diffutils-2.7).
 154          *
 155          * @param string[] $lines
 156          * @param string[] $changed
 157          * @param string[] $other_changed
 158          */
 159         private function shiftBoundaries( array $lines, array &$changed, array $other_changed ) {
 160                 $i = 0;
 161                 $j = 0;
 162
 163                 assert( count( $lines ) == count( $changed ) );
 164                 $len = count( $lines );
 165                 $other_len = count( $other_changed );
 166
 167                 while ( 1 ) {
 168                         /*
 169                          * Scan forwards to find beginning of another run of changes.
 170                          * Also keep track of the corresponding point in the other file.
 171                          *
 172                          * Throughout this code, $i and $j are adjusted together so that
 173                          * the first $i elements of $changed and the first $j elements
 174                          * of $other_changed both contain the same number of zeros
 175                          * (unchanged lines).
 176                          * Furthermore, $j is always kept so that $j == $other_len or
 177                          * $other_changed[$j] == false.
 178                          */
 179                         while ( $j < $other_len && $other_changed[$j] ) {
 180                                 $j++;
 181                         }
 182
 183                         while ( $i < $len && !$changed[$i] ) {
 184                                 assert( $j < $other_len && !$other_changed[$j] );
 185                                 $i++;
 186                                 $j++;
 187                                 while ( $j < $other_len && $other_changed[$j] ) {
 188                                         $j++;
 189                                 }
 190                         }
 191
 192                         if ( $i == $len ) {
 193                                 break;
 194                         }
 195
 196                         $start = $i;
 197
 198                         // Find the end of this run of changes.
 199                         while ( ++$i < $len && $changed[$i] ) {
 200                                 continue;
 201                         }
 202
 203                         do {
 204                                 /*
 205                                  * Record the length of this run of changes, so that
 206                                  * we can later determine whether the run has grown.
 207                                  */
 208                                 $runlength = $i - $start;
 209
 210                                 /*
 211                                  * Move the changed region back, so long as the
 212                                  * previous unchanged line matches the last changed one.
 213                                  * This merges with previous changed regions.
 214                                  */
 215                                 while ( $start > 0 && $lines[$start - 1] == $lines[$i - 1] ) {
 216                                         $changed[--$start] = 1;
 217                                         $changed[--$i] = false;
 218                                         while ( $start > 0 && $changed[$start - 1] ) {
 219                                                 $start--;
 220                                         }
 221                                         assert( $j > 0 );
 222                                         while ( $other_changed[--$j] ) {
 223                                                 continue;
 224                                         }
 225                                         assert( $j >= 0 && !$other_changed[$j] );
 226                                 }
 227
 228                                 /*
 229                                  * Set CORRESPONDING to the end of the changed run, at the last
 230                                  * point where it corresponds to a changed run in the other file.
 231                                  * CORRESPONDING == LEN means no such point has been found.
 232                                  */
 233                                 $corresponding = $j < $other_len ? $i : $len;
 234
 235                                 /*
 236                                  * Move the changed region forward, so long as the
 237                                  * first changed line matches the following unchanged one.
 238                                  * This merges with following changed regions.
 239                                  * Do this second, so that if there are no merges,
 240                                  * the changed region is moved forward as far as possible.
 241                                  */
 242                                 while ( $i < $len && $lines[$start] == $lines[$i] ) {
 243                                         $changed[$start++] = false;
 244                                         $changed[$i++] = 1;
 245                                         while ( $i < $len && $changed[$i] ) {
 246                                                 $i++;
 247                                         }
 248
 249                                         assert( $j < $other_len && !$other_changed[$j] );
 250                                         $j++;
 251                                         if ( $j < $other_len && $other_changed[$j] ) {
 252                                                 $corresponding = $i;
 253                                                 while ( $j < $other_len && $other_changed[$j] ) {
 254                                                         $j++;
 255                                                 }
 256                                         }
 257                                 }
 258                         } while ( $runlength != $i - $start );
 259
 260                         /*
 261                          * If possible, move the fully-merged run of changes
 262                          * back to a corresponding run in the other file.
 263                          */
 264                         while ( $corresponding < $i ) {
 265                                 $changed[--$start] = 1;
 266                                 $changed[--$i] = 0;
 267                                 assert( $j > 0 );
 268                                 while ( $other_changed[--$j] ) {
 269                                         continue;
 270                                 }
 271                                 assert( $j >= 0 && !$other_changed[$j] );
 272                         }
 273                 }
 274         }
 275
 276         /**
 277          * @param string[] $from
 278          * @param string[] $to
 279          * @throws ComplexityException
 280          */
 281         protected function diffInternal( array $from, array $to ) {
 282                 // remember initial lengths
 283                 $m = count( $from );
 284                 $n = count( $to );
 285
 286                 $this->heuristicUsed = false;
 287
 288                 // output
 289                 $removed = $m > 0 ? array_fill( 0, $m, true ) : [];
 290                 $added = $n > 0 ? array_fill( 0, $n, true ) : [];
 291
 292                 // reduce the complexity for the next step (intentionally done twice)
 293                 // remove common tokens at the start
 294                 $i = 0;
 295                 while ( $i < $m && $i < $n && $from[$i] === $to[$i] ) {
 296                         $removed[$i] = $added[$i] = false;
 297                         unset( $from[$i], $to[$i] );
 298                         ++$i;
 299                 }
 300
 301                 // remove common tokens at the end
 302                 $j = 1;
 303                 while ( $i + $j <= $m && $i + $j <= $n && $from[$m - $j] === $to[$n - $j] ) {
 304                         $removed[$m - $j] = $added[$n - $j] = false;
 305                         unset( $from[$m - $j], $to[$n - $j] );
 306                         ++$j;
 307                 }
 308
 309                 $this->from = $newFromIndex = $this->to = $newToIndex = [];
 310
 311                 // remove tokens not in both sequences
 312                 $shared = [];
 313                 foreach ( $from as $key ) {
 314                         $shared[$key] = false;
 315                 }
 316
 317                 foreach ( $to as $index => &$el ) {
 318                         if ( array_key_exists( $el, $shared ) ) {
 319                                 // keep it
 320                                 $this->to[] = $el;
 321                                 $shared[$el] = true;
 322                                 $newToIndex[] = $index;
 323                         }
 324                 }
 325                 foreach ( $from as $index => &$el ) {
 326                         if ( $shared[$el] ) {
 327                                 // keep it
 328                                 $this->from[] = $el;
 329                                 $newFromIndex[] = $index;
 330                         }
 331                 }
 332
 333                 unset( $shared, $from, $to );
 334
 335                 $this->m = count( $this->from );
 336                 $this->n = count( $this->to );
 337
 338                 if ( $this->bailoutComplexity > 0 && $this->m * $this->n > $this->bailoutComplexity ) {
 339                         throw new ComplexityException();
 340                 }
 341
 342                 $this->removed = $this->m > 0 ? array_fill( 0, $this->m, true ) : [];
 343                 $this->added = $this->n > 0 ? array_fill( 0, $this->n, true ) : [];
 344
 345                 if ( $this->m == 0 || $this->n == 0 ) {
 346                         $this->length = 0;
 347                 } else {
 348                         $this->maxDifferences = ceil( ( $this->m + $this->n ) / 2.0 );
 349                         if ( $this->m * $this->n > $this->tooLong ) {
 350                                 // limit complexity to D^POW_LIMIT for long sequences
 351                                 $this->maxDifferences = floor( pow( $this->maxDifferences, $this->powLimit - 1.0 ) );
 352                                 wfDebug( "Limiting max number of differences to $this->maxDifferences\n" );
 353                         }
 354
 355                         /*
 356                          * The common prefixes and suffixes are always part of some LCS, include
 357                          * them now to reduce our search space
 358                          */
 359                         $max = min( $this->m, $this->n );
 360                         for ( $forwardBound = 0; $forwardBound < $max
 361                                 && $this->from[$forwardBound] === $this->to[$forwardBound];
 362                                 ++$forwardBound
 363                         ) {
 364                                 $this->removed[$forwardBound] = $this->added[$forwardBound] = false;
 365                         }
 366
 367                         $backBoundL1 = $this->m - 1;
 368                         $backBoundL2 = $this->n - 1;
 369
 370                         while ( $backBoundL1 >= $forwardBound && $backBoundL2 >= $forwardBound
 371                                 && $this->from[$backBoundL1] === $this->to[$backBoundL2]
 372                         ) {
 373                                 $this->removed[$backBoundL1--] = $this->added[$backBoundL2--] = false;
 374                         }
 375
 376                         $temp = array_fill( 0, $this->m + $this->n + 1, 0 );
 377                         $V = [ $temp, $temp ];
 378                         $snake = [ 0, 0, 0 ];
 379
 380                         $this->length = $forwardBound + $this->m - $backBoundL1 - 1
 381                                 + $this->lcs_rec(
 382                                         $forwardBound,
 383                                         $backBoundL1,
 384                                         $forwardBound,
 385                                         $backBoundL2,
 386                                         $V,
 387                                         $snake
 388                         );
 389                 }
 390
 391                 $this->m = $m;
 392                 $this->n = $n;
 393
 394                 $this->length += $i + $j - 1;
 395
 396                 foreach ( $this->removed as $key => &$removed_elem ) {
 397                         if ( !$removed_elem ) {
 398                                 $removed[$newFromIndex[$key]] = false;
 399                         }
 400                 }
 401                 foreach ( $this->added as $key => &$added_elem ) {
 402                         if ( !$added_elem ) {
 403                                 $added[$newToIndex[$key]] = false;
 404                         }
 405                 }
 406                 $this->removed = $removed;
 407                 $this->added = $added;
 408         }
 409
 410         function diff_range( $from_lines, $to_lines ) {
 411                 // Diff and store locally
 412                 $this->diff( $from_lines, $to_lines );
 413                 unset( $from_lines, $to_lines );
 414
 415                 $ranges = [];
 416                 $xi = $yi = 0;
 417                 while ( $xi < $this->m || $yi < $this->n ) {
 418                         // Matching "snake".
 419                         while ( $xi < $this->m && $yi < $this->n
 420                                 && !$this->removed[$xi]
 421                                 && !$this->added[$yi]
 422                         ) {
 423                                 ++$xi;
 424                                 ++$yi;
 425                         }
 426                         // Find deletes & adds.
 427                         $xstart = $xi;
 428                         while ( $xi < $this->m && $this->removed[$xi] ) {
 429                                 ++$xi;
 430                         }
 431
 432                         $ystart = $yi;
 433                         while ( $yi < $this->n && $this->added[$yi] ) {
 434                                 ++$yi;
 435                         }
 436
 437                         if ( $xi > $xstart || $yi > $ystart ) {
 438                                 $ranges[] = new RangeDifference( $xstart, $xi, $ystart, $yi );
 439                         }
 440                 }
 441
 442                 return $ranges;
 443         }
 444
 445         private function lcs_rec( $bottoml1, $topl1, $bottoml2, $topl2, &$V, &$snake ) {
 446                 // check that both sequences are non-empty
 447                 if ( $bottoml1 > $topl1 || $bottoml2 > $topl2 ) {
 448                         return 0;
 449                 }
 450
 451                 $d = $this->find_middle_snake( $bottoml1, $topl1, $bottoml2,
 452                         $topl2, $V, $snake );
 453
 454                 // need to store these so we don't lose them when they're
 455                 // overwritten by the recursion
 456                 $len = $snake[2];
 457                 $startx = $snake[0];
 458                 $starty = $snake[1];
 459
 460                 // the middle snake is part of the LCS, store it
 461                 for ( $i = 0; $i < $len; ++$i ) {
 462                         $this->removed[$startx + $i] = $this->added[$starty + $i] = false;
 463                 }
 464
 465                 if ( $d > 1 ) {
 466                         return $len
 467                         + $this->lcs_rec( $bottoml1, $startx - 1, $bottoml2,
 468                                 $starty - 1, $V, $snake )
 469                         + $this->lcs_rec( $startx + $len, $topl1, $starty + $len,
 470                                 $topl2, $V, $snake );
 471                 } elseif ( $d == 1 ) {
 472                         /*
 473                          * In this case the sequences differ by exactly 1 line. We have
 474                          * already saved all the lines after the difference in the for loop
 475                          * above, now we need to save all the lines before the difference.
 476                          */
 477                         $max = min( $startx - $bottoml1, $starty - $bottoml2 );
 478                         for ( $i = 0; $i < $max; ++$i ) {
 479                                 $this->removed[$bottoml1 + $i] =
 480                                         $this->added[$bottoml2 + $i] = false;
 481                         }
 482
 483                         return $max + $len;
 484                 }
 485
 486                 return $len;
 487         }
 488
 489         private function find_middle_snake( $bottoml1, $topl1, $bottoml2, $topl2, &$V, &$snake ) {
 490                 $from = &$this->from;
 491                 $to = &$this->to;
 492                 $V0 = &$V[0];
 493                 $V1 = &$V[1];
 494                 $snake0 = &$snake[0];
 495                 $snake1 = &$snake[1];
 496                 $snake2 = &$snake[2];
 497                 $bottoml1_min_1 = $bottoml1 - 1;
 498                 $bottoml2_min_1 = $bottoml2 - 1;
 499                 $N = $topl1 - $bottoml1_min_1;
 500                 $M = $topl2 - $bottoml2_min_1;
 501                 $delta = $N - $M;
 502                 $maxabsx = $N + $bottoml1;
 503                 $maxabsy = $M + $bottoml2;
 504                 $limit = min( $this->maxDifferences, ceil( ( $N + $M ) / 2 ) );
 505
 506                 // value_to_add_forward: a 0 or 1 that we add to the start
 507                 // offset to make it odd/even
 508                 if ( ( $M & 1 ) == 1 ) {
 509                         $value_to_add_forward = 1;
 510                 } else {
 511                         $value_to_add_forward = 0;
 512                 }
 513
 514                 if ( ( $N & 1 ) == 1 ) {
 515                         $value_to_add_backward = 1;
 516                 } else {
 517                         $value_to_add_backward = 0;
 518                 }
 519
 520                 $start_forward = -$M;
 521                 $end_forward = $N;
 522                 $start_backward = -$N;
 523                 $end_backward = $M;
 524
 525                 $limit_min_1 = $limit - 1;
 526                 $limit_plus_1 = $limit + 1;
 527
 528                 $V0[$limit_plus_1] = 0;
 529                 $V1[$limit_min_1] = $N;
 530                 $limit = min( $this->maxDifferences, ceil( ( $N + $M ) / 2 ) );
 531
 532                 if ( ( $delta & 1 ) == 1 ) {
 533                         for ( $d = 0; $d <= $limit; ++$d ) {
 534                                 $start_diag = max( $value_to_add_forward + $start_forward, -$d );
 535                                 $end_diag = min( $end_forward, $d );
 536                                 $value_to_add_forward = 1 - $value_to_add_forward;
 537
 538                                 // compute forward furthest reaching paths
 539                                 for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
 540                                         if ( $k == -$d || ( $k < $d
 541                                                         && $V0[$limit_min_1 + $k] < $V0[$limit_plus_1 + $k] )
 542                                         ) {
 543                                                 $x = $V0[$limit_plus_1 + $k];
 544                                         } else {
 545                                                 $x = $V0[$limit_min_1 + $k] + 1;
 546                                         }
 547
 548                                         $absx = $snake0 = $x + $bottoml1;
 549                                         $absy = $snake1 = $x - $k + $bottoml2;
 550
 551                                         while ( $absx < $maxabsx && $absy < $maxabsy && $from[$absx] === $to[$absy] ) {
 552                                                 ++$absx;
 553                                                 ++$absy;
 554                                         }
 555                                         $x = $absx - $bottoml1;
 556
 557                                         $snake2 = $absx - $snake0;
 558                                         $V0[$limit + $k] = $x;
 559                                         if ( $k >= $delta - $d + 1 && $k <= $delta + $d - 1
 560                                                 && $x >= $V1[$limit + $k - $delta]
 561                                         ) {
 562                                                 return 2 * $d - 1;
 563                                         }
 564
 565                                         // check to see if we can cut down the diagonal range
 566                                         if ( $x >= $N && $end_forward > $k - 1 ) {
 567                                                 $end_forward = $k - 1;
 568                                         } elseif ( $absy - $bottoml2 >= $M ) {
 569                                                 $start_forward = $k + 1;
 570                                                 $value_to_add_forward = 0;
 571                                         }
 572                                 }
 573
 574                                 $start_diag = max( $value_to_add_backward + $start_backward, -$d );
 575                                 $end_diag = min( $end_backward, $d );
 576                                 $value_to_add_backward = 1 - $value_to_add_backward;
 577
 578                                 // compute backward furthest reaching paths
 579                                 for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
 580                                         if ( $k == $d
 581                                                 || ( $k != -$d && $V1[$limit_min_1 + $k] < $V1[$limit_plus_1 + $k] )
 582                                         ) {
 583                                                 $x = $V1[$limit_min_1 + $k];
 584                                         } else {
 585                                                 $x = $V1[$limit_plus_1 + $k] - 1;
 586                                         }
 587
 588                                         $y = $x - $k - $delta;
 589
 590                                         $snake2 = 0;
 591                                         while ( $x > 0 && $y > 0
 592                                                 && $from[$x + $bottoml1_min_1] === $to[$y + $bottoml2_min_1]
 593                                         ) {
 594                                                 --$x;
 595                                                 --$y;
 596                                                 ++$snake2;
 597                                         }
 598                                         $V1[$limit + $k] = $x;
 599
 600                                         // check to see if we can cut down our diagonal range
 601                                         if ( $x <= 0 ) {
 602                                                 $start_backward = $k + 1;
 603                                                 $value_to_add_backward = 0;
 604                                         } elseif ( $y <= 0 && $end_backward > $k - 1 ) {
 605                                                 $end_backward = $k - 1;
 606                                         }
 607                                 }
 608                         }
 609                 } else {
 610                         for ( $d = 0; $d <= $limit; ++$d ) {
 611                                 $start_diag = max( $value_to_add_forward + $start_forward, -$d );
 612                                 $end_diag = min( $end_forward, $d );
 613                                 $value_to_add_forward = 1 - $value_to_add_forward;
 614
 615                                 // compute forward furthest reaching paths
 616                                 for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
 617                                         if ( $k == -$d
 618                                                 || ( $k < $d && $V0[$limit_min_1 + $k] < $V0[$limit_plus_1 + $k] )
 619                                         ) {
 620                                                 $x = $V0[$limit_plus_1 + $k];
 621                                         } else {
 622                                                 $x = $V0[$limit_min_1 + $k] + 1;
 623                                         }
 624
 625                                         $absx = $snake0 = $x + $bottoml1;
 626                                         $absy = $snake1 = $x - $k + $bottoml2;
 627
 628                                         while ( $absx < $maxabsx && $absy < $maxabsy && $from[$absx] === $to[$absy] ) {
 629                                                 ++$absx;
 630                                                 ++$absy;
 631                                         }
 632                                         $x = $absx - $bottoml1;
 633                                         $snake2 = $absx - $snake0;
 634                                         $V0[$limit + $k] = $x;
 635
 636                                         // check to see if we can cut down the diagonal range
 637                                         if ( $x >= $N && $end_forward > $k - 1 ) {
 638                                                 $end_forward = $k - 1;
 639                                         } elseif ( $absy - $bottoml2 >= $M ) {
 640                                                 $start_forward = $k + 1;
 641                                                 $value_to_add_forward = 0;
 642                                         }
 643                                 }
 644
 645                                 $start_diag = max( $value_to_add_backward + $start_backward, -$d );
 646                                 $end_diag = min( $end_backward, $d );
 647                                 $value_to_add_backward = 1 - $value_to_add_backward;
 648
 649                                 // compute backward furthest reaching paths
 650                                 for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
 651                                         if ( $k == $d
 652                                                 || ( $k != -$d && $V1[$limit_min_1 + $k] < $V1[$limit_plus_1 + $k] )
 653                                         ) {
 654                                                 $x = $V1[$limit_min_1 + $k];
 655                                         } else {
 656                                                 $x = $V1[$limit_plus_1 + $k] - 1;
 657                                         }
 658
 659                                         $y = $x - $k - $delta;
 660
 661                                         $snake2 = 0;
 662                                         while ( $x > 0 && $y > 0
 663                                                 && $from[$x + $bottoml1_min_1] === $to[$y + $bottoml2_min_1]
 664                                         ) {
 665                                                 --$x;
 666                                                 --$y;
 667                                                 ++$snake2;
 668                                         }
 669                                         $V1[$limit + $k] = $x;
 670
 671                                         if ( $k >= -$delta - $d && $k <= $d - $delta
 672                                                 && $x <= $V0[$limit + $k + $delta]
 673                                         ) {
 674                                                 $snake0 = $bottoml1 + $x;
 675                                                 $snake1 = $bottoml2 + $y;
 676
 677                                                 return 2 * $d;
 678                                         }
 679
 680                                         // check to see if we can cut down our diagonal range
 681                                         if ( $x <= 0 ) {
 682                                                 $start_backward = $k + 1;
 683                                                 $value_to_add_backward = 0;
 684                                         } elseif ( $y <= 0 && $end_backward > $k - 1 ) {
 685                                                 $end_backward = $k - 1;
 686                                         }
 687                                 }
 688                         }
 689                 }
 690                 /*
 691                  * computing the true LCS is too expensive, instead find the diagonal
 692                  * with the most progress and pretend a midle snake of length 0 occurs
 693                  * there.
 694                  */
 695
 696                 $most_progress = self::findMostProgress( $M, $N, $limit, $V );
 697
 698                 $snake0 = $bottoml1 + $most_progress[0];
 699                 $snake1 = $bottoml2 + $most_progress[1];
 700                 $snake2 = 0;
 701                 wfDebug( "Computing the LCS is too expensive. Using a heuristic.\n" );
 702                 $this->heuristicUsed = true;
 703
 704                 return 5; /*
 705                 * HACK: since we didn't really finish the LCS computation
 706                 * we don't really know the length of the SES. We don't do
 707                 * anything with the result anyway, unless it's <=1. We know
 708                 * for a fact SES > 1 so 5 is as good a number as any to
 709                 * return here
 710                 */
 711         }
 712
 713         private static function findMostProgress( $M, $N, $limit, $V ) {
 714                 $delta = $N - $M;
 715
 716                 if ( ( $M & 1 ) == ( $limit & 1 ) ) {
 717                         $forward_start_diag = max( -$M, -$limit );
 718                 } else {
 719                         $forward_start_diag = max( 1 - $M, -$limit );
 720                 }
 721
 722                 $forward_end_diag = min( $N, $limit );
 723
 724                 if ( ( $N & 1 ) == ( $limit & 1 ) ) {
 725                         $backward_start_diag = max( -$N, -$limit );
 726                 } else {
 727                         $backward_start_diag = max( 1 - $N, -$limit );
 728                 }
 729
 730                 $backward_end_diag = -min( $M, $limit );
 731
 732                 $temp = [ 0, 0, 0 ];
 733
 734                 $max_progress = array_fill( 0, ceil( max( $forward_end_diag - $forward_start_diag,
 735                                 $backward_end_diag - $backward_start_diag ) / 2 ), $temp );
 736                 $num_progress = 0; // the 1st entry is current, it is initialized
 737                 // with 0s
 738
 739                 // first search the forward diagonals
 740                 for ( $k = $forward_start_diag; $k <= $forward_end_diag; $k += 2 ) {
 741                         $x = $V[0][$limit + $k];
 742                         $y = $x - $k;
 743                         if ( $x > $N || $y > $M ) {
 744                                 continue;
 745                         }
 746
 747                         $progress = $x + $y;
 748                         if ( $progress > $max_progress[0][2] ) {
 749                                 $num_progress = 0;
 750                                 $max_progress[0][0] = $x;
 751                                 $max_progress[0][1] = $y;
 752                                 $max_progress[0][2] = $progress;
 753                         } elseif ( $progress == $max_progress[0][2] ) {
 754                                 ++$num_progress;
 755                                 $max_progress[$num_progress][0] = $x;
 756                                 $max_progress[$num_progress][1] = $y;
 757                                 $max_progress[$num_progress][2] = $progress;
 758                         }
 759                 }
 760
 761                 $max_progress_forward = true; // initially the maximum
 762                 // progress is in the forward
 763                 // direction
 764
 765                 // now search the backward diagonals
 766                 for ( $k = $backward_start_diag; $k <= $backward_end_diag; $k += 2 ) {
 767                         $x = $V[1][$limit + $k];
 768                         $y = $x - $k - $delta;
 769                         if ( $x < 0 || $y < 0 ) {
 770                                 continue;
 771                         }
 772
 773                         $progress = $N - $x + $M - $y;
 774                         if ( $progress > $max_progress[0][2] ) {
 775                                 $num_progress = 0;
 776                                 $max_progress_forward = false;
 777                                 $max_progress[0][0] = $x;
 778                                 $max_progress[0][1] = $y;
 779                                 $max_progress[0][2] = $progress;
 780                         } elseif ( $progress == $max_progress[0][2] && !$max_progress_forward ) {
 781                                 ++$num_progress;
 782                                 $max_progress[$num_progress][0] = $x;
 783                                 $max_progress[$num_progress][1] = $y;
 784                                 $max_progress[$num_progress][2] = $progress;
 785                         }
 786                 }
 787
 788                 // return the middle diagonal with maximal progress.
 789                 return $max_progress[(int)floor( $num_progress / 2 )];
 790         }
 791
 792         /**
 793          * @return mixed
 794          */
 795         public function getLcsLength() {
 796                 if ( $this->heuristicUsed && !$this->lcsLengthCorrectedForHeuristic ) {
 797                         $this->lcsLengthCorrectedForHeuristic = true;
 798                         $this->length = $this->m - array_sum( $this->added );
 799                 }
 800
 801                 return $this->length;
 802         }
 803
 804 }
 805
 806 /**
 807  * Alternative representation of a set of changes, by the index
 808  * ranges that are changed.
 809  *
 810  * @ingroup DifferenceEngine
 811  */
 812 class RangeDifference {
 813
 814         /** @var int */
 815         public $leftstart;
 816
 817         /** @var int */
 818         public $leftend;
 819
 820         /** @var int */
 821         public $leftlength;
 822
 823         /** @var int */
 824         public $rightstart;
 825
 826         /** @var int */
 827         public $rightend;
 828
 829         /** @var int */
 830         public $rightlength;
 831
 832         function __construct( $leftstart, $leftend, $rightstart, $rightend ) {
 833                 $this->leftstart = $leftstart;
 834                 $this->leftend = $leftend;
 835                 $this->leftlength = $leftend - $leftstart;
 836                 $this->rightstart = $rightstart;
 837                 $this->rightend = $rightend;
 838                 $this->rightlength = $rightend - $rightstart;
 839         }
 840
 841 }