MemoryEfficientLongestCommonSubsequenceImplementation.php
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<?php
/*
* This file is part of the Diff package.
*
* (c) Sebastian Bergmann <sebastian@phpunit.de>
*
* For the full copyright and license information, please view the LICENSE
* file that was distributed with this source code.
*/
namespace SebastianBergmann\Diff\LCS;
/**
* Memory-efficient implementation of longest common subsequence calculation.
*/
class MemoryEfficientImplementation implements LongestCommonSubsequence
{
/**
* Calculates the longest common subsequence of two arrays.
*
* @param array $from
* @param array $to
*
* @return array
*/
public function calculate(array $from, array $to)
{
$cFrom = count($from);
$cTo = count($to);
if ($cFrom == 0) {
return array();
} elseif ($cFrom == 1) {
if (in_array($from[0], $to)) {
return array($from[0]);
} else {
return array();
}
} else {
$i = intval($cFrom / 2);
$fromStart = array_slice($from, 0, $i);
$fromEnd = array_slice($from, $i);
$llB = $this->length($fromStart, $to);
$llE = $this->length(array_reverse($fromEnd), array_reverse($to));
$jMax = 0;
$max = 0;
for ($j = 0; $j <= $cTo; $j++) {
$m = $llB[$j] + $llE[$cTo - $j];
if ($m >= $max) {
$max = $m;
$jMax = $j;
}
}
$toStart = array_slice($to, 0, $jMax);
$toEnd = array_slice($to, $jMax);
return array_merge(
$this->calculate($fromStart, $toStart),
$this->calculate($fromEnd, $toEnd)
);
}
}
/**
* @param array $from
* @param array $to
*
* @return array
*/
private function length(array $from, array $to)
{
$current = array_fill(0, count($to) + 1, 0);
$cFrom = count($from);
$cTo = count($to);
for ($i = 0; $i < $cFrom; $i++) {
$prev = $current;
for ($j = 0; $j < $cTo; $j++) {
if ($from[$i] == $to[$j]) {
$current[$j + 1] = $prev[$j] + 1;
} else {
$current[$j + 1] = max($current[$j], $prev[$j + 1]);
}
}
}
return $current;
}
}