34 lines
1.0 KiB
PHP
34 lines
1.0 KiB
PHP
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<?php
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$number = 600851475143;
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echo max(factorize($number));
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/**
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* Returns a sorted array of the prime factorization of $num
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* @staticvar array $aFactors
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* @param type int Number to factorize
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* @return type array Prime factors
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*/
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function factorize($num) {
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// Returns a sorted array of the prime factorization of $num
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// Caches prior results. Returns empty array for |$num|<2
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// eg. factorize(360) => [5, 3, 3, 2, 2, 2]
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static $aFactors = array();
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if (2 > $num = abs($num))
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return array(); // negatives, 1, 0
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if ($aFactors[$key = "x$num"]) { // handles doubles
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// Been there, done that
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if (($factor = $aFactors[$key]) == $num)
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return array($num);
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return array_merge(factorize($num / $factor), array($factor));
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}
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// Find a smallest factor
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for ($sqrt = sqrt($num), $factor = 2; $factor <= $sqrt; ++$factor)
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if (floor($num / $factor) == $num / $factor)
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return array_merge(factorize($num / $factor), array($aFactors[$key] = $factor));
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return (array($aFactors[$key] = $num));
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}
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