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run code in 150+ php & hhvm versions
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<?php // Returns the total number of $count-length strings generatable from $letters. function getPermCount($letters, $count) { $result = 1; // k characters from a set of n has n!/(n-k)! possible combinations for($i = strlen($letters) - $count + 1; $i <= strlen($letters); $i++) { $result *= $i; } return $result; } // Decodes $index to a $count-length string from $letters, no repeat chars. function getPerm($letters, $count, $index) { $result = array(); for($i = 0; $i < $count; $i++) { $pos = $index % strlen($letters); $result[] = $letters[$pos]; $index = ($index-$pos)/strlen($letters); $letters = substr($letters, 0, $pos) . substr($letters, $pos+1); } sort($result); return implode("", $result); } $r = array(); //$letters = 'abcdefghijklm'; $letters = 'abc'; $len = strlen($letters); for ($c = 1; $c <= $len; $c++) for($i = 0; $i < getPermCount($letters, $c); $i++) $r[] = getPerm($letters, $c, $i); print_r(array_unique($r, SORT_REGULAR));
Output for 5.2.9 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
Array ( [0] => a [1] => b [2] => c [3] => ab [5] => ac [7] => bc [9] => abc )
Output for 4.3.0 - 5.2.8
Warning: Wrong parameter count for array_unique() in /in/AvGAk on line 37