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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) { $result = ''; for($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); } return $result; } $letters = 'abcd'; echo '2 letters from 4: '; for($i = 0; $i < getPermCount($letters, 2); $i++) echo getPerm($letters, 2, $i).' '; echo ' 3 letters from 4: '; $r = array(); for($i = 0; $i < getPermCount($letters, 3); $i++) $r[] = getPerm($letters, 3, $i); print_r($r);
Output for 4.3.0 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
2 letters from 4: ab ba ca da ac bc cb db ad bd cd dc 3 letters from 4: Array ( [0] => abc [1] => bac [2] => cab [3] => dab [4] => acb [5] => bca [6] => cba [7] => dba [8] => adb [9] => bda [10] => cda [11] => dca [12] => abd [13] => bad [14] => cad [15] => dac [16] => acd [17] => bcd [18] => cbd [19] => dbc [20] => adc [21] => bdc [22] => cdb [23] => dcb )