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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; } $letters = 'abcd'; echo '2 letters from 4:<br>'; for($i = 0; $i < getPermCount($letters, 2); $i++) echo getPerm($letters, 2, $i).'<br>'; echo '<br>3 letters from 4:<br>'; for($i = 0; $i < getPermCount($letters, 3); $i++) echo getPerm($letters, 3, $i). " ";
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:<br>ab<br>ba<br>ca<br>da<br>ac<br>bc<br>cb<br>db<br>ad<br>bd<br>cd<br>dc<br><br>3 letters from 4:<br>abc bac cab dab acb bca cba dba adb bda cda dca abd bad cad dac acd bcd cbd dbc adc bdc cdb dcb