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@ 2014-05-15T18:14:07Z

<?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 = 'abcdef';
$len = strlen($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 git.master, git.master_jit, rfc.property-hooks: Array ( [0] => a [1] => b [2] => c [3] => d [4] => e [5] => f [6] => ab [8] => ac [9] => ad [10] => ae [11] => af [13] => bc [15] => bd [16] => be [17] => bf [20] => cd [22] => ce [23] => cf [27] => de [29] => df [34] => ef [36] => abc [39] => abd [40] => abe [41] => abf [50] => acd [52] => ace [53] => acf [57] => ade [59] => adf [64] => aef [73] => bcd [76] => bce [77] => bcf [87] => bde [89] => bdf [94] => bef [110] => cde [113] => cdf [124] => cef [147] => def [156] => abcd [160] => abce [161] => abcf [177] => abde [179] => abdf [184] => abef [230] => acde [233] => acdf [244] => acef [267] => adef [313] => bcde [317] => bcdf [334] => bcef [387] => bdef [470] => cdef [516] => abcde [521] => abcdf [544] => abcef [627] => abdef [830] => acdef [1033] => bcdef [1236] => abcdef )

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2020-03-21 RFC: Is_Literal - declined
2020-12-04 Enumerations - included in PHP 8.1
2018-06-15 Typed Properties v2 - implemented in PHP 7.4
2016-08-12 Object typehint - included in PHP 7.2
2016-08-14 Arrow functions - withdrawn
2016-05-29 array_change_keys() - withdrawn
2016-04-19 Lexical scope support for anonymous classes - suspended
2016-04-17 Functional Interfaces - declined
2016-03-16 Typed Properties - superseded in favor of Typed Properties 2.0
2015-08-27 Callable Prototypes - declined
2015-05-01 Short Closures - declined / withdrawn in favor of Arrow Functions
2015-02-18 Scalar type hints v5 - included in PHP 7
2015-02-14 Union Types - declined
2013-09-22 Anonymous classes - included in PHP 7

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