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Bugs & Features
<?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 = 'abcdefgh'; $len = strlen($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 - 7.1.0
Array ( [0] => a [1] => b [2] => c [3] => d [4] => e [5] => f [6] => g [7] => h [8] => ab [10] => ac [11] => ad [12] => ae [13] => af [14] => ag [15] => ah [17] => bc [19] => bd [20] => be [21] => bf [22] => bg [23] => bh [26] => cd [28] => ce [29] => cf [30] => cg [31] => ch [35] => de [37] => df [38] => dg [39] => dh [44] => ef [46] => eg [47] => eh [53] => fg [55] => fh [62] => gh [64] => abc [67] => abd [68] => abe [69] => abf [70] => abg [71] => abh [82] => acd [84] => ace [85] => acf [86] => acg [87] => ach [91] => ade [93] => adf [94] => adg [95] => adh [100] => aef [102] => aeg [103] => aeh [109] => afg [111] => afh [118] => agh [129] => bcd [132] => bce [133] => bcf [134] => bcg [135] => bch [147] => bde [149] => bdf [150] => bdg [151] => bdh [156] => bef [158] => beg [159] => beh [165] => bfg [167] => bfh [174] => bgh [194] => cde [197] => cdf [198] => cdg [199] => cdh [212] => cef [214] => ceg [215] => ceh [221] => cfg [223] => cfh [230] => cgh [259] => def [262] => deg [263] => deh [277] => dfg [279] => dfh [286] => dgh [324] => efg [327] => efh [342] => egh [389] => fgh [400] => abcd [404] => abce [405] => abcf [406] => abcg [407] => abch [427] => abde [429] => abdf [430] => abdg [431] => abdh [436] => abef [438] => abeg [439] => abeh [445] => abfg [447] => abfh [454] => abgh [530] => acde [533] => acdf [534] => acdg [535] => acdh [548] => acef [550] => aceg [551] => aceh [557] => acfg [559] => acfh [566] => acgh [595] => adef [598] => adeg [599] => adeh [613] => adfg [615] => adfh [622] => adgh [660] => aefg [663] => aefh [678] => aegh [725] => afgh [801] => bcde [805] => bcdf [806] => bcdg [807] => bcdh [828] => bcef [830] => bceg [831] => bceh [837] => bcfg [839] => bcfh [846] => bcgh [931] => bdef [934] => bdeg [935] => bdeh [949] => bdfg [951] => bdfh [958] => bdgh [996] => befg [999] => befh [1014] => begh [1061] => bfgh [1202] => cdef [1206] => cdeg [1207] => cdeh [1229] => cdfg [1231] => cdfh [1238] => cdgh [1332] => cefg [1335] => cefh [1350] => cegh [1397] => cfgh [1603] => defg [1607] => defh [1630] => degh [1733] => dfgh [2004] => efgh [2080] => abcde [2085] => abcdf [2086] => abcdg [2087] => abcdh [2116] => abcef [2118] => abceg [2119] => abceh [2125] => abcfg [2127] => abcfh [2134] => abcgh [2275] => abdef [2278] => abdeg [2279] => abdeh [2293] => abdfg [2295] => abdfh [2302] => abdgh [2340] => abefg [2343] => abefh [2358] => abegh [2405] => abfgh [2882] => acdef [2886] => acdeg [2887] => acdeh [2909] => acdfg [2911] => acdfh [2918] => acdgh [3012] => acefg [3015] => acefh [3030] => acegh [3077] => acfgh [3283] => adefg [3287] => adefh [3310] => adegh [3413] => adfgh [3684] => aefgh [4161] => bcdef [4166] => bcdeg [4167] => bcdeh [4197] => bcdfg [4199] => bcdfh [4206] => bcdgh [4356] => bcefg [4359] => bcefh [4374] => bcegh [4421] => bcfgh [4963] => bdefg [4967] => bdefh [4990] => bdegh [5093] => bdfgh [5364] => befgh [6242] => cdefg [6247] => cdefh [6278] => cdegh [6437] => cdfgh [7044] => cefgh [8323] => defgh [8800] => abcdef [8806] => abcdeg [8807] => abcdeh [8845] => abcdfg [8847] => abcdfh [8854] => abcdgh [9060] => abcefg [9063] => abcefh [9078] => abcegh [9125] => abcfgh [10003] => abdefg [10007] => abdefh [10030] => abdegh [10133] => abdfgh [10404] => abefgh [12962] => acdefg [12967] => acdefh [12998] => acdegh [13157] => acdfgh [13764] => acefgh [15043] => adefgh [17601] => bcdefg [17607] => bcdefh [17646] => bcdegh [17861] => bcdfgh [18804] => bcefgh [21763] => bdefgh [26402] => cdefgh [28960] => abcdefg [28967] => abcdefh [29014] => abcdegh [29285] => abcdfgh [30564] => abcefgh [35203] => abdefgh [46562] => acdefgh [57921] => bcdefgh [69280] => abcdefgh )
Output for 5.1.0 - 5.2.8
Warning: Wrong parameter count for array_unique() in /in/RqRd3 on line 37
Output for 5.0.4 - 5.0.5

Process exited with code 137.