How to generate all permutations of a string in PHP ?

Question

I need an algorithm that return all possible combination of all characters in one string.

I've tried:

$langd = strlen($input);
 for($i = 0;$i < $langd; $i++){
     $tempStrang = NULL;
     $tempStrang .= substr($input, $i, 1);
  for($j = $i+1, $k=0; $k < $langd; $k++, $j++){
   if($j > $langd) $j = 0;
   $tempStrang .= substr($input, $j, 1);
 }
 $myarray[] = $tempStrang;
}

But that only returns the same amount combination as the length of the string.

Say the $input is = "hey", the result would be: hey, hye, eyh, ehy, yhe, yeh.

Answer 1

I'd say this is a typical backtracking problem.

Answer 2

You can use a back tracking based approach to systematically generate all the permutations:

// function to generate and print all N! permutations of $str. (N = strlen($str)).
function permute($str,$i,$n) {
   if ($i == $n)
       print "$str\n";
   else {
        for ($j = $i; $j < $n; $j++) {
          swap($str,$i,$j);
          permute($str, $i+1, $n);
          swap($str,$i,$j); // backtrack.
       }
   }
}

// function to swap the char at pos $i and $j of $str.
function swap(&$str,$i,$j) {
    $temp = $str[$i];
    $str[$i] = $str[$j];
    $str[$j] = $temp;
}   

$str = "hey";
permute($str,0,strlen($str)); // call the function.

Output:

#php a.php
hey
hye
ehy
eyh
yeh
yhe

Answer 3

I would put all the characters in an array, and write a recursive function that will 'stripe out' all the remaining characters. If the array is empty, to a reference passed array.

<?php

$input = "hey";

function string_getpermutations($prefix, $characters, &$permutations)
{
    if (count($characters) == 1)
        $permutations[] = $prefix . array_pop($characters);
    else
    {
        for ($i = 0; $i < count($characters); $i++)
        {
            $tmp = $characters;
            unset($tmp[$i]);

            string_getpermutations($prefix . $characters[$i], array_values($tmp), $permutations);
        }
    }
}
$characters = array();
for ($i = 0; $i < strlen($input); $i++)
    $characters[] = $input[$i];
$permutations = array();

print_r($characters);
string_getpermutations("", $characters, $permutations);

print_r($permutations);

Prints out:

Array
(
    [0] => h
    [1] => e
    [2] => y
)
Array
(
    [0] => hey
    [1] => hye
    [2] => ehy
    [3] => eyh
    [4] => yhe
    [5] => yeh
)

Ah yes, combinations = order doens't matter. permutations = order does matter.

So hey, hye yeh are all the same combination, but 3 separate permutations as mentioned. Watch out that the scale of items goes up very fast. It's called factorial, and is written like 6! = 6*5*4*3*2*1 = 720 items (for a 6 character string). A 10 character string will be 10! = 3628800 permutations already, which is a very big array. In this example it's 3! = 3*2*1 = 6.