Question

I have a set of strings, each string has a variable number of segments separated by pipes (|), e.g.:

$string = 'abc|b|ac';

Each segment with more than one char should be expanded into all the possible one char combinations, for 3 segments the following “algorithm” works wonderfully:

$result = array();
$string = explode('|', 'abc|b|ac');

foreach (str_split($string[0]) as $i)
{
    foreach (str_split($string[1]) as $j)
    {
        foreach (str_split($string[2]) as $k)
        {
            $result[] = implode('|', array($i, $j, $k)); // more...
        }
    }
}

print_r($result);

Output:

$result = array('a|b|a', 'a|b|c', 'b|b|a', 'b|b|c', 'c|b|a', 'c|b|c');

Obviously, for more than 3 segments the code starts to get extremely messy, since I need to add (and check) more and more inner loops. I tried coming up with a dynamic solution but I can’t figure out how to generate the correct combination for all the segments (individually and as a whole). I also looked at some combinatorics source code but I’m unable to combine the different combinations of my segments.

I appreciate if anyone can point me in the right direction.

Answer 1

Recursion to the rescue (you might need to tweak a bit to cover edge cases, but it works):

function explodinator($str) {
    $segments = explode('|', $str);
    $pieces = array_map('str_split', $segments);

    return e_helper($pieces);
}

function e_helper($pieces) {

    if (count($pieces) == 1)
        return $pieces[0];

    $first = array_shift($pieces);
    $subs = e_helper($pieces);

    foreach($first as $char) {
        foreach ($subs as $sub) {
            $result[] = $char . '|' . $sub;
        }
    }

    return $result;
}

print_r(explodinator('abc|b|ac'));

Outputs:

Array
(
    [0] => a|b|a
    [1] => a|b|c
    [2] => b|b|a
    [3] => b|b|c
    [4] => c|b|a
    [5] => c|b|c
)

As seen on ideone.