"Unfolding" a String - php

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.

Answer 2

This looks like a job for recursive programming! :P
I first looked at this and thought it was going to be a on-liner (and probably is in perl).
There are other non-recursive ways (enumerate all combinations of indexes into segments then loop through, for example) but I think this is more interesting, and probably 'better'.
$str = explode('|', 'abc|b|ac');
$strlen = count( $str );
$results = array();
function splitAndForeach( $bchar , $oldindex, $tempthread) {
global $strlen, $str, $results;
$temp = $tempthread;
$newindex = $oldindex + 1;
if ( $bchar != '') { array_push($temp, $bchar ); }
if ( $newindex <= $strlen ){
print "starting foreach loop on string '".$str[$newindex-1]."' \n";
foreach(str_split( $str[$newindex - 1] ) as $c) {
print "Going into next depth ($newindex) of recursion on char $c \n";
splitAndForeach( $c , $newindex, $temp);
}
} else {
$found = implode('|', $temp);
print "Array length (max recursion depth) reached, result: $found \n";
array_push( $results, $found );
$temp = $tempthread;
$index = 0;
print "***************** Reset index to 0 *****************\n\n";
}
}
splitAndForeach('', 0, array() );
print "your results: \n";
print_r($results);

Answer 3

You could have two arrays: the alternatives and a current counter.
$alternatives = array(array('a', 'b', 'c'), array('b'), array('a', 'c'));
$counter = array(0, 0, 0);
Then, in a loop, you increment the "last digit" of the counter, and if that is equal to the number of alternatives for that position, you reset that "digit" to zero and increment the "digit" left to it. This works just like counting with decimal numbers.
The string for each step is built by concatenating the $alternatives[$i][$counter[$i]] for each digit.
You are finished when the "first digit" becomes as large as the number of alternatives for that digit.
Example: for the above variables, the counter would get the following values in the steps:
0,0,0
0,0,1
1,0,0 (overflow in the last two digit)
1,0,1
2,0,0 (overflow in the last two digits)
2,0,1
3,0,0 (finished, since the first "digit" has only 3 alternatives)