skilleo Squad Formation Curse

gistfile1.php

<?php

$n = 6;
$k = 3;
$squads = 0;

$pattern = 'R??L?R';

$curse = '';
for($i = 0; $i < $k; $i++) { $curse .= 'R'; }

$positions = substr_count($pattern, '?');
$replacements = [];
for($i = 0; $i <= pow(2, $n); $i++) {
   $tmp = substr(sprintf("%0".$positions."d", decbin($i)), $positions-($positions*2));
   $replacements[$tmp] = str_split($tmp);
}

foreach($replacements as $r) {
    $tmp = $pattern;
    
    $i = 0;
    while(($pos = strpos($tmp, '?')) !== false) {
        $tmp = substr_replace($tmp, ($r[$i] ? 'R' : 'L'), $pos, 1);
        $i++;
    }
    
    if (strpos($tmp, $curse) === false) {
        $squads++;
    }
}

echo $squads."\n";