gistfile1.php

#!/usr/bin/env php
<?php

// http://twitter.com/#!/Xopus/status/88187651705946112

// "We ask new devs to write program to find equation with 3, 6, 9, 12, 15
//  (all) and +, -, *, / (any) which equals 381. Many struggle. Can you do it?"

// Let's rock! This one is for classic PHP4 style. Very dumb.

function permutations($array) {
    if (count($array) == 1) {
        return array($array);
    } else {
        $result = array();
        foreach ($array as $key => $value) {
            $tail = $array;
            unset($tail[$key]);
            $tail = permutations($tail);
            foreach ($tail as $row) {
                $result[] = array_merge(array($value), $row);
            }
        }
        return $result;
    }
}

function exhaustion($variants, $depth) {
    $result = array();
    foreach ($variants as $variant) {
        if ($depth > 1) {
            $tail = exhaustion($variants, $depth-1);
            foreach ($tail as $row) {
                $result[] = array_merge(array($variant), $row);
            }
        } else {
            $result[] = array($variant);
        }
    }
    return $result;
}

function calculate_RPN($commands) {
    $stack = array();
    while (count($commands)) {
        $cmd = array_shift($commands);
        switch (TRUE) {
            case is_numeric($cmd):
                array_push($stack, $cmd);
                break;
            case in_array($cmd, array('+', '-', '*', '/')):
                if (count($stack) < 2) {
                    return 'Stack underflow';
                }
                $b = array_pop($stack);
                $a = array_pop($stack);
                switch ($cmd) {
                    case '+':
                        $c = $a + $b;
                        break;
                    case '-':
                        $c = $a - $b;
                        break;
                    case '*':
                        $c = $a * $b;
                        break;
                    case '/':
                        if (empty($b)) {
                            return 'Division by zero';
                        }
                        $c = $a / $b; break;
                }
                array_push($stack, $c);
                break;
            default:
                die('Wrong command: '.$cmd);
        }
    }
    return end($stack);
}

$source_numbers = array(3, 6, 9, 12, 15);
$operators = array('+', '-', '*', '/');

$count_row = $count_all = $count_valid = 0;

$commands = exhaustion($operators, count($source_numbers)-1); 
foreach ($commands as $command_set) {
    echo (++$count_row) . ' of ' . count($commands) . "\r";
    $equation_items = array_merge($source_numbers, $command_set);
    foreach (permutations($equation_items) as $equation) {
        $count_all++;
        $result = calculate_RPN($equation);
        if (is_numeric($result)) {
            $count_valid++;
            if ($result == 381) {
                echo join(' ', $equation) . ' = ' . $result . "\n";
            }
        }
    }
}

echo "Tested equations: $count_all, valid: $count_valid\n";