<?php
    function getRandomNumbers()
    {
        // We start with an empty array and add numbers until we hit 12.
        $result = array();

        // We choose either 3 or 4 as basic number.
        $x = mt_rand(3, 4);

        // Now, as long as we don't hit 12, we iterate through a loop,
        // adding either the basic number, or 2 times the basic number:
        while (array_sum($result) < 12) {

            // Randomly decide
            if (mt_rand(0, 1) > 0) {
                $newElement = 2 * $x; // This is either 6 or 8

                // However, always make sure not to exceed 12:
                if (array_sum($result) + $newElement > 12) {
                    $newElement = $x;
                }

            } else {
                $newElement = $x; // This is either 3 or 4
            }

            // Add the new number to the array:
            $result[] = $newElement;
        }

        // Return the resulting array
        return $result;
    }

    // BEGIN LOOP
    if(have_posts()) :

        // Get the initial array with random numbers
        $randomArray = getRandomNumbers();

        // Open the initial row.
        echo '<div>';

        // CHECK FOR POSTS
        while (have_posts()) : the_post();

            // CREATE ROWS

            if (count($randomArray) < 1) {
                // Close the row and start a new one:
                echo '</div><div>';
                // Get a fresh array with random numbers:
                $randomArray = getRandomNumbers();
            }

            $nextRandomNumber = array_pop($randomArray); // This takes the next number.

            echo '<div class="col-md-' . $nextRandomNumber . '">';
            the_content();
            echo '</div>';

        // END
        endwhile;

        // Close the final row.
        echo '</div>';

    // CLOSE LOOP
    endif;
?>