## Polyomino Combinatorics: How to count distinct n- letter long array permutations

Let’s say we have 3 arrays of following tetromino letters: J, L, O. Tetromino J is a 4 member long array (J[0],J[1],J[2],J[3]), signifying the number of positions J block can take. L tetromino is also a 4 member long array and O is a square tetromino that has only one position (1 member long array). How do we calculate the total number of all unique n-letter long permutations, without programmatically permuting through them? [Read more…]