The Tetris Randomizer algorithm implemented in Groovy. Specifically the TGM2 variant.

TetrisRandomizer2.groovy

def rng = new Random()

def blocks = ['T', 'I', 'L', 'J', 'S', 'Z', 'O']
def history = ['Z', 'Z', 'S', 'S'] as ArrayDeque

def first = true

def nextPiece = {

    def block

    if (first) {

        block = blocks[rng.nextInt(4)]

        history.removeFirst()
        history.addFirst(block)

        first = false
    }
    else {

        for (i in 1..6) {

            block = blocks[rng.nextInt(7)]

            if (!(block in history)) break
        }

        history.remove()
        history.add(block)
    }

    return block
}