Piece_RNG.md

Piece RNG

Though TETR.IO has five different types of randomizer, all of them are based upon the same pseudorandom number generator. This RNG is the Lehmer random number generator, specifically MINSTD (a = 16807 and m = 2147483647). The shuffleArray function uses the Fisher–Yates shuffle.

A JavaScript reference implementation closely matching that found in tetrio.js follows:

function rng(seed) {
    let t = seed % 2147483647;

    if (t <= 0) {
        t += 2147483646;
    }

    return {
        next: function () {
            return t = 16807 * t % 2147483647;
        },
        nextFloat: function () {
            return (this.next() - 1) / 2147483646;
        },
        shuffleArray: function (array) {
            if (array.length == 0) {
                return array;
            }

            for (let i = array.length - 1; i != 0; i--) {
                const r = Math.floor(this.nextFloat() * (i + 1));
                [array[i], array[r]] = [array[r], array[i]];
            }

            return array;
        }
    }
}

In singleplayer, replays with no_szo set to true (currently accessible via stride mode) will skip the first piece if it is any of S, Z, or O.

"7-bag"

The 7-bag randomization system generates a set of the seven tetrominos and shuffles them before putting them into the queue. This guarantees that there will never be a gap of more than 12 tetrominos between any two tetrominos of the same type. As with any shuffling algorithm, the order of the input array matters. TETR.IO uses the ordering ZLOSIJT (a rainbow, how cute!) for all purposes.

queue.push(...rng.shuffleArray(["Z", "L", "O", "S", "I", "J", "T"]));

"14-bag"

The 14-bag randomization system is a variant of the 7-bag system with less of a guarantee against droughts. The input ordering is ZLOSIJTZLOSIJT.

queue.push(...rng.shuffleArray(["Z", "L", "O", "S", "I", "J", "T", "Z", "L", "O", "S", "I", "J", "T"]));

"classic"

This is the randomizer used in Tetris for the NES. For each piece, the randomizer rolls an eight-sided die, with seven sides representing seven tetrominos and the eighth side representing a reroll. If a reroll or the same tetromino that was previously generated is rolled, then the randomizer selects randomly from the tetrominos.

const TETROMINOS = ["Z", "L", "O", "S", "I", "J", "T"];

let index = Math.floor(rng.nextFloat() * (TETROMINOS.length + 1));

if (index === lastGenerated || index >= TETROMINOS.length) {
    index = Math.floor(rng.nextFloat() * TETROMINOS.length);
}

lastGenerated = index;
queue.push(TETROMINOS[index]);

"pairs"

This randomizer picks pairs of tetromino types and gives three of each in a random order.

const s = rng.shuffleArray(["Z", "L", "O", "S", "I", "J", "T"]),
const pairs = [s[0], s[0], s[0], s[1], s[1], s[1]];
rng.shuffleArray(pairs);

queue.push(...pairs)

"total mayhem"

This randomizer is entirely random.

const TETROMINOS = ["Z", "L", "O", "S", "I", "J", "T"];
queue.push(TETROMINOS[Math.floor(rng.nextFloat() * TETROMINOS.length)]);