Rules and mechanics

How the Seven-Bag Randomiser Works

Pieces are not drawn at random. Every group of seven contains each shape exactly once, which makes the queue countable.

Modern stackers, TETERI included, do not pick each piece at random. They deal from a bag: take the seven shapes, shuffle them, hand them out one at a time, and when the bag is empty refill and shuffle again.

What this guarantees

Two useful properties fall straight out of the rule.

No droughts. With true randomness, the long straight piece might not appear for thirty turns. That happens rarely, but when it does it ends a run for reasons that have nothing to do with your play. With a bag, the longest possible gap between two of the same shape is twelve pieces — last in one bag, first in the next — and even that is uncommon.

No floods. The mirror problem is equally real: four S pieces in a row will wreck a clean stack. A bag makes it impossible to see three of anything in a row.

What it means for how you play

The important consequence is that the queue is countable. Because you know each bag contains exactly one of each shape, you can reason about what has not appeared yet.

If a bag has produced six pieces and none of them was the long piece, the seventh is the long piece. Not probably — certainly. Advanced players use this constantly: knowing an I is guaranteed within a known window is what makes it safe to keep a well open.

Counting in practice

You do not need to track all seven. The useful habit is narrower: keep a rough sense of whether the long piece has come out of the current bag yet.

If it has not, you can commit to a deep well because it is coming. If it came out early, you may be waiting up to twelve pieces for the next — that is the moment to stop deepening the well and clear something.

The implementation

The algorithm is short: fill an array with the seven shapes, shuffle it with a Fisher-Yates pass, and append it to the queue. TETERI keeps two bags' worth queued at all times so the preview never runs dry, and seeds the shuffle from a number the match provides, which is how both players in a duel receive an identical sequence.

In versus modes, an identical sequence is the whole point. Both sides get the same pieces in the same order, so the result is a test of what you do with them rather than who got luckier.

Common questions

What is the seven-bag randomiser?

A system where the seven piece shapes are shuffled and dealt one at a time until the bag is empty, then reshuffled. Each group of seven contains every shape exactly once.

What is the longest possible wait for an I piece?

Twelve pieces. If it is the last piece of one bag and the first of the next, you go twelve turns without seeing another.

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