# Difference between revisions of "TGM randomizer"

Most games in Arika's Tetris The Grand Master series randomize the order of tetrominoes using an algorithm that makes successive identical tetrominoes less common. It involves keeping a history of the four most recent tetrominoes and trying to choose a random tetromino not in the history. It "rolls the dice" a given number of times and takes the first tetromino that doesn't match any in the history. TGM1 uses 4 tries; subsequent games using the TGM randomizer use 6 tries.

The history is not a unique list. If the randomizer fails to generate a unique tetromino, which happens about 3.5 percent of the time in a 6-try system, then two or more of one tetromino may occupy elements of the history.

A few additional behaviors exist in the beginning of the game.

• The game never deals an S, Z or O as the first piece.
• The state of the history is initialized to a fixed state:
• In TGM1, the history begins filled with 4 Z pieces.
• In TGM2, the history begins with a Z,S,Z,S sequence.

Tetris The Grand Master Ace does not use the TGM randomizer; it uses TTC's Random Generator algorithm instead.

## Pseudocode

The finite-tries variation used in TGM works as follows:

Function tgmRandomize(history as list of 4 pieces by reference, numTries as integer by value) as piece:
For try = 1 to numTries:
candidatePiece = random element of set {I, J, L, O, S, T, Z}
If candidatePiece is not in history:
Exit loop
Move all tetrominoes in history back by one position
Put candidatePiece at front of history
Return candidatePiece

As numTries increases without bound, the algorithm's behavior approaches the following:

Function tgmRandomizeInf(history as list of 4 pieces by reference) as piece:
candidatePiece = random element of set {I, J, L, O, S, T, Z} minus those pieces that are in history
Move all tetrominoes in history back by one position
Put candidatePiece at front of history
Return candidatePiece