Playing forever
In games that follow the Tetris Guideline, it appears that the player can play forever by mentally dividing the playfield into three heaps, each of which can be considered a separate well. All S, Z, and T tetrominoes go to one side, and L, J, and O tetrominoes go to the other. I tetrominoes go to the center.
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S, T, and Z tetrominoes
Imagine playing Tetris in a 4-block-wide well, with only S, T, and Z tetrominoes using a "bag" randomizer. Each round would consist of three tetrominoes, and in each round the player would have only six different bags to deal with: STZ, SZT, TSZ, TZA, ZST, and ZTS. For each shape of the well floor, I describe how each bag would be played:
Flat
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Becomes 1210 or 0121.
TZS, ZTS:
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TSZ, STZ: mirror image of STZ, TSZ SZT, ZST: hold the first tetromino and play as above
1210
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Becomes 0112.
SZT, ZST, STZ:
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ZTS, TSZ, TZS: hold the T and play as above
0112
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Becomes 2101.
TSZ, TZS, ZTS:
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ZST, STZ, SZT: hold the S and play as above
2101
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Becomes 1021.
ZST, ZTS, SZT:
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TSZ, TZS, STZ: hold the T and play as above
1021
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Becomes 1210.
TSZ, TZS, ZTS:
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ZST, STZ, SZT: hold the Z and play as above
0121
Becomes 2110 through a mirror of the plays used for 1210.
2110
Becomes 1012 through a mirror of the plays used for 0112.
1012
Becomes 1201 through a mirror of the plays used for 2101.
1201
Becomes 0121 through a mirror of the plays used for 1021.
L, J, and O tetrominoes
Well starts and ends flat.
LJO, JLO:
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OLJ, OJL:
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JOL:
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LOJ: mirror image of JOL
I tetromino
The player puts I tetrominoes in the center columns and reaps tetrises.
After each 2 rounds of STZ and LJO, the STZ heap has grown by 6 rows, and the LJO heap has grown by 6 rows. However, the center has grown only by 4 rows.
If the STZ heap becomes too high, and its floor is shaped as 1210, 0121, 2101, or 1012, the player can borrow a T tetromino from the STZ heap, placed on its side, to create a single line in the center. This results in no change to the floor shape, and it works especially in cases where the player would otherwise have to "hold the T and play as above".
If the LJO heap becomes too high, the player can borrow two J tetrominoes to create single lines in the center in the same manner as the Ts. Then the next L and O tetrominoes form a combo square:
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