Playing forever

The following outlines a method of playing forever given the following conditions:

the Random Generator is used to generate piece sequences
the game has the Hold feature
at least 5 piece previews are available

Standard Procedure

The general method is achieved by dividing the screen into self contained sections as shown below. Specifically, the 4 left columns, the 4 right columns, and the 2 middle columns will be treated as distinct regions, with specific pieces assigned exclusively to each region. Because the random generator provides strings of bags containing each of the 7 pieces in a random order, it is possible to construct a strategy around the relatively small variation, with looping patterns.

S, T, and Z will be placed to the left, L, J, and O will be placed to the right, and I pieces will fill the middle.

The S, T, and Z piece loop

This pattern loops after 4 bags of pieces. Depending on the order of the pieces for each bag, you may need to use Hold to force a piece to come last. Piece previews are technically not required to play this pattern.

The Z piece must drop last, so use Hold to change the order if necessary:

The T piece must drop last, so use Hold to change the order if necessary:

The S piece must drop last, so use Hold to change the order if necessary:

The T piece must drop last, so use Hold to change the order if necessary:

The L, J, and O piece loop

This pattern loops after a single bag of pieces. Depending on the order of the pieces, you will need to use a different construction. 5 piece previews are required to play this pattern, because in the worst case scenario you will need to see the first 6 pieces (active piece + 5 piece previews) in order to determine whether the O piece is coming last.

O first (OJL, OLJ):

O last (JLO, LJO):

JOL: (mirror for LOJ)

Drop J

Soft-drop O

Slide O

Drop L

Alternate method for JOL:

Drop J

Drop O

Soft-drop L

Slide L

The I piece loop

This pattern loops every 2 bags. No use of Hold or piece previews is required. The player must simply alternate putting the I tetromino in columns 5 and 6 to reap tetrises.

Balancing the Stacks

Since the loops have a cycle of 4, 1, and 2 bags respectively, after playing 4 bags they will all be flat. However, the I loop will have only placed 8 rows compared to the 12 rows placed by both the STZ and LJO loops.

The rows stacked
after 4 bags.

Though of course,
lines have cleared.

Leaving us with a
surplus.

In order to offset this balance, it is necessary to change strategy. Continue to do the standard STZ loop, while using the following LO and IJ loops.

The L, O piece loop

This pattern loops after a 2 bags of pieces. Order is not important, and therefore neither Hold nor piece previews are strictly required for it.

The I, J piece loop

This pattern loops after 2 bags of pieces. Order is not important, and therefore neither Hold nor piece previews are strictly required for it.

Because of line clears, the order is not very important. Even if you place pieces in the opposite order than what is shown, the net change in screen geometry will be the same.

Putting it all together

If you play the standard method for 12 bags, the balancing method for 4 bags, and finally the reverse balancing method (with LO on the left and STZ on the right) for 4 bags, then you will clear the entire screen allowing you to start over again and play indefinitely.

The net result of
12 bags stacked
the standard way.

The rows stacked
after 4 bags of
balanced stacking.

Of course,
line clears
have actually...

...kept things
realistically short.

Where we left off.

4 bags balancing
the other side.

Back to
square one.

Open questions

Adapting this method to the following situations is left as an exercise for the reader:

All tetris line clears
High gravity
No hold piece
Fewer previews, as seen in Tetris Worlds for Game Boy Advance and Tetris The Grand Master Ace and Tetris Deluxe and Tetris Evolution
14-piece bag randomizer