# Playing forever

The following outlines a method of playing forever given the following conditions, which apply in many *Tetris* products since 2001:

- the Random Generator is used to generate piece sequences
- the game has the Hold feature
- at least 3 piece previews are available

## Contents

## 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 after T, so use Hold to change the order if necessary:

The T piece must not drop first, so use Hold to change the order if necessary:

Option 1 |
Option 2 |

The T piece must land diagonally adjacent to the other T, so use Hold to change the order if necessary:

Option 1 (needs |
Option 2 (needs |

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. At least 5 previews are required to choose the appropriate construction. Alternatively, it is possible to use 3 previews and clever use of Hold (that does not conflict with the STZ loop's Hold needs) to choose an approriate construction using some advanced techniques.

O first (OJL, OLJ):

O last (JLO, LJO):

JOL: (mirror for LOJ)

Drop J |
Soft-drop O |
Slide O |
Drop L |

#### Advanced techniques for when only 3 previews are available

Worst case bag distributions such as H?XX?X? and H?XXX?? deserve a special mention. The first piece 'H' denotes a piece which must be placed in Hold in order to follow the STZ loop procedure. Pieces from the LJO loop are denoted by '?', and the remaining pieces are denoted by 'X'. Using 3 previews and Hold, it is only possible to see the first 4 pieces of the bag before the second piece enters the screen. This means you only see H?XX, and only know the first piece of the LJO loop. Because H must be put in Hold, you are forced to make a decision without knowing the order of the rest of the LJO loop. If the O comes first, you can follow the procedure above without problems. The rest of the time you will run into complications like this:

Impossible O placement (eg. HLXXJXO, HLXXXJO):

Impossible J placement (eg. HLXXOXJ, HLXXXOJ):

When L or J come first it is impossible to determine which LJO pattern to use without knowing the order of the final 2 pieces. The solution to this problem is to wait until the first L or J piece enters the screen before making a decision. With the held piece, active piece, and 3 previews, you now see 5 pieces into the bag. This allows you to tell the LJO piece order for the case of H?XX?X?. However, in the case of H?XXX?? the order remains unknown. With this final worst case, the STZ loop is guaranteed to be finished before the second and third pieces of the LJO loop are dealt. This means Hold is available! You can start building whichever pattern you prefer, and use Hold to change the order of the final 2 pieces as necessary.

#### Tricky starts when only 3 previews are available

There is a worst case start that only complicates the very first bag when playing forever. With a sequence such as ?HXX?X? you can see a maximum of 4 pieces into the bag as you place the first piece. It is impossible to Hold the first piece (part of LJO) because the second piece (part of SZT) must be held. Additionally, because the second last piece is also part of the STZ loop, Hold cannot be used to change the order of the the final 2 LJO pieces as discussed above. So a decision must be made with only the first 4 pieces of the bag known. In this case, the solution is to note that the STZ pattern in the previous section requires the Z to be placed last. However, the mirror STZ pattern is equally feasible, and would require the S to be placed last. Because you have these 2 options, it is impossible that the first piece dealt for the STZ loop is required to be placed last. The worst case is now ?SZX?X?, which can be expressed as ?XHX?X?. By holding the first piece, placing the second, and holding the third, you can now see the first 6 pieces of the bag. This then allows you to choose an appropriate LJO pattern.

It should be noted that this affects only the very first bag of playing forever, which is not to be confused with the first bag of every 20-bag loop of playing forever. On subsequent loops, the first piece of the first bag is already in Hold. You can see the first 5 pieces of ?HXX?X? after you put H on Hold, which makes the first piece active, and sums to 5 with the 3 previews. It is only a problem for the first bag of the game because the player is restricted to a maximum of one use of Hold before placing a piece.

### 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.

Notice that the alternate JOL method will work only if the first I tetromino of the loop is placed away from the JLO heap.

## 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 |
Though of course, |
Leaving us with a |

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 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.

JIIJ:

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 |
The rows stacked |
Of course, |
...kept things |

Where we left off. |
4 bags balancing |
Back to |

### The Final Bag

The final bag requires some special consideration. Because the sides are getting low, the J piece will not necessarily clear. So I must come before J in order to follow the pattern. You can't rely on using Hold for this as a worst case scenario (eg. TJIxxxx) would also require you to Hold the T. To deal with this problem, you can follow the procedure below.

Stack the I on |
Play the |
The end of the |

But after the first bag of the next loop, we're exactly where we expect to be:

## 5 Bag Solution

A shorter but far more complicated algorithm for Playing Forever in 5 bags was discovered by QM in 2014. It requires 4 piece previews to guarantee success.

This is the minimum possible Playing Forever loop, as a Perfect Clear must use 35 pieces (as the well is 10x10). To avoid parity issues with 5 T pieces, the Bag 2 T is used to clear a single and skim.

### First Bag

Build this structure whenever possible.

If necessary, the JILO square can be built in a different order as long as it remains a 4x4 square.

If this structure was impossible (this is rare), place the I to the right.

### Second Bag

The second bag is built differently based on whether the JILO square was completed successfully or not. In either case, the J, L, O, S, and Z pieces are placed identically, like this:

Again, the J, L, and O can be flipped if necessary.

The I and T pieces will be placed differently based on the completion of the JILO square in Bag 1.

#### Case A: The JILO square in Bag 1 was completed.

The T and I pieces should be used to line clear, in this order:

Leaving the end result of the second bag as this:

The T and I pieces can be placed at any time as long as Hold is used to ensure that the T is placed before the I.

#### Case B: The I piece in Bag 1 was placed to the right.

The I piece should be placed first using Hold to perform a Tetris line clear.

Then, the T piece should be placed in one of these two ways, dependent on the order of Bag 3:

T facing to the left

T facing to the right

The T and I pieces can be placed at any time as long as Hold is used to ensure that the I is placed before the T.

##### Determining the direction of the T

With Hold and 4 piece previews, you can see the first 5 pieces of the third bag when placing the T. Based on the order of STZ in the third bag, decide which direction the T should be placed in using the first rule that applies:

1) If TZ comes first in {STZ}, the first two pieces of {IJLO} are OI, and Z comes before I, place facing left.

2) If T comes first, place facing right.

3) If Z comes first, place facing left.

4) If S comes first and is the only STZ piece in all 5 visible pieces, place facing right.

5) If S comes first and T comes second, place facing right.

6) If none of the previous rules apply, place facing left.

### Bag 3

This bag is highly situational based on the order of the pieces.

#### Case A: The JILO square in Bag 1 was successfully completed.

If this is the case, place the pieces like this. Make sure to use Hold on Z to be sure it is possible.

#### Case B: The JILO square in Bag 1 was not successfully completed.

If this is the case, the S, T, and Z pieces should be placed in one of these two ways:

Decide which one to build based on the T placement from Bag 2.

The I, J, L, and O pieces should be assembled to form a 4x4 square. Note that based on the order, this square can be assembled in whatever arrangement is necessary, including sideways, as long as it forms a square. These are just a few examples of how it could be built.

This can be done without hold unless the order is OIJL or OILJ. In these cases Hold should be used either to modify the piece order to IO{JL} or to O{JL}I. Decide which of these is necessary based on when in the bag Hold is used for the S, T, and Z pieces. This will rarely happen, though.

### Bag 4

The I, J, L, and O pieces in Bag 4 are placed into the holes left from the previous bags.

Occasionally, the O piece may be placed in before the J or L on the corresponding side, as long as the I and J/L piece are already in.

In some cases, an I-spin may be necessary if the I shows up late in the sequence.

#### Placing the S, T, and Z

The first of S/Z and T should be placed like this:

The last S/Z piece should be held to the end. 5 pieces of Bag 5 can then be seen in Hold + 4 piece previews. If T comes first in Bag 5's S, T, and Z, then place the S/Z piece like this:

If S or Z come first in Bag 5's S, T, and Z, place the S/Z piece like this:

### Bag 5

The T piece and one of the S/Z pieces are placed like this:

The other STZ piece is held until the end.

The 4x4 square on the side is filled with the I, J, L, and O pieces (This can be done in any way as long as the area is filled)

The final piece is placed in with an S/Z spin.

## Open questions

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

- All tetris line clears
- High gravity (solved)
- No hold piece
- 14-piece bag randomizer

## See also

- ST stacking, a method of making back-to-back T-spin doubles