Tetris DX

Tetris DX
;
Release Information
Developer	Nintendo
Publisher	Nintendo
Platform	Game Boy Color
Release Date	21, Oct 1998 (Japan); 31, Oct 1998 (USA); 1, Jul 1999 (Europe)
Gameplay Information
Next pieces	1
Playfield dimensions	10x18
Rotation system	Bounding box, wall kick, SRS predecessor
Hold piece	✗
Hard drop	✗
Has 180° rotation	N/A
Adjustable tuning	N/A
Websites;

Not to be confused with Tetris DS, a game released for the Nintendo DS.

Tetris DX is a Game Boy Color game that is an enhancement of Nintendo's original Game Boy Tetris, with two added gameplay modes (Ultra, lasting 3 minutes; and Vs. CPU, a match against the computer), a profile/save feature, and color graphics on Game Boy Color and above. In addition, the game's rotation system had a feature which lets a player move any tetromino except O upward by holding the direction against a wall while rotating in the opposite direction.

Modes

Marathon (score maxes at 9,999,999)
Ultra
True 40 lines mode (not 40 "points" like in Tetris Worlds)
Vs. CPU opponent

Rotation system

All rotation states of all seven tetrominoes in TDX. From top to bottom: I, J, L, O, S, T, Z. The circle doesn't appear in the game; it helps to illustrate the axis on which each tetromino rotates.

The rotation system is an early version of what would become SRS. Very interestingly, L, J, T spawn pointing down, while I, S, Z spawn pointing up. TDX first tries rotation about the center of the box (the blue circle). If this fails, it tries rotation about an alternate center displaced by one cell (the gray circle). For I, S, and Z, this center starts out one cell above the ordinary center; for L, J, or T, it starts out one cell below.

Wall climbing

This rotation system is exploitable, especially on low levels:

Charge DAS

Rotate left

Shift left (DAS)

Rotate right about
alternate center

This particular exploit was fixed in SRS, because it uses a different rotation system.

Infinity

Ordinarily, Tetris DX lock delay follows step reset behavior: only downward motion under gravity resets the lock delay timer. But rotation about the alternate center does reset the timer used for gravity. This can be exploited by placing a piece in a corner and rotating it back and forth about the alternate center:

Randomizer

Like in the original GameBoy version, some piece shapes show up more frequently than others - for different reasons though. A test involving around 10,000 pieces resulted in the following piece distributions:

Shape	Chances	Piece ID
I	12.2 %	0
T	24.4 %	1
Z	14.1 %	2
S	11.9 %	3
J	12.8 %	4
L	12.5 %	5
O	12.1 %	6

Almost every fourth dealt piece is T-shaped. According to people trying to read the assembly code, the algorithm works roughly like this:

currentPiece = nextPiece
pieceCount = (pieceCount + 1) % 7

// generate the piece
a = randInt(0..7)
if a == 7 {
    a = pieceCount
}
if a != lastPiece {
    // stick to choice
    lastPiece = a
    rerollCount = 0
} else {
    // change choice
    rerollCount += 1
    a = rerollCount
    if rerollCount >= 5 {
        rerollCount = 0
        go back to the "generate the piece" comment
    }
}
nextPiece = a

The final choice is noted by the variable a in this pseudo code. Each shape is represented by an integer number between 0 and 6, as listed in the table above (piece ID). At first, the algorithm rolls a random integer number between 0 and 7, and a becomes that number - except if the random number was 7, then it will chose 0 to 6 according to the total number of dealt pieces. The algorithm will stick to this choice, if a differs from the last dealt piece.

However, there's also some code with the purpose to reduce the likelihood of receiving the same shape twice in a row, and this part is flawed. There's a variable called rerollCount and it basically counts how often the algorithm will change its choice in a row. The chance to change the choice is slightly below 1/7 = 14.3 %, and rerollCount is almost geometrically distributed according to this chance. rerollCount is most likely 1 in this branch of the code; 2 is the second most common; and 5 is already so unlikely that this if statement can be ignored.

Anyway, if the algorithm changes its choice, then it will reroll the piece exactly once. And in case of the reroll it doesn't set a to a random integer number - it sets a to rerollCount instead. And as explained above, rerollCount is most likely 1 which corresponds to the T shape. And if it's not 1, then it's most likely 2 which corresponds to the Z shape. This explains why the T piece shows up almost twice as often as the other shapes, and why the Z piece is the second most common. It seems the S piece doesn't show up as often as it should according to this code - maybe the random number generation has a slight bias, too.

Power

Power is a value saved against each user profile and displayed on the file select screen. It updates after the player completes a Marathon, Ultra, or 40 Lines game (and finishes watching the celebratory cutscene if applicable).

While not a perfect description, power can be roughly described as a recent score-per-line average.

In Marathon and Ultra modes, power for a single game is calculated using this formula:

$P_{c}={S-D_{1}+D_{2} \over L}\times M$

P_c = Current Game Power
S = Score
D₁ = Soft-drop points % 65,536
D₂ = Soft-drop points % 16
L = Lines % 65,536
M = Line Multiplier
- If L is 0, M is 0
- If L is between 1 and 10, M is 0.25
- If L is between 11 and 15, M is 0.5
- If L is between 16 and 20, M is 0.75
- Else M is 1

This is then combined with your existing power using this formula:

$P_{n}={P_{c}+P_{p}\times N \over N+1}$

P_n = New Profile Power
P_p = Previous Profile Power
N = Number of games played previously, capped at 5

The score value used in this calculation is an internal 4-byte integer tracked separately from the BCD representation used by the score counter. This internal value will continue increasing even after the counter maxes out, up to 4,294,967,295 before overflowing.

The line value is an internal 2-byte integer, also tracked separately from the line counter. This will overflow at 65,536 lines, though this overflow will not be displayed in the line counter.

Bugs

The line counter is capped at 999,999 lines. However, it has a bug where six-digit values will prevent the highest digit from being displayed. For example, a value of 100,000 lines will be displayed in-game as "00000".

The power value displayed on the file select screen has a similar bug, only displaying the last four digits. However, the only way to achieve a five-digit power value is by overflowing the internal lines value in a single Marathon game.

Gallery

External links

Koryan's description of Sega Tetris, TGM, and Tetris DX rotation systems (Japanese language)
- English translation using Google Language Tools

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