Tetris AI

How the AI works

Every time a new piece spawns, the AI considers every legal placement — each combination of hold-swap (yes/no), column (0–9), and rotation (0–3). That's at most 80 candidates. For each candidate it simulates dropping the piece straight down onto the current board and scores the resulting position with a penalty function. The placement with the lowest penalty wins.

The search depth is 1 ply — it doesn't look ahead at the next piece. Two hand-coded shortcuts come first: the very first piece is always swapped into the hold slot, and an I-piece is dropped vertically into column 8 whenever every other column is already at least 4 rows higher (preserving a "Tetris well" on the right edge).

The penalty function

The score is a sum of penalty terms, each multiplied by a weight. Lower is better — a perfectly flat empty board scores 0.

• Height difference — the gap between the tallest and shortest column (ignoring the rightmost). Encourages a flat surface.
• Covered cells — empty cells with at least one block above them in the same column. These are unreachable holes; weighted heavily.
• Covered (rightmost column) — same idea but penalized harder, because the right column is reserved for the I-well.
• Height (rightmost column) — small penalty for any height in the rightmost column. Keeps the well open.
• Consecutive change — penalizes two adjacent column-height changes in a row (a zigzag surface).
• Small pit — a single column that is at least 2 rows lower than both its neighbours (the minimum gap is exactly 2). Hard to fill without an S/Z mistake.
• Big pit — same shape but the gap is 3+ rows deep. Only an I-piece can fill it.

Current weights

Weight	Value
HEIGHT_DIFFERENCE_FACTOR	5.27
COVERED_PENALTY	19.65
COVER_RIGHTMOST_PENALTY	50.00
HEIGHT_RIGHTMOST_PENALTY	5.00
CONSECUTIVE_CHANGE_PENALTY	1.71
SMALL_PIT_PENALTY	5.60
BIG_PIT_PENALTY	22.46

These weights were tuned by the original author with random search. The Penalty readout in the stats panel shows the current board's score using these weights — green is good, red is risky.

Close