Implement 2048 tilt move

Q: Implement 2048 tilt move

This question evaluates a candidate's ability to implement array and matrix manipulation, deterministic state-transition logic, and rule-based merging behavior in a grid-based simulation.

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This is a hard difficulty Coding & Algorithms question, commonly asked during Technical Screen rounds at Glean.

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This question is commonly asked for Software Engineer candidates at Glean during technical interviews.

Question

Problem: Implement a 2048 “tilt” (move) operation

You are given a 4×4 board for the game 2048. Each cell contains either 0 (empty) or a power of two (2, 4, 8, ...). When the player swipes in one of four directions (up, down, left, right), the board updates as follows:

Shift: All non-zero tiles move as far as possible in the swipe direction.
Merge: If two tiles with the same value collide during the move, they merge into one tile with value equal to the sum (i.e., doubled).
Single-merge rule: A tile can be merged at most once per move .
After merges, tiles continue shifting to fill gaps in the swipe direction.

Task

Implement a function that applies exactly one swipe (tilt) to the board.

Function signature (language-agnostic)

Input:
- board : 4×4 integer matrix
- dir : one of {up, down, left, right}
Output:
- The updated 4×4 board after the move

Examples

Swipe left:
- [2, 0, 2, 4] → [4, 4, 0, 0]
Swipe left (single-merge rule):
- [2, 2, 2, 0] → [4, 2, 0, 0] (not [8, 0, 0, 0] )
Swipe left:
- [4, 4, 4, 4] → [8, 8, 0, 0]

Constraints / Notes

Board size is always 4×4.
Only implement the tilt/move logic (no random tile spawning, no scoring required unless you choose to return it as an extra).
Be careful with edge cases around multiple merges in a line and ensuring each tile merges at most once per move.