13

2019, Day 13: Care Package

As you ponder the solitude of space and the ever-increasing three-hour roundtrip for messages between you and Earth, you notice that the Space Mail Indicator Light is blinking. To help keep you sane, the Elves have sent you a care package.

It's a new game for the ship's arcade cabinet! Unfortunately, the arcade is all the way on the other end of the ship. Surely, it won't be hard to build your own - the care package even comes with schematics.

Part 1

The arcade cabinet runs Intcode software like the game the Elves sent (your puzzle input). It has a primitive screen capable of drawing square tiles on a grid. The software draws tiles to the screen with output instructions: every three output instructions specify the x position (distance from the left), y position (distance from the top), and tile id. The tile id is interpreted as follows:

• 0 is an empty tile. No game object appears in this tile.
• 1 is a wall tile. Walls are indestructible barriers.
• 2 is a block tile. Blocks can be broken by the ball.
• 3 is a horizontal paddle tile. The paddle is indestructible.
• 4 is a ball tile. The ball moves diagonally and bounces off objects.

For example, a sequence of output values like 1,2,3,6,5,4 would draw a horizontal paddle tile (1 tile from the left and 2 tiles from the top) and a ball tile (6 tiles from the left and 5 tiles from the top).

Start the game. How many block tiles are on the screen when the game exits?

Your puzzle answer was 320.

Part 2

The game didn't run because you didn't put in any quarters. Unfortunately, you did not bring any quarters. Memory address 0 represents the number of quarters that have been inserted; set it to 2 to play for free.

The arcade cabinet has a joystick that can move left and right. The software reads the position of the joystick with input instructions:

• If the joystick is in the neutral position, provide 0.
• If the joystick is tilted to the left, provide -1.
• If the joystick is tilted to the right, provide 1.

The arcade cabinet also has a segment display capable of showing a single number that represents the player's current score. When three output instructions specify X=-1, Y=0, the third output instruction is not a tile; the value instead specifies the new score to show in the segment display. For example, a sequence of output values like -1,0,12345 would show 12345 as the player's current score.

Beat the game by breaking all the blocks. What is your score after the last block is broken?

Your puzzle answer was 15156.

Solution Notes

This is a proper simulation of a Breakout game! The hard (or rather: hard to explain in prose) problems like ball/paddle/brick physics are encoded in the Intcode program, and all the user has to do is provide input. Playing the game as a human is nearly impossible; it's too long and requires very precisely timed inputs. So, the actual task is to write a ... well, let's say "AI" for it: Track the ball and paddle positions and have the paddle follow the ball all the time.

For golfing, some shortcuts can be made: Part 1 draws the playfield exactly once, without any overdraw, so counting the drawn block tiles is sufficient. Part 2 doesn't require any kind of display or framebuffer either: The ball and paddle's X positions are simply latched whenever the objects are drawn, and the score is stored if something with a negative X coordinate is output. The Y coordinate isn't used at all.

The golf version expands on this and stores just two dictionaries: The first (w) is a map from X coordinates to values; for X >= 0, it doesn't contain anything meaningful (it's the tile ID of the last painted tile in that column), but w[-1] contains the current score. The other dictionary (z) maps tile IDs to their most recently seen X coordinate, which can then be used to look up the position of the paddle (z[3]) and ball (z[4]). This dictionary also contains a lot of bogus entries for each score that has been seen in the past (as score updates are just writes of arbitrary large values); in theory, a score of 3 or 4 could thus disturb the ball/paddle logic. In practice, the score increments per cleared block are around 50 though, so such a situation doesn't occur.

The non-golf implementation contains a console visualization:

./intcode.py 13.2-vis

• Part 1, Python: 432 bytes, ~100 ms
• Part 2, Python: 517 bytes, ~3 s