Source: Day 9: Disk Fragmenter

Full solution for today (spoilers!).

Part 1

Given a disk layout alternating between files and empty spaces, move all files as early on the disk is possible, splitting into multiple blocks. Return a checksum on the disk.

Alternating means: 23331 would mean a 2 block file, 3 empty, a 3 block file, 3 empty, and a 1 block file.

The checksum is the sum of file_id * block_index for all occupied blocks. File IDs are assigned sequentially on initial generation.

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Source: Day 8: Resonant Collinearity

Full solution for today (spoilers!).

Part 1

Given a grid with either open tiles (.) or towers (anything else), for each pair of towers, there is an antinode at each of the points that is 2x as far from one tower as the other. How many antinodes are there still within the bounds of the map?

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Source: Rope Bridge

Part 1

Simulate two connected links such that whenever the first link (head) moves, the tail moves to follow according to the following rules:

• If the tail is at the same location as head, don’t move
• If the tail is adjacent to the head (orthogonal or diagonal), don’t move
• If the tail is in the same row/column as the head, move one directly towards it orthogonally
• If the tail is in neither the same row nor column, move one towards diagonally

Count how many unique spaces are visited by the tail of the link.

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