These are my solutions to Advent of Code 2023. They are all coded in Java, needing Java 21 and --enable-preview (for string templates).

Index

• Day 1: solution (extracting integers from strings).
• Day 2: solution (possible draws from a bag of coloured cubes).
• Day 3: solution (extracting integers from a grid with adjacent characters).
• Day 4: solution (evaluating scratch cards).
• Day 5: solution (tracing number ranges through a sequence of maps).
• Day 6: solution (timing races).
• Day 7: solution (evaluating poker hands).
• Day 8: solution (sequences of left-right choices through a graph).
• Day 9: solution (successive differences).
• Day 10: solution (length and enclosed area of a path).
• Day 11: solution (cosmic expansion).
• Day 12: solution (matching wildcards against counts).
• Day 13: solution (searching for reflections).
• Day 14: solution (cycles of rolling rocks).
• Day 15: solution (hashing).
• Day 16: solution (tracing a beam with splitting).
• Day 17: solution (least-cost path with constraints).
• Day 18: solution (area enclosed by a path).
• Day 19: solution (DFA with conditions on 4 variables).
• Day 20: solution (cycling behaviour of circuits).
• Day 21: solution (counting paths in a graph).
• Day 22: solution (3D bricks falling vertically).
• Day 23: solution (longest path through a graph).
• Day 24: solution (intersections of 3D lines).
• Day 25: solution (finding a 3-cut in a large graph).

Acknowledgements

All solutions are entirely the work of Éamonn McManus except as noted below.

• Day 17 (least-cost path with constraints on moves)

  I gave up before finding the right Dynamic Programming approach. I ended up copying my approach from David Brownman.

• Day 20 (pulse propagation)

  I had a strong suspicion of what the right approach might be for Part 2 but would have needed to investigate in detail to confirm. Instead I looked online and found this description by Dazbo, which confirmed my suspicion. Then solving was straightforward.

• Day 21 (counting paths in an infinite graph)

  I was not really motivated to put in the work for Part 2 so I outright cheated, by copying this solution by Michiel Graat.

• Day 24 (intersections of 3-dimensional lines)

  This curious puzzle was entirely solvable with algebra, the computer only serving to solve simultaneous equations. I did the first part myself, but did not find the right approach for the second part on my own. The excellent explanation and solution by @dirk527 showed me the right path, but then there wasn't much for me to write.