Advent of Code 2025

Problems

• Day 01 day01.c:
  • Part 1: Simulate rotations on a circular dial based on a sequence of left/right instructions, counting how many times the dial points at 0 after each rotation.
  • Part 2: Count how many times the dial points at 0 during any click of the dial, not just at the end of a rotation.
• Day 02 day02.c:
  • Part 1: Find all product IDs in given ranges that are made of a sequence of digits repeated exactly twice, and sum them.
  • Part 2: Find all product IDs in given ranges that are made of a sequence of digits repeated at least twice, and sum them.
• Day 03 day03.c:
  • Part 1: For each bank of batteries, select two batteries to form the largest possible two-digit joltage, and sum these maximums across all banks.
  • Part 2: For each bank, select twelve batteries to form the largest possible twelve-digit joltage, and sum these maximums across all banks.
• Day 04 day04.c:
  • Part 1: Count rolls of paper on a grid that can be accessed by a forklift (fewer than four adjacent rolls).
  • Part 2: Iteratively remove accessible rolls, updating the grid, and count the total removed.
• Day 05 day05.c:
  • Part 1: Count how many available ingredient IDs are fresh by checking if they fall within any of the given fresh ID ranges.
  • Part 2: Count the total number of unique ingredient IDs considered fresh by all the fresh ID ranges, regardless of availability.
• Day 06 day06.c:
  • Part 1: Solve math problems arranged in vertical columns, applying the specified operation to each group, and sum all results.
  • Part 2: Read problems right-to-left by columns, reconstruct numbers, apply the operation, and sum all results.
• Day 07 day07.c:
  • Part 1: Simulate a downward-moving beam in a grid, splitting at each splitter, and count total splits.
  • Part 2: Count all possible timelines by recursively following every left/right path at each splitter.
• Day 08 day08.c:
  • Part 1: Connect the closest pairs of junction boxes in 3D space, track circuit sizes, and multiply the sizes of the three largest circuits after a set number of connections.
  • Part 2: Continue connecting closest pairs until all boxes are in one circuit, then multiply the X coordinates of the last connected pair.
• Day 09 day09.c:
  • Part 1: Find the largest rectangle in a grid using two red tiles as opposite corners.
  • Part 2: Find the largest rectangle using only red and green tiles, with red tiles as opposite corners.
• Day 10 day10.c/day10.jl:
  • Part 1: Find the minimum button presses to match indicator light diagrams using button wiring schematics for each machine.
  • Part 2: Find the minimum button presses to match joltage requirements using button wiring schematics for each machine.
• Day 11 day11.c
  • Part 1: Find all paths between two nodes in a graph.
  • Part 2: Find all paths between two nodes in a graph that go through two types of marked nodes.
• Day 12 day12.c
  • Part 1: Tiling.
  • Part 2: -

Solution approaches

• Day 01 day01.c:
  • Part 1: (C) Modular arithmetic.
  • Part 2: (C) Modular arithmetic.
• Day 02: day02.c:
  • Part 1: (C) Pattern matching.
  • Part 2: (C) Pattern matching.
• Day 03: day03.c:
  • Part 1: (C) Greedy.
  • Part 2: (C) Greedy.
• Day 04: day04.c:
  • Part 1: (C) Grid simulation.
  • Part 2: (C) Grid simulation.
• Day 05: day05.c:
  • Part 1: (C) Checking intervals.
  • Part 2: (C) Merging intervals.
• Day 06: day06.c:
  • Part 1: (C) Grid simulation.
  • Part 2: (C) Matrix transpose.
• Day 07: day07.c:
  • Part 1: (C) Recursion.
  • Part 2: (C) Recursion.
• Day 08: day08.c:
  • Part 1: (C) Union find.
  • Part 2: (C) Union find.
• Day 09: day09.c
  • Part 1: (C) Sorting, Matrix compression.
  • Part 2: (C) Sorting, Matrix compression, Flood fill.
• Day 10: day10.c/day10.jl:
  • Part 1: (C) Bit mask, Linear algebra in GF(2).
  • Part 2: (Julia) Linear programming. Gave up on trying to implement it in C.
• Day 11 day11.c
  • Part 1: (C) Depth-First Search, Memoization/Dynamic programming
  • Part 2: (C) Depth-First Search, Memoization/Dynamic programming, Bitmask
• Day 12 day12.c
  • Part 1: (C) Heuristic
  • Part 2: -