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AdventOfCode_C

Advent of Code solutions written in C, with a small framework for fetching inputs and timing each part.

Requirements

  • CMake 3.16+
  • A C11 compiler (GCC or Clang)
  • libcurl (for fetching puzzle inputs)

Build

cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build

This produces two binaries in build/:

Binary Purpose
aoc Run a solution
fetch_input Download a puzzle input from adventofcode.com

Running a solution

./run.sh <year> <day>

run.sh fetches the input automatically if it is not already present, then runs the solution. The build directory defaults to ./build and can be overridden:

BUILD_DIR=/path/to/build ./run.sh 2025 1

To run directly:

./build/aoc run 2025 1

Fetching inputs manually

Create a .env file in the project root with your session cookie:

AOC_SESSION=your_session_cookie_here

Then:

./build/fetch_input 2025 1

Inputs are saved to inputs/year<YYYY>/day<DD>.txt.

Adding a new solution

  1. Create src/solutions/year<YYYY>/day<DD>.c and day<DD>.h following the pattern in year2025/day01.*.
  2. Register the function in src/solutions/year<YYYY>/year<YYYY>.c by appending it to the days[] array.
  3. Re-run CMake (the build system picks up new .c files via GLOB_RECURSE automatically after reconfiguring).

To add a new year, create year<YYYY>.c/year<YYYY>.h and add an entry to the years[] table in src/solutions/solutions.c.

Project structure

src/
  common.c/h          — shared utilities (I/O, timing, string helpers)
  main.c              — entry point
  runner.c/h          — CLI argument parsing and solution dispatch
  fetch_input.c/h     — input downloader (libcurl)
  fetch_main.c        — entry point for fetch_input binary
  solutions/
    solutions.c/h     — year dispatch table
    year2025/
      year2025.c/h    — day dispatch table for 2025
      day01.c/h       — solution for day 1
inputs/
  year2025/
    day01.txt         — puzzle inputs (not tracked in git)