Advent of Code 2025 in Gleam

Solutions for days 1-12 of Advent of Code 2025. Each module in src/aoc_2025/ exposes part1 and part2 functions that accept puzzle input as a string.

Build and test

Install Gleam and Erlang/OTP, then run:

gleam deps download
gleam build
gleam test
gleam format --check src test

The example tests run in a fresh checkout. Regression tests also run when inputs are present at input/2025/01.txt through input/2025/12.txt. If a file is missing, its regression tests are skipped. The input/ and puzzles/ directories are ignored by Git and must never be committed.

download_puzzle.py can download a personal input and puzzle description. It requires Python 3, advent-of-code-data, requests, and the own Advent of Code session cookie in ~/.config/aocd/token or AOC_SESSION:

pip install advent-of-code-data requests
python download_puzzle.py 2025 1

The puzzle inputs and descriptions are personal data and are not included in this repository.

Regression tests only have my valid solutions, if you want to test your inputs, you need to modify the expected solutions. This is not automated.