Advent of Code 2025 - Elixir Solutions

Solutions to Advent of Code 2025 implemented in Elixir, with a focus on educational value and thorough documentation.

🤖 The Experiment: 100% AI-Generated

This year's approach was an experiment: use Claude Code to solve 100% of Advent of Code 2025.

Every line of code in this repository was generated by Claude (Opus 4.5), including:

• The initial project setup and framework
• All 12 day solutions
• All test files
• All documentation and writeups
• Even this README

My role was purely supervisory: reading puzzle descriptions, providing inputs, verifying answers on the AoC website, and occasionally providing guidance when Claude needed clarification. The goal was to explore what's possible when AI handles the entire implementation while a human provides oversight.

Results

Metric	Value
Stars earned	24/24 ⭐
Human-written code	0 lines
Total cost (tracked)	$34.45
Cost per star	~$1.44
Hardest day (by cost)	Day 10: $13.10

Claude Contribution Analysis

Commit Statistics

Metric	Value
Total commits	53
Claude co-authored	35 (66%)
Merge commits (GitHub)	18 (34%)
Human-only commits	0

Lines of Code Generated

Category	Lines
Solution code (lib/aoc2025/days/)	2,725
Test files	770
Solution writeups	2,164
Framework/templates	~1,500
Total	~7,159

Cost Breakdown

Day	Cost	Notes
1-3	-	Pre-tracking
4	$1.87
5	$2.71
6	$1.52
7	$1.34
8	$2.14
9	$4.07
10	$13.10	Complex state machine simulation
11	$2.20
12	$5.50	2D bin packing
Total	$34.45

Cost efficiency: ~$0.005 per line of code (half a cent per line)

Goals

• Explore AI Capabilities: Test Claude's ability to solve algorithmic challenges
• Learn Elixir: Use AoC as an opportunity to explore Elixir's functional patterns
• Document Everything: Each solution includes detailed reasoning and explanations
• Share Knowledge: Make solutions accessible to others learning Elixir or algorithms

Quick Start

# Install dependencies
mix deps.get

# Run a specific day
mix day 1

# Run with example input (for testing)
mix example 1

# Run tests for a specific day
mix test.day 1

# Run all example tests
mix test

# Run all tests including full puzzle inputs
mix test --include solution

Project Structure

lib/
  aoc2025.ex              # Main module
  aoc2025/
    day.ex                # Behaviour and helpers for solutions
    input.ex              # Input loading utilities
    days/                 # Daily solutions
      day01.ex
      day02.ex
      ...

priv/
  inputs/                 # Full puzzle inputs
    day01.txt
    examples/             # Example inputs from problem descriptions
      day01.txt

solutions/                # Detailed solution writeups
  day01.md
  day02.md
  ...

test/
  days/                   # Tests for each day
    day01_test.exs

Solution Format

Each day includes:

1. Solution Module (lib/aoc2025/days/dayXX.ex)

   • Implements part1/1 and part2/1
   • Includes parsing logic and helper functions
   • Documented with @moduledoc and @doc

2. Tests (test/days/dayXX_test.exs)

   • Verifies example inputs match expected outputs
   • Tests full puzzle inputs (tagged :solution)

3. Writeup (solutions/dayXX.md)

   • Problem summary
   • Solution approach and reasoning
   • Complexity analysis
   • Lessons learned

Progress

Day	Stars	Solution	Writeup	Cost
1	⭐⭐	day01.ex	day01.md	-
2	⭐⭐	day02.ex	day02.md	-
3	⭐⭐	day03.ex	day03.md	-
4	⭐⭐	day04.ex	day04.md	$1.87
5	⭐⭐	day05.ex	day05.md	$2.71
6	⭐⭐	day06.ex	day06.md	$1.52
7	⭐⭐	day07.ex	day07.md	$1.34
8	⭐⭐	day08.ex	day08.md	$2.14
9	⭐⭐	day09.ex	day09.md	$4.07
10	⭐⭐	day10.ex	day10.md	$13.10
11	⭐⭐	day11.ex	day11.md	$2.20
12	⭐⭐	day12.ex	day12.md	$5.50

Total: 24 stars - Advent of Code 2025 Complete! 🎄

License

MIT