Advent of Code 2025 - Haskell Solutions

My Advent of Code 2025 solutions, written in literate Haskell and published as a beautiful, readable web book.

📖 Read the full book with explanations

What Makes This Special?

This isn't just code—it's a literate programming book where prose and code live side-by-side. Each day's solution includes:

• Clear problem explanations with concrete examples
• Step-by-step algorithm walkthroughs
• Insights into why certain approaches were chosen
• Clean, functional Haskell code

The solutions prioritize readability first, with performance as a strong second. The goal is code you can understand and learn from, not just code that runs fast.

Project Structure

• 📖 Web Book - Full explanations and solutions
• src/ - Literate Haskell source files (.lhs)
• app/Main.hs - Executable to run and benchmark solutions
• book/ - Pandoc tooling for generating the web book

Performance

While readability is the priority, performance is still important. All solutions run efficiently on modern hardware.

Benchmarks measured using Criterion on an Apple M1 Pro:

Instalation

Requirements

• GHC with compatible base (see package.yaml)
• Z3 4.8.17 (macOS instructions)

stack build

Running Solutions

Warning

This repository doesn't include puzzle inputs, as Advent of Code does not allow redistribution. You'll need to download your inputs and place them in ./assets/dayXX-input.txt.

# Run a specific day and part
stack run aoc -- --day 1 --part 1

# Run benchmarks
scripts/runBenchmarks dry

Building the Web Book

The literate Haskell files are transformed into a beautiful two-column web book where explanations appear on the left and code appears on the right.

Tech Stack:

• Standard Literate Haskell - Uses the markdown subset compatible with GHC and the existing Haskell toolchain (no custom preprocessors needed)
• Pandoc - Converts .lhs files to HTML
• Custom Lua filter - Creates the two-column layout and adds utilities like [h3] Title for custom headers within prose
• Single HTML output - All days compiled into one cohesive document
• Custom templates - Tweaked HTML and CSS for responsive design with dark/light mode support

Build Commands:

# Build the book once (generates docs/index.html)
book/build.sh

# Watch mode (rebuilds on file changes - requires entr)
book/watch.sh

The generated site is published to GitHub Pages from the docs/ directory.

Credits

The two-column layout and Pandoc + Lua filter approach is inspired by @oppi.li's excellent Book of Solves. This implementation differs by:

• Using standard literate Haskell (GHC-compatible) rather than custom markdown
• Enhanced Lua filter with additional parsing utilities
• Single-file HTML output instead of multiple pages
• Custom HTML/CSS templates tailored for this project

AI Disclaimer

An LLM was used to help improve the clarity of the explanations, but none was used to produce code at any point.