Advent of Code 2025 in Zig

My solutions for Advent of Code 2025, written in Zig.

Goals

As an added challenge to myself, I aim to implement each solution with the following constraints:

• Zero allocation: Avoid runtime memory allocation (if possible).
• Utilize Comptime: Use (and abuse) Zig's comptime capabilities to shift work to compile-time.
• No external dependencies: Only the Zig standard library.

Quick Start

Dev Container (Recommended)

This project includes a Dev Container for a pre-configured development environment. Open the project in VS Code and select "Reopen in Container" to get started instantly with Zig and all dependencies ready.

Prerequisites

• Zig (0.16.0-dev.1484+d0ba6642b or later)

Build & Run

# Build the project
zig build

# Run all days
zig build run

# Run a specific day (e.g., day 2)
zig build run -- 2

# Run tests
zig build test

Project Structure

src/
├── main.zig           # CLI entry point
├── root.zig           # Shared utilities (solutions, obfuscation, lookup tables)
├── day_discovery.zig  # Build-time day auto-discovery
├── embed_input.zig    # Build-time input file embedding
├── days/              # Daily solution files
│   ├── one.zig
│   ├── two.zig
│   └── ...
└── inputs/            # Puzzle input files
    ├── one.txt
    ├── two.txt
    └── ...

Architecture

As part of the build we auto-discover and register daily solutions and inputs:

1. Day Discovery (src/day_discovery.zig): Scans src/days/ for .zig files and generates a DayLocator module with all solutions registered automatically.
2. Input Embedding (src/embed_input.zig): Scans src/inputs/ for .txt files and embeds them directly into the binary, making puzzle inputs available at runtime without external file access.
3. Solution Interface: Each day module can export Solution_Part_One and Solution_Part_Two functions that return a numeric or string result.

Adding a New Day

1. Create a solution file: src/days/<day_name>.zig (e.g., four.zig)
2. Create an input file: src/inputs/<day_name>.txt (e.g., four.txt)
3. Implement your solution:

const std = @import("std");
const inputs = @import("input").Input;

const input = inputs.<day_name>.data();

pub fn Solution_Part_One() !u64 {
    // Your solution here
    return 0;
}

pub fn Solution_Part_Two() !u64 {
    // Your solution here
    return 0;
}

Testing

Tests are automatically discovered alongside solutions. Add test blocks in your day files and they'll be included when running zig build test.

Test Patterns

Each day typically includes multiple types of tests:

1. Example Tests: Verify logic against the example inputs provided in the puzzle description
2. Solution Tests: Verify the actual solution against the obfuscated expected answer

// Test with example data from the puzzle description
test "Example Pattern Part 1" {
    const result = try solvePart1(exampleInput);
    try std.testing.expectEqual(42, result);
}

// Verify the actual solution using obfuscated expected values
test "Solution Part One" {
    const result = try Solution_Part_One();
    try std.testing.expectEqual(solutions.DayOne_PartOne.value(), result);
}

Adding Solutions to the Registry

When you solve a new day, add the obfuscated answer to src/root.zig:

1. Run zig build run to get the obfuscated value from the output:

   info: Solution to Day 4 🐦🐦🐦🐦
   info:          Part 1: 12345 (obf: 54321)
   info:          Part 2: 67890 (obf: 98765)
   

2. Add new enum variants (e.g., DayFour_PartOne, DayFour_PartTwo)
3. Add the obfuscated values in the value() switch:

   .DayFour => switch (part) {
       .PartOne => obf(54321),
       .PartTwo => obf(98765),
   },