Advent of code 2025 solved using Zig

Requirements

• Zig 0.15.x

How to run

> cd day<N>
> zig build run

Day 1

A circular dial puzzle where we track how many times the dial points to zero after executing movement commands.

Input

Commands are read from src/commands.txt. Each line contains a command in the format:

• R<number> - Move right (clockwise) by the specified number of steps
• L<number> - Move left (counter-clockwise) by the specified number of steps

The dial has values 0-99 and wraps around (like a clock with 100 positions).

Output

The program outputs how many times the dial points exactly to zero after executing all commands.

Day 2

Find invalid IDs within given ranges based on repeating digit patterns.

Input

A comma-separated list of ID ranges in the format start-end (e.g., 11-22,95-115,998-1012).

Rules

Part 1: An ID is invalid if the first half of its digits equals the second half.

• Examples: 11, 1010, 446446, 1188511885

Part 2: An ID is invalid if it consists of any repeating sequence of digits (at least 2 repetitions).

• Examples: 111 (1 repeated 3 times), 1010 (10 repeated 2 times), 565656 (56 repeated 3 times), 123123123 (123 repeated 3 times)

Output

The program outputs the sum of all invalid IDs for each part.

Day 3

Select batteries with maximum voltage from each row of a battery grid.

Input

Battery data is read from src/batteries.txt. Each line contains a sequence of digits where each digit represents the voltage of a battery.

Rules

Part 1: Select 2 batteries from each row to maximize voltage.

• Choose the first battery with the maximum digit value
• Choose the second battery with the maximum digit value from positions after the first battery
• Concatenate the two digits as a string to form the voltage (e.g., digits 9 and 8 → voltage 98)

Part 2: Select 12 batteries from each row using the same greedy approach.

• At each step, pick the maximum digit that still allows enough batteries to be selected from remaining positions
• Concatenate all 12 digits to form the voltage

Output

The program outputs the sum of all voltages for each part.

Day 4

Remove lonely rolls from a map iteratively.

Input

A map is read from src/map.txt. The map contains @ characters representing rolls and other characters representing empty space.

Rules

Part 1: Count rolls that have 3 or fewer neighboring rolls (in all 8 directions).

Part 2: Iteratively remove all rolls with 3 or fewer neighbors until no more can be removed.

• In each iteration, find all "lonely" rolls (≤3 neighbors)
• Remove them all simultaneously
• Repeat until no lonely rolls remain
• Count total removed rolls across all iterations

Output

• Part 1: Number of lonely rolls in the initial map
• Part 2: Total number of rolls removed across all iterations

Day 5

Determine fresh ingredients based on ID ranges.

Input

Data is read from db.txt. The file contains two sections separated by a blank line:

1. Fresh ID ranges in format start-end (inclusive)
2. Ingredient IDs to check

Rules

Part 1: Count how many of the listed ingredient IDs fall within any fresh range.

• An ID is fresh if it falls into at least one range
• Ranges can overlap

Part 2: Count the total number of unique fresh IDs across all ranges.

• Merge overlapping/adjacent ranges to avoid double-counting
• Example: ranges 3-5 and 4-7 together cover IDs 3,4,5,6,7 = 5 unique IDs

Output

• Part 1: Number of fresh ingredients from the ID list
• Part 2: Total count of unique fresh IDs across all ranges

Day 8

Connect junction boxes with wires to form circuits based on proximity.

Input

Coordinates of boxes are read from boxes.txt. Each line contains X,Y,Z coordinates.

Rules

Part 1: Connect the 1000 closest pairs of boxes.

• Calculate Euclidean distance between all pairs.
• Sort connections by distance.
• Connect the top 1000 pairs.
• Calculate the product of the sizes of the three largest resulting circuits.

Part 2: Connect boxes until they form a single circuit (Minimum Spanning Tree).

• Continue connecting closest pairs until all boxes are in the same component.
• Multiply the X coordinates of the two boxes connected by the final wire that merges everything into one circuit.

Visualization

A Python script visualize.py is provided to visualize the connection process in 3D.

Requirements: python3, matplotlib, numpy.

# Visualize Part 1 (1000 closest connections)
python3 visualize.py part1

# Visualize Part 2 (Building the MST)
python3 visualize.py part2

# Save as GIF
python3 visualize.py part1 --save