AdventOfCode/AoC_2025/Day01.cs

using System;
using System.IO;
using System.Linq;
using AdvenOfCode.Contracts;

namespace AoC_2025;

public class Day01 : IPuzzleSolver<int>
{
    private string[] ParsePuzzleInput(string path)
    {
        var puzzleInput = File.ReadAllLines(path)
            .Where(l => !string.IsNullOrWhiteSpace(l))
            .ToArray();
        return puzzleInput;
    }

    public int SolvePart1(string pathToPuzzleInput)
    {
        var rotationInstructions = ParsePuzzleInput(pathToPuzzleInput);
        var dialPosition = 50;
        var timesZeroWasHit = 0;
        foreach (var rotationAmount in rotationInstructions.Select(ParseRotationAmount))
        {
            dialPosition = (dialPosition + rotationAmount) % 100;
            if (dialPosition == 0)
                timesZeroWasHit++;
        }
        return timesZeroWasHit;
    }
    
    public int SolvePart2(string pathToPuzzleInput)
    {
        var rotationInstructions = ParsePuzzleInput(pathToPuzzleInput);
        var dialPosition = 50;
        var timesZeroWasHit = 0;
        foreach (var rotationAmount in rotationInstructions.Select(ParseRotationAmount))
        {
            var actualRotationAmount = 0;
            if (rotationAmount < 0)
            {
                (var fullTurns, actualRotationAmount) = DivMod(rotationAmount, -100);
                timesZeroWasHit += fullTurns;
                if (dialPosition != 0 && dialPosition + actualRotationAmount <= 0)
                    timesZeroWasHit++;
            }
            else
            {
                (var fullTurns, actualRotationAmount) = DivMod(rotationAmount, 100);
                timesZeroWasHit += fullTurns;
                if (dialPosition + actualRotationAmount >= 100)
                    timesZeroWasHit++;
            }

            dialPosition = Mod(dialPosition + actualRotationAmount, 100);
        }
        return timesZeroWasHit;
    }

    private (int div, int mod) DivMod(int number, int divisor)
    {
        return (number / divisor, Mod(number, divisor));
    }

    private int Mod(int number, int divident)
    {
        var mod = number % divident;
        if (number < 0 && divident > 0)
        {
            mod = divident + mod;
        }

        return mod;
    }

    private static int ParseRotationAmount(string puzzleInput)
    {
        var direction = puzzleInput[..1];
        var rotationAmount = int.Parse(puzzleInput[1..]);
        if (direction == "L")
        {
            rotationAmount *= -1;
        }

        return rotationAmount;
    }
}