AdventOfCode/Days/Day3.cs

using Spectre.Console;

namespace AdventOfCode.Days;

public class Day3 : Day
{
    public override int Number => 3;
    public override string Name => "Mull It Over";

    public override void RunPart1(bool display = true)
    {
        var result = 0;

        var input = Input.AsSpan();
        var mulStart = input.IndexOf("mul(");

        while (mulStart != -1)
        {
            input = input[(mulStart + 4)..];

            // Start of a mul operation, search the ending parenthesis and make sure it's before the next mul
            var mulEnd = input.IndexOf(")");
            var nextMulStart = input.IndexOf("mul(");

            if (mulEnd != -1 && (nextMulStart == -1 || mulEnd < nextMulStart))
            {
                var operands = input[..mulEnd];
                var separator = operands.IndexOf(",");

                if (separator != -1
                    && int.TryParse(operands[..separator], out var left)
                    && int.TryParse(operands[(separator + 1)..], out var right))
                {
                    result += left * right;
                }
            }

            mulStart = nextMulStart;
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Result: [yellow]{result}[/][/]");
        }
    }

    public override void RunPart2(bool display = true)
    {
        var result = 0;

        var input = Input.AsSpan();
        var mulStart = input.IndexOf("mul(");

        while (mulStart != -1)
        {
            // Check if there is a "don't()" before next mul
            var dontStart = input.IndexOf("don't()");

            if (dontStart < mulStart)
            {
                // Search for next "do()"
                input = input[(dontStart + 7)..];

                var doStart = input.IndexOf("do()");

                if (doStart == -1)
                {
                    break;
                }

                input = input[(doStart + 4)..];

                // Update next mult position
                mulStart = input.IndexOf("mul(");

                if (mulStart == -1)
                {
                    break;
                }
            }

            input = input[(mulStart + 4)..];

            // Start of a mul operation, search the ending parenthesis and make sure it's before the next mul
            var mulEnd = input.IndexOf(")");
            var nextMulStart = input.IndexOf("mul(");

            if (mulEnd != -1 && (nextMulStart == -1 || mulEnd < nextMulStart))
            {
                var operands = input[..mulEnd];
                var separator = operands.IndexOf(",");

                if (separator != -1
                    && int.TryParse(operands[..separator], out var left)
                    && int.TryParse(operands[(separator + 1)..], out var right))
                {
                    result += left * right;
                }
            }

            mulStart = nextMulStart;
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Result: [yellow]{result}[/][/]");
        }
    }
}