AdventOfCode/Days/Day6.cs

using System.Buffers;
using System.Numerics;
using Spectre.Console;

namespace AdventOfCode.Days;

public class Day6 : Day
{
    public override int Number => 6;
    public override string Name => "Trash Compactor";

    private static SearchValues<char> DigitsSearchValue = SearchValues.Create(
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9'
    );

    public override void RunPart1(bool display = true)
    {
        long grandTotal = 0;
        var numberRows = new List<List<long>>(4);
        var symbolRow = new List<char>();

        var rowIndex = 0;
        foreach (var line in Input.EnumerateLines())
        {
            if (rowIndex <= 3)
            {
                numberRows.Add([]);
            }

            foreach (var range in line.Split(' '))
            {
                var span = line[range];

                // Ignore split columns
                if (span.IsWhiteSpace())
                {
                    continue;
                }

                // Numbers
                if (rowIndex <= 3)
                {
                    numberRows[rowIndex].Add(int.Parse(span));
                }
                // Symbols
                else
                {
                    symbolRow.Add(span.Trim()[0]);
                }
            }

            rowIndex++;
        }

        // Do the calculations
        var count = numberRows[0].Count;
        for (var i = 0; i < count; i++)
        {
            checked
            {
                grandTotal += symbolRow[i] switch
                {
                    '+' => numberRows[0][i] + numberRows[1][i] + numberRows[2][i] + numberRows[3][i],
                    '*' => numberRows[0][i] * numberRows[1][i] * numberRows[2][i] * numberRows[3][i],
                    _ => throw new ArgumentOutOfRangeException()
                };
            }
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]The grand total is: [yellow]{grandTotal}[/][/]");
        }
    }

    public override void RunPart2(bool display = true)
    {
        long grandTotal = 0L;
        var numberRows = new List<List<long>>(4);
        var symbolRow = new List<char>();

        var lines = Input.Split(Environment.NewLine);

        for (var i = 0; i < 4; i++)
        {
            numberRows.Add([]);
        }

        var offset = 0;
        while (offset < lines[0].Length)
        {
            // Find offset at which the next set of numbers stop
            var numbersEnd = offset + 1;
            var foundSpaceColumn = false;
            while (!foundSpaceColumn)
            {
                foreach (var line in lines)
                {
                    // If any line does not contain a space, we can go next offset
                    if (line[numbersEnd] is not ' ')
                    {
                        numbersEnd++;
                        foundSpaceColumn = false;

                        break;
                    }

                    foundSpaceColumn = true;
                }
            }

            for (var i = 0; i < numberRows.Count; i++)
            {
                var span = lines[i].AsSpan()[offset..numbersEnd];

                // Parse number with correct offset
                var lastPaddingIndex = span.LastIndexOfAnyExcept(DigitsSearchValue);
                var lastDigitIndex = span.LastIndexOfAny(DigitsSearchValue);

                if (lastPaddingIndex > lastDigitIndex)
                {
                    numberRows[i].Add(long.Parse(span) * (int)Math.Pow(10, lastPaddingIndex - lastDigitIndex));
                }
                else
                {
                    numberRows[i].Add(long.Parse(span));
                }
            }

            // Also get the symbol
            symbolRow.Add(lines[4][offset]);

            // Next pack of numbers starts after the space column
            offset = numbersEnd + 1;
        }

        // Do the calculations
        Span<long> numbersBuffer = stackalloc long[4];
        var count = numberRows[0].Count;
        for (var i = 0; i < count; i++)
        {
            numbersBuffer[0] = numberRows[0][i];
            numbersBuffer[1] = numberRows[1][i];
            numbersBuffer[2] = numberRows[2][i];
            numbersBuffer[3] = numberRows[3][i];

            NumbersToRtl(numbersBuffer);

            grandTotal += symbolRow[i] switch
            {
                '+' => numbersBuffer[0] + numbersBuffer[1] + numbersBuffer[2] + numbersBuffer[3],
                '*' => ValueOrIdentity(numbersBuffer[0]) * ValueOrIdentity(numbersBuffer[1]) * ValueOrIdentity(numbersBuffer[2]) * ValueOrIdentity(numbersBuffer[3]),
                _ => throw new ArgumentOutOfRangeException()
            };
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]The grand total is: [yellow]{grandTotal}[/][/]");
        }

        return;

        void NumbersToRtl(Span<long> numbers)
        {
            Span<char> number0 = stackalloc char[4];
            Span<char> number1 = stackalloc char[4];
            Span<char> number2 = stackalloc char[4];
            Span<char> number3 = stackalloc char[4];

            numbers[0].TryFormat(number0, out _, "D4");
            numbers[1].TryFormat(number1, out _, "D4");
            numbers[2].TryFormat(number2, out _, "D4");
            numbers[3].TryFormat(number3, out _, "D4");

            // Reconstruct numbers
            for (var i = 0; i < 4; i++)
            {
                var number = 0;

                if (number0[i] - '0' is not 0 and var digit0)
                {
                    number = digit0;
                }

                if (number1[i] - '0' is not 0 and var digit1)
                {
                    number = (number * 10) + digit1;
                }

                if (number2[i] - '0' is not 0 and var digit2)
                {
                    number = (number * 10) + digit2;
                }

                if (number3[i] - '0' is not 0 and var digit3)
                {
                    number = (number * 10) + digit3;
                }

                numbers[i] = number;
            }
        }

        long ValueOrIdentity(long value) => value is 0
            ? 1
            : value;
    }
}