AdventOfCode/Days/Day7.cs

using Spectre.Console;

namespace AdventOfCode.Days;

public class Day7 : Day
{
    public override int Number => 7;
    public override string Name => "Bridge Repair";

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

        foreach (var line in Input.AsSpan().EnumerateLines())
        {
            // Read input equation
            var equationStart = line.IndexOf(':');

            var target = long.Parse(line[..equationStart]);

            var numbers = new List<long>();
            var equationNumbers = line[(equationStart + 2)..];

            foreach (var range in equationNumbers.Split(' '))
            {
                numbers.Add(int.Parse(equationNumbers[range]));
            }

            // Check if equation is possible
            if (CheckEquation(target, numbers.ToArray().AsSpan()[1..], numbers[0]))
            {
                calibrationResult += target;
            }
        }

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

        return;

        bool CheckEquation(long target, ReadOnlySpan<long> numbers, long result)
        {
            // Early return when result is greater than target because result is increasing
            if (result > target)
            {
                return false;
            }

            if (numbers.IsEmpty)
            {
                return result == target;
            }

            var sumResult = result + numbers[0];
            var multiplyResult = result * numbers[0];

            return CheckEquation(target, numbers[1..], sumResult)
                   || CheckEquation(target, numbers[1..], multiplyResult);
        }
    }

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

        foreach (var line in Input.AsSpan().EnumerateLines())
        {
            // Read input equation
            var equationStart = line.IndexOf(':');

            var target = long.Parse(line[..equationStart]);

            var numbers = new List<long>();
            var equationNumbers = line[(equationStart + 2)..];

            foreach (var range in equationNumbers.Split(' '))
            {
                numbers.Add(int.Parse(equationNumbers[range]));
            }

            // Check if equation is possible
            if (CheckEquation(target, numbers.ToArray().AsSpan()[1..], numbers[0]))
            {
                calibrationResult += target;
            }
        }

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

        return;

        bool CheckEquation(long target, ReadOnlySpan<long> numbers, long result)
        {
            // Early return when result is greater than target because result is increasing
            if (result > target)
            {
                return false;
            }

            if (numbers.IsEmpty)
            {
                return result == target;
            }

            var sumResult = result + numbers[0];
            var multiplyResult = result * numbers[0];
            var concatResult = result * (int)Math.Pow(10, (int)Math.Log10(numbers[0]) + 1) + numbers[0];

            return CheckEquation(target, numbers[1..], sumResult)
                   || CheckEquation(target, numbers[1..], multiplyResult)
                   || CheckEquation(target, numbers[1..], concatResult);
        }
    }
}