Program.cs

namespace BrainFudge;
using System;
using System.Collections.Generic;

internal class Program
{
    private static string _code;
    private static readonly List<int> _cells = new();
    private static int _currentCell = 0, _currentInstruction = 0, _instructionsRan = 0;

    private static void Main()
    {
        while (true)
        {
            _cells.Clear();
            _cells.Add(0);
            _currentCell = _currentInstruction = _instructionsRan = 0;
            _code = string.Empty;

            Console.WriteLine("Enter BrainFudge code: (press enter twice to run)");

            string input;
            do
            {
                input = Console.ReadLine().Trim();
                _code += input;
            }
            while (input != string.Empty);

            while (_currentInstruction < _code.Length)
            {
                Interpret(_code[_currentInstruction]);

                _currentInstruction++;
                _instructionsRan++;
            }

            Console.WriteLine($"{_instructionsRan} instructions ran.");
            Console.WriteLine("Cell Data:");

            foreach (var x in _cells) Console.WriteLine(x);

            Console.WriteLine();
        }
    }

    private static void Interpret(char c)
    {
        switch (c)
        {
            case '>':
            {
                _currentCell++;
                Expand(_currentCell);
                break;
            }
            case '<':
            {
                _currentCell--;
                Expand(_currentCell);
                break;
            }
            case '+':
            {
                _cells[_currentCell]++;
                break;
            }
            case '-':
            {
                _cells[_currentCell]--;
                break;
            }
            case '.':
            {
                Console.Write((char)_cells[_currentCell]);
                break;
            }
            case ',':
            {
                _cells[_currentCell] = Console.ReadLine()[0];
                break;
            }
            case '[':
            {
                if (_cells[_currentCell] == 0) JumpForward();
                break;
            }
            case ']':
            {
                if (_cells[_currentCell] != 0) JumpBack();
                break;
            }
            default:
            {
                _instructionsRan--;
                break;
            }
        }
    }

    private static void Expand(int cell)
    {
        for (int i = _cells.Count; i <= cell; i++) _cells.Add(0);
    }

    private static void JumpForward()
    {
        _currentInstruction++;

        for (int open = 1; open > 0; _currentInstruction++)
        {
            if (_code[_currentInstruction] == '[') open++;
            else if (_code[_currentInstruction] == ']') open--;
        }

        _currentInstruction--;
    }

    private static void JumpBack()
    {
        _currentInstruction--;

        for (int open = 1; open > 0; _currentInstruction--)
        {
            if (_code[_currentInstruction] == '[') open--;
            else if (_code[_currentInstruction] == ']') open++;
        }

        _currentInstruction++;
    }
}