/// <summary>
/// https://adventofcode.com/2020/day/8#part2
/// </summary>
public static void Part2()
{
var program = HandheldProgram.Parse(Input.Get(8));
for (var i = 0; i < program.Instructions.Count(); i++)
{
var modified = ModifyProgram(program, i);
if (modified is null)
{
continue;
}
var computer = new HandheldComputer(modified);
var visited = new HashSet <int>();
while (!computer.IsHalted && !visited.Contains(computer.ProgramCounter))
{
visited.Add(computer.ProgramCounter);
computer.Step();
}
if (computer.IsHalted)
{
Console.WriteLine(computer.Accumulator);
return;
}
}
}
public static HandheldProgram?ModifyProgram(HandheldProgram program, int line)
{
var instruction = program.Instructions.ElementAt(line);
return(instruction.Code switch
{
InstructionCode.Acc => null,
InstructionCode.Jmp => new HandheldProgram(program.Instructions.Take(line)
.Concat(new[] { new Instruction(InstructionCode.Nop, instruction.Value) })
.Concat(program.Instructions.Skip(line + 1))),
InstructionCode.Nop => new HandheldProgram(program.Instructions.Take(line)
.Concat(new[] { new Instruction(InstructionCode.Jmp, instruction.Value) })
.Concat(program.Instructions.Skip(line + 1))),
_ => throw new NotImplementedException(),
});
/// <summary>
/// https://adventofcode.com/2020/day/8
/// </summary>
public static void Part1()
{
var program = HandheldProgram.Parse(Input.Get(8));
var computer = new HandheldComputer(program);
var accumulator = computer.Accumulator;
var visited = new HashSet <int>();
while (!visited.Contains(computer.ProgramCounter))
{
accumulator = computer.Accumulator;
visited.Add(computer.ProgramCounter);
computer.Step();
}
Console.WriteLine(accumulator);
}
public HandheldComputer(HandheldProgram program)
{
Program = program;
}
