public static int Part1(string[] input)
{
var vm = new HandheldVm(input.Select(HandheldInstruction.Parse));
var executed = new HashSet <int>();
for (;;)
{
var accumBefore = vm.Accumulator;
if (!executed.Add(vm.Step()))
{
return(accumBefore);
}
}
}
public static int Part2(string[] input)
{
var program = input.Select(HandheldInstruction.Parse).ToImmutableList();
for (var line = 0; line < program.Count; line++)
{
var instruction = program[line];
if (instruction.Opcode == Opcode.acc)
{
continue;
}
var variantInstr = instruction.WithOpcode(instruction.Opcode == Opcode.jmp ? Opcode.nop : Opcode.jmp);
var variantProgram = program.SetItem(line, variantInstr);
// Run the program until it terminates (fail on loop)
var vm = new HandheldVm(variantProgram);
var executed = new HashSet <int>();
var didLoop = false;
while (!vm.Terminated)
{
if (!executed.Add(vm.Step()))
{
didLoop = true;
break;
}
}
if (!didLoop)
{
return(vm.Accumulator);
}
}
throw new ApplicationException("No solution found");
}
