static int SolveAlternate()
{
var machine = new Machine(TextFile.ReadStringList("input.txt"));
var target = machine.Program.Count;
var originalRun = machine.RunFrom(0);
foreach (var i in originalRun)
{
var originalInstruction = machine.Program[i];
var flippedInstruction = InstructionFactory.Flip(originalInstruction);
machine.Program[i] = flippedInstruction;
var visited = machine.RunFrom(i);
if (visited.Contains(target))
{
machine.AC = 0;
machine.PC = 0;
machine.RunFrom(0);
return(machine.AC);
}
machine.Program[i] = originalInstruction;
}
return(-1);
}
public Machine(List <string> program)
{
Program = program.Select(l => InstructionFactory.Parse(l)).ToList();
}
static void Main(string[] args)
{
int acWithoutFlip = 0;
int acAfterFlip = 0;
Performance.TimeRun("Read and solve", () =>
{
var machine = new Machine(TextFile.ReadStringList("input.txt"));
var visited = machine.RunFrom(0);
acWithoutFlip = machine.AC;
var target = machine.Program.Count;
HashSet <int> knownGood = new HashSet <int>();
HashSet <int> knownBad = new HashSet <int>(visited);
for (int i = 1; i < machine.Program.Count; i++)
{
if (knownBad.Contains(i) || knownGood.Contains(i))//this instruction has already been visited in a previous path
{
continue;
}
visited = machine.RunFrom(i);
if (visited.Contains(target))
{
knownGood.UnionWith(visited);
}
else
{
knownBad.UnionWith(visited);
}
}
machine.AC = 0;
machine.PC = 0;
bool flipped = false;
while (machine.PC < machine.Program.Count)
{
if (!flipped)
{
var candidate = NextIfFlipped(machine);
if (knownGood.Contains(candidate)) //flipping this instructions brings us to a "winning" path
{
var curr = machine.Program[machine.PC];
var flippedInstruction = InstructionFactory.Flip(curr);
machine.Program[machine.PC] = flippedInstruction;
flipped = true;
}
}
machine.Step();
}
acAfterFlip = machine.AC;
}, 100, 10);
Performance.TimeRun("Alternate solve", () =>
{
acAfterFlip = SolveAlternate();
}, 100, 10);
Console.WriteLine($"AC before flip: {acWithoutFlip}");
Console.WriteLine($"AC after flip: {acAfterFlip}");
}
