// Flips exactly one(1) instruction from Nop->Jmp or Jmp->Nop so the program can run all the way through.
// Prints the accumulator after that.
// Correct answer: 1703
public int SolutionPart2()
{
// list of all indices of Instructions that can be reached if starting from the last instruction and working backwards
List <int> vmReversed = InstructionMethods.BackTravelFromIndex(allInstructions, allInstructions.Count - 1);
int accumulator = 0;
// VM should run from the top
int currentIndex = 0;
// Current instruction in the loop and a tempInstruction used for switching
Instruction currInstr;
Instruction tempInstr = null;
// if the switch is found, no reason to test anymore
bool switchFound = false;
// Program should stop when the index reaches the end
while (currentIndex != allInstructions.Count)
{
currInstr = allInstructions[currentIndex];
// test for Instruction type and makes a temp swapped version of it.
if (!switchFound)
{
if (currInstr is Nop)
{
tempInstr = InstructionMethods.SwapType <Jmp>(currInstr);
}
else if (currInstr is Jmp)
{
tempInstr = InstructionMethods.SwapType <Nop>(currInstr);
}
}
// if the next index of the swapped instruction is in the vmreversed list -
// then the switch will cause the program to complete
if (tempInstr != null && vmReversed.Contains(tempInstr.NextIndex))
{
// current instruction is set to the swapped
currInstr = tempInstr;
switchFound = true;
}
// current instruction runs its action against index and accumulator
currInstr.Action(ref currentIndex, ref accumulator);
// the temp instruction is removed
tempInstr = null;
}
return(accumulator);
}
//
// From a List of Instructions and a Index, travels backwards and makes a list of all instructions visited that way.
public static List <int> BackTravelFromIndex(List <Instruction> instructions, int fromIndex)
{
// Uses Graph traversal logic - the list of instructions never change.
List <int> visitedInstructions = new List <int>();
Instruction currInstr = instructions[fromIndex];
// always add current index to the list
visitedInstructions.Add(currInstr.Index);
// returns from here, if no instructions leads to the current instruction
if (currInstr.FormerIndices.Count == 0)
{
return(visitedInstructions);
}
foreach (int index in currInstr.FormerIndices)
{
visitedInstructions.AddRange(InstructionMethods.BackTravelFromIndex(instructions, index));
}
return(visitedInstructions);
}
private void CreateInstructions()
{
allInstructions = InstructionMethods.CreateVM(dataList);
}