public static BootCode ParseInputLines(IList <string> inputLines)
{
var bootCodeInstructions = new List <BootCodeInstruction>();
foreach (var inputLine in inputLines)
{
var instruction = ParseBootCodeInstruction(inputLine);
bootCodeInstructions.Add(instruction);
}
var result = new BootCode(bootCodeInstructions);
return(result);
}
public static bool TryFindFixedProgram(BootCode originalBootCode, out BootCode fixedBootCode, out int accumulator)
{
fixedBootCode = null;
accumulator = 0;
var possiblyFixedBootCodes = GetPossiblyFixedBootCodes(originalBootCode);
foreach (var possiblyFixedBootCode in possiblyFixedBootCodes)
{
var isSuccessfulTermination = TryRunProgram(possiblyFixedBootCode, out int possibleAccumulator);
if (isSuccessfulTermination)
{
fixedBootCode = possiblyFixedBootCode;
accumulator = possibleAccumulator;
return(true);
}
}
return(false);
}
public static IList <BootCode> GetPossiblyFixedBootCodes(BootCode bootCode)
{
var result = new List <BootCode>();
for (int i = 0; i < bootCode.Instructions.Count; i++)
{
var currentInstruction = bootCode.Instructions[i];
if ("jmp".Equals(currentInstruction.Instruction) ||
"nop".Equals(currentInstruction.Instruction))
{
var instructionsCopy = bootCode.Instructions.ToList();
var newInstruction = "jmp".Equals(currentInstruction.Instruction) ? "nop" : "jmp";
instructionsCopy[i] = new BootCodeInstruction(newInstruction, currentInstruction.Value);
var newBootCode = new BootCode(instructionsCopy);
result.Add(newBootCode);
}
}
return(result);
}
/// <summary>
/// Runs the given boot code program.
/// Returns true if the program terminates correctly.
/// Otherwise, returns false.
/// </summary>
/// <param name="bootCode"></param>
/// <param name="accumulator"></param>
/// <returns></returns>
public static bool TryRunProgram(BootCode bootCode, out int accumulator)
{
accumulator = 0;
var instructionsExecuted = new HashSet <int>();
var currentInstructionIndex = 0;
while (!instructionsExecuted.Contains(currentInstructionIndex))
{
instructionsExecuted.Add(currentInstructionIndex);
if (currentInstructionIndex == bootCode.Instructions.Count)
{
// Terminate when trying to execute the instruction immediately after the last one
return(true);
}
if (currentInstructionIndex < 0 || currentInstructionIndex > bootCode.Instructions.Count)
{
throw new Exception($"Instruction index out of range - Current index: {currentInstructionIndex}, # Instructions: {bootCode.Instructions.Count}");
}
var instruction = bootCode.Instructions[currentInstructionIndex];
if ("acc".Equals(instruction.Instruction))
{
accumulator += instruction.Value;
currentInstructionIndex++;
}
else if ("jmp".Equals(instruction.Instruction))
{
currentInstructionIndex += instruction.Value;
}
else if ("nop".Equals(instruction.Instruction))
{
currentInstructionIndex++;
}
else
{
throw new Exception($"Invalid instruction: {instruction.Instruction}");
}
}
return(false);
}
