adventofcode/2020/8/Program.cs

using System;
using System.IO;
using System.Collections.Generic;

namespace day8 {
class Program {

static void Main(string[] args) {
	//PartOne();
	PartTwo();
}

static void PartOne() { 
	List<string> lines = new List<string>(File.ReadAllLines("input")) { "hlt" };

	int acc = 0, i = 0;

	// Halt when reaching HLT
	while(lines[i] != "hlt") {
		var parts = lines[i].Split(" ");
		var instruction = parts[0];
		var intval = int.Parse(parts[1]);

		// Halt when we go back to this instruction
		lines[i] = "hlt";

		// Increase accumilator
		if(instruction == "acc") acc += intval;

		// Do the jump
		if(instruction == "jmp") i += intval;
		else i++;
	}

	Console.WriteLine($"The value of the accumilator is {acc}");
}

static void PartTwo() {
	var lines = File.ReadAllLines("input");
	
	for(var i=0; i<lines.Length; i++) {
		var parts = lines[i].Split(" ");
		var instruction = parts[0];
		var intval = int.Parse(parts[1]);

		if(instruction == "acc") 
			continue;

		var clone = new List<string>(lines);
		clone[i] = instruction == "jmp" ? $"nop {intval}" : $"jmp {intval}";
	
		try {
			var result = DoTheThing(clone);
			Console.WriteLine($"Found that instruction {i} fixes the loop, accumilator end up being {result}.");
			return;
		} catch { }
	}
}

static int DoTheThing(List<string> lines) {
	int acc = 0, i = 0;

	// Halt when reaching HLT
	while(lines[i] != "hlt") {
		var parts = lines[i].Split(" ");
		var instruction = parts[0];
		var intval = int.Parse(parts[1]);

		// Halt when we go back to this instruction
		lines[i] = "hlt";

		// Increase accumilator
		if(instruction == "acc") acc += intval;

		// Do the jump
		if(instruction == "jmp") i += intval;
		else i++;

		if(i >= lines.Count) return acc;
	}

	throw new Exception("Lmaoo this isn't it");
}

} // class
} // namespace