Day23.java

import java.util.ArrayList;
import java.util.List;

public class Day23 {

	private final static String FILENAME = "./InputDay23.txt";

	private List<String> input;

	private List<Instruction> program;

	private Register a;
	private Register b;
	private int ip; // Instruction Pointer

	private enum Cmd {
		hlf, tpl, inc, jmp, jie, jio;
	}

	class Instruction {
		Cmd cmd;
		Register register;
		int offset;

		public Instruction(String line) {
			String[] data = line.split("[ ,]+");
			cmd = Cmd.valueOf(data[0]);
			switch (cmd) {
			case hlf:
			case tpl:
			case inc:
				register = getRegister(data[1]);
				offset = 0;
				break;
			case jmp:
				register = a;
				offset = Integer.parseInt(data[1]);
				break;
			case jie:
			case jio:
				register = getRegister(data[1]);
				offset = Integer.parseInt(data[2]);
				break;
			default:
				break;
			}
		}

		private Register getRegister(String data) {
			return "a".equals(data) ? a : "b".equals(data) ? b : a;
		}

		public void execute() {
			switch (cmd) {
			case hlf:
				register.value = register.value / 2;
				ip++;
				break;
			case tpl:
				register.value = register.value * 3;
				ip++;
				break;
			case inc:
				register.value++;
				ip++;
				break;
			case jmp:
				ip += offset;
				break;
			case jie:
				ip += (register.value % 2 == 0) ? offset : 1;
				break;
			case jio:
				ip += (register.value == 1) ? offset : 1;
				break;
			default:
				break;
			}
		}

	}

	class Register {
		int value = 0;
	}

	public Day23() {
		input = FileIO.getFileAsList(Day23.FILENAME);

		a = new Register();
		b = new Register();

		program = new ArrayList<>();

		for (String line : input) {
			program.add(new Instruction(line));
		}
		ip = 0;
	}

	public void reset() {
		ip = 0;
		a.value = 0;
		b.value = 0;
	}

	public void run() {
		while (ip < program.size()) {
			Instruction cmd = program.get(ip);
			cmd.execute();
			if (ip >= program.size())
				break;
		}
	}

	public static void main(String[] args) {

		Day23 day23 = new Day23();
		day23.reset();

		day23.run();

		System.out.println("Part 1: Value of Register b is: " + day23.b.value);

		day23.reset();
		day23.a.value++;

		day23.run();

		System.out.println("Part 2: Value of Register b is: " + day23.b.value);

	}

}