Day18.java

package com.sbaars.adventofcode.year17.days;

import com.sbaars.adventofcode.year17.Day2017;
import java.util.HashMap;
import java.util.Map;

public class Day18 extends Day2017 {
  public Day18() {
    super(18);
  }

  public static void main(String[] args) {
    new Day18().printParts();
  }

  @Override
  public Object part1() {
    String[] instructions = dayStrings();
    Map<String, Long> registers = new HashMap<>();
    long lastSound = 0;
    int ip = 0;  // instruction pointer

    while (ip >= 0 && ip < instructions.length) {
      String[] parts = instructions[ip].split(" ");
      String cmd = parts[0];
      String x = parts[1];
      String y = parts.length > 2 ? parts[2] : null;

      long xVal = getValue(registers, x);
      long yVal = y != null ? getValue(registers, y) : 0;

      switch (cmd) {
        case "snd":
          lastSound = xVal;
          break;
        case "set":
          registers.put(x, yVal);
          break;
        case "add":
          registers.put(x, xVal + yVal);
          break;
        case "mul":
          registers.put(x, xVal * yVal);
          break;
        case "mod":
          registers.put(x, xVal % yVal);
          break;
        case "rcv":
          if (xVal != 0) {
            return lastSound;
          }
          break;
        case "jgz":
          if (xVal > 0) {
            ip += yVal - 1;  // -1 because we'll add 1 at the end
          }
          break;
      }

      ip++;
    }

    return lastSound;
  }

  private static long getValue(Map<String, Long> registers, String value) {
    try {
      return Long.parseLong(value);
    } catch (NumberFormatException e) {
      return registers.getOrDefault(value, 0L);
    }
  }

  private static class Program {
    private final Map<String, Long> registers = new HashMap<>();
    private final java.util.Queue<Long> messageQueue = new java.util.LinkedList<>();
    private final String[] instructions;
    private int ip = 0;  // instruction pointer
    private int sendCount = 0;
    private boolean isWaiting = false;

    Program(String[] instructions, int id) {
      this.instructions = instructions;
      registers.put("p", (long) id);
    }

    boolean isTerminated() {
      return ip < 0 || ip >= instructions.length;
    }

    void addToQueue(long value) {
      messageQueue.add(value);
      isWaiting = false;
    }

    boolean execute(Program other) {
      if (isTerminated()) return false;

      String[] parts = instructions[ip].split(" ");
      String cmd = parts[0];
      String x = parts[1];
      String y = parts.length > 2 ? parts[2] : null;

      long xVal = getValue(registers, x);
      long yVal = y != null ? getValue(registers, y) : 0;

      switch (cmd) {
        case "snd":
          other.addToQueue(xVal);
          sendCount++;
          break;
        case "set":
          registers.put(x, yVal);
          break;
        case "add":
          registers.put(x, xVal + yVal);
          break;
        case "mul":
          registers.put(x, xVal * yVal);
          break;
        case "mod":
          registers.put(x, xVal % yVal);
          break;
        case "rcv":
          if (messageQueue.isEmpty()) {
            isWaiting = true;
            return false;
          }
          registers.put(x, messageQueue.poll());
          break;
        case "jgz":
          if (xVal > 0) {
            ip += yVal - 1;  // -1 because we'll add 1 at the end
          }
          break;
      }

      ip++;
      return true;
    }
  }

  @Override
  public Object part2() {
    String[] instructions = dayStrings();
    Program prog0 = new Program(instructions, 0);
    Program prog1 = new Program(instructions, 1);

    while (true) {
      boolean prog0Moved = prog0.execute(prog1);
      boolean prog1Moved = prog1.execute(prog0);

      // Check for deadlock or termination
      if ((!prog0Moved && !prog1Moved) || 
          (prog0.isTerminated() && prog1.isTerminated())) {
        break;
      }
    }

    return prog1.sendCount;
  }
}