Watcher.java

// Prints deltas each tick; exits on first backward step or after N samples.
public class Watcher {
    public static void main(String[] args) throws Exception {
        final long INTERVAL_MS = 1000;
        final int  SAMPLES = 30; // ~30s total
        final long BACKWARD_THRESHOLD_MS = 2;

        System.out.println("Java watcher: printing deltas; waiting for backward step...");

        long prevWall = System.currentTimeMillis();
        long prevMono = System.nanoTime();

        for (int i = 0; i < SAMPLES; i++) {
            Thread.sleep(INTERVAL_MS);

            long nowWall = System.currentTimeMillis();
            long nowMono = System.nanoTime();

            long dWall = nowWall - prevWall;
            long dMonoMs = (nowMono - prevMono) / 1_000_000L;

            if (dWall < -BACKWARD_THRESHOLD_MS) {
                System.out.printf("OK: wall BACKWARD %d ms | Δwall=%d ms, Δmono=%d ms. Exit.%n",
                        -dWall, dWall, dMonoMs);
                System.exit(0);
            }
            System.out.printf("Δwall=%d ms, Δmono=%d ms%n", dWall, dMonoMs);

            prevWall = nowWall;
            prevMono = nowMono;
        }
        System.out.println("No backward step observed within samples. Exit(1).");
        System.exit(1);
    }
}