mancala/apps/backend/src/main/java/app/playmancala/loop/GameLoop.java

package app.playmancala.loop;

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

import org.jboss.logging.Logger;

// The one thread that owns all live game state: games, matchmaking, previews and the WebSocket registry. Like the
// Node backend's event loop, it lets that state go without locks. Never block it on I/O.
public final class GameLoop implements Scheduler, AutoCloseable {
    private static final Logger LOG = Logger.getLogger(GameLoop.class);

    private final ScheduledExecutorService executor;
    private volatile Thread thread;

    public GameLoop() {
        executor = Executors.newSingleThreadScheduledExecutor(runnable -> {
            Thread loopThread = new Thread(runnable, "game-loop");
            loopThread.setDaemon(true);
            thread = loopThread;
            return loopThread;
        });
    }

    @Override
    public long now() {
        return System.currentTimeMillis();
    }

    @Override
    public void execute(Runnable task) {
        executor.execute(guarded(task));
    }

    @Override
    public Cancellable schedule(Runnable task, long delayMs) {
        Future<?> future = executor.schedule(guarded(task), delayMs, TimeUnit.MILLISECONDS);
        return () -> future.cancel(false);
    }

    @Override
    public Cancellable scheduleAtFixedRate(Runnable task, long periodMs) {
        Future<?> future = executor.scheduleAtFixedRate(guarded(task), periodMs, periodMs, TimeUnit.MILLISECONDS);
        return () -> future.cancel(false);
    }

    // Runs the task on the loop and waits for its result; for threads that need to read live state.
    public <T> T call(Callable<T> task) {
        if (Thread.currentThread() == thread) {
            try {
                return task.call();
            } catch (RuntimeException e) {
                throw e;
            } catch (Exception e) {
                throw new IllegalStateException(e);
            }
        }
        try {
            return executor.submit(task).get();
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
            throw new IllegalStateException("Interrupted while waiting for the game loop", e);
        } catch (ExecutionException e) {
            if (e.getCause() instanceof RuntimeException runtime) throw runtime;
            throw new IllegalStateException(e.getCause());
        }
    }

    @Override
    public void close() {
        executor.shutdownNow();
    }

    // An exception must not stop a repeating task or reach the executor, which would swallow it.
    private static Runnable guarded(Runnable task) {
        return () -> {
            try {
                task.run();
            } catch (Throwable e) {
                LOG.error("(GameLoop) Task failed", e);
            }
        };
    }
}