@RequestMapping
@RestController
class TaskController {
    /**
     * Starts a heavy task running in the background. Will block
     * the common thread pool for 60 seconds.
     * <p>
     * In a real example, this might have been a @Scheduled method.
     */
    @PostMapping("/heavyAsyncWork")
    void heavyAsyncWork() {
        CompletableFuture.runAsync(() -> {
            var work = range(0, 60 * (ForkJoinPool.commonPool().getParallelism() - 1));
            work.parallelStream()
                    .forEach(i -> {
                        try {
                            Thread.sleep(1000);
                        } catch (InterruptedException e) {
                            // ignore
                        }
                    });

        });
    }

    /**
     * Starts a minor async task using CompletableFuture. Would normally
     * be run in the background. But to make it easy to try
     * out, it returns a value computed async.
     */
    @PostMapping("/minorAsyncWork")
    String minorAsyncWork() throws Exception {
        Future<String> future = CompletableFuture.supplyAsync(() -> {
            return "work done";
        });
        return future.get();
    }

    /**
     * Calculates the primes less than 1000. Also returns the threads
     * that was used in the calculation. Uses a parallelStream to do
     * the calculation.
     */
    @GetMapping("/parallelStreamWork")
    List<Object> parallelStreamWork() {
        var calculatingThreads = new ConcurrentHashMap<String, Boolean>();
        var primes = range(2, 1000).parallelStream()
                .filter(n -> {
                    calculatingThreads.put(Thread.currentThread().getName(), true);
                    return isPrime(n);
                })
                .collect((toList()));
        var result = new ArrayList<Object>();
        result.addAll(calculatingThreads.keySet());
        result.addAll(primes);
        return result;
    }

    private List<Integer> range(int start, int end) {
        List<Integer> maybePrime = new ArrayList<>();
        for (int i = start; i < end; i++) {
            maybePrime.add(i);
        }
        return maybePrime;
    }

    private boolean isPrime(int n) {
        int lim = (int) Math.sqrt(n);
        for (int i = 2; i <= lim; i++) {
            if (n % i == 0) {
                return false;
            }
        }
        return true;
    }
}