package futures;

import java.util.concurrent.*;
import java.util.function.Supplier;

/**
 * Goal: write parallel Fibonacci without using any blocking calls (except at the very end).
 */
public class FibExec3 {
    final static ExecutorService exec = new MyExecutor(8);
//    final static ExecutorService exec = new ForkJoinPool(8);

    public static <T> CompletableFuture<T> supplyAsyncFut(Supplier<CompletableFuture<T>> supplier, Executor executor) {
        final CompletableFuture<T> result = new CompletableFuture<>();
        CompletableFuture.runAsync(() -> {
            supplier.get().thenAccept((v) -> {
                result.complete(v);
            });
        }, executor);
        return result;
    }

    static CompletableFuture<Integer> fcall(int n) {
        if (n < 2) return CompletableFuture.completedFuture(n);
        CompletableFuture<Integer> f1 = supplyAsyncFut(() -> fcall(n - 1), exec);
        CompletableFuture<Integer> f2 = fcall(n - 2);
        return f1.thenCombine(f2, (a, b) -> a + b);
    }

    public static void main(String[] args) throws Exception {
        for (int n = 0; n <= 47; n++) {
            long t1 = System.currentTimeMillis();
            long f = fcall(n).join();
            long t2 = System.currentTimeMillis();
            double tm = 1e-3 * (t2 - t1);
            System.out.printf("fib(%d)=%d, time=%f%n", n, f, tm);
        }
        exec.shutdown();
    }
}