#![allow(non_snake_case)]

use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
use std::thread;
use std::time::{Duration, Instant};

struct Delay {
    when: Instant,
}

impl Future for Delay {
    type Output = &'static str;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<&'static str> {
        if Instant::now() >= self.when {
            println!("Hello world");
            Poll::Ready("done")
        } else {
            // Get a handle to the waker for the current task
            let waker = cx.waker().clone();
            let when = self.when;

            // Spawn a timer thread.
            thread::spawn(move || {
                let now = Instant::now();

                if now < when {
                    thread::sleep(when - now);
                }

                waker.wake();
            });

            Poll::Pending
        }
    }
}

#[tokio::main]
async fn main() {
    let when = Instant::now() + Duration::from_millis(100);
    let future = Delay { when };

    let out = future.await;
    assert_eq!(out, "done");
}