/// A watchdog timer with a timeout that needs to be regularly pinged
/// to prevent it from firing.
///
/// **Note**: the behavior of the watchdog is undefined (but will not
/// result in a crash or put the watchdog in an invalid state) if it
/// is pet at the last possible moment, for example, like this:
///
/// ```no_compile
/// let (watchdog, pet) = /* ... */
/// tokio::select! {
///     tokio::time::sleep(watchdog.timeout).then(move |()| pet.pet()) = _ => { eprintln!("foo"); },
///     watchdog.wait() = _ => { eprintln!("bar"); }
/// ```
pub struct Watchdog {
    rx: tokio::sync::mpsc::Receiver<()>,
    /// Timeout of this watchdog.
    pub timeout: std::time::Duration,
}
#[derive(Clone)]
pub(crate) struct Pet(tokio::sync::mpsc::Sender<()>);

pub(crate) fn watchdog(timeout: std::time::Duration) -> (Watchdog, Pet) {
    let (tx, rx) = tokio::sync::mpsc::channel::<()>(1);
    let pet = Pet(tx);
    let watchdog = Watchdog { rx, timeout };

    (watchdog, pet)
}

impl Watchdog {
    /// Wait until the watchdog fires from not being pet in a while.
    ///
    /// The watchdog doesn't start waiting for pets and scritches
    /// until you await the future returned from this method.
    pub async fn wait(mut self) {
        while let Ok(Some(())) = tokio::time::timeout(self.timeout, self.rx.recv()).await {}
    }
}

impl Pet {
    pub(crate) async fn pet_owned(self) -> Result<(), tokio::sync::mpsc::error::SendError<()>> {
        let tx = self.0.clone();
        tx.send(()).await
    }
}