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use async_once_cell::Lazy;
use async_trait::async_trait;
use std::future::Future;
use std::pin::Pin;

/// Trait that abstracts getting a value asynchrously.
///
/// It returns a result with a reference counted pointer to some value.
#[async_trait(?Send)]
pub trait AsyncAccessor {
    type Value;
    type Error;

    async fn get_value(&self) -> Result<Self::Value, Self::Error>;
}

/// StaticValue that implements AsyncAccessor.
///
/// It can be used to hard code a value that doesn't need to be dynamically updated.
pub struct StaticValue<T, E> {
    value: T,
    __error: std::marker::PhantomData<E>,
}

impl<T, E> StaticValue<T, E> {
    pub fn new(value: T) -> Self {
        Self {
            value,
            __error: Default::default(),
        }
    }
}

#[async_trait(?Send)]
impl<T: Clone, E> AsyncAccessor for StaticValue<T, E> {
    type Value = T;
    type Error = E;

    async fn get_value(&self) -> Result<Self::Value, Self::Error> {
        Ok(self.value.clone())
    }
}

type BoxedResultFuture<T, E> = Pin<Box<dyn Future<Output = Result<T, E>>>>;

/// A lazy async value created from a future.
///
/// The internal future is only run when `get_value` is called
pub struct LazyValue<T, E> {
    value: Lazy<Result<T, E>, BoxedResultFuture<T, E>>,
}

impl<T, E> LazyValue<T, E> {
    pub fn new(f: impl Future<Output = Result<T, E>> + 'static) -> Self {
        Self {
            value: Lazy::new(Box::pin(f)),
        }
    }
}

#[async_trait(?Send)]
impl<T: Clone + Unpin, E: Clone + Unpin> AsyncAccessor for LazyValue<T, E> {
    type Value = T;
    type Error = E;

    async fn get_value(&self) -> Result<Self::Value, Self::Error> {
        (*Lazy::get(Pin::new(&self.value)).await).clone()
    }
}

#[async_trait(?Send)]
impl<T, E> AsyncAccessor for Box<dyn AsyncAccessor<Value = T, Error = E>> {
    type Value = T;
    type Error = E;

    async fn get_value(&self) -> Result<Self::Value, Self::Error> {
        (**self).get_value().await
    }
}

#[cfg(test)]
mod tests {
    use std::{
        sync::{
            atomic::{AtomicBool, Ordering},
            Arc,
        },
        time::Duration,
    };

    use super::{AsyncAccessor, LazyValue};

    #[tokio::test]
    async fn test_lazy_value_only_runs_when_called() {
        let called = Arc::new(AtomicBool::new(false));

        let value: LazyValue<usize, ()> = LazyValue::new({
            let called = called.clone();

            async move {
                called.store(true, Ordering::Relaxed);

                Ok(10)
            }
        });

        tokio::time::sleep(Duration::from_millis(10)).await;

        assert_eq!(called.load(Ordering::Relaxed), false);

        let value = value.get_value().await;

        assert_eq!(value, Ok(10));
        assert!(called.load(Ordering::Relaxed));
    }
}