use super::assert_stream;

use crate::unfold_state::UnfoldState;

use core::fmt;

use core::pin::Pin;

use futures_core::future::Future;

use futures_core::ready;

use futures_core::stream::{FusedStream, Stream};

use futures_core::task::{Context, Poll};

use pin_project_lite::pin_project;

/// Creates a `Stream` from a seed and a closure returning a `Future`.

///

/// This function is the dual for the `Stream::fold()` adapter: while

/// `Stream::fold()` reduces a `Stream` to one single value, `unfold()` creates a

/// `Stream` from a seed value.

///

/// `unfold()` will call the provided closure with the provided seed, then wait

/// for the returned `Future` to complete with `(a, b)`. It will then yield the

/// value `a`, and use `b` as the next internal state.

///

/// If the closure returns `None` instead of `Some(Future)`, then the `unfold()`

/// will stop producing items and return `Poll::Ready(None)` in future

/// calls to `poll()`.

///

/// This function can typically be used when wanting to go from the "world of

/// futures" to the "world of streams": the provided closure can build a

/// `Future` using other library functions working on futures, and `unfold()`

/// will turn it into a `Stream` by repeating the operation.

///

/// # Example

///

/// ```

/// # futures::executor::block_on(async {

/// use futures::stream::{self, StreamExt};

///

/// let stream = stream::unfold(0, |state| async move {

///     if state <= 2 {

///         let next_state = state + 1;

///         let yielded = state  * 2;

///         Some((yielded, next_state))

///     } else {

///         None

///     }

/// });

///

/// let result = stream.collect::<Vec<i32>>().await;

/// assert_eq!(result, vec![0, 2, 4]);

/// # });

/// ```

pub fn unfold<T, F, Fut, Item>(init: T, f: F) -> Unfold<T, F, Fut>

where

    F: FnMut(T) -> Fut,

    Fut: Future<Output = Option<(Item, T)>>,

{

    assert_stream::<Item, _>(Unfold { f, state: UnfoldState::Value { value: init } })

}

pin_project! {

    /// Stream for the [`unfold`] function.

    #[must_use = "streams do nothing unless polled"]

    pub struct Unfold<T, F, Fut> {

        f: F,

        #[pin]

        state: UnfoldState<T, Fut>,

    }

}

impl<T, F, Fut> fmt::Debug for Unfold<T, F, Fut>

where

    T: fmt::Debug,

    Fut: fmt::Debug,

{

    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {

        f.debug_struct("Unfold").field("state", &self.state).finish()

    }

}

impl<T, F, Fut, Item> FusedStream for Unfold<T, F, Fut>

where

    F: FnMut(T) -> Fut,

    Fut: Future<Output = Option<(Item, T)>>,

{

    fn is_terminated(&self) -> bool {

        if let UnfoldState::Empty = self.state {

            true

        } else {

            false

        }

    }

}

impl<T, F, Fut, Item> Stream for Unfold<T, F, Fut>

where

    F: FnMut(T) -> Fut,

    Fut: Future<Output = Option<(Item, T)>>,

{

    type Item = Item;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {

        let mut this = self.project();

        if let Some(state) = this.state.as_mut().take_value() {

            this.state.set(UnfoldState::Future { future: (this.f)(state) });

        }

        let step = match this.state.as_mut().project_future() {

            Some(fut) => ready!(fut.poll(cx)),

            None => panic!("Unfold must not be polled after it returned `Poll::Ready(None)`"),

        };

        if let Some((item, next_state)) = step {

            this.state.set(UnfoldState::Value { value: next_state });

            Poll::Ready(Some(item))

        } else {

            this.state.set(UnfoldState::Empty);

            Poll::Ready(None)

        }

    }

}