use std::{
    fmt,
    pin::Pin,
    sync::{Arc, Mutex},
    task::{Context as TaskContext, Poll},
};

use anyhow::Result;
use bincode::{
    BorrowDecode, Decode, Encode,
    de::{BorrowDecoder, Decoder},
    enc::Encoder,
    error::{DecodeError, EncodeError},
};
use futures::{Stream as StreamTrait, StreamExt};

/// Streams allow for streaming values from source to sink.
///
/// A Stream implements both a reader (which implements the Stream trait), and a
/// writer (which can be sent to another thread). As new values are written, any
/// pending readers will be woken up to receive the new value.
#[derive(Clone, Debug)]
pub struct Stream<T: Clone + Send> {
    inner: Arc<Mutex<StreamState<T>>>,
}

/// The StreamState actually holds the data of a Stream.
struct StreamState<T> {
    source: Option<Pin<Box<dyn StreamTrait<Item = T> + Send>>>,
    pulled: Vec<T>,
}

impl<T: Clone + Send> Stream<T> {
    /// Constructs a new Stream, and immediately closes it with only the passed
    /// values.
    pub fn new_closed(pulled: Vec<T>) -> Self {
        Self {
            inner: Arc::new(Mutex::new(StreamState {
                source: None,
                pulled,
            })),
        }
    }

    /// Creates a new Stream, which will lazily pull from the source stream.
    pub fn new_open(
        pulled: Vec<T>,
        source: Box<dyn StreamTrait<Item = T> + Send + 'static>,
    ) -> Self {
        Self {
            inner: Arc::new(Mutex::new(StreamState {
                source: Some(Box::into_pin(source)),
                pulled,
            })),
        }
    }

    /// Returns a [StreamTrait] implementation to poll values out of our Stream.
    pub fn read(&self) -> StreamRead<T> {
        StreamRead {
            source: self.clone(),
            index: 0,
        }
    }

    pub async fn into_single(&self) -> SingleValue<T> {
        let mut stream = self.read();
        let Some(first) = stream.next().await else {
            return SingleValue::None;
        };

        if stream.next().await.is_some() {
            return SingleValue::Multiple;
        }

        SingleValue::Single(first)
    }
}

pub enum SingleValue<T> {
    /// The Stream did not hold a value.
    None,

    /// The Stream held multiple values.
    Multiple,

    /// The held only a single value.
    Single(T),
}

impl<T: fmt::Debug> fmt::Debug for SingleValue<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            SingleValue::None => f.debug_struct("SingleValue::None").finish(),
            SingleValue::Multiple => f.debug_struct("SingleValue::Multiple").finish(),
            SingleValue::Single(v) => f.debug_tuple("SingleValue::Single").field(v).finish(),
        }
    }
}

impl<T: Clone + Send, S: StreamTrait<Item = T> + Send + Unpin + 'static> From<S> for Stream<T> {
    fn from(source: S) -> Self {
        Self::new_open(vec![], Box::new(source))
    }
}

impl<T: Clone + Send> Default for Stream<T> {
    fn default() -> Self {
        Self::new_closed(vec![])
    }
}

impl<T: Clone + PartialEq + Send> PartialEq for Stream<T> {
    // A Stream is equal if it's the same internal pointer, or both streams are
    // closed with equivalent values.
    fn eq(&self, other: &Self) -> bool {
        Arc::ptr_eq(&self.inner, &other.inner) || {
            let left = self.inner.lock().unwrap();
            let right = other.inner.lock().unwrap();

            match (&*left, &*right) {
                (
                    StreamState {
                        pulled: a,
                        source: None,
                    },
                    StreamState {
                        pulled: b,
                        source: None,
                    },
                ) => a == b,
                _ => false,
            }
        }
    }
}
impl<T: Clone + Eq + Send> Eq for Stream<T> {}

impl<T: Clone + Encode + Send> Encode for Stream<T> {
    fn encode<E: Encoder>(&self, encoder: &mut E) -> Result<(), EncodeError> {
        let lock = self
            .inner
            .lock()
            .map_err(|_| EncodeError::Other("failed to lock stream, lock is poisoned?"))?;
        match &*lock {
            StreamState {
                pulled,
                source: None,
            } => Encode::encode(pulled, encoder),
            _ => Err(EncodeError::Other("cannot encode open stream")),
        }
    }
}

impl<Context, T: Clone + Send + Decode<Context>> Decode<Context> for Stream<T> {
    fn decode<D: Decoder<Context = Context>>(decoder: &mut D) -> Result<Self, DecodeError> {
        let data = <Vec<T>>::decode(decoder)?;
        Ok(Stream::new_closed(data))
    }
}

impl<'de, Context, T: Clone + Send + BorrowDecode<'de, Context>> BorrowDecode<'de, Context>
    for Stream<T>
{
    fn borrow_decode<D: BorrowDecoder<'de, Context = Context>>(
        decoder: &mut D,
    ) -> Result<Self, DecodeError> {
        let data = <Vec<T>>::borrow_decode(decoder)?;
        Ok(Stream::new_closed(data))
    }
}

impl<T: Clone + fmt::Debug> fmt::Debug for StreamState<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("StreamState")
            .field("pulled", &self.pulled)
            .finish()
    }
}

/// Implements [StreamTrait] over our Stream.
#[derive(Debug)]
pub struct StreamRead<T: Clone + Send> {
    index: usize,
    source: Stream<T>,
}

impl<T: Clone + Send> StreamTrait for StreamRead<T> {
    type Item = T;

    fn poll_next(self: Pin<&mut Self>, cx: &mut TaskContext<'_>) -> Poll<Option<Self::Item>> {
        let this = self.get_mut();
        let index = this.index;
        let mut inner = this.source.inner.lock().unwrap();

        if let Some(v) = inner.pulled.get(index) {
            // If the current reader can be satisfied by a value we've already pulled, then
            // just do that.
            this.index += 1;
            return Poll::Ready(Some(v.clone()));
        };

        let Some(source) = &mut inner.source else {
            // If the source has been closed, there's nothing left to pull.
            return Poll::Ready(None);
        };

        match source.poll_next_unpin(cx) {
            // If the source stream is ready to give us a new value, we can immediately store that
            // and return it to the caller. Any other readers will be able to read the value from
            // the already-pulled data.
            Poll::Ready(Some(v)) => {
                this.index += 1;
                inner.pulled.push(v.clone());
                Poll::Ready(Some(v))
            }
            // If the source stream is finished, then we can transition to the closed state
            // to drop the source stream.
            Poll::Ready(None) => {
                inner.source.take();
                Poll::Ready(None)
            }
            // Else, we need to wait for the source stream to give us a new value. The
            // source stream will be responsible for waking the TaskContext.
            Poll::Pending => Poll::Pending,
        }
    }
}