qubit-io 0.18.1

// =============================================================================
//    Copyright (c) 2026 Haixing Hu.
//
//    SPDX-License-Identifier: Apache-2.0
//
//    Licensed under the Apache License, Version 2.0.
// =============================================================================

use std::pin::Pin;
use std::task::Context;
use std::task::Poll;

use futures_io::AsyncRead;

use crate::AsyncInput;

/// Exposes a Qubit byte [`AsyncInput`] as a futures-io [`AsyncRead`].
///
/// # Type Parameters
///
/// - `I`: Qubit byte input type.
///
/// # Examples
///
/// ```
/// use qubit_io::AsyncInput;
/// use qubit_io::FuturesAsyncRead;
/// use futures_io::AsyncRead;
/// use std::io::Result;
/// use std::pin::Pin;
/// use std::sync::Arc;
/// use std::task::Context;
/// use std::task::Poll;
/// use std::task::Wake;
/// use std::task::Waker;
///
/// struct EmptyInput;
///
/// impl AsyncInput for EmptyInput {
///     type Item = u8;
///
///     unsafe fn poll_read_unchecked(
///         self: Pin<&mut Self>,
///         _cx: &mut Context<'_>,
///         _output: &mut [u8],
///         _index: usize,
///         _count: usize,
///     ) -> Poll<Result<usize>> {
///         Poll::Ready(Ok(0))
///     }
/// }
///
/// struct NoopWake;
///
/// impl Wake for NoopWake {
///     fn wake(self: Arc<Self>) {}
/// }
///
/// let mut reader = FuturesAsyncRead::new(EmptyInput);
/// let waker = Waker::from(Arc::new(NoopWake));
/// let mut context = Context::from_waker(&waker);
/// let mut output = [0; 1];
/// assert!(matches!(
///     AsyncRead::poll_read(Pin::new(&mut reader), &mut context, &mut output),
///     Poll::Ready(Ok(0)),
/// ));
/// ```
#[must_use]
#[repr(transparent)]
pub struct FuturesAsyncRead<I> {
    /// Qubit byte input exposed through futures-io.
    inner: I,
}

impl<I> FuturesAsyncRead<I> {
    /// Creates a futures-io reader around a Qubit byte input.
    ///
    /// # Parameters
    ///
    /// - `inner`: Qubit byte input to expose.
    ///
    /// # Returns
    ///
    /// Returns a futures-io reader adapter that owns `inner`.
    #[inline(always)]
    pub const fn new(inner: I) -> Self {
        Self { inner }
    }

    /// Returns a shared reference to the wrapped input.
    ///
    /// # Returns
    ///
    /// Returns the wrapped Qubit input.
    #[inline(always)]
    #[must_use]
    pub const fn get_ref(&self) -> &I {
        &self.inner
    }

    /// Returns a mutable reference to the wrapped input.
    ///
    /// # Returns
    ///
    /// Returns the wrapped Qubit input with mutable access.
    #[inline(always)]
    #[must_use]
    pub const fn get_mut(&mut self) -> &mut I {
        &mut self.inner
    }

    /// Projects a pinned adapter to its pinned wrapped input.
    ///
    /// # Returns
    ///
    /// Returns a pinned mutable reference to the wrapped Qubit input without
    /// moving it.
    #[inline(always)]
    #[must_use]
    pub fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut I> {
        // SAFETY: The projection does not move `inner`, and the transparent
        // adapter never exposes a way to replace a pinned inner value.
        unsafe { self.map_unchecked_mut(|this| &mut this.inner) }
    }

    /// Consumes the adapter and returns the wrapped input.
    ///
    /// # Returns
    ///
    /// Returns the owned Qubit input.
    #[inline(always)]
    #[must_use]
    pub fn into_inner(self) -> I {
        self.inner
    }
}

impl<I> AsyncRead for FuturesAsyncRead<I>
where
    I: AsyncInput<Item = u8>,
{
    /// Polls one futures-io read through the wrapped Qubit input.
    ///
    /// An empty destination completes immediately without polling `inner`.
    ///
    /// # Parameters
    ///
    /// - `cx`: Task context used to register a wake-up.
    /// - `output`: Destination byte slice.
    ///
    /// # Returns
    ///
    /// Returns [`Poll::Pending`] when the input is not ready. A ready result
    /// contains the number of bytes read.
    ///
    /// # Errors
    ///
    /// Returns an I/O error reported by the wrapped input.
    #[inline]
    fn poll_read(self: Pin<&mut Self>, cx: &mut Context<'_>, output: &mut [u8]) -> Poll<std::io::Result<usize>> {
        if output.is_empty() {
            return Poll::Ready(Ok(0));
        }
        self.get_pin_mut().poll_read(cx, output)
    }
}