qubit-io 0.14.1

// =============================================================================
//    Copyright (c) 2025 - 2026 Haixing Hu.
//
//    SPDX-License-Identifier: Apache-2.0
//
//    Licensed under the Apache License, Version 2.0.
// =============================================================================

use std::future::Future;
use std::io::{
    Error,
    ErrorKind,
    Result,
};
use std::pin::Pin;
use std::task::{
    Context,
    Poll,
};

use crate::AsyncInput;

use super::read_fully_future::MAX_READY_OPERATIONS_PER_POLL;

/// Future that reads until its destination is full.
///
/// Progress is retained in the destination and is observable through
/// [`Self::items_read`] if the future is cancelled.
///
/// To preserve executor fairness, one outer poll performs a bounded number of
/// successful inner reads. The future self-wakes and returns [`Poll::Pending`]
/// when that budget is exhausted before completion.
///
/// # Panics
///
/// [`Future::poll`] panics when called again after this future has returned
/// [`Poll::Ready`].
///
/// # Type Parameters
///
/// - `'a`: Shared lifetime of the input borrow and destination slice.
/// - `I`: Asynchronous input type.
#[must_use = "futures do nothing unless polled"]
pub struct ReadExactFuture<'a, I>
where
    I: AsyncInput + ?Sized,
{
    /// Input being read.
    input: Pin<&'a mut I>,
    /// Destination that must be filled.
    output: &'a mut [I::Item],
    /// Number of items read so far.
    read: usize,
    /// Whether the exact-read operation has completed.
    completed: bool,
}

impl<'a, I> ReadExactFuture<'a, I>
where
    I: AsyncInput + ?Sized,
{
    /// Creates an exact-read future from a pinned input.
    ///
    /// # Parameters
    ///
    /// - `input`: Pinned asynchronous input.
    /// - `output`: Destination slice that must be filled.
    ///
    /// # Returns
    ///
    /// Returns a future that completes after filling `output`.
    #[inline(always)]
    pub const fn new(input: Pin<&'a mut I>, output: &'a mut [I::Item]) -> Self {
        Self {
            input,
            output,
            read: 0,
            completed: false,
        }
    }

    /// Returns the number of items read so far.
    ///
    /// # Returns
    ///
    /// Returns the progress retained across polls.
    #[inline(always)]
    #[must_use]
    pub const fn items_read(&self) -> usize {
        self.read
    }
}

impl<I> Future for ReadExactFuture<'_, I>
where
    I: AsyncInput + ?Sized,
{
    /// Result produced when the exact-read operation becomes ready.
    type Output = Result<()>;

    /// Polls the exact-read operation.
    ///
    /// # Parameters
    ///
    /// - `cx`: Task context used to register a wake-up.
    ///
    /// # Returns
    ///
    /// Returns [`Poll::Pending`] while more input is needed, or
    /// [`Poll::Ready`] after the destination is filled or an error occurs.
    ///
    /// # Errors
    ///
    /// Returns [`ErrorKind::UnexpectedEof`] when EOF is reached before the
    /// destination is full. Other errors are propagated from the input.
    ///
    /// # Panics
    ///
    /// Panics when polled after returning [`Poll::Ready`].
    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let this = self.get_mut();
        assert!(!this.completed, "ReadExactFuture polled after completion");
        let mut ready_operations = 0_usize;
        while this.read < this.output.len() {
            let remaining = &mut this.output[this.read..];
            match this.input.as_mut().poll_read(cx, remaining) {
                Poll::Ready(Ok(0)) => {
                    this.completed = true;
                    return Poll::Ready(Err(Error::new(
                        ErrorKind::UnexpectedEof,
                        "failed to fill whole input range",
                    )));
                }
                Poll::Ready(Ok(read)) => {
                    this.read += read;
                    ready_operations += 1;
                    if this.read < this.output.len()
                        && ready_operations >= MAX_READY_OPERATIONS_PER_POLL
                    {
                        cx.waker().wake_by_ref();
                        return Poll::Pending;
                    }
                }
                Poll::Ready(Err(error)) => {
                    this.completed = true;
                    return Poll::Ready(Err(error));
                }
                Poll::Pending => return Poll::Pending,
            }
        }
        this.completed = true;
        Poll::Ready(Ok(()))
    }
}