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use std::pin::Pin;

use cfg_if::cfg_if;

use crate::future::Future;
use crate::io::{self, SeekFrom};
use crate::task::{Context, Poll};

cfg_if! {
    if #[cfg(feature = "docs")] {
        use std::ops::{Deref, DerefMut};

        #[doc(hidden)]
        pub struct ImplFuture<T>(std::marker::PhantomData<T>);

        /// Allows seeking through a byte stream.
        ///
        /// This trait is a re-export of [`futures::io::AsyncSeek`] and is an async version of
        /// [`std::io::Seek`].
        ///
        /// The [provided methods] do not really exist in the trait itself, but they become
        /// available when the prelude is imported:
        ///
        /// ```
        /// # #[allow(unused_imports)]
        /// use async_std::prelude::*;
        /// ```
        ///
        /// [`std::io::Seek`]: https://doc.rust-lang.org/std/io/trait.Seek.html
        /// [`futures::io::AsyncSeek`]:
        /// https://docs.rs/futures-preview/0.3.0-alpha.17/futures/io/trait.AsyncSeek.html
        /// [provided methods]: #provided-methods
        pub trait Seek {
            /// Attempt to seek to an offset, in bytes, in a stream.
            fn poll_seek(
                self: Pin<&mut Self>,
                cx: &mut Context<'_>,
                pos: SeekFrom,
            ) -> Poll<io::Result<u64>>;

            /// Seeks to a new position in a byte stream.
            ///
            /// Returns the new position in the byte stream.
            ///
            /// A seek beyond the end of stream is allowed, but behavior is defined by the
            /// implementation.
            ///
            /// # Examples
            ///
            /// ```no_run
            /// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
            /// #
            /// use async_std::fs::File;
            /// use async_std::io::SeekFrom;
            /// use async_std::prelude::*;
            ///
            /// let mut file = File::open("a.txt").await?;
            ///
            /// let file_len = file.seek(SeekFrom::End(0)).await?;
            /// #
            /// # Ok(()) }) }
            /// ```
            fn seek(&mut self, pos: SeekFrom) -> ImplFuture<io::Result<u64>>
            where
                Self: Unpin
            {
                unreachable!()
            }
        }

        impl<T: Seek + Unpin + ?Sized> Seek for Box<T> {
            fn poll_seek(
                self: Pin<&mut Self>,
                cx: &mut Context<'_>,
                pos: SeekFrom,
            ) -> Poll<io::Result<u64>> {
                unreachable!()
            }
        }

        impl<T: Seek + Unpin + ?Sized> Seek for &mut T {
            fn poll_seek(
                self: Pin<&mut Self>,
                cx: &mut Context<'_>,
                pos: SeekFrom,
            ) -> Poll<io::Result<u64>> {
                unreachable!()
            }
        }

        impl<P> Seek for Pin<P>
        where
            P: DerefMut + Unpin,
            <P as Deref>::Target: Seek,
        {
            fn poll_seek(
                self: Pin<&mut Self>,
                cx: &mut Context<'_>,
                pos: SeekFrom,
            ) -> Poll<io::Result<u64>> {
                unreachable!()
            }
        }
    } else {
        pub use futures_io::AsyncSeek as Seek;
    }
}

#[doc(hidden)]
pub trait SeekExt: futures_io::AsyncSeek {
    fn seek(&mut self, pos: SeekFrom) -> SeekFuture<'_, Self>
    where
        Self: Unpin,
    {
        SeekFuture { seeker: self, pos }
    }
}

impl<T: futures_io::AsyncSeek + ?Sized> SeekExt for T {}

#[doc(hidden)]
#[allow(missing_debug_implementations)]
pub struct SeekFuture<'a, T: Unpin + ?Sized> {
    seeker: &'a mut T,
    pos: SeekFrom,
}

impl<T: SeekExt + Unpin + ?Sized> Future for SeekFuture<'_, T> {
    type Output = io::Result<u64>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let pos = self.pos;
        Pin::new(&mut *self.seeker).poll_seek(cx, pos)
    }
}