// Copyright (c) 2022 Tokio Contributors

// Permission is hereby granted, free of charge, to any
// person obtaining a copy of this software and associated
// documentation files (the "Software"), to deal in the
// Software without restriction, including without
// limitation the rights to use, copy, modify, merge,
// publish, distribute, sublicense, and/or sell copies of
// the Software, and to permit persons to whom the Software
// is furnished to do so, subject to the following
// conditions:

// The above copyright notice and this permission notice
// shall be included in all copies or substantial portions
// of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
// ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
// TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
// PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
// SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
// CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
// IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.

use std::future;
use std::io;
use std::net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr, SocketAddrV4, SocketAddrV6};

/// Performs a DNS resolution.
pub async fn lookup_host(host: impl ToSocketAddrs) -> io::Result<impl Iterator<Item = SocketAddr>> {
    to_socket_addrs(host).await
}

/// Converts or resolves without blocking to one or more `SocketAddr` values.
///
/// # DNS
///
/// Implementations of `ToSocketAddrs` for string types require a DNS lookup.
///
/// # Calling
///
/// Currently, this trait is only used as an argument to Tokio functions that
/// need to reference a target socket address. To perform a `SocketAddr`
/// conversion directly, use [`lookup_host()`](super::lookup_host()).
///
/// This trait is sealed and is intended to be opaque. The details of the trait
/// will change. Stabilization is pending enhancements to the Rust language.
pub trait ToSocketAddrs: sealed::ToSocketAddrsPriv {}

type ReadyFuture<T> = future::Ready<io::Result<T>>;

pub(crate) fn to_socket_addrs<T>(arg: T) -> T::Future
where
    T: ToSocketAddrs,
{
    arg.to_socket_addrs(sealed::Internal)
}

// ===== impl &impl ToSocketAddrs =====

impl<T: ToSocketAddrs + ?Sized> ToSocketAddrs for &T {}

impl<T> sealed::ToSocketAddrsPriv for &T
where
    T: sealed::ToSocketAddrsPriv + ?Sized,
{
    type Iter = T::Iter;
    type Future = T::Future;

    fn to_socket_addrs(&self, _: sealed::Internal) -> Self::Future {
        (**self).to_socket_addrs(sealed::Internal)
    }
}

// ===== impl SocketAddr =====

impl ToSocketAddrs for SocketAddr {}

impl sealed::ToSocketAddrsPriv for SocketAddr {
    type Iter = std::option::IntoIter<SocketAddr>;
    type Future = ReadyFuture<Self::Iter>;

    fn to_socket_addrs(&self, _: sealed::Internal) -> Self::Future {
        let iter = Some(*self).into_iter();
        future::ready(Ok(iter))
    }
}

// ===== impl SocketAddrV4 =====

impl ToSocketAddrs for SocketAddrV4 {}

impl sealed::ToSocketAddrsPriv for SocketAddrV4 {
    type Iter = std::option::IntoIter<SocketAddr>;
    type Future = ReadyFuture<Self::Iter>;

    fn to_socket_addrs(&self, _: sealed::Internal) -> Self::Future {
        SocketAddr::V4(*self).to_socket_addrs(sealed::Internal)
    }
}

// ===== impl SocketAddrV6 =====

impl ToSocketAddrs for SocketAddrV6 {}

impl sealed::ToSocketAddrsPriv for SocketAddrV6 {
    type Iter = std::option::IntoIter<SocketAddr>;
    type Future = ReadyFuture<Self::Iter>;

    fn to_socket_addrs(&self, _: sealed::Internal) -> Self::Future {
        SocketAddr::V6(*self).to_socket_addrs(sealed::Internal)
    }
}

// ===== impl (IpAddr, u16) =====

impl ToSocketAddrs for (IpAddr, u16) {}

impl sealed::ToSocketAddrsPriv for (IpAddr, u16) {
    type Iter = std::option::IntoIter<SocketAddr>;
    type Future = ReadyFuture<Self::Iter>;

    fn to_socket_addrs(&self, _: sealed::Internal) -> Self::Future {
        let iter = Some(SocketAddr::from(*self)).into_iter();
        future::ready(Ok(iter))
    }
}

// ===== impl (Ipv4Addr, u16) =====

impl ToSocketAddrs for (Ipv4Addr, u16) {}

impl sealed::ToSocketAddrsPriv for (Ipv4Addr, u16) {
    type Iter = std::option::IntoIter<SocketAddr>;
    type Future = ReadyFuture<Self::Iter>;

    fn to_socket_addrs(&self, _: sealed::Internal) -> Self::Future {
        let (ip, port) = *self;
        SocketAddrV4::new(ip, port).to_socket_addrs(sealed::Internal)
    }
}

// ===== impl (Ipv6Addr, u16) =====

impl ToSocketAddrs for (Ipv6Addr, u16) {}

impl sealed::ToSocketAddrsPriv for (Ipv6Addr, u16) {
    type Iter = std::option::IntoIter<SocketAddr>;
    type Future = ReadyFuture<Self::Iter>;

    fn to_socket_addrs(&self, _: sealed::Internal) -> Self::Future {
        let (ip, port) = *self;
        SocketAddrV6::new(ip, port, 0, 0).to_socket_addrs(sealed::Internal)
    }
}

// ===== impl &[SocketAddr] =====

impl ToSocketAddrs for &[SocketAddr] {}

impl sealed::ToSocketAddrsPriv for &[SocketAddr] {
    type Iter = std::vec::IntoIter<SocketAddr>;
    type Future = ReadyFuture<Self::Iter>;

    fn to_socket_addrs(&self, _: sealed::Internal) -> Self::Future {
        #[inline]
        fn slice_to_vec(addrs: &[SocketAddr]) -> Vec<SocketAddr> {
            addrs.to_vec()
        }

        // This uses a helper method because clippy doesn't like the `to_vec()`
        // call here (it will allocate, whereas `self.iter().copied()` would
        // not), but it's actually necessary in order to ensure that the
        // returned iterator is valid for the `'static` lifetime, which the
        // borrowed `slice::Iter` iterator would not be.
        //
        // Note that we can't actually add an `allow` attribute for
        // `clippy::unnecessary_to_owned` here, as Tokio's CI runs clippy lints
        // on Rust 1.52 to avoid breaking LTS releases of Tokio. Users of newer
        // Rust versions who see this lint should just ignore it.
        let iter = slice_to_vec(self).into_iter();
        future::ready(Ok(iter))
    }
}

// ===== impl str =====

impl ToSocketAddrs for str {}

impl sealed::ToSocketAddrsPriv for str {
    type Iter = sealed::OneOrMore;
    type Future = sealed::MaybeReady;

    fn to_socket_addrs(&self, _: sealed::Internal) -> Self::Future {
        use crate::task::spawn_blocking;
        use sealed::MaybeReady;

        // First check if the input parses as a socket address
        let res: Result<SocketAddr, _> = self.parse();

        if let Ok(addr) = res {
            return MaybeReady(sealed::State::Ready(Some(addr)));
        }

        // Run DNS lookup on the blocking pool
        let s = self.to_owned();

        MaybeReady(sealed::State::Blocking(spawn_blocking(move || {
            std::net::ToSocketAddrs::to_socket_addrs(&s)
        })))
    }
}

// ===== impl (&str, u16) =====

impl ToSocketAddrs for (&str, u16) {}

impl sealed::ToSocketAddrsPriv for (&str, u16) {
    type Iter = sealed::OneOrMore;
    type Future = sealed::MaybeReady;

    fn to_socket_addrs(&self, _: sealed::Internal) -> Self::Future {
        use crate::task::spawn_blocking;
        use sealed::MaybeReady;

        let (host, port) = *self;

        // try to parse the host as a regular IP address first
        if let Ok(addr) = host.parse::<Ipv4Addr>() {
            let addr = SocketAddrV4::new(addr, port);
            let addr = SocketAddr::V4(addr);

            return MaybeReady(sealed::State::Ready(Some(addr)));
        }

        if let Ok(addr) = host.parse::<Ipv6Addr>() {
            let addr = SocketAddrV6::new(addr, port, 0, 0);
            let addr = SocketAddr::V6(addr);

            return MaybeReady(sealed::State::Ready(Some(addr)));
        }

        let host = host.to_owned();

        MaybeReady(sealed::State::Blocking(spawn_blocking(move || {
            std::net::ToSocketAddrs::to_socket_addrs(&(&host[..], port))
        })))
    }
}

// ===== impl (String, u16) =====

impl ToSocketAddrs for (String, u16) {}

impl sealed::ToSocketAddrsPriv for (String, u16) {
    type Iter = sealed::OneOrMore;
    type Future = sealed::MaybeReady;

    fn to_socket_addrs(&self, _: sealed::Internal) -> Self::Future {
        (self.0.as_str(), self.1).to_socket_addrs(sealed::Internal)
    }
}

// ===== impl String =====

impl ToSocketAddrs for String {}

impl sealed::ToSocketAddrsPriv for String {
    type Iter = <str as sealed::ToSocketAddrsPriv>::Iter;
    type Future = <str as sealed::ToSocketAddrsPriv>::Future;

    fn to_socket_addrs(&self, _: sealed::Internal) -> Self::Future {
        (&self[..]).to_socket_addrs(sealed::Internal)
    }
}

pub(crate) mod sealed {
    //! The contents of this trait are intended to remain private and __not__
    //! part of the `ToSocketAddrs` public API. The details will change over
    //! time.

    use std::future::Future;
    use std::io;
    use std::net::SocketAddr;

    #[doc(hidden)]
    pub trait ToSocketAddrsPriv {
        type Iter: Iterator<Item = SocketAddr> + Send + 'static;
        type Future: Future<Output = io::Result<Self::Iter>> + Send + 'static;

        fn to_socket_addrs(&self, internal: Internal) -> Self::Future;
    }

    #[allow(missing_debug_implementations)]
    pub struct Internal;

    use crate::task::JoinHandle;

    use std::option;
    use std::pin::Pin;
    use std::task::{Context, Poll};
    use std::vec;

    #[doc(hidden)]
    #[derive(Debug)]
    pub struct MaybeReady(pub(super) State);

    #[derive(Debug)]
    pub(super) enum State {
        Ready(Option<SocketAddr>),
        Blocking(JoinHandle<io::Result<vec::IntoIter<SocketAddr>>>),
    }

    #[doc(hidden)]
    #[derive(Debug)]
    pub enum OneOrMore {
        One(option::IntoIter<SocketAddr>),
        More(vec::IntoIter<SocketAddr>),
    }

    impl Future for MaybeReady {
        type Output = io::Result<OneOrMore>;

        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            match self.0 {
                State::Ready(ref mut i) => {
                    let iter = OneOrMore::One(i.take().into_iter());
                    Poll::Ready(Ok(iter))
                }
                State::Blocking(ref mut rx) => {
                    let res = match Pin::new(rx).poll(cx) {
                        std::task::Poll::Ready(t) => t?.map(OneOrMore::More),
                        std::task::Poll::Pending => return std::task::Poll::Pending,
                    };

                    Poll::Ready(res)
                }
            }
        }
    }

    impl Iterator for OneOrMore {
        type Item = SocketAddr;

        fn next(&mut self) -> Option<Self::Item> {
            match self {
                OneOrMore::One(i) => i.next(),
                OneOrMore::More(i) => i.next(),
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            match self {
                OneOrMore::One(i) => i.size_hint(),
                OneOrMore::More(i) => i.size_hint(),
            }
        }
    }
}