//! The `Resolve` trait, support types, and some basic implementations.
//!
//! This module contains:
//!
//! - A [`GaiResolver`](dns::GaiResolver) that is the default resolver for the
//!   `HttpConnector`.
//! - The [`Resolve`](dns::Resolve) trait and related types to build a custom
//!   resolver for use with the `HttpConnector`.
use std::{fmt, io, vec};
use std::error::Error;
use std::net::{
    IpAddr, Ipv4Addr, Ipv6Addr,
    SocketAddr, ToSocketAddrs,
    SocketAddrV4, SocketAddrV6,
};
use std::str::FromStr;
use std::sync::Arc;

use futures_util::{FutureExt, StreamExt};
use tokio_executor::TypedExecutor;
use tokio_sync::{mpsc, oneshot};
use tokio_threadpool;

use crate::common::{Future, Never, Pin, Poll, Unpin, task};

/// Resolve a hostname to a set of IP addresses.
pub trait Resolve: Unpin {
    /// The set of IP addresses to try to connect to.
    type Addrs: Iterator<Item=IpAddr>;
    /// A Future of the resolved set of addresses.
    type Future: Future<Output=Result<Self::Addrs, io::Error>> + Unpin;
    /// Resolve a hostname.
    fn resolve(&self, name: Name) -> Self::Future;
}

/// A domain name to resolve into IP addresses.
#[derive(Clone, Hash, Eq, PartialEq)]
pub struct Name {
    host: String,
}

/// A resolver using blocking `getaddrinfo` calls in a threadpool.
#[derive(Clone)]
pub struct GaiResolver {
    tx: tokio_threadpool::Sender,
    /// A handle to keep the threadpool alive until all `GaiResolver` clones
    /// have been dropped.
    _threadpool_keep_alive: ThreadPoolKeepAlive,
}

#[derive(Clone)]
struct ThreadPoolKeepAlive(mpsc::Sender<Never>);

/// An iterator of IP addresses returned from `getaddrinfo`.
pub struct GaiAddrs {
    inner: IpAddrs,
}

/// A future to resole a name returned by `GaiResolver`.
pub struct GaiFuture {
    rx: oneshot::Receiver<Result<IpAddrs, io::Error>>,
    _threadpool_keep_alive: ThreadPoolKeepAlive,
}

impl Name {
    pub(super) fn new(host: String) -> Name {
        Name {
            host,
        }
    }

    /// View the hostname as a string slice.
    pub fn as_str(&self) -> &str {
        &self.host
    }
}

impl fmt::Debug for Name {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        fmt::Debug::fmt(&self.host, f)
    }
}

impl fmt::Display for Name {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        fmt::Display::fmt(&self.host, f)
    }
}

impl FromStr for Name {
    type Err = InvalidNameError;

    fn from_str(host: &str) -> Result<Self, Self::Err> {
        // Possibly add validation later
        Ok(Name::new(host.to_owned()))
    }
}

/// Error indicating a given string was not a valid domain name.
#[derive(Debug)]
pub struct InvalidNameError(());

impl fmt::Display for InvalidNameError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.write_str("Not a valid domain name")
    }
}

impl Error for InvalidNameError {}


impl GaiResolver {
    /// Construct a new `GaiResolver`.
    ///
    /// Takes number of DNS worker threads.
    pub fn new(threads: usize) -> Self {
        let pool = tokio_threadpool::Builder::new()
            .name_prefix("hyper-dns-gai-resolver")
            // not for CPU tasks, so only spawn workers
            // in blocking mode
            .pool_size(1)
            .max_blocking(threads)
            .build();

        let tx = pool.sender().clone();

        // The pool will start to shutdown once `pool` is dropped,
        // so to keep it alive, we spawn a future onto the pool itself
        // that will only resolve once all `GaiResolver` requests
        // are finished.
        let (shutdown_tx, shutdown_rx) = mpsc::channel(1);

        let on_shutdown = shutdown_rx
            .into_future()
            .map(move |(next, _rx)| {
                match next {
                    Some(never) => match never {},
                    None => (),
                }

                drop(pool)
            });
        tx.spawn(on_shutdown);

        GaiResolver {
            tx,
            _threadpool_keep_alive: ThreadPoolKeepAlive(shutdown_tx),
        }
    }
}

impl Resolve for GaiResolver {
    type Addrs = GaiAddrs;
    type Future = GaiFuture;

    fn resolve(&self, name: Name) -> Self::Future {
        let (tx, rx) = oneshot::channel();
        self.tx.spawn(GaiBlocking {
            host: name.host,
            tx: Some(tx),
        });
        GaiFuture {
            rx,
            _threadpool_keep_alive: self._threadpool_keep_alive.clone(),
        }
    }
}

impl fmt::Debug for GaiResolver {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.pad("GaiResolver")
    }
}

impl Future for GaiFuture {
    type Output = Result<GaiAddrs, io::Error>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
        Pin::new(&mut self.rx).poll(cx).map(|res| match res {
            Ok(Ok(addrs)) => Ok(GaiAddrs { inner: addrs }),
            Ok(Err(err)) => Err(err),
            Err(_canceled) => unreachable!("GaiResolver threadpool shutdown"),
        })
    }
}

impl fmt::Debug for GaiFuture {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.pad("GaiFuture")
    }
}

impl Iterator for GaiAddrs {
    type Item = IpAddr;

    fn next(&mut self) -> Option<Self::Item> {
        self.inner.next().map(|sa| sa.ip())
    }
}

impl fmt::Debug for GaiAddrs {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.pad("GaiAddrs")
    }
}


pub(super) struct GaiBlocking {
    host: String,
    tx: Option<oneshot::Sender<io::Result<IpAddrs>>>,
}

impl GaiBlocking {
    fn block(&self) -> io::Result<IpAddrs> {
        debug!("resolving host={:?}", self.host);
        (&*self.host, 0).to_socket_addrs()
            .map(|i| IpAddrs { iter: i })

    }
}

impl Future for GaiBlocking {
    type Output = ();

    fn poll(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
        if self.tx.as_mut().expect("polled after complete").poll_closed(cx).is_ready() {
            trace!("resolve future canceled for {:?}", self.host);
            return Poll::Ready(());
        }

        debug!("resolving host={:?}", self.host);
        let res = (&*self.host, 0).to_socket_addrs()
            .map(|i| IpAddrs { iter: i });

        let tx = self.tx.take().expect("polled after complete");
        let _ = tx.send(res);

        Poll::Ready(())
    }
}

pub(super) struct IpAddrs {
    iter: vec::IntoIter<SocketAddr>,
}

impl IpAddrs {
    pub(super) fn new(addrs: Vec<SocketAddr>) -> Self {
        IpAddrs { iter: addrs.into_iter() }
    }

    pub(super) fn try_parse(host: &str, port: u16) -> Option<IpAddrs> {
        if let Ok(addr) = host.parse::<Ipv4Addr>() {
            let addr = SocketAddrV4::new(addr, port);
            return Some(IpAddrs { iter: vec![SocketAddr::V4(addr)].into_iter() })
        }
        let host = {
            // trim_left/trim_right deprecated...
            // TODO: use trim_start/trim_end in Rust 1.30
            #[allow(deprecated)]
            {
                host
                .trim_left_matches('[')
                .trim_right_matches(']')
            }
        };
        if let Ok(addr) = host.parse::<Ipv6Addr>() {
            let addr = SocketAddrV6::new(addr, port, 0, 0);
            return Some(IpAddrs { iter: vec![SocketAddr::V6(addr)].into_iter() })
        }
        None
    }

    pub(super) fn split_by_preference(self) -> (IpAddrs, IpAddrs) {
        let preferring_v6 = self.iter
            .as_slice()
            .first()
            .map(SocketAddr::is_ipv6)
            .unwrap_or(false);

        let (preferred, fallback) = self.iter
            .partition::<Vec<_>, _>(|addr| addr.is_ipv6() == preferring_v6);

        (IpAddrs::new(preferred), IpAddrs::new(fallback))
    }

    pub(super) fn is_empty(&self) -> bool {
        self.iter.as_slice().is_empty()
    }
}

impl Iterator for IpAddrs {
    type Item = SocketAddr;
    #[inline]
    fn next(&mut self) -> Option<SocketAddr> {
        self.iter.next()
    }
}

/// A resolver using `getaddrinfo` calls via the `tokio_threadpool::blocking` API.
///
/// Unlike the `GaiResolver` this will not spawn dedicated threads, but only works when running on the
/// multi-threaded Tokio runtime.
#[derive(Clone, Debug)]
pub struct TokioThreadpoolGaiResolver(());

/// The future returned by `TokioThreadpoolGaiResolver`.
#[derive(Debug)]
pub struct TokioThreadpoolGaiFuture {
    name: Name,
}

impl TokioThreadpoolGaiResolver {
    /// Creates a new DNS resolver that will use tokio threadpool's blocking
    /// feature.
    ///
    /// **Requires** its futures to be run on the threadpool runtime.
    pub fn new() -> Self {
        TokioThreadpoolGaiResolver(())
    }
}

impl Resolve for TokioThreadpoolGaiResolver {
    type Addrs = GaiAddrs;
    type Future = TokioThreadpoolGaiFuture;

    fn resolve(&self, name: Name) -> TokioThreadpoolGaiFuture {
        TokioThreadpoolGaiFuture { name }
    }
}

impl Future for TokioThreadpoolGaiFuture {
    type Output = Result<GaiAddrs, io::Error>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
        match ready!(tokio_threadpool::blocking(|| (self.name.as_str(), 0).to_socket_addrs())) {
            Ok(Ok(iter)) => Poll::Ready(Ok(GaiAddrs { inner: IpAddrs { iter } })),
            Ok(Err(e)) => Poll::Ready(Err(e)),
            // a BlockingError, meaning not on a tokio_threadpool :(
            Err(e) => Poll::Ready(Err(io::Error::new(io::ErrorKind::Other, e))),
        }
    }
}

#[cfg(test)]
mod tests {
    use std::net::{Ipv4Addr, Ipv6Addr};
    use super::*;

    #[test]
    fn test_ip_addrs_split_by_preference() {
        let v4_addr = (Ipv4Addr::new(127, 0, 0, 1), 80).into();
        let v6_addr = (Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1), 80).into();

        let (mut preferred, mut fallback) =
            IpAddrs { iter: vec![v4_addr, v6_addr].into_iter() }.split_by_preference();
        assert!(preferred.next().unwrap().is_ipv4());
        assert!(fallback.next().unwrap().is_ipv6());

        let (mut preferred, mut fallback) =
            IpAddrs { iter: vec![v6_addr, v4_addr].into_iter() }.split_by_preference();
        assert!(preferred.next().unwrap().is_ipv6());
        assert!(fallback.next().unwrap().is_ipv4());
    }

    #[test]
    fn test_name_from_str() {
        const DOMAIN: &str = "test.example.com";
        let name = Name::from_str(DOMAIN).expect("Should be a valid domain");
        assert_eq!(name.as_str(), DOMAIN);
        assert_eq!(name.to_string(), DOMAIN);
    }

    #[test]
    fn ip_addrs_try_parse_v6() {
        let uri = ::http::Uri::from_static("http://[::1]:8080/");
        let dst = super::super::Destination { uri };

        let mut addrs = IpAddrs::try_parse(
            dst.host(),
            dst.port().expect("port")
        ).expect("try_parse");

        let expected = "[::1]:8080".parse::<SocketAddr>().expect("expected");

        assert_eq!(addrs.next(), Some(expected));
    }
}