use std::cell::Cell;
use std::cmp;
use std::collections::VecDeque;
use std::fmt;
use std::io;

use bytes::{Buf, BufMut, Bytes, BytesMut};
use futures::{Async, Poll};
use iovec::IoVec;
use tokio_io::{AsyncRead, AsyncWrite};

use super::{Http1Transaction, ParseContext, ParsedMessage};

/// The initial buffer size allocated before trying to read from IO.
pub(crate) const INIT_BUFFER_SIZE: usize = 8192;

/// The minimum value that can be set to max buffer size.
pub const MINIMUM_MAX_BUFFER_SIZE: usize = INIT_BUFFER_SIZE;

/// The default maximum read buffer size. If the buffer gets this big and
/// a message is still not complete, a `TooLarge` error is triggered.
// Note: if this changes, update server::conn::Http::max_buf_size docs.
pub(crate) const DEFAULT_MAX_BUFFER_SIZE: usize = 8192 + 4096 * 100;

/// The maximum number of distinct `Buf`s to hold in a list before requiring
/// a flush. Only affects when the buffer strategy is to queue buffers.
///
/// Note that a flush can happen before reaching the maximum. This simply
/// forces a flush if the queue gets this big.
const MAX_BUF_LIST_BUFFERS: usize = 16;

pub struct Buffered<T, B> {
    flush_pipeline: bool,
    io: T,
    read_blocked: bool,
    read_buf: BytesMut,
    read_buf_strategy: ReadStrategy,
    write_buf: WriteBuf<B>,
}

impl<T, B> fmt::Debug for Buffered<T, B>
where
    B: Buf,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("Buffered")
            .field("read_buf", &self.read_buf)
            .field("write_buf", &self.write_buf)
            .finish()
    }
}

impl<T, B> Buffered<T, B>
where
    T: AsyncRead + AsyncWrite,
    B: Buf,
{
    pub fn new(io: T) -> Buffered<T, B> {
        Buffered {
            flush_pipeline: false,
            io: io,
            read_blocked: false,
            read_buf: BytesMut::with_capacity(0),
            read_buf_strategy: ReadStrategy::default(),
            write_buf: WriteBuf::new(),
        }
    }

    pub fn set_flush_pipeline(&mut self, enabled: bool) {
        debug_assert!(!self.write_buf.has_remaining());
        self.flush_pipeline = enabled;
        if enabled {
            self.set_write_strategy_flatten();
        }
    }

    pub fn set_max_buf_size(&mut self, max: usize) {
        assert!(
            max >= MINIMUM_MAX_BUFFER_SIZE,
            "The max_buf_size cannot be smaller than {}.",
            MINIMUM_MAX_BUFFER_SIZE,
        );
        self.read_buf_strategy = ReadStrategy::with_max(max);
        self.write_buf.max_buf_size = max;
    }

    pub fn set_read_buf_exact_size(&mut self, sz: usize) {
        self.read_buf_strategy = ReadStrategy::Exact(sz);
    }

    pub fn set_write_strategy_flatten(&mut self) {
        // this should always be called only at construction time,
        // so this assert is here to catch myself
        debug_assert!(self.write_buf.queue.bufs.is_empty());
        self.write_buf.set_strategy(WriteStrategy::Flatten);
    }

    pub fn read_buf(&self) -> &[u8] {
        self.read_buf.as_ref()
    }

    #[cfg(test)]
    #[cfg(feature = "nightly")]
    pub(super) fn read_buf_mut(&mut self) -> &mut BytesMut {
        &mut self.read_buf
    }

    pub fn headers_buf(&mut self) -> &mut Vec<u8> {
        let buf = self.write_buf.headers_mut();
        &mut buf.bytes
    }

    pub(super) fn write_buf(&mut self) -> &mut WriteBuf<B> {
        &mut self.write_buf
    }

    pub fn buffer<BB: Buf + Into<B>>(&mut self, buf: BB) {
        self.write_buf.buffer(buf)
    }

    pub fn can_buffer(&self) -> bool {
        self.flush_pipeline || self.write_buf.can_buffer()
    }

    pub fn consume_leading_lines(&mut self) {
        if !self.read_buf.is_empty() {
            let mut i = 0;
            while i < self.read_buf.len() {
                match self.read_buf[i] {
                    b'\r' | b'\n' => i += 1,
                    _ => break,
                }
            }
            self.read_buf.split_to(i);
        }
    }

    pub(super) fn parse<S>(&mut self, ctx: ParseContext)
        -> Poll<ParsedMessage<S::Incoming>, ::Error>
    where
        S: Http1Transaction,
    {
        loop {
            match S::parse(&mut self.read_buf, ParseContext {
                cached_headers: ctx.cached_headers,
                req_method: ctx.req_method,
            })? {
                Some(msg) => {
                    debug!("parsed {} headers", msg.head.headers.len());
                    return Ok(Async::Ready(msg))
                },
                None => {
                    let max = self.read_buf_strategy.max();
                    if self.read_buf.len() >= max {
                        debug!("max_buf_size ({}) reached, closing", max);
                        return Err(::Error::new_too_large());
                    }
                },
            }
            match try_ready!(self.read_from_io().map_err(::Error::new_io)) {
                0 => {
                    trace!("parse eof");
                    return Err(::Error::new_incomplete());
                }
                _ => {},
            }
        }
    }

    pub fn read_from_io(&mut self) -> Poll<usize, io::Error> {
        self.read_blocked = false;
        let next = self.read_buf_strategy.next();
        if self.read_buf.remaining_mut() < next {
            self.read_buf.reserve(next);
        }
        self.io.read_buf(&mut self.read_buf).map(|ok| {
            match ok {
                Async::Ready(n) => {
                    debug!("read {} bytes", n);
                    self.read_buf_strategy.record(n);
                    Async::Ready(n)
                },
                Async::NotReady => {
                    self.read_blocked = true;
                    Async::NotReady
                }
            }
        })
    }

    pub fn into_inner(self) -> (T, Bytes) {
        (self.io, self.read_buf.freeze())
    }

    pub fn io_mut(&mut self) -> &mut T {
        &mut self.io
    }

    pub fn is_read_blocked(&self) -> bool {
        self.read_blocked
    }

    pub fn flush(&mut self) -> Poll<(), io::Error> {
        if self.flush_pipeline && !self.read_buf.is_empty() {
            //Ok(())
        } else if self.write_buf.remaining() == 0 {
            try_nb!(self.io.flush());
        } else {
            match self.write_buf.strategy {
                WriteStrategy::Flatten => return self.flush_flattened(),
                _ => (),
            }
            loop {
                let n = try_ready!(self.io.write_buf(&mut self.write_buf.auto()));
                debug!("flushed {} bytes", n);
                if self.write_buf.remaining() == 0 {
                    break;
                } else if n == 0 {
                    trace!("write returned zero, but {} bytes remaining", self.write_buf.remaining());
                    return Err(io::ErrorKind::WriteZero.into())
                }
            }
            try_nb!(self.io.flush())
        }
        Ok(Async::Ready(()))
    }

    /// Specialized version of `flush` when strategy is Flatten.
    ///
    /// Since all buffered bytes are flattened into the single headers buffer,
    /// that skips some bookkeeping around using multiple buffers.
    fn flush_flattened(&mut self) -> Poll<(), io::Error> {
        loop {
            let n = try_nb!(self.io.write(self.write_buf.headers.bytes()));
            debug!("flushed {} bytes", n);
            self.write_buf.headers.advance(n);
            if self.write_buf.headers.remaining() == 0 {
                self.write_buf.headers.reset();
                break;
            } else if n == 0 {
                trace!("write returned zero, but {} bytes remaining", self.write_buf.remaining());
                return Err(io::ErrorKind::WriteZero.into())
            }
        }
        try_nb!(self.io.flush());
        Ok(Async::Ready(()))
    }
}

pub trait MemRead {
    fn read_mem(&mut self, len: usize) -> Poll<Bytes, io::Error>;
}

impl<T, B> MemRead for Buffered<T, B> 
where
    T: AsyncRead + AsyncWrite,
    B: Buf,
{
    fn read_mem(&mut self, len: usize) -> Poll<Bytes, io::Error> {
        if !self.read_buf.is_empty() {
            let n = ::std::cmp::min(len, self.read_buf.len());
            Ok(Async::Ready(self.read_buf.split_to(n).freeze()))
        } else {
            let n = try_ready!(self.read_from_io());
            Ok(Async::Ready(self.read_buf.split_to(::std::cmp::min(len, n)).freeze()))
        }
    }
}

#[derive(Clone, Copy, Debug)]
enum ReadStrategy {
    Adaptive {
        decrease_now: bool,
        next: usize,
        max: usize
    },
    Exact(usize),
}

impl ReadStrategy {
    fn with_max(max: usize) -> ReadStrategy {
        ReadStrategy::Adaptive {
            decrease_now: false,
            next: INIT_BUFFER_SIZE,
            max,
        }
    }

    fn next(&self) -> usize {
        match *self {
            ReadStrategy::Adaptive { next, .. } => next,
            ReadStrategy::Exact(exact) => exact,
        }
    }

    fn max(&self) -> usize {
        match *self {
            ReadStrategy::Adaptive { max, .. } => max,
            ReadStrategy::Exact(exact) => exact,
        }
    }

    fn record(&mut self, bytes_read: usize) {
        match *self {
            ReadStrategy::Adaptive { ref mut decrease_now, ref mut next, max, .. } => {
                if bytes_read >= *next {
                    *next = cmp::min(incr_power_of_two(*next), max);
                    *decrease_now = false;
                } else {
                    let decr_to = prev_power_of_two(*next);
                    if bytes_read < decr_to {
                        if *decrease_now {
                            *next = cmp::max(decr_to, INIT_BUFFER_SIZE);
                            *decrease_now = false;
                        } else {
                            // Decreasing is a two "record" process.
                            *decrease_now = true;
                        }
                    } else {
                        // A read within the current range should cancel
                        // a potential decrease, since we just saw proof
                        // that we still need this size.
                        *decrease_now = false;
                    }
                }
            },
            _ => (),
        }
    }
}

fn incr_power_of_two(n: usize) -> usize {
    n.saturating_mul(2)
}

fn prev_power_of_two(n: usize) -> usize {
    // Only way this shift can underflow is if n is less than 4.
    // (Which would means `usize::MAX >> 64` and underflowed!)
    debug_assert!(n >= 4);
    (::std::usize::MAX >> (n.leading_zeros() + 2)) + 1
}

impl Default for ReadStrategy {
    fn default() -> ReadStrategy {
        ReadStrategy::with_max(DEFAULT_MAX_BUFFER_SIZE)
    }
}

#[derive(Clone)]
pub struct Cursor<T> {
    bytes: T,
    pos: usize,
}

impl<T: AsRef<[u8]>> Cursor<T> {
    #[inline]
    pub(crate) fn new(bytes: T) -> Cursor<T> {
        Cursor {
            bytes: bytes,
            pos: 0,
        }
    }
}

impl Cursor<Vec<u8>> {
    fn reset(&mut self) {
        self.pos = 0;
        self.bytes.clear();
    }
}

impl<T: AsRef<[u8]>> fmt::Debug for Cursor<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("Cursor")
            .field("pos", &self.pos)
            .field("len", &self.bytes.as_ref().len())
            .finish()
    }
}

impl<T: AsRef<[u8]>> Buf for Cursor<T> {
    #[inline]
    fn remaining(&self) -> usize {
        self.bytes.as_ref().len() - self.pos
    }

    #[inline]
    fn bytes(&self) -> &[u8] {
        &self.bytes.as_ref()[self.pos..]
    }

    #[inline]
    fn advance(&mut self, cnt: usize) {
        debug_assert!(self.pos + cnt <= self.bytes.as_ref().len());
        self.pos += cnt;
    }
}

// an internal buffer to collect writes before flushes
pub(super) struct WriteBuf<B> {
    /// Re-usable buffer that holds message headers
    headers: Cursor<Vec<u8>>,
    max_buf_size: usize,
    /// Deque of user buffers if strategy is Queue
    queue: BufDeque<B>,
    strategy: WriteStrategy,
}

impl<B> WriteBuf<B> {
    fn new() -> WriteBuf<B> {
        WriteBuf {
            headers: Cursor::new(Vec::with_capacity(INIT_BUFFER_SIZE)),
            max_buf_size: DEFAULT_MAX_BUFFER_SIZE,
            queue: BufDeque::new(),
            strategy: WriteStrategy::Auto,
        }
    }
}


impl<B> WriteBuf<B>
where
    B: Buf,
{
    fn set_strategy(&mut self, strategy: WriteStrategy) {
        self.strategy = strategy;
    }

    #[inline]
    fn auto(&mut self) -> WriteBufAuto<B> {
        WriteBufAuto::new(self)
    }

    pub(super) fn buffer<BB: Buf + Into<B>>(&mut self, mut buf: BB) {
        debug_assert!(buf.has_remaining());
        match self.strategy {
            WriteStrategy::Flatten => {
                let head = self.headers_mut();
                //perf: This is a little faster than <Vec as BufMut>>::put,
                //but accomplishes the same result.
                loop {
                    let adv = {
                        let slice = buf.bytes();
                        if slice.is_empty() {
                            return;
                        }
                        head.bytes.extend_from_slice(slice);
                        slice.len()
                    };
                    buf.advance(adv);
                }
            },
            WriteStrategy::Auto | WriteStrategy::Queue => {
                self.queue.bufs.push_back(buf.into());
            },
        }
    }

    fn can_buffer(&self) -> bool {
        match self.strategy {
            WriteStrategy::Flatten => {
                self.remaining() < self.max_buf_size
            },
            WriteStrategy::Auto | WriteStrategy::Queue => {
                self.queue.bufs.len() < MAX_BUF_LIST_BUFFERS
                    && self.remaining() < self.max_buf_size
            },
        }
    }

    fn headers_mut(&mut self) -> &mut Cursor<Vec<u8>> {
        debug_assert!(!self.queue.has_remaining());
        &mut self.headers
    }
}

impl<B: Buf> fmt::Debug for WriteBuf<B> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("WriteBuf")
            .field("remaining", &self.remaining())
            .field("strategy", &self.strategy)
            .finish()
    }
}

impl<B: Buf> Buf for WriteBuf<B> {
    #[inline]
    fn remaining(&self) -> usize {
        self.headers.remaining() + self.queue.remaining()
    }

    #[inline]
    fn bytes(&self) -> &[u8] {
        let headers = self.headers.bytes();
        if !headers.is_empty() {
            headers
        } else {
            self.queue.bytes()
        }
    }

    #[inline]
    fn advance(&mut self, cnt: usize) {
        let hrem = self.headers.remaining();
        if hrem == cnt {
            self.headers.reset();
        } else if hrem > cnt {
            self.headers.advance(cnt);
        } else {
            let qcnt = cnt - hrem;
            self.headers.reset();
            self.queue.advance(qcnt);
        }
    }

    #[inline]
    fn bytes_vec<'t>(&'t self, dst: &mut [&'t IoVec]) -> usize {
        let n = self.headers.bytes_vec(dst);
        self.queue.bytes_vec(&mut dst[n..]) + n
    }
}

/// Detects when wrapped `WriteBuf` is used for vectored IO, and
/// adjusts the `WriteBuf` strategy if not.
struct WriteBufAuto<'a, B: Buf + 'a> {
    bytes_called: Cell<bool>,
    bytes_vec_called: Cell<bool>,
    inner: &'a mut WriteBuf<B>,
}

impl<'a, B: Buf> WriteBufAuto<'a, B> {
    fn new(inner: &'a mut WriteBuf<B>) -> WriteBufAuto<'a, B> {
        WriteBufAuto {
            bytes_called: Cell::new(false),
            bytes_vec_called: Cell::new(false),
            inner: inner,
        }
    }
}

impl<'a, B: Buf> Buf for WriteBufAuto<'a, B> {
    #[inline]
    fn remaining(&self) -> usize {
        self.inner.remaining()
    }

    #[inline]
    fn bytes(&self) -> &[u8] {
        self.bytes_called.set(true);
        self.inner.bytes()
    }

    #[inline]
    fn advance(&mut self, cnt: usize) {
        self.inner.advance(cnt)
    }

    #[inline]
    fn bytes_vec<'t>(&'t self, dst: &mut [&'t IoVec]) -> usize {
        self.bytes_vec_called.set(true);
        self.inner.bytes_vec(dst)
    }
}

impl<'a, B: Buf + 'a> Drop for WriteBufAuto<'a, B> {
    fn drop(&mut self) {
        if let WriteStrategy::Auto = self.inner.strategy {
            if self.bytes_vec_called.get() {
                self.inner.strategy = WriteStrategy::Queue;
            } else if self.bytes_called.get() {
                trace!("detected no usage of vectored write, flattening");
                self.inner.strategy = WriteStrategy::Flatten;
                self.inner.headers.bytes.put(&mut self.inner.queue);
            }
        }
    }
}


#[derive(Debug)]
enum WriteStrategy {
    Auto,
    Flatten,
    Queue,
}

struct BufDeque<T> {
    bufs: VecDeque<T>,
}


impl<T> BufDeque<T> {
    fn new() -> BufDeque<T> {
        BufDeque {
            bufs: VecDeque::new(),
        }
    }
}

impl<T: Buf> Buf for BufDeque<T> {
    #[inline]
    fn remaining(&self) -> usize {
        self.bufs.iter()
            .map(|buf| buf.remaining())
            .sum()
    }

    #[inline]
    fn bytes(&self) -> &[u8] {
        for buf in &self.bufs {
            return buf.bytes();
        }
        &[]
    }

    #[inline]
    fn advance(&mut self, mut cnt: usize) {
        while cnt > 0 {
            {
                let front = &mut self.bufs[0];
                let rem = front.remaining();
                if rem > cnt {
                    front.advance(cnt);
                    return;
                } else {
                    front.advance(rem);
                    cnt -= rem;
                }
            }
            self.bufs.pop_front();
        }
    }

    #[inline]
    fn bytes_vec<'t>(&'t self, dst: &mut [&'t IoVec]) -> usize {
        if dst.is_empty() {
            return 0;
        }
        let mut vecs = 0;
        for buf in &self.bufs {
            vecs += buf.bytes_vec(&mut dst[vecs..]);
            if vecs == dst.len() {
                break;
            }
        }
        vecs
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::io::Read;
    use mock::AsyncIo;

    #[cfg(feature = "nightly")]
    use test::Bencher;

    #[cfg(test)]
    impl<T: Read> MemRead for ::mock::AsyncIo<T> {
        fn read_mem(&mut self, len: usize) -> Poll<Bytes, io::Error> {
            let mut v = vec![0; len];
            let n = try_nb!(self.read(v.as_mut_slice()));
            Ok(Async::Ready(BytesMut::from(&v[..n]).freeze()))
        }
    }

    #[test]
    fn iobuf_write_empty_slice() {
        let mut mock = AsyncIo::new_buf(vec![], 256);
        mock.error(io::Error::new(io::ErrorKind::Other, "logic error"));

        let mut io_buf = Buffered::<_, Cursor<Vec<u8>>>::new(mock);

        // underlying io will return the logic error upon write,
        // so we are testing that the io_buf does not trigger a write
        // when there is nothing to flush
        io_buf.flush().expect("should short-circuit flush");
    }

    #[test]
    fn parse_reads_until_blocked() {
        // missing last line ending
        let raw = "HTTP/1.1 200 OK\r\n";

        let mock = AsyncIo::new_buf(raw, raw.len());
        let mut buffered = Buffered::<_, Cursor<Vec<u8>>>::new(mock);
        let ctx = ParseContext {
            cached_headers: &mut None,
            req_method: &mut None,
        };
        assert!(buffered.parse::<::proto::h1::ClientTransaction>(ctx).unwrap().is_not_ready());
        assert!(buffered.io.blocked());
    }

    #[test]
    fn read_strategy_adaptive_increments() {
        let mut strategy = ReadStrategy::default();
        assert_eq!(strategy.next(), 8192);

        // Grows if record == next
        strategy.record(8192);
        assert_eq!(strategy.next(), 16384);

        strategy.record(16384);
        assert_eq!(strategy.next(), 32768);

        // Enormous records still increment at same rate
        strategy.record(::std::usize::MAX);
        assert_eq!(strategy.next(), 65536);

        let max = strategy.max();
        while strategy.next() < max {
            strategy.record(max);
        }

        assert_eq!(strategy.next(), max, "never goes over max");
        strategy.record(max + 1);
        assert_eq!(strategy.next(), max, "never goes over max");
    }

    #[test]
    fn read_strategy_adaptive_decrements() {
        let mut strategy = ReadStrategy::default();
        strategy.record(8192);
        assert_eq!(strategy.next(), 16384);

        strategy.record(1);
        assert_eq!(strategy.next(), 16384, "first smaller record doesn't decrement yet");
        strategy.record(8192);
        assert_eq!(strategy.next(), 16384, "record was with range");

        strategy.record(1);
        assert_eq!(strategy.next(), 16384, "in-range record should make this the 'first' again");

        strategy.record(1);
        assert_eq!(strategy.next(), 8192, "second smaller record decrements");

        strategy.record(1);
        assert_eq!(strategy.next(), 8192, "first doesn't decrement");
        strategy.record(1);
        assert_eq!(strategy.next(), 8192, "doesn't decrement under minimum");
    }

    #[test]
    fn read_strategy_adaptive_stays_the_same() {
        let mut strategy = ReadStrategy::default();
        strategy.record(8192);
        assert_eq!(strategy.next(), 16384);

        strategy.record(8193);
        assert_eq!(strategy.next(), 16384, "first smaller record doesn't decrement yet");

        strategy.record(8193);
        assert_eq!(strategy.next(), 16384, "with current step does not decrement");
    }

    #[test]
    fn read_strategy_adaptive_max_fuzz() {
        fn fuzz(max: usize) {
            let mut strategy = ReadStrategy::with_max(max);
            while strategy.next() < max {
                strategy.record(::std::usize::MAX);
            }
            let mut next = strategy.next();
            while next > 8192 {
                strategy.record(1);
                strategy.record(1);
                next = strategy.next();
                assert!(
                    next.is_power_of_two(),
                    "decrement should be powers of two: {} (max = {})",
                    next,
                    max,
                );
            }
        }

        let mut max = 8192;
        while max < ::std::usize::MAX {
            fuzz(max);
            max = (max / 2).saturating_mul(3);
        }
        fuzz(::std::usize::MAX);
    }

    #[test]
    #[should_panic]
    fn write_buf_requires_non_empty_bufs() {
        let mock = AsyncIo::new_buf(vec![], 1024);
        let mut buffered = Buffered::<_, Cursor<Vec<u8>>>::new(mock);

        buffered.buffer(Cursor::new(Vec::new()));
    }

    #[test]
    fn write_buf_queue() {
        extern crate pretty_env_logger;
        let _ = pretty_env_logger::try_init();

        let mock = AsyncIo::new_buf(vec![], 1024);
        let mut buffered = Buffered::<_, Cursor<Vec<u8>>>::new(mock);


        buffered.headers_buf().extend(b"hello ");
        buffered.buffer(Cursor::new(b"world, ".to_vec()));
        buffered.buffer(Cursor::new(b"it's ".to_vec()));
        buffered.buffer(Cursor::new(b"hyper!".to_vec()));
        assert_eq!(buffered.write_buf.queue.bufs.len(), 3);
        buffered.flush().unwrap();

        assert_eq!(buffered.io, b"hello world, it's hyper!");
        assert_eq!(buffered.io.num_writes(), 1);
        assert_eq!(buffered.write_buf.queue.bufs.len(), 0);
    }

    #[test]
    fn write_buf_flatten() {
        extern crate pretty_env_logger;
        let _ = pretty_env_logger::try_init();

        let mock = AsyncIo::new_buf(vec![], 1024);
        let mut buffered = Buffered::<_, Cursor<Vec<u8>>>::new(mock);
        buffered.write_buf.set_strategy(WriteStrategy::Flatten);

        buffered.headers_buf().extend(b"hello ");
        buffered.buffer(Cursor::new(b"world, ".to_vec()));
        buffered.buffer(Cursor::new(b"it's ".to_vec()));
        buffered.buffer(Cursor::new(b"hyper!".to_vec()));
        assert_eq!(buffered.write_buf.queue.bufs.len(), 0);

        buffered.flush().unwrap();

        assert_eq!(buffered.io, b"hello world, it's hyper!");
        assert_eq!(buffered.io.num_writes(), 1);
    }

    #[test]
    fn write_buf_auto_flatten() {
        extern crate pretty_env_logger;
        let _ = pretty_env_logger::try_init();

        let mut mock = AsyncIo::new_buf(vec![], 1024);
        mock.max_read_vecs(0); // disable vectored IO
        let mut buffered = Buffered::<_, Cursor<Vec<u8>>>::new(mock);

        // we have 4 buffers, but hope to detect that vectored IO isn't
        // being used, and switch to flattening automatically,
        // resulting in only 2 writes
        buffered.headers_buf().extend(b"hello ");
        buffered.buffer(Cursor::new(b"world, ".to_vec()));
        buffered.buffer(Cursor::new(b"it's ".to_vec()));
        buffered.buffer(Cursor::new(b"hyper!".to_vec()));
        assert_eq!(buffered.write_buf.queue.bufs.len(), 3);
        buffered.flush().unwrap();

        assert_eq!(buffered.io, b"hello world, it's hyper!");
        assert_eq!(buffered.io.num_writes(), 2);
        assert_eq!(buffered.write_buf.queue.bufs.len(), 0);
    }

    #[test]
    fn write_buf_queue_disable_auto() {
        extern crate pretty_env_logger;
        let _ = pretty_env_logger::try_init();

        let mut mock = AsyncIo::new_buf(vec![], 1024);
        mock.max_read_vecs(0); // disable vectored IO
        let mut buffered = Buffered::<_, Cursor<Vec<u8>>>::new(mock);
        buffered.write_buf.set_strategy(WriteStrategy::Queue);

        // we have 4 buffers, and vec IO disabled, but explicitly said
        // don't try to auto detect (via setting strategy above)

        buffered.headers_buf().extend(b"hello ");
        buffered.buffer(Cursor::new(b"world, ".to_vec()));
        buffered.buffer(Cursor::new(b"it's ".to_vec()));
        buffered.buffer(Cursor::new(b"hyper!".to_vec()));
        assert_eq!(buffered.write_buf.queue.bufs.len(), 3);
        buffered.flush().unwrap();

        assert_eq!(buffered.io, b"hello world, it's hyper!");
        assert_eq!(buffered.io.num_writes(), 4);
        assert_eq!(buffered.write_buf.queue.bufs.len(), 0);
    }

    #[cfg(feature = "nightly")]
    #[bench]
    fn bench_write_buf_flatten_buffer_chunk(b: &mut Bencher) {
        let s = "Hello, World!";
        b.bytes = s.len() as u64;

        let mut write_buf = WriteBuf::<::Chunk>::new();
        write_buf.set_strategy(WriteStrategy::Flatten);
        b.iter(|| {
            let chunk = ::Chunk::from(s);
            write_buf.buffer(chunk);
            ::test::black_box(&write_buf);
            write_buf.headers.bytes.clear();
        })
    }
}