// Copyright (c) 2018-2019 Parity Technologies (UK) Ltd.
//
// Licensed under the Apache License, Version 2.0 or MIT license, at your option.
//
// A copy of the Apache License, Version 2.0 is included in the software as
// LICENSE-APACHE and a copy of the MIT license is included in the software
// as LICENSE-MIT. You may also obtain a copy of the Apache License, Version 2.0
// at https://www.apache.org/licenses/LICENSE-2.0 and a copy of the MIT license
// at https://opensource.org/licenses/MIT.

//! This crate implements the [Yamux specification][1].
//!
//! It multiplexes independent I/O streams over reliable, ordered connections,
//! such as TCP/IP.
//!
//! The three primary objects, clients of this crate interact with, are:
//!
//! - [`Connection`], which wraps the underlying I/O resource, e.g. a socket,
//! - [`Stream`], which implements [`futures::io::AsyncRead`] and
//!   [`futures::io::AsyncWrite`], and
//! - [`Control`], to asynchronously control the [`Connection`].
//!
//! [1]: https://github.com/hashicorp/yamux/blob/master/spec.md

#![forbid(unsafe_code)]

mod chunks;
mod error;
mod frame;
mod pause;

#[cfg(test)]
mod tests;

pub(crate) mod connection;

pub use crate::connection::{into_stream, Connection, Control, Mode, Packet, Stream};
pub use crate::error::ConnectionError;
pub use crate::frame::{
    header::{HeaderDecodeError, StreamId},
    FrameDecodeError,
};

const DEFAULT_CREDIT: u32 = 256 * 1024; // as per yamux specification

/// Default maximum number of bytes a Yamux data frame might carry as its
/// payload when being send. Larger Payloads will be split.
///
/// The data frame payload size is not restricted by the yamux specification.
/// Still, this implementation restricts the size to:
///
/// 1. Reduce delays sending time-sensitive frames, e.g. window updates.
/// 2. Minimize head-of-line blocking across streams.
/// 3. Enable better interleaving of send and receive operations, as each is
///    carried out atomically instead of concurrently with its respective
///    counterpart.
///
/// For details on why this concrete value was chosen, see
/// https://github.com/paritytech/yamux/issues/100.
const DEFAULT_SPLIT_SEND_SIZE: usize = 16 * 1024;

/// Specifies when window update frames are sent.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum WindowUpdateMode {
    /// Send window updates as soon as a [`Stream`]'s receive window drops to 0.
    ///
    /// This ensures that the sender can resume sending more data as soon as possible
    /// but a slow reader on the receiving side may be overwhelmed, i.e. it accumulates
    /// data in its buffer which may reach its limit (see `set_max_buffer_size`).
    /// In this mode, window updates merely prevent head of line blocking but do not
    /// effectively exercise back pressure on senders.
    OnReceive,

    /// Send window updates only when data is read on the receiving end.
    ///
    /// This ensures that senders do not overwhelm receivers and keeps buffer usage
    /// low. However, depending on the protocol, there is a risk of deadlock, namely
    /// if both endpoints want to send data larger than the receivers window and they
    /// do not read before finishing their writes. Use this mode only if you are sure
    /// that this will never happen, i.e. if
    ///
    /// - Endpoints *A* and *B* never write at the same time, *or*
    /// - Endpoints *A* and *B* write at most *n* frames concurrently such that the sum
    ///   of the frame lengths is less or equal to the available credit of *A* and *B*
    ///   respectively.
    OnRead,
}

/// Yamux configuration.
///
/// The default configuration values are as follows:
///
/// - receive window = 256 KiB
/// - max. buffer size (per stream) = 1 MiB
/// - max. number of streams = 8192
/// - window update mode = on read
/// - read after close = true
/// - split send size = 16 KiB
#[derive(Debug, Clone)]
pub struct Config {
    receive_window: u32,
    max_buffer_size: usize,
    max_num_streams: usize,
    window_update_mode: WindowUpdateMode,
    read_after_close: bool,
    split_send_size: usize,
}

impl Default for Config {
    fn default() -> Self {
        Config {
            receive_window: DEFAULT_CREDIT,
            max_buffer_size: 1024 * 1024,
            max_num_streams: 8192,
            window_update_mode: WindowUpdateMode::OnRead,
            read_after_close: true,
            split_send_size: DEFAULT_SPLIT_SEND_SIZE,
        }
    }
}

impl Config {
    /// Set the receive window per stream (must be >= 256 KiB).
    ///
    /// # Panics
    ///
    /// If the given receive window is < 256 KiB.
    pub fn set_receive_window(&mut self, n: u32) -> &mut Self {
        assert!(n >= DEFAULT_CREDIT);
        self.receive_window = n;
        self
    }

    /// Set the max. buffer size per stream.
    pub fn set_max_buffer_size(&mut self, n: usize) -> &mut Self {
        self.max_buffer_size = n;
        self
    }

    /// Set the max. number of streams.
    pub fn set_max_num_streams(&mut self, n: usize) -> &mut Self {
        self.max_num_streams = n;
        self
    }

    /// Set the window update mode to use.
    pub fn set_window_update_mode(&mut self, m: WindowUpdateMode) -> &mut Self {
        self.window_update_mode = m;
        self
    }

    /// Allow or disallow streams to read from buffered data after
    /// the connection has been closed.
    pub fn set_read_after_close(&mut self, b: bool) -> &mut Self {
        self.read_after_close = b;
        self
    }

    /// Set the max. payload size used when sending data frames. Payloads larger
    /// than the configured max. will be split.
    pub fn set_split_send_size(&mut self, n: usize) -> &mut Self {
        self.split_send_size = n;
        self
    }
}

// Check that we can safely cast a `usize` to a `u64`.
static_assertions::const_assert! {
    std::mem::size_of::<usize>() <= std::mem::size_of::<u64>()
}

// Check that we can safely cast a `u32` to a `usize`.
static_assertions::const_assert! {
    std::mem::size_of::<u32>() <= std::mem::size_of::<usize>()
}