use std::{cmp, mem};
use std::collections::VecDeque;

use netutils::{n16, Ipv4Addr, Checksum, Ipv4Header, Ipv4};
use resource_scheme::Resource;
use syscall;
use syscall::error::*;

/// Max number of bytes in a packet.
const MAX_PACKET_LENGTH : usize = 65536;

/// A IP (internet protocol) resource.
///
/// Each instance represents a connection (~ a IP socket).
pub struct IpResource {
    /// The underlying mechanism ensured to connect to the peer.
    pub connection: Connection,

    /// The IP address of the host (i.e. this machine).
    pub host_addr: Ipv4Addr,

    /// The IP address of the peer (i.e. the other machine).
    pub peer_addr: Ipv4Addr,

    /// The IP protocol used by this connection. See
    /// http://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml
    /// for the list of valid protocols.
    pub proto: u8,

    /// The id of the next packet being sent.
    /// See https://en.wikipedia.org/wiki/IPv4#Identification .
    pub id: u16,
}

pub enum Connection {
    Device {
        /// Link to the underlying device (typically, an Ethernet card).
        link: usize,

        /// If this connection was opened waiting for a peer (i.e. `ip:/protocol`),
        /// the data received when the peer actually connected. Otherwise, empty.
        /// Emptied during the first call to `read()`.
        init_data: Vec<u8>,
    },
    Loopback {
        /// FIFO queue of packets written to the connection and waiting to be read.
        ///
        /// The data stored contains the exact data that has been added by the client
        /// calling `write()`, without adding any headers.
        packets: VecDeque<Vec<u8>>
    }
}

impl Resource for IpResource {
    /// Duplicate the connection.
    ///
    /// This duplicates both `self.link` and `self.init_data`.
    ///
    /// # Errors
    ///
    /// Fails if the `link` to the underlying device cannot be
    /// duplicated.
    fn dup(&self) -> Result<Box<Self>> {
        use self::Connection::*;
        let connection = match self.connection {
            Loopback { ref packets } => Loopback {
                packets: packets.clone()
            },
            Device { link, ref init_data } => {
                let link = try!(syscall::dup(link));
                let init_data = init_data.clone();
                Device {
                    link: link,
                    init_data: init_data
                }
            }
        };
        Ok(Box::new(IpResource {
            host_addr: self.host_addr,
            peer_addr: self.peer_addr,
            proto: self.proto,
            id: self.id,
            connection: connection,
        }))
    }

    /// Get the current path, as `ip:peer/protocol`, where `peer`
    /// is the IPv4 address of the peer and `protocol` is the hex-based
    /// number of the IP protocol used.
    ///
    /// Note that the `peer` is specified even if the connection was initially
    /// created as `ip:/protocol`.
    fn path(&self, buf: &mut [u8]) -> Result<usize> {
        let path_string = format!("ip:{}/{:X}", self.peer_addr.to_string(), self.proto);
        let path = path_string.as_bytes();

        for (b, p) in buf.iter_mut().zip(path.iter()) {
            *b = *p;
        }

        Ok(cmp::min(buf.len(), path.len()))
    }

    /// Read data from the device.
    ///
    /// If some data has already been made available during the establishment
    /// of the connection, this data is (entirely) read during the first call
    /// to `read()`, without attempting to actually read from the device. This
    /// can happen only if the connection was waiting for a remote peer to connect, i.e.
    /// with a url `ip:/protocol`, without host.
    ///
    /// If this connection is a loopback, oldest unread packet written is read.
    ///
    /// # Errors
    ///
    /// Fails if the call to `syscall::read()` fails for this device.
    ///
    /// # Data loss
    ///
    /// If `buf` is too small, *exceeding data is discarded*. To be sure that you read
    /// all data, you should provide a 64kb `buf`.
    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
        use self::Connection::*;
        match self.connection {
            Loopback { ref mut packets } => {
                match packets.pop_front() {
                    None => Ok(0),
                    Some(data) => {
                        for (b, d) in buf.iter_mut().zip(data.iter()) {
                            *b = *d;
                        }
                        // Note: We're discarding excess `data`.
                        Ok(cmp::min(buf.len(), data.len()))
                    }
                }
            }
            Device { ref mut init_data, link } => {
                if !init_data.is_empty() {
                    let mut data: Vec<u8> = Vec::new();
                    mem::swap(init_data, &mut data);

                    for (b, d) in buf.iter_mut().zip(data.iter()) {
                        *b = *d;
                    }

                    return Ok(cmp::min(buf.len(), data.len()));
                }

                let mut bytes = [0; MAX_PACKET_LENGTH];
                let count = try!(syscall::read(link, &mut bytes));

                if let Some(packet) = Ipv4::from_bytes(&bytes[..count]) {
                    if packet.header.proto == self.proto &&
                       (packet.header.dst.equals(self.host_addr) || packet.header.dst.equals(Ipv4Addr::BROADCAST)) &&
                       (packet.header.src.equals(self.peer_addr) || self.peer_addr.equals(Ipv4Addr::BROADCAST)) {
                        for (b, d) in buf.iter_mut().zip(packet.data.iter()) {
                            *b = *d;
                        }

                        return Ok(cmp::min(buf.len(), packet.data.len()));
                    }
                }

                Ok(0)
            }
        }
    }

    /// Send data to the peer.
    ///
    /// # Errors
    ///
    /// Fails if the call to `syscall::write()` fails for this device.
    fn write(&mut self, buf: &[u8]) -> Result<usize> {
        use self::Connection::*;

        let ip_data = Vec::from(buf);
        match self.connection {
            Loopback { ref mut packets } => {
                // Make sure that we're not going to store data that can't be read.
                let buf =
                    if buf.len() > MAX_PACKET_LENGTH {
                        &buf[0..MAX_PACKET_LENGTH]
                    } else {
                        buf
                    };
                packets.push_back(buf.to_vec());
                return Ok(buf.len())
            }
            Device { link, .. } => {
                self.id += 1;
                let mut ip = Ipv4 {
                    header: Ipv4Header {
                        ver_hlen: 0x40 | (mem::size_of::<Ipv4Header>() / 4 & 0xF) as u8, // No Options
                        services: 0,
                        len: n16::new((mem::size_of::<Ipv4Header>() + ip_data.len()) as u16), // No Options
                        id: n16::new(self.id),
                        flags_fragment: n16::new(0),
                        ttl: 128,
                        proto: self.proto,
                        checksum: Checksum { data: 0 },
                        src: self.host_addr,
                        dst: self.peer_addr,
                    },
                    options: Vec::new(),
                    data: ip_data,
                };

                unsafe {
                    let header_ptr: *const Ipv4Header = &ip.header;
                    ip.header.checksum.data =
                        Checksum::compile(Checksum::sum(header_ptr as usize, mem::size_of::<Ipv4Header>()) +
                                          Checksum::sum(ip.options.as_ptr() as usize, ip.options.len()));
                }

                match syscall::write(link, &ip.to_bytes()) {
                    Ok(_) => Ok(buf.len()),
                    Err(err) => Err(err),
                }
            }
        }
    }

    fn sync(&mut self) -> Result<usize> {
        if let Connection::Device { link, .. } = self.connection {
            syscall::fsync(link)
        } else {
            Ok(0)
        }
    }
}

impl Drop for IpResource {
    fn drop(&mut self) {
        if let Connection::Device { link, .. } = self.connection {
            let _ = syscall::close(link);
        }
    }
}