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use std::convert::{TryFrom, TryInto};
use anyhow::{anyhow, Error};
use super::echo::Echo;

pub const HEADER_SIZE: usize = 8;

pub const ECHO_REPLY:    u8 = 0;
pub const UNREACHABLE:   u8 = 3;
pub const ECHO_REQUEST:  u8 = 8;
pub const TIME_EXCEEDED: u8 = 11;

#[derive(Debug)]
pub enum IcmpV4Packet<'a> {
    EchoRequest(Echo<'a>),
    EchoReply(Echo<'a>),
    Unreachable(Unreachable<'a>),
    TimeExceeded(&'a [u8]),
    Other(u8, u8, &'a [u8]),
}

#[derive(Debug)]
pub enum Unreachable<'a> {
    Net(&'a [u8]),
    Host(&'a [u8]),
    Protocol(&'a [u8]),
    Port(&'a [u8]),
    Other(u8, &'a [u8]),
}

impl<'a> TryFrom<&'a [u8]> for IcmpV4Packet<'a> {
    type Error = Error;

    fn try_from(slice: &'a [u8]) -> Result<Self, Self::Error> {
        if slice.len() < HEADER_SIZE {
            return Err(anyhow!("invalid slice"));
        }

        let kind = slice[0];
        let code = slice[1];
        let rest = &slice[4..];

        Ok(match (kind, code) {
            (ECHO_REPLY,    0) => IcmpV4Packet::EchoReply(rest.try_into()?),
            (UNREACHABLE,   _) => IcmpV4Packet::Unreachable((code, rest).try_into()?),
            (ECHO_REQUEST,  0) => IcmpV4Packet::EchoRequest(rest.try_into()?),
            (TIME_EXCEEDED, _) => IcmpV4Packet::TimeExceeded(&rest[4..]),
            _                  => IcmpV4Packet::Other(kind, code, rest),
        })
    }
}


impl<'a> TryFrom<(u8, &'a [u8])> for Unreachable<'a> {
    type Error = Error;

    fn try_from((code, slice): (u8, &'a [u8])) -> Result<Self, Self::Error> {
        let data = &slice[4..];
        Ok(match code {
            0 => Unreachable::Net(data),
            1 => Unreachable::Host(data),
            2 => Unreachable::Protocol(data),
            3 => Unreachable::Port(data),
            c => Unreachable::Other(c, data),
        })
    }
}

pub fn checksum(pkt: &[u8]) -> u16 {
    let mut sum = 0u32;

    for chunk in pkt.chunks(2) {
        let word = match chunk {
            [x, y] => u16::from_be_bytes([*x, *y]),
            [x]    => u16::from_be_bytes([*x, 0]),
            _      => unreachable!(),
        } as u32;
        sum = sum.wrapping_add(word);
    }

    while (sum >> 16) > 0 {
        sum = (sum & 0xffff) + (sum >> 16);
    }

    !sum as u16
}