//! Raw Ethernet I/O over `AF_PACKET` with a classic BPF filter on ethertype
//! `0x6969`. No eBPF authoring: the filter is a hand-assembled cBPF program
//! installed via `SO_ATTACH_FILTER`. The kernel may translate it to eBPF
//! internally at attach time; that is transparent and not our concern.

use std::io::{self, ErrorKind};
use std::os::unix::io::{AsRawFd, RawFd};
use tokio::io::unix::{AsyncFd, AsyncFdReadyGuard};

use crate::frame::{ETHERTYPE, ETH_HEADER_LEN};

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct MacAddr(pub [u8; 6]);

impl MacAddr {
    #[allow(dead_code)]
    pub fn broadcast() -> Self {
        MacAddr([0xff; 6])
    }
}

pub(crate) struct LinkFd(RawFd);
impl AsRawFd for LinkFd {
    fn as_raw_fd(&self) -> RawFd {
        self.0
    }
}
impl Drop for LinkFd {
    fn drop(&mut self) {
        unsafe {
            libc::close(self.0);
        }
    }
}

pub struct Link {
    fd: AsyncFd<LinkFd>,
    ifindex: libc::c_int,
    pub our_mac: MacAddr,
}

// cBPF program for `ether proto 0x6969`:
//   0: ldh [12]                       load ethertype (host-order u16)
//   1: jeq #host(ETHERTYPE), 1, 0     match -> skip to accept; else fall to drop
//   2: ret #0                         drop
//   3: ret #0xFFFF                    accept (return whole packet)
//
// The ethertype is carried big-endian on the wire; `ldh` loads it in host
// byte order, so the comparison constant must be `u16::from_be(ETHERTYPE)`.
fn bpf_program() -> [libc::sock_filter; 4] {
    const BPF_LD_H_ABS: u16 = 0x28;
    const BPF_JMP_JEQ_K: u16 = 0x15;
    const BPF_RET_K: u16 = 0x06;
    let k = u16::from_be(ETHERTYPE) as u32;
    [
        libc::sock_filter { code: BPF_LD_H_ABS, jt: 0, jf: 0, k: 12 },
        libc::sock_filter { code: BPF_JMP_JEQ_K, jt: 1, jf: 0, k },
        libc::sock_filter { code: BPF_RET_K, jt: 0, jf: 0, k: 0 },
        libc::sock_filter { code: BPF_RET_K, jt: 0, jf: 0, k: 0xFFFF },
    ]
}

fn htons(v: u16) -> u16 {
    v.to_be()
}

fn lookup_mac(iface: &str) -> io::Result<MacAddr> {
    for ni in pnet_datalink::interfaces() {
        if ni.name == iface {
            if let Some(mac) = ni.mac {
                return Ok(MacAddr(mac.octets()));
            }
        }
    }
    Err(io::Error::new(
        ErrorKind::NotFound,
        format!("could not determine MAC for interface {iface}"),
    ))
}

impl Link {
    pub fn open(iface: &str) -> io::Result<Link> {
        unsafe {
            let fd = libc::socket(
                libc::AF_PACKET,
                libc::SOCK_RAW,
                htons(libc::ETH_P_ALL as u16) as libc::c_int,
            );
            if fd < 0 {
                return Err(io::Error::last_os_error());
            }

            // Non-blocking for AsyncFd.
            let flags = libc::fcntl(fd, libc::F_GETFL);
            if flags < 0 {
                let e = io::Error::last_os_error();
                libc::close(fd);
                return Err(e);
            }
            if libc::fcntl(fd, libc::F_SETFL, flags | libc::O_NONBLOCK) < 0 {
                let e = io::Error::last_os_error();
                libc::close(fd);
                return Err(e);
            }

            let ciface = match std::ffi::CString::new(iface) {
                Ok(s) => s,
                Err(_) => {
                    libc::close(fd);
                    return Err(io::Error::new(
                        ErrorKind::InvalidInput,
                        "interface name contains an interior NUL",
                    ));
                }
            };
            let ifindex = libc::if_nametoindex(ciface.as_ptr());
            if ifindex == 0 {
                let e = io::Error::last_os_error();
                libc::close(fd);
                return Err(e);
            }

            // Bind to the interface so we only receive frames on it.
            let mut sll: libc::sockaddr_ll = std::mem::zeroed();
            sll.sll_family = libc::AF_PACKET as u16;
            sll.sll_protocol = htons(libc::ETH_P_ALL as u16);
            sll.sll_ifindex = ifindex as libc::c_int;
            let r = libc::bind(
                fd,
                &sll as *const _ as *const libc::sockaddr,
                std::mem::size_of::<libc::sockaddr_ll>() as libc::socklen_t,
            );
            if r < 0 {
                let e = io::Error::last_os_error();
                libc::close(fd);
                return Err(e);
            }

            // Classic BPF: only our ethertype reaches userspace.
            let filt = bpf_program();
            let prog = libc::sock_fprog {
                len: filt.len() as u16,
                filter: filt.as_ptr() as *mut libc::sock_filter,
            };
            let r = libc::setsockopt(
                fd,
                libc::SOL_SOCKET,
                libc::SO_ATTACH_FILTER,
                &prog as *const _ as *const libc::c_void,
                std::mem::size_of::<libc::sock_fprog>() as libc::socklen_t,
            );
            if r < 0 {
                let e = io::Error::last_os_error();
                libc::close(fd);
                return Err(e);
            }

            let our_mac = lookup_mac(iface)?;

            let async_fd = AsyncFd::new(LinkFd(fd))?;
            Ok(Link {
                fd: async_fd,
                ifindex: ifindex as libc::c_int,
                our_mac,
            })
        }
    }

    pub async fn readable(&self) -> io::Result<AsyncFdReadyGuard<'_, LinkFd>> {
        self.fd.readable().await
    }

    pub async fn writable(&self) -> io::Result<AsyncFdReadyGuard<'_, LinkFd>> {
        self.fd.writable().await
    }

    /// Receive one frame. Returns:
    /// - `Ok(Some((src, payload)))` for a frame we should process.
    /// - `Ok(None)` for an ignorable frame (our own outgoing frame, short
    ///   frame, ethertype mismatch) — the caller should keep draining without
    ///   clearing readiness.
    /// - `Err(WouldBlock)` when no more frames are available — the caller
    ///   should clear readiness and await again.
    pub fn recv<'a>(&self, buf: &'a mut [u8]) -> io::Result<Option<(MacAddr, &'a [u8])>> {
        let mut sll: libc::sockaddr_ll = unsafe { std::mem::zeroed() };
        let mut slen = std::mem::size_of::<libc::sockaddr_ll>() as libc::socklen_t;
        let n = unsafe {
            libc::recvfrom(
                self.fd.as_raw_fd(),
                buf.as_mut_ptr() as *mut libc::c_void,
                buf.len(),
                0,
                &mut sll as *mut _ as *mut libc::sockaddr,
                &mut slen,
            )
        };
        if n < 0 {
            return Err(io::Error::last_os_error());
        }
        // Skip frames we transmitted (AF_PACKET with ETH_P_ALL loops them back).
        if sll.sll_pkttype == libc::PACKET_OUTGOING {
            return Ok(None);
        }
        let pkt = &buf[..n as usize];
        if pkt.len() < ETH_HEADER_LEN {
            return Ok(None);
        }
        let src = MacAddr([pkt[6], pkt[7], pkt[8], pkt[9], pkt[10], pkt[11]]);
        // BPF already filtered; double-check for safety against the ethertype.
        let et = u16::from_be_bytes([pkt[12], pkt[13]]);
        if et != ETHERTYPE {
            return Ok(None);
        }
        Ok(Some((src, &pkt[ETH_HEADER_LEN..])))
    }

    /// Send `payload` (our protocol frame) wrapped in an Ethernet header to
    /// `dst`. The ethertype is `0x6969`. Returns `WouldBlock` if the kernel
    /// buffer is full; the caller is expected to await writability and retry.
    pub fn send(&self, dst: MacAddr, payload: &[u8]) -> io::Result<()> {
        let mut frame = Vec::with_capacity(ETH_HEADER_LEN + payload.len());
        frame.extend_from_slice(&dst.0);
        frame.extend_from_slice(&self.our_mac.0);
        frame.extend_from_slice(&ETHERTYPE.to_be_bytes());
        frame.extend_from_slice(payload);

        let mut sll: libc::sockaddr_ll = unsafe { std::mem::zeroed() };
        sll.sll_family = libc::AF_PACKET as u16;
        sll.sll_protocol = htons(libc::ETH_P_ALL as u16);
        sll.sll_ifindex = self.ifindex;
        sll.sll_hatype = 1; // ARPHRD_ETHER
        sll.sll_halen = 6;
        sll.sll_addr[..6].copy_from_slice(&dst.0);

        let r = unsafe {
            libc::sendto(
                self.fd.as_raw_fd(),
                frame.as_ptr() as *const libc::c_void,
                frame.len(),
                0,
                &sll as *const _ as *const libc::sockaddr,
                std::mem::size_of::<libc::sockaddr_ll>() as libc::socklen_t,
            )
        };
        if r < 0 {
            return Err(io::Error::last_os_error());
        }
        Ok(())
    }
}