// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use std::{
    ffi::CStr,
    io::{Error, Read as _, Result, Write as _},
    net::IpAddr,
    num::TryFromIntError,
    ptr, slice,
};

use libc::{
    AF_NETLINK, ARPHRD_NONE, IFLA_IFNAME, IFLA_MTU, NETLINK_ROUTE, RT_SCOPE_UNIVERSE,
    RT_TABLE_MAIN, RTA_DST, RTA_OIF, RTM_GETLINK, RTM_GETROUTE, RTM_NEWLINK, RTM_NEWROUTE,
    RTN_UNICAST, c_int,
};
use static_assertions::{const_assert, const_assert_eq};

use crate::{aligned_by, default_err, routesocket::RouteSocket, unlikely_err};

#[allow(
    clippy::allow_attributes,
    clippy::allow_attributes_without_reason,
    clippy::struct_field_names,
    non_camel_case_types,
    clippy::too_many_lines,
    reason = "Bindgen-generated code"
)]
mod bindings {
    include!(env!("BINDINGS"));
}

use bindings::{ifinfomsg, nlmsghdr, rtattr, rtmsg};

asserted_const_with_type!(AF_INET, u8, libc::AF_INET, i32);
asserted_const_with_type!(AF_INET6, u8, libc::AF_INET6, i32);
asserted_const_with_type!(AF_UNSPEC, u8, libc::AF_UNSPEC, i32);
asserted_const_with_type!(NLM_F_REQUEST, u16, libc::NLM_F_REQUEST, c_int);
asserted_const_with_type!(NLM_F_ACK, u16, libc::NLM_F_ACK, c_int);
asserted_const_with_type!(NLMSG_ERROR, u16, libc::NLMSG_ERROR, c_int);

const_assert!(size_of::<nlmsghdr>() <= u8::MAX as usize);
const_assert!(size_of::<rtmsg>() <= u8::MAX as usize);
const_assert!(size_of::<rtattr>() <= u8::MAX as usize);
const_assert!(size_of::<ifinfomsg>() <= u8::MAX as usize);

const NETLINK_BUFFER_SIZE: usize = 8192; // See netlink(7) man page.

#[repr(C)]
enum AddrBytes {
    V4([u8; 4]),
    V6([u8; 16]),
}

impl AddrBytes {
    const fn new(ip: IpAddr) -> Self {
        match ip {
            IpAddr::V4(ip) => Self::V4(ip.octets()),
            IpAddr::V6(ip) => Self::V6(ip.octets()),
        }
    }

    const fn len(&self) -> usize {
        match self {
            Self::V4(_) => 4,
            Self::V6(_) => 16,
        }
    }
}

impl From<AddrBytes> for [u8; 16] {
    fn from(addr: AddrBytes) -> Self {
        match addr {
            AddrBytes::V4(bytes) => {
                let mut v6 = [0; 16];
                v6[..4].copy_from_slice(&bytes);
                v6
            }
            AddrBytes::V6(bytes) => bytes,
        }
    }
}

#[repr(C)]
#[derive(Default)]
struct IfIndexMsg {
    nlmsg: nlmsghdr,
    rtm: rtmsg,
    rt: rtattr,
    addr: [u8; 16],
}

impl IfIndexMsg {
    fn new(remote: IpAddr, nlmsg_seq: u32) -> Self {
        let addr = AddrBytes::new(remote);
        #[expect(
            clippy::cast_possible_truncation,
            reason = "Structs lens are <= u8::MAX per `const_assert!`s above; `addr_bytes` is max. 16 for IPv6."
        )]
        let nlmsg_len =
            (size_of::<nlmsghdr>() + size_of::<rtmsg>() + size_of::<rtattr>() + addr.len()) as u32;
        Self {
            nlmsg: nlmsghdr {
                nlmsg_len,
                nlmsg_type: RTM_GETROUTE,
                nlmsg_flags: NLM_F_REQUEST | NLM_F_ACK,
                nlmsg_seq,
                ..Default::default()
            },
            rtm: rtmsg {
                rtm_family: match remote {
                    IpAddr::V4(_) => AF_INET,
                    IpAddr::V6(_) => AF_INET6,
                },
                rtm_dst_len: match remote {
                    IpAddr::V4(_) => 32,
                    IpAddr::V6(_) => 128,
                },
                rtm_table: RT_TABLE_MAIN,
                rtm_scope: RT_SCOPE_UNIVERSE,
                rtm_type: RTN_UNICAST,
                ..Default::default()
            },
            rt: rtattr {
                #[expect(
                    clippy::cast_possible_truncation,
                    reason = "Structs len is <= u8::MAX per `const_assert!` above; `addr_bytes` is max. 16 for IPv6."
                )]
                rta_len: (size_of::<rtattr>() + addr.len()) as u16,
                rta_type: RTA_DST,
            },
            addr: addr.into(),
        }
    }

    const fn len(&self) -> usize {
        let len = self.nlmsg.nlmsg_len as usize;
        debug_assert!(len <= size_of::<Self>());
        len
    }
}

impl From<&IfIndexMsg> for &[u8] {
    fn from(value: &IfIndexMsg) -> Self {
        unsafe { slice::from_raw_parts(ptr::from_ref(value).cast(), value.len()) }
    }
}

impl TryFrom<&[u8]> for nlmsghdr {
    type Error = Error;

    fn try_from(value: &[u8]) -> Result<Self> {
        if value.len() < size_of::<Self>() {
            return Err(default_err());
        }
        Ok(unsafe { ptr::read_unaligned(value.as_ptr().cast()) })
    }
}

fn parse_c_int(buf: &[u8]) -> Result<c_int> {
    if buf.len() < size_of::<c_int>() {
        return Err(default_err());
    }
    let bytes = <&[u8] as TryInto<[u8; size_of::<c_int>()]>>::try_into(&buf[..size_of::<c_int>()])
        .map_err(|_| default_err())?;
    Ok(c_int::from_ne_bytes(bytes))
}

fn read_msg_with_seq(fd: &mut RouteSocket, seq: u32, kind: u16) -> Result<(nlmsghdr, Vec<u8>)> {
    loop {
        let buf = &mut [0u8; NETLINK_BUFFER_SIZE];
        let len = fd.read(buf.as_mut_slice())?;
        let mut next = &buf[..len];
        while size_of::<nlmsghdr>() <= next.len() {
            let (hdr, mut msg) = next.split_at(size_of::<nlmsghdr>());
            let hdr: nlmsghdr = hdr.try_into()?;
            // `msg` has the remainder of this message plus any following messages.
            // Strip those it off and assign them to `next`.
            debug_assert!(size_of::<nlmsghdr>() <= hdr.nlmsg_len as usize);
            (msg, next) = msg.split_at(hdr.nlmsg_len as usize - size_of::<nlmsghdr>());

            if hdr.nlmsg_seq != seq {
                continue;
            }

            if hdr.nlmsg_type == NLMSG_ERROR {
                // Extract the error code and return it.
                let err = parse_c_int(msg)?;
                if err != 0 {
                    return Err(Error::from_raw_os_error(-err));
                }
            } else if hdr.nlmsg_type == kind {
                // Return the header and the message.
                return Ok((hdr, msg.to_vec()));
            }
        }
    }
}

impl TryFrom<&[u8]> for rtattr {
    type Error = Error;

    fn try_from(value: &[u8]) -> Result<Self> {
        if value.len() < size_of::<Self>() {
            return Err(default_err());
        }
        Ok(unsafe { ptr::read_unaligned(value.as_ptr().cast()) })
    }
}

struct RtAttr<'a> {
    hdr: rtattr,
    msg: &'a [u8],
}

impl<'a> RtAttr<'a> {
    fn new(bytes: &'a [u8]) -> Result<Self> {
        debug_assert!(bytes.len() >= size_of::<rtattr>());
        let (hdr, mut msg) = bytes.split_at(size_of::<rtattr>());
        let hdr: rtattr = hdr.try_into()?;
        let aligned_len = aligned_by(hdr.rta_len.into(), 4);
        debug_assert!(size_of::<rtattr>() <= aligned_len);
        (msg, _) = msg.split_at(aligned_len - size_of::<rtattr>());
        Ok(Self { hdr, msg })
    }
}

struct RtAttrs<'a>(&'a [u8]);

impl<'a> Iterator for RtAttrs<'a> {
    type Item = RtAttr<'a>;

    fn next(&mut self) -> Option<Self::Item> {
        if size_of::<rtattr>() <= self.0.len() {
            let attr = RtAttr::new(self.0).ok()?;
            let aligned_len = aligned_by(attr.hdr.rta_len.into(), 4);
            debug_assert!(self.0.len() >= aligned_len);
            self.0 = self.0.split_at(aligned_len).1;
            Some(attr)
        } else {
            None
        }
    }
}

fn if_index(remote: IpAddr, fd: &mut RouteSocket) -> Result<i32> {
    // Send RTM_GETROUTE message to get the interface index associated with the destination.
    let msg_seq = RouteSocket::new_seq();
    let msg = IfIndexMsg::new(remote, msg_seq);
    fd.write_all((&msg).into())?;

    // Receive RTM_GETROUTE response.
    let (_hdr, mut buf) = read_msg_with_seq(fd, msg_seq, RTM_NEWROUTE)?;
    debug_assert!(size_of::<rtmsg>() <= buf.len());
    let buf = buf.split_off(size_of::<rtmsg>());

    // Parse through the attributes to find the interface index.
    for attr in RtAttrs(buf.as_slice()).by_ref() {
        if attr.hdr.rta_type == RTA_OIF {
            // We have our interface index.
            return parse_c_int(attr.msg);
        }
    }
    Err(default_err())
}

#[repr(C)]
struct IfInfoMsg {
    nlmsg: nlmsghdr,
    ifim: ifinfomsg,
}

impl IfInfoMsg {
    fn new(if_index: i32, nlmsg_seq: u32) -> Self {
        #[expect(
            clippy::cast_possible_truncation,
            reason = "Structs lens are <= u8::MAX per `const_assert!`s above."
        )]
        let nlmsg_len = (size_of::<nlmsghdr>() + size_of::<ifinfomsg>()) as u32;
        Self {
            nlmsg: nlmsghdr {
                nlmsg_len,
                nlmsg_type: RTM_GETLINK,
                nlmsg_flags: NLM_F_REQUEST | NLM_F_ACK,
                nlmsg_seq,
                ..Default::default()
            },
            ifim: ifinfomsg {
                ifi_family: AF_UNSPEC,
                ifi_type: ARPHRD_NONE,
                ifi_index: if_index,
                ..Default::default()
            },
        }
    }

    const fn len(&self) -> usize {
        self.nlmsg.nlmsg_len as usize
    }
}

impl From<&IfInfoMsg> for &[u8] {
    fn from(value: &IfInfoMsg) -> Self {
        debug_assert!(value.len() >= size_of::<Self>());
        unsafe { slice::from_raw_parts(ptr::from_ref(value).cast(), value.len()) }
    }
}

fn if_name_mtu(if_index: i32, fd: &mut RouteSocket) -> Result<(String, usize)> {
    // Send RTM_GETLINK message to get interface information for the given interface index.
    let msg_seq = RouteSocket::new_seq();
    let msg = IfInfoMsg::new(if_index, msg_seq);
    fd.write_all((&msg).into())?;

    // Receive RTM_GETLINK response.
    let (_hdr, mut buf) = read_msg_with_seq(fd, msg_seq, RTM_NEWLINK)?;
    debug_assert!(size_of::<ifinfomsg>() <= buf.len());
    let buf = buf.split_off(size_of::<ifinfomsg>());

    // Parse through the attributes to find the interface name and MTU.
    let mut ifname = None;
    let mut mtu = None;
    for attr in RtAttrs(buf.as_slice()).by_ref() {
        match attr.hdr.rta_type {
            IFLA_IFNAME => {
                let name = CStr::from_bytes_until_nul(attr.msg).map_err(Error::other)?;
                ifname = Some(name.to_str().map_err(Error::other)?.to_string());
            }
            IFLA_MTU => {
                mtu = Some(
                    parse_c_int(attr.msg)?
                        .try_into()
                        .map_err(|e: TryFromIntError| unlikely_err(e.to_string()))?,
                );
            }
            _ => (),
        }
        if let (Some(ifname), Some(mtu)) = (ifname.as_ref(), mtu.as_ref()) {
            return Ok((ifname.clone(), *mtu));
        }
    }

    Err(default_err())
}

pub fn interface_and_mtu_impl(remote: IpAddr) -> Result<(String, usize)> {
    // Create a netlink socket.
    let mut fd = RouteSocket::new(AF_NETLINK, NETLINK_ROUTE)?;
    let if_index = if_index(remote, &mut fd)?;
    if_name_mtu(if_index, &mut fd)
}

#[cfg(test)]
#[cfg_attr(coverage_nightly, coverage(off))]
mod test {
    use std::net::{Ipv4Addr, Ipv6Addr};

    use super::*;

    #[test]
    fn nlmsghdr_try_from() {
        assert!(nlmsghdr::try_from([0u8; 4].as_slice()).is_err());
        let mut buf = [0u8; 32];
        buf[0..4].copy_from_slice(&20u32.to_ne_bytes());
        assert_eq!(nlmsghdr::try_from(buf.as_slice()).unwrap().nlmsg_len, 20);
    }

    #[test]
    fn rtattr_try_from() {
        assert!(rtattr::try_from([0u8; 2].as_slice()).is_err());
        let mut buf = [0u8; 8];
        buf[0..2].copy_from_slice(&8u16.to_ne_bytes());
        buf[2..4].copy_from_slice(&3u16.to_ne_bytes());
        let attr = rtattr::try_from(buf.as_slice()).unwrap();
        assert_eq!((attr.rta_len, attr.rta_type), (8, 3));
    }

    #[test]
    fn rtattrs_iteration() {
        assert_eq!(RtAttrs(&[]).count(), 0);
        assert_eq!(RtAttrs(&[0u8; 2]).count(), 0);

        let mut buf = [0u8; 16];
        for (offset, rta_type) in [(0, 1u16), (8, 2u16)] {
            buf[offset..offset + 2].copy_from_slice(&8u16.to_ne_bytes());
            buf[offset + 2..offset + 4].copy_from_slice(&rta_type.to_ne_bytes());
        }
        let types: Vec<_> = RtAttrs(&buf).map(|a| a.hdr.rta_type).collect();
        assert_eq!(types, [1, 2]);
    }

    #[test]
    fn addr_bytes() {
        assert_eq!(AddrBytes::new(IpAddr::V4(Ipv4Addr::LOCALHOST)).len(), 4);
        assert_eq!(AddrBytes::new(IpAddr::V6(Ipv6Addr::LOCALHOST)).len(), 16);

        let v4: [u8; 16] = AddrBytes::new(IpAddr::V4(Ipv4Addr::new(1, 2, 3, 4))).into();
        assert_eq!(v4, [1, 2, 3, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);
    }

    #[test]
    fn parse_c_int_valid_and_invalid() {
        assert!(parse_c_int(&[0u8; 2]).is_err());
        assert_eq!(parse_c_int(&42i32.to_ne_bytes()).unwrap(), 42);
    }

    #[test]
    fn if_index_msg_len_and_slice() {
        let msg = IfIndexMsg::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 1);
        assert!(msg.len() > 0);
        let slice: &[u8] = (&msg).into();
        assert_eq!(slice.len(), msg.len());
    }

    #[test]
    fn if_info_msg_len_and_slice() {
        let msg = IfInfoMsg::new(1, 1);
        assert!(msg.len() > 0);
        let slice: &[u8] = (&msg).into();
        assert_eq!(slice.len(), msg.len());
    }
}