//! IPIP Tunnel (IP-in-IP) — mirrors Linux 7.1's `net/ipv4/ipip.c`.
//!
//! Reference files:
//! - `net/ipv4/ipip.c:184` — `ipip_tunnel_xmit()` — build outer header, transmit
//! - `net/ipv4/ip_tunnel.c:326` — `ip_tunnel_rcv()` — receive and decapsulate
//! - `include/uapi/linux/in.h:30` — `IPPROTO_IPIP = 4`
//!
//! The IPIP tunnel wraps an inner IP packet in an outer IP header, routing
//! it through a parent link-layer device to a remote endpoint. On reception,
//! the outer header is stripped and the inner packet is delivered to the
//! network stack.

use std::cell::RefCell;
use std::collections::VecDeque;
use std::rc::Rc;

use smoltcp::time::Instant;
use smoltcp::wire::{
    EthernetAddress, IpAddress, IpCidr, Ipv4Address, Ipv4Packet, Ipv4Repr, IpProtocol,
};

use super::{DeviceList, LinkDevice};

const IPIP_PROTO: u8 = 4;

pub struct IpipDevice {
    name: Rc<str>,
    parent_name: Rc<str>,
    devices: Rc<RefCell<DeviceList>>,
    local_ip: Ipv4Address,
    remote_ip: Ipv4Address,
    send_buffer: Vec<u8>,
    recv_buffer: Vec<u8>,
    recv_queue: VecDeque<Vec<u8>>,
    ip_address: Option<IpCidr>,
}

impl IpipDevice {
    pub fn new(name: &str, parent_name: &str, local_ip: Ipv4Address, remote_ip: Ipv4Address, devices: Rc<RefCell<DeviceList>>) -> Self {
        Self {
            name: name.into(),
            parent_name: parent_name.into(),
            devices,
            local_ip,
            remote_ip,
            send_buffer: Vec::with_capacity(1500),
            recv_buffer: Vec::with_capacity(1500),
            recv_queue: VecDeque::new(),
            ip_address: None,
        }
    }

    pub fn push_received(&mut self, packet: Vec<u8>) {
        self.recv_queue.push_back(packet);
    }

    fn build_outer_header(&mut self, inner_packet: &[u8]) -> &[u8] {
        self.send_buffer.clear();
        let outer_header = Ipv4Repr {
            src_addr: self.local_ip,
            dst_addr: self.remote_ip,
            next_header: IpProtocol::from(IPIP_PROTO),
            payload_len: inner_packet.len(),
            hop_limit: 64,
        };
        let header_len = outer_header.buffer_len();
        self.send_buffer.resize(header_len + inner_packet.len(), 0);
        let mut ip_pkt = Ipv4Packet::new_unchecked(&mut self.send_buffer);
        outer_header.emit(&mut ip_pkt, &smoltcp::phy::ChecksumCapabilities::ignored());
        self.send_buffer[header_len..].copy_from_slice(inner_packet);
        &self.send_buffer
    }

    fn matches_endpoint(&self, outer_packet: &[u8]) -> bool {
        if outer_packet.len() < 20 {
            return false;
        }
        let Ok(ipv4) = Ipv4Packet::new_checked(outer_packet) else {
            return false;
        };
        u8::from(ipv4.next_header()) == IPIP_PROTO && ipv4.dst_addr() == self.local_ip
    }
}

impl LinkDevice for IpipDevice {
    fn send(&mut self, next_hop: IpAddress, packet: &[u8], now: Instant) {
        let encapsulated = self.build_outer_header(packet).to_vec();
        if let Some(parent) = self.devices.borrow_mut().get_mut(&self.parent_name) {
            parent.send(next_hop, &encapsulated, now);
        } else {
            log::debug!("ipip: parent {} not found, dropping {} byte frame",
                self.parent_name, packet.len());
        }
    }

    fn recv(&mut self, _now: Instant) -> Option<&[u8]> {
        let packet = self.recv_queue.pop_front()?;
        if !self.matches_endpoint(&packet) {
            return None;
        }
        let Ok(ipv4) = Ipv4Packet::new_checked(&packet) else {
            return None;
        };
        let header_len = 20;
        let payload = &packet[header_len..];
        self.recv_buffer.clear();
        self.recv_buffer.extend_from_slice(payload);
        Some(&self.recv_buffer)
    }

    fn name(&self) -> &Rc<str> {
        &self.name
    }

    fn can_recv(&self) -> bool {
        !self.recv_queue.is_empty()
    }

    fn mac_address(&self) -> Option<EthernetAddress> {
        None
    }

    fn set_mac_address(&mut self, _addr: EthernetAddress) {}

    fn ip_address(&self) -> Option<IpCidr> {
        self.ip_address
    }

    fn set_ip_address(&mut self, addr: IpCidr) {
        self.ip_address = Some(addr);
    }
}