//! Spanning Tree Protocol (IEEE 802.1D) — mirrors Linux 7.1's `net/bridge/`.
//!
//! Reference files:
//! - `net/bridge/br_stp.c` — STP state machine (`br_set_state`, `br_become_root_bridge`)
//! - `net/bridge/br_stp_bpdu.c` — BPDU handling (`br_stp_rcv`, `br_send_config_bpdu`)
//! - `net/bridge/br_stp_timer.c` — timers (`br_hello_timer_expired`, `br_tcn_timer_expired`)
//! - `net/bridge/br_private_stp.h` — port roles/states
//!
//! Prevents Ethernet loops by selectively blocking ports. Uses BPDU messages
//! (multicast to 01:80:c2:00:00:00) to elect a root bridge and compute the
//! spanning tree. Ports are either Forwarding or Blocking (simplified from the
//! full 802.1D state machine: Blocking→Listening→Learning→Forwarding).

use smoltcp::time::{Duration, Instant};
use smoltcp::wire::EthernetAddress;

pub const BPDU_MAC: EthernetAddress = EthernetAddress([0x01, 0x80, 0xc2, 0x00, 0x00, 0x00]);
pub const DEFAULT_PRIORITY: u16 = 32768;
pub const DEFAULT_HELLO: Duration = Duration::from_secs(2);
pub const DEFAULT_MAX_AGE: Duration = Duration::from_secs(20);
pub const DEFAULT_FORWARD_DELAY: Duration = Duration::from_secs(15);

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum PortState {
    Blocking,
    Forwarding,
}

#[derive(Debug, Clone)]
pub struct StpState {
    pub bridge_priority: u16,
    pub bridge_mac: EthernetAddress,
    pub root_id: u64,
    pub root_path_cost: u32,
    pub root_port: Option<usize>,
    pub hello_timer: Instant,
    pub port_states: Vec<PortState>,
}

impl StpState {
    pub fn new(priority: u16, mac: EthernetAddress, port_count: usize) -> Self {
        let root_id = ((priority as u64) << 48) | mac_to_u64(mac);
        Self {
            bridge_priority: priority,
            bridge_mac: mac,
            root_id,
            root_path_cost: 0,
            root_port: None,
            hello_timer: Instant::from_millis(0),
            port_states: vec![PortState::Forwarding; port_count],
        }
    }

    pub fn bridge_id(&self) -> u64 {
        ((self.bridge_priority as u64) << 48) | mac_to_u64(self.bridge_mac)
    }

    pub fn send_hello(&mut self, now: Instant) -> bool {
        if now < self.hello_timer + DEFAULT_HELLO {
            return false;
        }
        self.hello_timer = now;
        true
    }

    pub fn process_bpdu(
        &mut self,
        port_idx: usize,
        data: &[u8],
        now: Instant,
    ) -> Option<Vec<u8>> {
        let bpdu = BpduMessage::parse(data)?;

        if bpdu.root_id < self.root_id {
            self.root_id = bpdu.root_id;
            self.root_path_cost = bpdu.root_path_cost + self.port_cost();
            self.root_port = Some(port_idx);
            return Some(self.build_bpdu());
        }

        if bpdu.root_id == self.root_id {
            if port_idx == self.root_port.unwrap_or(usize::MAX) {
                self.root_path_cost = bpdu.root_path_cost + self.port_cost();
            } else if bpdu.root_path_cost < self.root_path_cost {
                self.port_states[port_idx] = PortState::Blocking;
            }
        }

        if self.bridge_id() < bpdu.bridge_id && bpdu.root_id == self.root_id {
            self.root_id = self.bridge_id();
            self.root_path_cost = 0;
            self.root_port = None;
            for state in &mut self.port_states {
                *state = PortState::Forwarding;
            }
            return Some(self.build_bpdu());
        }

        None
    }

    pub fn is_blocked(&self, port_idx: usize) -> bool {
        port_idx < self.port_states.len() && self.port_states[port_idx] == PortState::Blocking
    }

    fn port_cost(&self) -> u32 {
        4
    }

    pub fn build_bpdu(&self) -> Vec<u8> {
        let mut buf = vec![0u8; 35];
        buf[0..2].copy_from_slice(&[0x00, 0x00]);
        buf[2] = 0x00;
        buf[3] = 0x00;
        buf[4] = 0x00;
        buf[5..13].copy_from_slice(&self.root_id.to_be_bytes());
        buf[13..17].copy_from_slice(&self.root_path_cost.to_be_bytes());
        buf[17..25].copy_from_slice(&self.bridge_id().to_be_bytes());
        buf[25..27].copy_from_slice(&0u16.to_be_bytes());
        buf[27..29].copy_from_slice(&0u16.to_be_bytes());
// IEEE 802.1D timer fields are in units of 1/256 second.
// 1 second = 256 ticks. So seconds * 256 = ticks.
        let to_ticks = |d: Duration| -> u16 {
            // total_millis returns i64; convert to u16 ticks (1s = 256 ticks).
            // Clamp to u16::MAX to avoid overflow.
            ((d.total_millis() as u64).wrapping_mul(256).wrapping_div(1000) as u16)
        };
        buf[29..31].copy_from_slice(&to_ticks(DEFAULT_MAX_AGE).to_be_bytes());
        buf[31..33].copy_from_slice(&to_ticks(DEFAULT_HELLO).to_be_bytes());
        buf[33..35].copy_from_slice(&to_ticks(DEFAULT_FORWARD_DELAY).to_be_bytes());
        buf
    }
}

struct BpduMessage {
    root_id: u64,
    root_path_cost: u32,
    bridge_id: u64,
}

impl BpduMessage {
    fn parse(data: &[u8]) -> Option<Self> {
        if data.len() < 35 || data[0..2] != [0x00, 0x00] || data[2] != 0x00 {
            return None;
        }
        let root_id = u64::from_be_bytes(data[5..13].try_into().ok()?);
        let root_path_cost = u32::from_be_bytes(data[13..17].try_into().ok()?);
        let bridge_id = u64::from_be_bytes(data[17..25].try_into().ok()?);
        Some(Self { root_id, root_path_cost, bridge_id })
    }
}

fn mac_to_u64(mac: EthernetAddress) -> u64 {
    let b = mac.as_bytes();
    ((b[0] as u64) << 40) | ((b[1] as u64) << 32) | ((b[2] as u64) << 24)
        | ((b[3] as u64) << 16) | ((b[4] as u64) << 8) | (b[5] as u64)
}
#[cfg(test)]
mod tests {
    use super::*;

    fn make_minimal_bpdu() -> Vec<u8> {
        // Minimal config BPDU: protocol=0x0000, version=0, type=0, flags=0,
        // root_id(8) + cost(4) + bridge_id(8) + port(2) + age(2) + max_age(2) +
        // hello(2) + fwd(2) = 35 bytes.
        let mut p = vec![0u8; 35];
        p[0] = 0x00; p[1] = 0x00; // protocol id
        p[2] = 0x00; // version
        p[3] = 0x00; // type = config
        p
    }

    #[test]
    fn bpdu_minimal_parses() {
        let p = make_minimal_bpdu();
        let m = BpduMessage::parse(&p);
        assert!(m.is_some(), "Valid config BPDU should parse");
    }

    #[test]
    fn bpdu_short_returns_none() {
        let p = vec![0u8; 10];
        assert!(BpduMessage::parse(&p).is_none());
    }

    #[test]
    fn bpdu_wrong_protocol_returns_none() {
        let mut p = make_minimal_bpdu();
        p[0] = 0x80;
        assert!(BpduMessage::parse(&p).is_none());
    }

    #[test]
    fn build_bpdu_does_not_panic() {
        // Regression test: build_bpdu used to panic because
        // Duration::total_millis() returns i64 (8 bytes) but the
        // destination buffer was only 2 bytes.
        let stp = StpState::new(32768, EthernetAddress([0,0,0,0,0,0]), 1);
        let buf = stp.build_bpdu();
        // Must produce 35 bytes
        assert_eq!(buf.len(), 35);
        // And must be a valid BPDU
        assert_eq!(&buf[0..2], &[0x00, 0x00]);
        assert_eq!(buf[2], 0x00); // version
        assert_eq!(buf[3], 0x00); // type
    }
}