// spec, раздел "Сетевой уровень → Обфускация транспорта → Identity-Bound Tunnel (IBT)"
//
// Online IBT proof:
//   proof = ML-DSA-65_sign(client_sk, "mt-tunnel" || server_node_id || floor(W / 2))
//
// Mesh IBT proof (Mesh transport IBT extension):
//   proof = ML-DSA-65_sign(client_sk,
//                          "mt-tunnel-mesh" || peer_node_id ||
//                          floor(cached_W / 2) || mesh_session_nonce)

use alloc::vec::Vec;

use mt_codec::domain::{TUNNEL_MESH, TUNNEL_ONLINE};
use mt_codec::{write_bytes, write_u64};
use mt_crypto::{sign, verify, CryptoError, PublicKey, SecretKey, Signature};

use crate::error::NetError;

// Re-export для backwards compatibility public API mt-net до Phase B.0 callsites.
pub use mt_codec::domain::TUNNEL_MESH as DOMAIN_TUNNEL_MESH;
pub use mt_codec::domain::TUNNEL_ONLINE as DOMAIN_TUNNEL_ONLINE;

pub const MESH_NONCE_SIZE: usize = 32;
pub const MESH_STALENESS_BOUND_TAU1: u64 = 7;

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum IbtError {
    InvalidSignature,
    StalenessBoundExceeded,
    ReplayedNonce,
    CryptoFailure,
}

impl From<CryptoError> for IbtError {
    fn from(_: CryptoError) -> Self {
        IbtError::CryptoFailure
    }
}

impl From<IbtError> for NetError {
    fn from(_: IbtError) -> Self {
        NetError::InvalidPayloadField
    }
}

#[inline]
fn window_slot(window_index: u64) -> u64 {
    window_index / 2
}

pub fn ibt_online_message(server_node_id: &[u8; 32], window_index: u64) -> Vec<u8> {
    let mut buf = Vec::with_capacity(TUNNEL_ONLINE.len() + 32 + 8);
    write_bytes(&mut buf, TUNNEL_ONLINE);
    write_bytes(&mut buf, server_node_id);
    write_u64(&mut buf, window_slot(window_index));
    buf
}

pub fn ibt_online_proof(
    client_sk: &SecretKey,
    server_node_id: &[u8; 32],
    window_index: u64,
) -> Result<Signature, IbtError> {
    let msg = ibt_online_message(server_node_id, window_index);
    sign(client_sk, &msg).map_err(IbtError::from)
}

pub fn ibt_online_verify(
    client_pk: &PublicKey,
    server_node_id: &[u8; 32],
    current_window: u64,
    proof: &Signature,
) -> Result<u64, IbtError> {
    // spec: window slot = current ИЛИ previous (acceptable bound)
    let candidates = [current_window, current_window.saturating_sub(2)];
    for &w in &candidates {
        let msg = ibt_online_message(server_node_id, w);
        if verify(client_pk, &msg, proof) {
            return Ok(w);
        }
    }
    Err(IbtError::InvalidSignature)
}

pub fn ibt_mesh_message(
    peer_node_id: &[u8; 32],
    cached_window_index: u64,
    mesh_session_nonce: &[u8; MESH_NONCE_SIZE],
) -> Vec<u8> {
    let mut buf = Vec::with_capacity(TUNNEL_MESH.len() + 32 + 8 + MESH_NONCE_SIZE);
    write_bytes(&mut buf, TUNNEL_MESH);
    write_bytes(&mut buf, peer_node_id);
    write_u64(&mut buf, window_slot(cached_window_index));
    write_bytes(&mut buf, mesh_session_nonce);
    buf
}

pub fn ibt_mesh_proof(
    client_sk: &SecretKey,
    peer_node_id: &[u8; 32],
    cached_window_index: u64,
    mesh_session_nonce: &[u8; MESH_NONCE_SIZE],
) -> Result<Signature, IbtError> {
    let msg = ibt_mesh_message(peer_node_id, cached_window_index, mesh_session_nonce);
    sign(client_sk, &msg).map_err(IbtError::from)
}

pub fn ibt_mesh_verify_with_window(
    client_pk: &PublicKey,
    peer_node_id: &[u8; 32],
    known_window: u64,
    mesh_session_nonce: &[u8; MESH_NONCE_SIZE],
    proof: &Signature,
    tau1_windows: u64,
) -> Result<u64, IbtError> {
    // Backwards-compat fallback API: searches full bound. Prefer
    // ibt_mesh_verify_explicit с явным cached_W из mesh advertisement
    // (per Pass 21 algorithmic complexity audit — O(1) verify path).
    let span = MESH_STALENESS_BOUND_TAU1 * tau1_windows;
    let lo = known_window.saturating_sub(span);
    let hi = known_window;
    let mut w = hi;
    loop {
        let msg = ibt_mesh_message(peer_node_id, w, mesh_session_nonce);
        if verify(client_pk, &msg, proof) {
            return Ok(w);
        }
        if w <= lo {
            break;
        }
        w -= 1;
    }
    Err(IbtError::InvalidSignature)
}

pub fn ibt_mesh_verify_explicit(
    client_pk: &PublicKey,
    peer_node_id: &[u8; 32],
    cached_window_index: u64,
    known_window: u64,
    mesh_session_nonce: &[u8; MESH_NONCE_SIZE],
    proof: &Signature,
    tau1_windows: u64,
) -> Result<u64, IbtError> {
    // O(1) path: cached_W пришёл от sender в plain mesh advertisement;
    // verifier проверяет staleness bound и одну ML-DSA-65 verify.
    let span = MESH_STALENESS_BOUND_TAU1 * tau1_windows;
    if cached_window_index > known_window || cached_window_index < known_window.saturating_sub(span)
    {
        return Err(IbtError::StalenessBoundExceeded);
    }
    let msg = ibt_mesh_message(peer_node_id, cached_window_index, mesh_session_nonce);
    if verify(client_pk, &msg, proof) {
        Ok(cached_window_index)
    } else {
        Err(IbtError::InvalidSignature)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use mt_crypto::keypair;

    #[test]
    fn online_proof_roundtrip() {
        let (pk, sk) = keypair();
        let server_node_id = [0x42u8; 32];
        let proof = ibt_online_proof(&sk, &server_node_id, 1000).unwrap();
        let w = ibt_online_verify(&pk, &server_node_id, 1000, &proof).unwrap();
        assert_eq!(w, 1000);
    }

    #[test]
    fn online_proof_previous_window_accepted() {
        let (pk, sk) = keypair();
        let server_node_id = [0x42u8; 32];
        // Sign at slot 500 (window 1000 либо 1001 → slot 500)
        let proof = ibt_online_proof(&sk, &server_node_id, 1000).unwrap();
        // Verify при current=1002 (slot 501) — ожидание fail так как replay
        // window = current ИЛИ previous (sub 2 windows = 1000, slot 500)
        let w = ibt_online_verify(&pk, &server_node_id, 1002, &proof).unwrap();
        assert_eq!(w, 1000);
    }

    #[test]
    fn online_proof_too_old_rejected() {
        let (pk, sk) = keypair();
        let server_node_id = [0x42u8; 32];
        let proof = ibt_online_proof(&sk, &server_node_id, 100).unwrap();
        // current = 1000 — slot 500; old slot 50 → reject (вне 2-window window)
        assert_eq!(
            ibt_online_verify(&pk, &server_node_id, 1000, &proof),
            Err(IbtError::InvalidSignature)
        );
    }

    #[test]
    fn online_proof_wrong_server_id_rejected() {
        let (pk, sk) = keypair();
        let server_a = [0x42u8; 32];
        let server_b = [0x33u8; 32];
        let proof = ibt_online_proof(&sk, &server_a, 1000).unwrap();
        assert_eq!(
            ibt_online_verify(&pk, &server_b, 1000, &proof),
            Err(IbtError::InvalidSignature)
        );
    }

    #[test]
    fn mesh_proof_roundtrip_within_staleness_window() {
        let (pk, sk) = keypair();
        let peer = [0x33u8; 32];
        let nonce = [0x77u8; 32];
        let cached_w = 5000;
        let known_w = 5000;
        let tau1 = 60;
        let proof = ibt_mesh_proof(&sk, &peer, cached_w, &nonce).unwrap();
        let w = ibt_mesh_verify_with_window(&pk, &peer, known_w, &nonce, &proof, tau1).unwrap();
        // window_slot(5000) == window_slot(5001), so verifier may accept even
        // window
        assert!(w == 5000 || w == 5001);
    }

    #[test]
    fn mesh_proof_outside_staleness_window_rejected() {
        let (pk, sk) = keypair();
        let peer = [0x33u8; 32];
        let nonce = [0x77u8; 32];
        let tau1 = 60;
        let proof = ibt_mesh_proof(&sk, &peer, 1000, &nonce).unwrap();
        // known_window = 1000 + 8·τ₁ — outside 7·τ₁ acceptable bound
        let known_w = 1000 + 8 * tau1;
        assert_eq!(
            ibt_mesh_verify_with_window(&pk, &peer, known_w, &nonce, &proof, tau1),
            Err(IbtError::InvalidSignature)
        );
    }

    #[test]
    fn cross_context_domain_separation() {
        let server = [0x42u8; 32];
        let nonce = [0x77u8; 32];
        let online = ibt_online_message(&server, 1000);
        let mesh = ibt_mesh_message(&server, 1000, &nonce);
        assert_ne!(online, mesh, "online vs mesh domain separator must differ");
    }

    #[test]
    fn online_message_byte_layout() {
        let server = [0x42u8; 32];
        let msg = ibt_online_message(&server, 1000);
        assert_eq!(msg.len(), TUNNEL_ONLINE.len() + 32 + 8);
        assert_eq!(&msg[..TUNNEL_ONLINE.len()], TUNNEL_ONLINE);
        assert_eq!(&msg[TUNNEL_ONLINE.len()..TUNNEL_ONLINE.len() + 32], &server);
        assert_eq!(&msg[TUNNEL_ONLINE.len() + 32..], &500u64.to_le_bytes());
    }

    #[test]
    fn mesh_message_byte_layout() {
        let peer = [0x33u8; 32];
        let nonce = [0x77u8; 32];
        let msg = ibt_mesh_message(&peer, 5000, &nonce);
        let exp_len = DOMAIN_TUNNEL_MESH.len() + 32 + 8 + 32;
        assert_eq!(msg.len(), exp_len);
        assert_eq!(&msg[..TUNNEL_MESH.len()], TUNNEL_MESH);
        let off = TUNNEL_MESH.len();
        assert_eq!(&msg[off..off + 32], &peer);
        assert_eq!(&msg[off + 32..off + 40], &2500u64.to_le_bytes());
        assert_eq!(&msg[off + 40..], &nonce);
    }
}