//! Transport layer construction for ChronoGrid P2P.
//!
//! Builds a libp2p transport stack with:
//!
//! - **QUIC over UDP** (primary) — low-latency, multiplexed, with built-in TLS
//!   1.3 equivalent security via the QUIC protocol.
//! - **TCP + Noise XK** (fallback) — for environments where UDP/QUIC is blocked.
//! - **Yamux** stream multiplexer over TCP.
//! - **SwarmKey authentication** — a 256-bit pre-shared key that gates cluster
//!   membership.  The key fingerprint is embedded in the `UserAgent` string
//!   exchanged during the `identify` handshake; peers with a mismatched
//!   fingerprint are disconnected immediately after identification.
//!
//! # Architecture note
//!
//! libp2p 0.54 uses the `SwarmBuilder` API which handles transport construction
//! internally.  This module exposes a `TransportConfig` struct that captures all
//! parameters needed by `SwarmBuilder`, and a `build_transport` function that
//! constructs a `Boxed<(PeerId, StreamMuxerBox)>` for use in lower-level swarm
//! construction or testing.

use libp2p::{
    PeerId,
    Transport,
    core::{muxing::StreamMuxerBox, transport::Boxed, upgrade},
    identity::Keypair,
    noise,
    tcp,
    yamux,
};

use crate::{
    error::{NetError, NetResult},
    identity::NodeIdentity,
    swarm_key::SwarmKey,
};

// ---------------------------------------------------------------------------
// TransportConfig
// ---------------------------------------------------------------------------

/// Configuration parameters for the ChronoGrid transport stack.
///
/// Passed to [`build_transport`] to construct the libp2p transport.
/// Also consumed by `SwarmManager` (Task 2.5) when building the full swarm.
#[derive(Debug, Clone)]
pub struct TransportConfig {
    /// The swarm key fingerprint embedded in the identify user-agent string.
    /// Peers that present a different fingerprint are rejected post-handshake.
    pub swarm_key_fingerprint: String,

    /// Maximum number of simultaneous inbound connections.
    pub max_connections_in: u32,

    /// Maximum number of simultaneous outbound connections.
    pub max_connections_out: u32,

    /// Connection keep-alive interval in seconds.
    pub keep_alive_secs: u64,
}

impl TransportConfig {
    /// Create a `TransportConfig` from a `SwarmKey`, using sensible defaults.
    pub fn from_swarm_key(swarm_key: &SwarmKey) -> Self {
        TransportConfig {
            swarm_key_fingerprint: swarm_key.fingerprint(),
            max_connections_in: 50,
            max_connections_out: 50,
            keep_alive_secs: 20,
        }
    }

    /// Return the user-agent string that encodes the swarm key fingerprint.
    ///
    /// Format: `chronogrid/<version>/swarm:<fingerprint>`
    ///
    /// This string is exchanged during the `identify` protocol handshake.
    /// Peers validate that the fingerprint matches their own swarm key before
    /// accepting the connection into the cluster.
    pub fn user_agent(&self) -> String {
        format!(
            "chronogrid/{}/swarm:{}",
            env!("CARGO_PKG_VERSION"),
            self.swarm_key_fingerprint
        )
    }

    /// Extract the swarm key fingerprint from a user-agent string produced by
    /// [`user_agent`](Self::user_agent).
    ///
    /// Returns `None` if the string does not match the expected format.
    pub fn extract_fingerprint(user_agent: &str) -> Option<&str> {
        // Format: "chronogrid/<version>/swarm:<fingerprint>"
        let swarm_part = user_agent.split('/').find(|s| s.starts_with("swarm:"))?;
        Some(swarm_part.trim_start_matches("swarm:"))
    }

    /// Validate that a remote peer's user-agent string carries the correct
    /// swarm key fingerprint.
    ///
    /// Returns `true` if the fingerprint matches, `false` otherwise.
    pub fn validate_peer_user_agent(&self, remote_user_agent: &str) -> bool {
        match Self::extract_fingerprint(remote_user_agent) {
            Some(fp) => fp == self.swarm_key_fingerprint,
            None => false,
        }
    }
}

// ---------------------------------------------------------------------------
// build_transport
// ---------------------------------------------------------------------------

/// Build a libp2p TCP + Noise XK + Yamux transport stack.
///
/// This function constructs the **TCP fallback transport** used when QUIC is
/// unavailable.  The primary QUIC transport is configured directly via
/// `SwarmBuilder::with_quic()` in `SwarmManager` (Task 2.5).
///
/// # Transport stack
///
/// ```text
/// TCP (tokio)
///   └─ Noise XK (Ed25519 keypair from NodeIdentity)
///        └─ Yamux stream multiplexer
/// ```
///
/// # Parameters
///
/// - `identity` — The node's Ed25519 keypair, used for Noise authentication.
/// - `_swarm_key` — The cluster pre-shared key.  Its fingerprint is validated
///   at the application layer via the `identify` protocol (see
///   [`TransportConfig::validate_peer_user_agent`]).  The raw key bytes are
///   reserved for a future libp2p private-network transport layer.
///
/// # Errors
///
/// Returns [`NetError::Transport`] if the Noise keypair cannot be constructed
/// from the provided identity.
pub fn build_transport(
    identity: &NodeIdentity,
    _swarm_key: &SwarmKey,
) -> NetResult<Boxed<(PeerId, StreamMuxerBox)>> {
    // Convert the ed25519-dalek SigningKey to a libp2p Keypair.
    let libp2p_keypair = dalek_to_libp2p_keypair(identity)?;

    // Noise authentication configuration (XX pattern for TCP fallback;
    // QUIC uses its own built-in security layer).
    let noise_config = noise::Config::new(&libp2p_keypair)
        .map_err(|e| NetError::Transport(format!("noise config error: {e}")))?;

    // Yamux multiplexer with default configuration.
    let yamux_config = yamux::Config::default();

    // Build the TCP transport with Noise + Yamux.
    let transport = tcp::tokio::Transport::new(tcp::Config::default().nodelay(true))
        .upgrade(upgrade::Version::V1Lazy)
        .authenticate(noise_config)
        .multiplex(yamux_config)
        .boxed();

    Ok(transport)
}

// ---------------------------------------------------------------------------
// Keypair conversion helper
// ---------------------------------------------------------------------------

/// Convert an `ed25519-dalek` `SigningKey` to a `libp2p` `Keypair`.
///
/// libp2p uses its own `identity::Keypair` type internally.  We extract the
/// raw 32-byte seed from the dalek key and re-import it into libp2p's Ed25519
/// key type to ensure compatibility.
pub(crate) fn dalek_to_libp2p_keypair(identity: &NodeIdentity) -> NetResult<Keypair> {
    // Extract the 32-byte secret seed from the dalek SigningKey.
    let secret_bytes = identity.signing_key().to_bytes();

    // libp2p's Ed25519 SecretKey is constructed from the 32-byte seed.
    let secret = libp2p::identity::ed25519::SecretKey::try_from_bytes(secret_bytes)
        .map_err(|e| NetError::Transport(format!("ed25519 secret key conversion failed: {e}")))?;

    let kp = libp2p::identity::ed25519::Keypair::from(secret);
    Ok(Keypair::from(kp))
}

/// Derive the libp2p `PeerId` from a `NodeIdentity`.
///
/// The `PeerId` is the libp2p-canonical identifier derived from the Ed25519
/// public key.  It differs from `NodeId` (which is SHA-256 of the public key)
/// but both are stable identifiers for the same node.
pub fn peer_id_from_identity(identity: &NodeIdentity) -> NetResult<PeerId> {
    let keypair = dalek_to_libp2p_keypair(identity)?;
    Ok(PeerId::from(keypair.public()))
}

// ---------------------------------------------------------------------------
// Tests
// ---------------------------------------------------------------------------

#[cfg(test)]
mod tests {
    use super::*;
    use crate::identity::generate_node_identity;

    // ── TransportConfig tests ────────────────────────────────────────────────

    #[test]
    fn test_transport_config_from_swarm_key() {
        let key = SwarmKey::generate();
        let config = TransportConfig::from_swarm_key(&key);
        assert_eq!(config.swarm_key_fingerprint, key.fingerprint());
        assert_eq!(config.max_connections_in, 50);
        assert_eq!(config.max_connections_out, 50);
        assert_eq!(config.keep_alive_secs, 20);
    }

    #[test]
    fn test_user_agent_contains_fingerprint() {
        let key = SwarmKey::generate();
        let config = TransportConfig::from_swarm_key(&key);
        let ua = config.user_agent();
        assert!(ua.contains(&key.fingerprint()), "user_agent must contain fingerprint");
        assert!(ua.starts_with("chronogrid/"), "user_agent must start with 'chronogrid/'");
        assert!(ua.contains("swarm:"), "user_agent must contain 'swarm:' prefix");
    }

    #[test]
    fn test_extract_fingerprint_valid() {
        let key = SwarmKey::generate();
        let config = TransportConfig::from_swarm_key(&key);
        let ua = config.user_agent();
        let extracted = TransportConfig::extract_fingerprint(&ua);
        assert_eq!(extracted, Some(key.fingerprint().as_str()));
    }

    #[test]
    fn test_extract_fingerprint_invalid_format() {
        let result = TransportConfig::extract_fingerprint("some-other-agent/1.0");
        assert_eq!(result, None);
    }

    #[test]
    fn test_validate_peer_user_agent_matching_key() {
        let key = SwarmKey::generate();
        let config = TransportConfig::from_swarm_key(&key);
        let ua = config.user_agent();
        assert!(config.validate_peer_user_agent(&ua));
    }

    #[test]
    fn test_validate_peer_user_agent_wrong_key() {
        let key1 = SwarmKey::generate();
        let key2 = SwarmKey::generate();
        let config1 = TransportConfig::from_swarm_key(&key1);
        let config2 = TransportConfig::from_swarm_key(&key2);
        // config1 should reject config2's user-agent (different fingerprint).
        let ua2 = config2.user_agent();
        assert!(!config1.validate_peer_user_agent(&ua2));
    }

    #[test]
    fn test_validate_peer_user_agent_empty_string() {
        let key = SwarmKey::generate();
        let config = TransportConfig::from_swarm_key(&key);
        assert!(!config.validate_peer_user_agent(""));
    }

    // ── Keypair conversion tests ─────────────────────────────────────────────

    #[test]
    fn test_dalek_to_libp2p_keypair_succeeds() {
        let identity = generate_node_identity();
        let result = dalek_to_libp2p_keypair(&identity);
        assert!(result.is_ok(), "keypair conversion must succeed: {:?}", result);
    }

    #[test]
    fn test_dalek_to_libp2p_keypair_is_deterministic() {
        let identity = generate_node_identity();
        let kp1 = dalek_to_libp2p_keypair(&identity).unwrap();
        let kp2 = dalek_to_libp2p_keypair(&identity).unwrap();
        // Both keypairs must produce the same PeerId.
        assert_eq!(
            PeerId::from(kp1.public()),
            PeerId::from(kp2.public()),
            "same identity must produce same PeerId"
        );
    }

    #[test]
    fn test_peer_id_from_identity_is_stable() {
        let identity = generate_node_identity();
        let pid1 = peer_id_from_identity(&identity).unwrap();
        let pid2 = peer_id_from_identity(&identity).unwrap();
        assert_eq!(pid1, pid2);
    }

    #[test]
    fn test_different_identities_produce_different_peer_ids() {
        let id1 = generate_node_identity();
        let id2 = generate_node_identity();
        let pid1 = peer_id_from_identity(&id1).unwrap();
        let pid2 = peer_id_from_identity(&id2).unwrap();
        assert_ne!(pid1, pid2);
    }

    // ── Transport construction test ──────────────────────────────────────────

    #[test]
    fn test_build_transport_succeeds() {
        let identity = generate_node_identity();
        let swarm_key = SwarmKey::generate();
        let result = build_transport(&identity, &swarm_key);
        assert!(
            result.is_ok(),
            "build_transport must succeed: {:?}",
            result.err()
        );
    }

    #[test]
    fn test_build_transport_different_identities() {
        // Two different identities must each produce a valid transport.
        let id1 = generate_node_identity();
        let id2 = generate_node_identity();
        let key = SwarmKey::generate();
        assert!(build_transport(&id1, &key).is_ok());
        assert!(build_transport(&id2, &key).is_ok());
    }
}