//! Transport abstraction for pluggable mesh backends.
//!
//! Every mesh transport (iroh QUIC, TCP, LoRa, Serial) implements the
//! [`MeshTransport`] trait. The [`TransportAddr`] enum provides a
//! transport-agnostic address type.

use std::fmt;

use anyhow::Result;

/// Transport-agnostic peer address.
#[derive(Clone, Debug, PartialEq, Eq, Hash)]
pub enum TransportAddr {
    /// iroh node ID (32-byte public key) with optional relay info.
    Iroh(Vec<u8>),
    /// IP socket address for TCP/UDP transports.
    Socket(std::net::SocketAddr),
    /// LoRa device address (4 bytes).
    LoRa([u8; 4]),
    /// Serial port identifier.
    Serial(String),
    /// Opaque bytes for unknown/future transports.
    Raw(Vec<u8>),
}

impl fmt::Display for TransportAddr {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::Iroh(id) => write!(f, "iroh:{}", hex::encode(&id[..4.min(id.len())])),
            Self::Socket(addr) => write!(f, "tcp:{addr}"),
            Self::LoRa(addr) => write!(f, "lora:{}", hex::encode(addr)),
            Self::Serial(port) => write!(f, "serial:{port}"),
            Self::Raw(data) => write!(f, "raw:{}", hex::encode(&data[..4.min(data.len())])),
        }
    }
}

/// Transport capability level for crypto mode selection.
///
/// Ordered from worst to best so max_by_key picks the best transport.
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub enum TransportCapability {
    /// Very low bandwidth, severely duty-cycled (LoRa SF11-SF12, serial).
    /// MLS-Lite without signature preferred.
    SeverelyConstrained = 0,
    /// Low bandwidth, duty-cycled (LoRa SF7-SF10).
    /// Classical MLS marginal, prefer MLS-Lite with sig.
    Constrained = 1,
    /// Medium bandwidth (BLE, slower WiFi).
    /// Supports full MLS with classical crypto.
    Medium = 2,
    /// High-bandwidth, low-latency (QUIC, TCP, WiFi).
    /// Supports full MLS with PQ-KEM, large KeyPackages.
    Unconstrained = 3,
}

impl TransportCapability {
    /// Determine capability from bitrate and MTU.
    pub fn from_metrics(bitrate_bps: u64, mtu: usize) -> Self {
        match (bitrate_bps, mtu) {
            (b, _) if b >= 1_000_000 => Self::Unconstrained, // ≥1 Mbps
            (b, m) if b >= 10_000 && m >= 200 => Self::Medium, // ≥10 kbps, decent MTU
            (b, m) if b >= 1_000 || m >= 100 => Self::Constrained, // ≥1 kbps
            _ => Self::SeverelyConstrained,
        }
    }

    /// Recommended crypto mode for this capability level.
    pub fn recommended_crypto(&self) -> CryptoMode {
        match self {
            Self::Unconstrained => CryptoMode::MlsHybrid,
            Self::Medium => CryptoMode::MlsClassical,
            Self::Constrained => CryptoMode::MlsLiteSigned,
            Self::SeverelyConstrained => CryptoMode::MlsLiteUnsigned,
        }
    }

    /// Whether full MLS is viable on this transport.
    pub fn supports_mls(&self) -> bool {
        matches!(self, Self::Unconstrained | Self::Medium)
    }
}

/// Crypto mode for mesh messaging.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum CryptoMode {
    /// Full MLS with X25519 + ML-KEM-768 hybrid.
    MlsHybrid,
    /// Full MLS with classical X25519 only.
    MlsClassical,
    /// MLS-Lite with Ed25519 signature.
    MlsLiteSigned,
    /// MLS-Lite without signature (smallest overhead).
    MlsLiteUnsigned,
}

impl CryptoMode {
    /// Approximate overhead in bytes for this mode.
    pub fn overhead_bytes(&self) -> usize {
        match self {
            Self::MlsHybrid => 2700,      // PQ KeyPackage alone
            Self::MlsClassical => 400,    // Classical KeyPackage + message
            Self::MlsLiteSigned => 262,   // MLS-Lite with sig
            Self::MlsLiteUnsigned => 129, // MLS-Lite minimal
        }
    }
}

/// Metadata about a transport's capabilities.
#[derive(Clone, Debug)]
pub struct TransportInfo {
    /// Human-readable transport name.
    pub name: String,
    /// Maximum transmission unit in bytes.
    pub mtu: usize,
    /// Estimated bitrate in bits/second.
    pub bitrate: u64,
    /// Whether this transport supports bidirectional communication.
    pub bidirectional: bool,
}

impl TransportInfo {
    /// Compute capability level from this transport's metrics.
    pub fn capability(&self) -> TransportCapability {
        TransportCapability::from_metrics(self.bitrate, self.mtu)
    }

    /// Recommended crypto mode for this transport.
    pub fn recommended_crypto(&self) -> CryptoMode {
        self.capability().recommended_crypto()
    }
}

/// Received packet from a transport.
#[derive(Clone, Debug)]
pub struct TransportPacket {
    /// Source address of the sender.
    pub from: TransportAddr,
    /// Raw packet data.
    pub data: Vec<u8>,
}

/// A pluggable mesh transport backend.
///
/// Implementations provide send/receive over a specific medium (QUIC, TCP, LoRa, etc).
#[async_trait::async_trait]
pub trait MeshTransport: Send + Sync {
    /// Transport metadata (name, MTU, bitrate).
    fn info(&self) -> TransportInfo;

    /// Send raw bytes to a destination.
    async fn send(&self, dest: &TransportAddr, data: &[u8]) -> Result<()>;

    /// Receive the next incoming packet. Blocks until data arrives.
    async fn recv(&self) -> Result<TransportPacket>;

    /// Discover reachable peers on this transport.
    /// Returns an empty vec if discovery is not supported.
    async fn discover(&self) -> Result<Vec<TransportAddr>> {
        Ok(Vec::new())
    }

    /// Gracefully shut down this transport.
    async fn close(&self) -> Result<()> {
        Ok(())
    }
}

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

    #[test]
    fn transport_addr_display_iroh() {
        let addr = TransportAddr::Iroh(vec![0xDE, 0xAD, 0xBE, 0xEF, 0x01, 0x02]);
        assert_eq!(addr.to_string(), "iroh:deadbeef");
    }

    #[test]
    fn transport_addr_display_iroh_short() {
        let addr = TransportAddr::Iroh(vec![0xAB, 0xCD]);
        assert_eq!(addr.to_string(), "iroh:abcd");
    }

    #[test]
    fn transport_addr_display_socket() {
        let addr = TransportAddr::Socket("127.0.0.1:9000".parse().unwrap());
        assert_eq!(addr.to_string(), "tcp:127.0.0.1:9000");
    }

    #[test]
    fn transport_addr_display_lora() {
        let addr = TransportAddr::LoRa([0x01, 0x02, 0x03, 0x04]);
        assert_eq!(addr.to_string(), "lora:01020304");
    }

    #[test]
    fn transport_addr_display_serial() {
        let addr = TransportAddr::Serial("/dev/ttyUSB0".to_string());
        assert_eq!(addr.to_string(), "serial:/dev/ttyUSB0");
    }

    #[test]
    fn transport_addr_display_raw() {
        let addr = TransportAddr::Raw(vec![0xFF, 0xEE, 0xDD, 0xCC, 0xBB]);
        assert_eq!(addr.to_string(), "raw:ffeeddcc");
    }

    #[test]
    fn transport_addr_display_raw_short() {
        let addr = TransportAddr::Raw(vec![0x01]);
        assert_eq!(addr.to_string(), "raw:01");
    }

    #[test]
    fn transport_addr_equality() {
        let a = TransportAddr::Socket("127.0.0.1:8080".parse().unwrap());
        let b = TransportAddr::Socket("127.0.0.1:8080".parse().unwrap());
        let c = TransportAddr::Socket("127.0.0.1:9090".parse().unwrap());
        assert_eq!(a, b);
        assert_ne!(a, c);
    }

    #[test]
    fn capability_ordering() {
        // Higher value = better capability
        assert!(TransportCapability::Unconstrained > TransportCapability::Medium);
        assert!(TransportCapability::Medium > TransportCapability::Constrained);
        assert!(TransportCapability::Constrained > TransportCapability::SeverelyConstrained);

        // max_by_key should pick the best
        let caps = vec![
            TransportCapability::Constrained,
            TransportCapability::Unconstrained,
            TransportCapability::Medium,
        ];
        let best = caps.into_iter().max().unwrap();
        assert_eq!(best, TransportCapability::Unconstrained);
    }

    #[test]
    fn capability_recommended_crypto() {
        assert_eq!(
            TransportCapability::Unconstrained.recommended_crypto(),
            CryptoMode::MlsHybrid
        );
        assert_eq!(
            TransportCapability::Medium.recommended_crypto(),
            CryptoMode::MlsClassical
        );
        assert_eq!(
            TransportCapability::Constrained.recommended_crypto(),
            CryptoMode::MlsLiteSigned
        );
        assert_eq!(
            TransportCapability::SeverelyConstrained.recommended_crypto(),
            CryptoMode::MlsLiteUnsigned
        );
    }

    #[test]
    fn transport_info_capability() {
        let tcp_info = TransportInfo {
            name: "tcp".to_string(),
            mtu: 1500,
            bitrate: 100_000_000, // 100 Mbps
            bidirectional: true,
        };
        assert_eq!(tcp_info.capability(), TransportCapability::Unconstrained);
        assert_eq!(tcp_info.recommended_crypto(), CryptoMode::MlsHybrid);

        let lora_info = TransportInfo {
            name: "lora".to_string(),
            mtu: 51,
            bitrate: 300,
            bidirectional: true,
        };
        assert_eq!(lora_info.capability(), TransportCapability::SeverelyConstrained);
        assert_eq!(lora_info.recommended_crypto(), CryptoMode::MlsLiteUnsigned);
    }

    #[test]
    fn crypto_mode_overhead() {
        assert!(CryptoMode::MlsHybrid.overhead_bytes() > 2000);
        assert!(CryptoMode::MlsClassical.overhead_bytes() < 500);
        assert!(CryptoMode::MlsLiteSigned.overhead_bytes() < 300);
        assert!(CryptoMode::MlsLiteUnsigned.overhead_bytes() < 150);
    }
}