use std::{
    borrow::Cow,
    ffi::CString,
    net::{Ipv4Addr, Ipv6Addr},
};
use talpid_types::net::wireguard::{PeerConfig, PrivateKey};
use talpid_types::net::{GenericTunnelOptions, obfuscation::Obfuscators, wireguard};

/// Name to use for the tunnel device
#[cfg(target_os = "linux")]
pub(crate) const MULLVAD_INTERFACE_NAME: &str = "wg0-mullvad";

/// Config required to set up a single WireGuard tunnel
#[derive(Debug, Clone)]
pub struct Config {
    /// Contains tunnel endpoint specific config
    pub tunnel: wireguard::TunnelConfig,
    /// Entry peer
    pub entry_peer: wireguard::PeerConfig,
    /// Multihop exit peer
    pub exit_peer: Option<wireguard::PeerConfig>,
    /// IPv4 gateway
    pub ipv4_gateway: Ipv4Addr,
    /// IPv6 gateway
    pub ipv6_gateway: Option<Ipv6Addr>,
    /// Maximum transmission unit for the tunnel
    pub mtu: u16,
    /// Firewall mark
    #[cfg(target_os = "linux")]
    pub fwmark: Option<u32>,
    /// Enable IPv6 routing rules
    #[cfg(target_os = "linux")]
    pub enable_ipv6: bool,
    /// Obfuscator config to be used for reaching the relay.
    pub obfuscator_config: Option<Obfuscators>,
    /// Enable quantum-resistant PSK exchange
    pub quantum_resistant: bool,
    /// Enable DAITA
    pub daita: bool,
}

/// Configuration errors
#[derive(thiserror::Error, Debug)]
pub enum Error {
    /// Supplied parameters don't contain a valid tunnel IP
    #[error("No valid tunnel IP")]
    InvalidTunnelIpError,

    /// Peer has no valid IPs
    #[error("Supplied peer has no valid IPs")]
    InvalidPeerIpError,
}

impl Config {
    /// Constructs a Config from parameters
    pub fn from_parameters(
        params: &wireguard::TunnelParameters,
        default_mtu: u16,
    ) -> Result<Config, Error> {
        Self::new(
            &params.connection,
            &params.options,
            &params.generic_options,
            &params.obfuscation,
            default_mtu,
        )
    }

    /// Constructs a new Config struct
    fn new(
        connection: &wireguard::ConnectionConfig,
        wg_options: &wireguard::TunnelOptions,
        generic_options: &GenericTunnelOptions,
        obfuscator_config: &Option<Obfuscators>,
        default_mtu: u16,
    ) -> Result<Config, Error> {
        let mut tunnel = connection.tunnel.clone();

        let mtu = wg_options.mtu.unwrap_or(default_mtu);

        if tunnel.addresses.is_empty() {
            return Err(Error::InvalidTunnelIpError);
        }
        tunnel
            .addresses
            .retain(|ip| ip.is_ipv4() || generic_options.enable_ipv6);

        let ipv6_gateway = connection
            .ipv6_gateway
            .filter(|_opt| generic_options.enable_ipv6);

        let mut config = Config {
            tunnel,
            entry_peer: connection.peer.clone(),
            exit_peer: connection.exit_peer.clone(),
            ipv4_gateway: connection.ipv4_gateway,
            ipv6_gateway,
            mtu,
            #[cfg(target_os = "linux")]
            fwmark: connection.fwmark,
            #[cfg(target_os = "linux")]
            enable_ipv6: generic_options.enable_ipv6,
            obfuscator_config: obfuscator_config.to_owned(),
            quantum_resistant: wg_options.quantum_resistant,
            #[cfg(daita)]
            daita: wg_options.daita,
            #[cfg(not(daita))]
            daita: false,
        };

        for peer in config.peers_mut() {
            peer.allowed_ips
                .retain(|ip| ip.is_ipv4() || generic_options.enable_ipv6);
            if peer.allowed_ips.is_empty() {
                return Err(Error::InvalidPeerIpError);
            }
        }

        Ok(config)
    }

    /// Returns a CString with the appropriate config for WireGuard-go
    // TODO: Consider outputting both overriding and additive configs
    pub fn to_userspace_format(&self) -> CString {
        userspace_format(
            &self.tunnel.private_key,
            self.peers(),
            #[cfg(target_os = "linux")]
            self.fwmark,
        )
    }

    /// Return whether the config connects to an exit peer from another remote peer.
    pub fn is_multihop(&self) -> bool {
        self.exit_peer.is_some()
    }

    /// Return the exit peer. `exit_peer` if it is set, otherwise `entry_peer`.
    pub fn exit_peer(&self) -> &wireguard::PeerConfig {
        self.exit_peer.as_ref().unwrap_or(&self.entry_peer)
    }

    /// Return the exit peer. `exit_peer` if it is set, otherwise `entry_peer`.
    pub fn exit_peer_mut(&mut self) -> &mut wireguard::PeerConfig {
        self.exit_peer.as_mut().unwrap_or(&mut self.entry_peer)
    }

    /// Return an iterator over all peers.
    pub fn peers(&self) -> impl Iterator<Item = &wireguard::PeerConfig> {
        self.exit_peer
            .as_ref()
            .into_iter()
            .chain(std::iter::once(&self.entry_peer))
    }

    /// Return a mutable iterator over all peers.
    pub fn peers_mut(&mut self) -> impl Iterator<Item = &mut wireguard::PeerConfig> {
        self.exit_peer
            .as_mut()
            .into_iter()
            .chain(std::iter::once(&mut self.entry_peer))
    }

    /// Return routes for all allowed IPs.
    pub fn get_tunnel_destinations(&self) -> impl Iterator<Item = ipnetwork::IpNetwork> + '_ {
        self.peers()
            .flat_map(|peer| peer.allowed_ips.iter())
            .cloned()
    }
}

enum ConfValue<'a> {
    String(&'a str),
    Bytes(&'a [u8]),
}

impl<'a> From<&'a str> for ConfValue<'a> {
    fn from(s: &'a str) -> ConfValue<'a> {
        ConfValue::String(s)
    }
}

impl<'a> From<&'a [u8]> for ConfValue<'a> {
    fn from(s: &'a [u8]) -> ConfValue<'a> {
        ConfValue::Bytes(s)
    }
}

impl<'a> ConfValue<'a> {
    fn to_bytes(&self) -> Cow<'a, [u8]> {
        match self {
            ConfValue::String(s) => s.as_bytes().into(),
            ConfValue::Bytes(bytes) => Cow::Owned(hex::encode(bytes).into_bytes()),
        }
    }
}

struct WgConfigBuffer {
    buf: Vec<u8>,
}

impl WgConfigBuffer {
    pub fn new() -> WgConfigBuffer {
        WgConfigBuffer { buf: Vec::new() }
    }

    pub fn add<'a, C: Into<ConfValue<'a>> + 'a>(&mut self, key: &str, value: C) -> &mut Self {
        self.buf.extend(key.as_bytes());
        self.buf.extend(b"=");
        self.buf.extend(value.into().to_bytes().as_ref());
        self.buf.extend(b"\n");
        self
    }

    pub fn into_config(mut self) -> Vec<u8> {
        self.buf.push(b'\n');
        self.buf
    }
}

/// Returns a CString with the appropriate config for WireGuard-go
#[allow(single_use_lifetimes)]
pub fn userspace_format<'a>(
    private_key: &PrivateKey,
    peers: impl Iterator<Item = &'a PeerConfig>,
    #[cfg(target_os = "linux")] fwmark: Option<u32>,
) -> CString {
    // the order of insertion matters, public key entry denotes a new peer entry
    let mut wg_conf = WgConfigBuffer::new();
    wg_conf
        .add::<&[u8]>("private_key", private_key.to_bytes().as_ref())
        .add("listen_port", "0");

    #[cfg(target_os = "linux")]
    if let Some(fwmark) = fwmark {
        wg_conf.add("fwmark", fwmark.to_string().as_str());
    }

    wg_conf.add("replace_peers", "true");

    for peer in peers {
        write_peer_to_config(&mut wg_conf, peer)
    }

    let bytes = wg_conf.into_config();
    CString::new(bytes).expect("null bytes inside config")
}

fn write_peer_to_config(wg_conf: &mut WgConfigBuffer, peer: &PeerConfig) {
    wg_conf
        .add::<&[u8]>("public_key", peer.public_key.as_bytes().as_ref())
        .add("endpoint", peer.endpoint.to_string().as_str())
        .add("replace_allowed_ips", "true");
    if let Some(ref psk) = peer.psk {
        wg_conf.add::<&[u8]>("preshared_key", psk.as_bytes().as_ref());
    }
    for addr in &peer.allowed_ips {
        wg_conf.add("allowed_ip", addr.to_string().as_str());
    }
    #[cfg(daita)]
    if peer.constant_packet_size {
        wg_conf.add("constant_packet_size", "true");
    }
}

/// Replace `0.0.0.0/0`/`::/0` with the gateway IPs.
/// Used to block traffic to other destinations while connecting on Android.
#[cfg(target_os = "android")]
pub(crate) fn patch_allowed_ips(mut config: Config) -> Config {
    use ipnetwork::IpNetwork;
    use std::net::IpAddr;

    let gateway_net_v4 = IpNetwork::from(IpAddr::from(config.ipv4_gateway));
    let gateway_net_v6 = config
        .ipv6_gateway
        .map(|net| IpNetwork::from(IpAddr::from(net)));
    for peer in config.peers_mut() {
        for allowed_ips in &mut peer.allowed_ips {
            if allowed_ips.prefix() == 0 {
                match (allowed_ips.is_ipv4(), gateway_net_v6) {
                    (true, _) => *allowed_ips = gateway_net_v4,
                    (_, Some(net)) => *allowed_ips = net,
                    _ => continue,
                }
            }
        }
    }
    config
}