use std::net::IpAddr;

#[cfg(feature = "crypto")]
use std::collections::HashMap;
#[cfg(feature = "crypto")]
use std::collections::HashSet;
#[cfg(feature = "crypto")]
use std::collections::VecDeque;
#[cfg(feature = "crypto")]
use std::pin::Pin;
#[cfg(feature = "crypto")]
use std::sync::{Arc, Mutex};
#[cfg(feature = "crypto")]
use std::time::{Duration as StdDuration, Instant};
#[cfg(feature = "crypto")]
use tokio::time::Duration;

pub use mtp_transport::Policy;

/// Callback that looks up a registered client by ID.
///
/// Called during login to retrieve a client's public key bundle for signature
/// verification, and also during guest ID generation to check whether a random
/// candidate collides with a registered client.  When used for collision
/// checking the `description` argument is `None`.
#[cfg(feature = "crypto")]
pub type GetExistingClient = Box<
    dyn Fn(
            u64,
            Option<String>,
        )
            -> Pin<Box<dyn std::future::Future<Output = Option<mtp_crypto::PublicKeyBundle>> + Send>>
        + Send
        + Sync,
>;

/// Callback that assigns a guest (unauthenticated) client ID.
///
/// Return `Some(id)` to accept the guest with the given full-width `u64` ID, or
/// `None` to reject the connection.
///
/// When set to `None` on `HostConfig`, the built-in generator produces a random
/// full-width non-zero ID that avoids collisions with registered clients and
/// currently connected guests.
#[cfg(feature = "crypto")]
pub type GuestIdGenerator =
    Box<dyn Fn() -> Pin<Box<dyn std::future::Future<Output = Option<u64>> + Send>> + Send + Sync>;

#[cfg(feature = "crypto")]
/// Callback that commits a new registration and returns its non-zero ID.
///
/// The host serializes registration commits and remembers successful identity
/// assignments for the lifetime of the host. Applications that need retry
/// recovery across a host restart should also configure [`FindRegisteredClient`]
/// to look up the public identity in persistent storage.
pub type CompleteRegister = Box<
    dyn Fn(
            mtp_crypto::PublicKeyBundle,
            Option<String>,
        ) -> Pin<Box<dyn std::future::Future<Output = u64> + Send>>
        + Send
        + Sync,
>;

/// Callback that recovers an existing registration by its public identity.
///
/// Returning an ID makes a registration retry idempotent: the host can send
/// the same final response when the original response was lost after the
/// application committed the registration. Returning `None` asks the host to
/// invoke [`CompleteRegister`] for a new registration.
#[cfg(feature = "crypto")]
pub type FindRegisteredClient = Box<
    dyn Fn(
            mtp_crypto::PublicKeyBundle,
            Option<String>,
        ) -> Pin<Box<dyn std::future::Future<Output = Option<u64>> + Send>>
        + Send
        + Sync,
>;

#[cfg(feature = "crypto")]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum AuthenticationPolicy {
    ForceAuthentication,
    AllowAuthentication,
    Unauthenticated,
}

/// Transport-supplied identity used to scope authentication attempt limits.
/// Concrete hosts should populate these fields from the accepted connection;
/// the zero/empty defaults exist only for transport-neutral callers.
#[derive(Debug, Clone, Default, PartialEq, Eq)]
pub struct AuthenticationContext {
    pub peer_network_identity: Option<String>,
    pub connection_id: u64,
}

#[cfg(feature = "crypto")]
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct AuthenticationAttempt {
    pub peer_network_identity: Option<String>,
    pub connection_id: u64,
    pub claimed_client_id: Option<u64>,
    pub registration: bool,
}

#[cfg(feature = "crypto")]
#[derive(Debug, thiserror::Error)]
pub enum AuthenticationLimitError {
    #[error("authentication limiter storage is unavailable")]
    Store,
}

#[cfg(feature = "crypto")]
pub trait AuthenticationAttemptLimiter: Send + Sync {
    fn allow(&self, context: &AuthenticationAttempt) -> Result<bool, AuthenticationLimitError>;
}

#[cfg(feature = "crypto")]
#[derive(Debug, Clone, Hash, PartialEq, Eq)]
enum AuthenticationLimitKey {
    Peer(String),
    Connection(u64),
    Client(u64),
    Registration,
    Global,
}

#[cfg(feature = "crypto")]
#[derive(Debug)]
pub struct InMemoryAuthenticationAttemptLimiter {
    max_attempts: usize,
    window: StdDuration,
    max_keys: usize,
    by_peer: bool,
    by_connection: bool,
    by_client: bool,
    by_registration: bool,
    attempts: Mutex<HashMap<AuthenticationLimitKey, VecDeque<Instant>>>,
}

#[cfg(feature = "crypto")]
impl InMemoryAuthenticationAttemptLimiter {
    pub fn new(max_attempts: usize, window: StdDuration) -> Self {
        Self {
            max_attempts,
            window,
            max_keys: 100_000,
            by_peer: true,
            by_connection: true,
            by_client: true,
            by_registration: true,
            attempts: Mutex::new(HashMap::new()),
        }
    }

    pub fn with_keys(
        mut self,
        by_peer: bool,
        by_connection: bool,
        by_client: bool,
        by_registration: bool,
    ) -> Self {
        self.by_peer = by_peer;
        self.by_connection = by_connection;
        self.by_client = by_client;
        self.by_registration = by_registration;
        self
    }

    pub fn with_max_keys(mut self, max_keys: usize) -> Self {
        self.max_keys = max_keys.max(1);
        self
    }

    fn keys(&self, context: &AuthenticationAttempt) -> Vec<AuthenticationLimitKey> {
        let mut keys = Vec::with_capacity(5);
        if self.by_peer
            && let Some(peer) = context.peer_network_identity.as_ref()
        {
            keys.push(AuthenticationLimitKey::Peer(peer.clone()));
        }
        if self.by_connection && context.connection_id != 0 {
            keys.push(AuthenticationLimitKey::Connection(context.connection_id));
        }
        if self.by_client
            && let Some(client_id) = context.claimed_client_id
        {
            keys.push(AuthenticationLimitKey::Client(client_id));
        }
        if self.by_registration && context.registration {
            keys.push(AuthenticationLimitKey::Registration);
        }
        // Keep one global bucket as a backstop when an attacker varies the
        // claimed client ID or presents no peer/connection identity.
        keys.push(AuthenticationLimitKey::Global);
        keys
    }
}

#[cfg(feature = "crypto")]
impl AuthenticationAttemptLimiter for InMemoryAuthenticationAttemptLimiter {
    fn allow(&self, context: &AuthenticationAttempt) -> Result<bool, AuthenticationLimitError> {
        if self.max_attempts == 0 {
            return Ok(false);
        }
        let now = Instant::now();
        let cutoff = now.checked_sub(self.window);
        let keys = self.keys(context);
        let mut attempts = self
            .attempts
            .lock()
            .map_err(|_| AuthenticationLimitError::Store)?;

        for key in &keys {
            if let Some(history) = attempts.get_mut(key) {
                while history
                    .front()
                    .is_some_and(|timestamp| cutoff.is_some_and(|cutoff| *timestamp <= cutoff))
                {
                    history.pop_front();
                }
                if history.len() >= self.max_attempts {
                    return Ok(false);
                }
            }
        }

        for key in keys {
            if !attempts.contains_key(&key) && attempts.len() >= self.max_keys {
                if let Some(oldest) = attempts.keys().next().cloned() {
                    attempts.remove(&oldest);
                }
            }
            attempts.entry(key).or_default().push_back(now);
        }
        Ok(true)
    }
}

pub struct HostConfig {
    pub ip: IpAddr,
    pub port: u16,
    pub tls_fullchain: Vec<u8>,
    pub tls_key: Vec<u8>,

    pub policy: Policy,
    pub send_pongs: bool,

    #[cfg(feature = "crypto")]
    pub authentication_policy: AuthenticationPolicy,
    #[cfg(feature = "crypto")]
    pub auth_timeout: Duration,
    #[cfg(feature = "crypto")]
    pub require_pq: bool,
    #[cfg(feature = "crypto")]
    pub host_keyring: mtp_crypto::Keyring,
    #[cfg(feature = "crypto")]
    pub get_existing_client: GetExistingClient,
    #[cfg(feature = "crypto")]
    pub(crate) active_guest_ids: Arc<Mutex<HashSet<u64>>>,
    #[cfg(feature = "crypto")]
    pub(crate) registration_ids: Arc<Mutex<HashMap<Vec<u8>, u64>>>,
    #[cfg(feature = "crypto")]
    pub(crate) registration_lock: Arc<tokio::sync::Mutex<()>>,
    #[cfg(feature = "crypto")]
    pub guest_id_generator: Option<GuestIdGenerator>,
    #[cfg(feature = "crypto")]
    pub complete_register: CompleteRegister,
    #[cfg(feature = "crypto")]
    pub find_registered_client: Option<FindRegisteredClient>,
    #[cfg(feature = "crypto")]
    pub auth_limiter: Arc<dyn AuthenticationAttemptLimiter>,
    #[cfg(feature = "crypto")]
    pub conceal_authentication_identities: bool,
}

impl HostConfig {
    pub fn new(ip: IpAddr, port: u16, tls_fullchain: Vec<u8>, tls_key: Vec<u8>) -> Self {
        Self {
            ip,
            port,
            tls_fullchain,
            tls_key,
            policy: Policy::default(),
            send_pongs: true,
            #[cfg(feature = "crypto")]
            authentication_policy: AuthenticationPolicy::Unauthenticated,
            #[cfg(feature = "crypto")]
            auth_timeout: Duration::from_secs(30),
            #[cfg(feature = "crypto")]
            require_pq: true,
            #[cfg(feature = "crypto")]
            host_keyring: mtp_crypto::Keyring::new(
                mtp_crypto::KemPublicKey::new(Vec::new()),
                mtp_crypto::KemPrivateKey::new(Vec::new()),
                mtp_crypto::SignaturePqPublicKey::new(Vec::new()),
                mtp_crypto::SignaturePqPrivateKey::new(Vec::new()),
                mtp_crypto::SignaturePublicKey::new(Vec::new()),
                mtp_crypto::SignaturePrivateKey::new(Vec::new()),
            ),
            #[cfg(feature = "crypto")]
            get_existing_client: Box::new(|_, _| Box::pin(async { None })),
            #[cfg(feature = "crypto")]
            active_guest_ids: Arc::new(Mutex::new(HashSet::new())),
            #[cfg(feature = "crypto")]
            registration_ids: Arc::new(Mutex::new(HashMap::new())),
            #[cfg(feature = "crypto")]
            registration_lock: Arc::new(tokio::sync::Mutex::new(())),
            #[cfg(feature = "crypto")]
            guest_id_generator: None,
            #[cfg(feature = "crypto")]
            complete_register: Box::new(|_, _| Box::pin(async { 0 })),
            #[cfg(feature = "crypto")]
            find_registered_client: None,
            #[cfg(feature = "crypto")]
            auth_limiter: Arc::new(InMemoryAuthenticationAttemptLimiter::new(
                32,
                StdDuration::from_secs(60),
            )),
            #[cfg(feature = "crypto")]
            conceal_authentication_identities: true,
        }
    }

    pub fn with_policy(mut self, policy: Policy) -> Self {
        self.policy = policy;
        self
    }

    pub fn with_pongs(mut self, send_pongs: bool) -> Self {
        self.send_pongs = send_pongs;
        self
    }

    #[cfg(feature = "crypto")]
    pub fn with_authentication(
        mut self,
        host_keyring: mtp_crypto::Keyring,
        get_existing_client: GetExistingClient,
        complete_register: CompleteRegister,
    ) -> Self {
        self.authentication_policy = AuthenticationPolicy::ForceAuthentication;
        self.host_keyring = host_keyring;
        self.get_existing_client = Box::new(get_existing_client);
        self.complete_register = Box::new(complete_register);
        self
    }

    #[cfg(feature = "crypto")]
    pub fn with_authentication_policy(mut self, policy: AuthenticationPolicy) -> Self {
        self.authentication_policy = policy;
        self
    }

    #[cfg(feature = "crypto")]
    pub fn with_auth_timeout(mut self, timeout: Duration) -> Self {
        self.auth_timeout = timeout;
        self
    }

    #[cfg(feature = "crypto")]
    pub fn with_require_pq(mut self, require_pq: bool) -> Self {
        self.require_pq = require_pq;
        self
    }

    #[cfg(feature = "crypto")]
    pub fn with_guest_id_generator(mut self, generator: GuestIdGenerator) -> Self {
        self.guest_id_generator = Some(generator);
        self
    }

    /// Configure the lookup used to make registration retries idempotent.
    #[cfg(feature = "crypto")]
    pub fn with_registration_lookup(mut self, lookup: FindRegisteredClient) -> Self {
        self.find_registered_client = Some(lookup);
        self
    }

    #[cfg(feature = "crypto")]
    pub fn with_authentication_limiter(
        mut self,
        limiter: Arc<dyn AuthenticationAttemptLimiter>,
    ) -> Self {
        self.auth_limiter = limiter;
        self
    }

    #[cfg(feature = "crypto")]
    pub fn with_authentication_identity_concealment(mut self, conceal: bool) -> Self {
        self.conceal_authentication_identities = conceal;
        self
    }
}

#[cfg(all(test, feature = "crypto"))]
mod tests {
    use super::*;

    #[test]
    fn authentication_attempt_limiter_rejects_repeated_attempts() {
        let limiter = InMemoryAuthenticationAttemptLimiter::new(1, StdDuration::from_secs(60))
            .with_keys(false, true, false, false);
        let attempt = AuthenticationAttempt {
            peer_network_identity: None,
            connection_id: 9,
            claimed_client_id: Some(42),
            registration: false,
        };

        assert!(limiter.allow(&attempt).expect("first attempt decision"));
        assert!(!limiter.allow(&attempt).expect("second attempt decision"));
    }

    #[test]
    fn authentication_attempt_limiter_can_scope_registration_separately() {
        let limiter = InMemoryAuthenticationAttemptLimiter::new(2, StdDuration::from_secs(60))
            .with_keys(false, false, false, true);
        let login = AuthenticationAttempt {
            peer_network_identity: None,
            connection_id: 1,
            claimed_client_id: None,
            registration: false,
        };
        let registration = AuthenticationAttempt {
            registration: true,
            ..login.clone()
        };

        assert!(limiter.allow(&login).expect("login attempt decision"));
        assert!(
            limiter
                .allow(&registration)
                .expect("registration attempt decision")
        );
        assert!(
            !limiter
                .allow(&registration)
                .expect("repeated registration decision")
        );
    }
}