use anyhow::Context;
use chrono::Utc;
use deadpool_redis::cluster::Pool;
use redis::AsyncCommands;
use serde_json;
use thiserror::Error;
use uuid::Uuid;

use crate::types::UserSession;

#[derive(Error, Debug)]
pub enum SessionStorageError {
    #[error("Redis error: {0}")]
    Redis(#[from] anyhow::Error),

    #[error("session not found: {0}")]
    NotFound(Uuid),
}

const KEY_CONN: &str = "user:conn:";
const KEY_USER_SESSIONS: &str = "user:user_sessions:";
const KEY_WORKSPACE_SESSIONS: &str = "user:workspace_sessions:";

#[derive(Clone)]
pub struct SessionStorage {
    pool: Pool,
    heartbeat_ttl_secs: u64,
}

impl SessionStorage {
    pub fn new(pool: Pool) -> Self {
        Self {
            pool,
            heartbeat_ttl_secs: 120,
        }
    }

    pub fn with_heartbeat_ttl(mut self, ttl_secs: u64) -> Self {
        self.heartbeat_ttl_secs = ttl_secs;
        self
    }

    async fn get_conn(&self) -> Result<deadpool_redis::cluster::Connection, SessionStorageError> {
        self.pool
            .get()
            .await
            .context("failed to get Redis connection from pool")
            .map_err(SessionStorageError::Redis)
    }

    fn conn_key(session_id: &Uuid) -> String {
        format!("{KEY_CONN}{session_id}")
    }

    fn user_sessions_key(user_id: &Uuid) -> String {
        format!("{KEY_USER_SESSIONS}{user_id}")
    }

    fn workspace_sessions_key(workspace_id: &Uuid) -> String {
        format!("{KEY_WORKSPACE_SESSIONS}{workspace_id}")
    }

    fn to_err<E: std::error::Error + Send + Sync + 'static>(e: E) -> SessionStorageError {
        SessionStorageError::Redis(anyhow::anyhow!(e))
    }

    /// Store a new user session and associate it with user + workspace indexes.
    pub async fn save_session(&self, session: &UserSession) -> Result<(), SessionStorageError> {
        let mut conn = self.get_conn().await?;
        let key = Self::conn_key(&session.session_id);
        let user_key = Self::user_sessions_key(&session.user_id);
        let ws_key = Self::workspace_sessions_key(&session.workspace_id);

        let value = serde_json::to_string(session)
            .context("serialize UserSession")
            .map_err(SessionStorageError::Redis)?;

        let ttl = self.heartbeat_ttl_secs;

        let _: () = redis::pipe()
            .set_ex(&key, &value, ttl)
            .sadd(&user_key, session_id_str(&session.session_id))
            .expire(&user_key, 0)
            .sadd(&ws_key, session_id_str(&session.session_id))
            .expire(&ws_key, 0)
            .query_async(&mut conn)
            .await
            .map_err(Self::to_err)?;

        Ok(())
    }

    /// Get a session by its ID.
    pub async fn get_session(&self, session_id: &Uuid) -> Result<Option<UserSession>, SessionStorageError> {
        let mut conn = self.get_conn().await?;
        let key = Self::conn_key(session_id);

        let value: Option<String> = conn
            .get(&key)
            .await
            .map_err(Self::to_err)?;

        match value {
            Some(v) => {
                let session: UserSession = serde_json::from_str(&v)
                    .context("deserialize UserSession")
                    .map_err(SessionStorageError::Redis)?;
                Ok(Some(session))
            }
            None => Ok(None),
        }
    }

    /// Update the heartbeat timestamp and refresh TTL.
    pub async fn heartbeat(&self, session_id: &Uuid) -> Result<(), SessionStorageError> {
        let mut conn = self.get_conn().await?;
        let key = Self::conn_key(session_id);
        let ttl = self.heartbeat_ttl_secs;
        let updated = Utc::now();

        let script = redis::Script::new(
            r#"
            local v = redis.call('GET', KEYS[1])
            if not v then return 0 end
            local session = cjson.decode(v)
            session.last_heartbeat = ARGV[1]
            redis.call('SETEX', KEYS[1], ARGV[2], cjson.encode(session))
            return 1
            "#,
        );

        let result: i64 = script
            .key(&key)
            .arg(updated.to_rfc3339())
            .arg(ttl)
            .invoke_async(&mut conn)
            .await
            .map_err(Self::to_err)?;

        if result == 0 {
            return Err(SessionStorageError::NotFound(*session_id));
        }

        Ok(())
    }

    /// Delete a session by ID and clean up indexes.
    pub async fn delete_session(&self, session_id: &Uuid) -> Result<(), SessionStorageError> {
        let session = self.get_session(session_id).await?;

        let key = Self::conn_key(session_id);

        let _: () = self
            .get_conn()
            .await?
            .del(&key)
            .await
            .map_err(Self::to_err)?;

        if let Some(ref s) = session {
            let mut conn = self.get_conn().await?;
            let user_key = Self::user_sessions_key(&s.user_id);
            let ws_key = Self::workspace_sessions_key(&s.workspace_id);
            let id_str = session_id_str(session_id);
            let _: () = conn.srem::<_, _, ()>(&user_key, &id_str).await.map_err(Self::to_err)?;
            let _: () = conn.srem::<_, _, ()>(&ws_key, &id_str).await.map_err(Self::to_err)?;
        }

        Ok(())
    }

    /// Delete all sessions for a specific user.
    pub async fn delete_user_sessions(&self, user_id: &Uuid) -> Result<Vec<Uuid>, SessionStorageError> {
        let mut conn = self.get_conn().await?;
        let user_key = Self::user_sessions_key(user_id);

        let session_ids: Vec<String> = conn
            .smembers(&user_key)
            .await
            .map_err(Self::to_err)?;

        let mut deleted = Vec::new();
        for id_str in &session_ids {
            if let Ok(sid) = Uuid::parse_str(id_str) {
                let conn_key = Self::conn_key(&sid);
                let _: () = conn.del(&conn_key).await.map_err(Self::to_err)?;
                deleted.push(sid);
            }
        }

        let _: () = conn.del(&user_key).await.map_err(Self::to_err)?;

        Ok(deleted)
    }

    /// Delete all sessions for a user within a specific workspace.
    pub async fn delete_user_workspace_sessions(
        &self,
        user_id: &Uuid,
        workspace_id: &Uuid,
    ) -> Result<Vec<Uuid>, SessionStorageError> {
        let mut conn = self.get_conn().await?;
        let ws_key = Self::workspace_sessions_key(workspace_id);
        let user_key = Self::user_sessions_key(user_id);

        let ws_session_ids: Vec<String> = conn
            .smembers(&ws_key)
            .await
            .map_err(Self::to_err)?;

        let mut deleted = Vec::new();
        for id_str in &ws_session_ids {
            if let Ok(sid) = Uuid::parse_str(id_str) {
                let session = self.get_session(&sid).await?;
                if let Some(ref s) = session {
                    if s.user_id == *user_id {
                        let conn_key = Self::conn_key(&sid);
                        let _: () = conn.del(&conn_key).await.map_err(Self::to_err)?;
                        let _: () = conn.srem::<_, _, ()>(&user_key, id_str).await.map_err(Self::to_err)?;
                        deleted.push(sid);
                    }
                }
            }
        }

        Ok(deleted)
    }

    /// Get all active sessions for a user.
    pub async fn get_user_sessions(&self, user_id: &Uuid) -> Result<Vec<UserSession>, SessionStorageError> {
        let mut conn = self.get_conn().await?;
        let user_key = Self::user_sessions_key(user_id);

        let session_ids: Vec<String> = conn
            .smembers(&user_key)
            .await
            .map_err(Self::to_err)?;

        if session_ids.is_empty() {
            return Ok(Vec::new());
        }

        // Batch fetch all sessions in a single MGET instead of N individual GET calls.
        let keys: Vec<String> = session_ids
            .iter()
            .map(|id| format!("{}{}", KEY_CONN, id))
            .collect();

        let values: Vec<Option<String>> = redis::cmd("MGET")
            .arg(&keys)
            .query_async(&mut conn)
            .await
            .map_err(Self::to_err)?;

        let sessions: Vec<UserSession> = values
            .into_iter()
            .flatten()
            .filter_map(|v| serde_json::from_str(&v).ok())
            .collect();

        Ok(sessions)
    }

    /// Get all active sessions in a workspace.
    pub async fn get_workspace_sessions(
        &self,
        workspace_id: &Uuid,
    ) -> Result<Vec<UserSession>, SessionStorageError> {
        let mut conn = self.get_conn().await?;
        let ws_key = Self::workspace_sessions_key(workspace_id);

        let session_ids: Vec<String> = conn
            .smembers(&ws_key)
            .await
            .map_err(Self::to_err)?;

        if session_ids.is_empty() {
            return Ok(Vec::new());
        }

        // Batch fetch all sessions in a single MGET instead of N individual GET calls.
        let keys: Vec<String> = session_ids
            .iter()
            .map(|id| format!("{}{}", KEY_CONN, id))
            .collect();

        let values: Vec<Option<String>> = redis::cmd("MGET")
            .arg(&keys)
            .query_async(&mut conn)
            .await
            .map_err(Self::to_err)?;

        let sessions: Vec<UserSession> = values
            .into_iter()
            .flatten()
            .filter_map(|v| serde_json::from_str(&v).ok())
            .collect();

        Ok(sessions)
    }

    /// Get distinct user IDs active in a workspace.
    pub async fn get_workspace_online_users(
        &self,
        workspace_id: &Uuid,
    ) -> Result<Vec<Uuid>, SessionStorageError> {
        let sessions = self.get_workspace_sessions(workspace_id).await?;
        let mut seen = std::collections::HashSet::new();
        let mut result = Vec::new();
        for s in sessions {
            if seen.insert(s.user_id) {
                result.push(s.user_id);
            }
        }
        Ok(result)
    }

    /// Get the count of online sessions for a user.
    pub async fn get_user_session_count(&self, user_id: &Uuid) -> Result<usize, SessionStorageError> {
        let sessions = self.get_user_sessions(user_id).await?;
        Ok(sessions.len())
    }

    /// Check if a user has any active sessions (online status).
    pub async fn is_user_online(&self, user_id: &Uuid) -> Result<bool, SessionStorageError> {
        let count = self.get_user_session_count(user_id).await?;
        Ok(count > 0)
    }

    /// Returns a reference to the underlying Redis pool.
    pub fn pool(&self) -> &deadpool_redis::cluster::Pool {
        &self.pool
    }

    /// Get online status for a user.
    pub async fn get_user_status(&self, user_id: &Uuid) -> Result<crate::types::OnlineStatus, SessionStorageError> {
        let sessions = self.get_user_sessions(user_id).await?;
        if sessions.is_empty() {
            return Ok(crate::types::OnlineStatus::Offline);
        }

        let now = Utc::now();
        let idle_threshold = chrono::Duration::minutes(5);

        if sessions.iter().any(|s| now - s.last_heartbeat < idle_threshold) {
            Ok(crate::types::OnlineStatus::Online)
        } else {
            Ok(crate::types::OnlineStatus::Idle)
        }
    }
}

fn session_id_str(id: &Uuid) -> String {
    id.to_string()
}