use std::time::{Duration, Instant};

use bson::Bson;
use dashmap::DashMap;
use tracing::{debug, warn};

use crate::error::{TransactionError, TransactionResult};

/// Represents a logical session.
#[derive(Debug, Clone)]
pub struct Session {
    pub id: String,
    pub created_at: Instant,
    pub last_activity_at: Instant,
    pub txn_id: Option<String>,
    pub in_transaction: bool,
}

/// Engine that manages logical sessions with timeout and cleanup.
pub struct SessionEngine {
    sessions: DashMap<String, Session>,
    timeout: Duration,
    _cleanup_interval: Duration,
}

impl SessionEngine {
    /// Create a new session engine.
    ///
    /// * `timeout_ms` - Session timeout in milliseconds (default: 30 minutes = 1_800_000).
    /// * `cleanup_interval_ms` - How often to run the cleanup task in milliseconds (default: 60_000).
    pub fn new(timeout_ms: u64, cleanup_interval_ms: u64) -> Self {
        Self {
            sessions: DashMap::new(),
            timeout: Duration::from_millis(timeout_ms),
            _cleanup_interval: Duration::from_millis(cleanup_interval_ms),
        }
    }

    /// Get an existing session or create a new one. Returns the session id.
    pub fn get_or_create_session(&self, id: &str) -> String {
        if let Some(mut session) = self.sessions.get_mut(id) {
            session.last_activity_at = Instant::now();
            return session.id.clone();
        }

        let now = Instant::now();
        let session = Session {
            id: id.to_string(),
            created_at: now,
            last_activity_at: now,
            txn_id: None,
            in_transaction: false,
        };
        self.sessions.insert(id.to_string(), session);
        debug!(session_id = %id, "created new session");
        id.to_string()
    }

    /// Update the last activity timestamp for a session.
    pub fn touch_session(&self, id: &str) {
        if let Some(mut session) = self.sessions.get_mut(id) {
            session.last_activity_at = Instant::now();
        }
    }

    /// End a session. If a transaction is active, it will be marked for abort.
    pub fn end_session(&self, id: &str) {
        if let Some((_, session)) = self.sessions.remove(id) {
            if session.in_transaction {
                warn!(
                    session_id = %id,
                    txn_id = ?session.txn_id,
                    "ending session with active transaction, transaction should be aborted"
                );
            }
            debug!(session_id = %id, "session ended");
        }
    }

    /// Associate a transaction with a session.
    pub fn start_transaction(&self, session_id: &str, txn_id: &str) -> TransactionResult<()> {
        let mut session = self
            .sessions
            .get_mut(session_id)
            .ok_or_else(|| TransactionError::NotFound(format!("session {}", session_id)))?;

        if session.in_transaction {
            return Err(TransactionError::AlreadyActive(session_id.to_string()));
        }

        session.txn_id = Some(txn_id.to_string());
        session.in_transaction = true;
        session.last_activity_at = Instant::now();
        Ok(())
    }

    /// Disassociate the transaction from a session (after commit or abort).
    pub fn end_transaction(&self, session_id: &str) {
        if let Some(mut session) = self.sessions.get_mut(session_id) {
            session.txn_id = None;
            session.in_transaction = false;
            session.last_activity_at = Instant::now();
        }
    }

    /// Check whether a session is currently in a transaction.
    pub fn is_in_transaction(&self, session_id: &str) -> bool {
        self.sessions
            .get(session_id)
            .map(|s| s.in_transaction)
            .unwrap_or(false)
    }

    /// Get the active transaction id for a session, if any.
    pub fn get_transaction_id(&self, session_id: &str) -> Option<String> {
        self.sessions
            .get(session_id)
            .and_then(|s| s.txn_id.clone())
    }

    /// Extract a session id from a BSON `lsid` value.
    ///
    /// Handles the following formats:
    /// - `{ "id": UUID }` (standard driver format)
    /// - `{ "id": "string" }` (string shorthand)
    /// - `{ "id": Binary(base64) }` (binary UUID)
    pub fn extract_session_id(lsid: &Bson) -> Option<String> {
        match lsid {
            Bson::Document(doc) => {
                if let Some(id_val) = doc.get("id") {
                    match id_val {
                        Bson::Binary(bin) => {
                            // UUID stored as Binary subtype 4.
                            let bytes = &bin.bytes;
                            if bytes.len() == 16 {
                                let uuid = uuid::Uuid::from_slice(bytes).ok()?;
                                Some(uuid.to_string())
                            } else {
                                // Fall back to base64 representation.
                                Some(base64_encode(bytes))
                            }
                        }
                        Bson::String(s) => Some(s.clone()),
                        _ => Some(format!("{}", id_val)),
                    }
                } else {
                    None
                }
            }
            Bson::String(s) => Some(s.clone()),
            _ => None,
        }
    }

    /// Clean up expired sessions. Returns the number of sessions removed.
    pub fn cleanup_expired(&self) -> usize {
        let now = Instant::now();
        let timeout = self.timeout;
        let expired: Vec<String> = self
            .sessions
            .iter()
            .filter(|entry| now.duration_since(entry.last_activity_at) > timeout)
            .map(|entry| entry.id.clone())
            .collect();

        let count = expired.len();
        for id in &expired {
            debug!(session_id = %id, "cleaning up expired session");
            self.sessions.remove(id);
        }
        count
    }
}

impl Default for SessionEngine {
    fn default() -> Self {
        // 30 minutes timeout, 60 seconds cleanup interval.
        Self::new(1_800_000, 60_000)
    }
}

/// Simple base64 encoding for binary data (no external dependency needed).
fn base64_encode(data: &[u8]) -> String {
    const CHARS: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    let mut result = String::with_capacity((data.len() + 2) / 3 * 4);
    for chunk in data.chunks(3) {
        let b0 = chunk[0] as u32;
        let b1 = if chunk.len() > 1 { chunk[1] as u32 } else { 0 };
        let b2 = if chunk.len() > 2 { chunk[2] as u32 } else { 0 };
        let triple = (b0 << 16) | (b1 << 8) | b2;
        result.push(CHARS[((triple >> 18) & 0x3F) as usize] as char);
        result.push(CHARS[((triple >> 12) & 0x3F) as usize] as char);
        if chunk.len() > 1 {
            result.push(CHARS[((triple >> 6) & 0x3F) as usize] as char);
        } else {
            result.push('=');
        }
        if chunk.len() > 2 {
            result.push(CHARS[(triple & 0x3F) as usize] as char);
        } else {
            result.push('=');
        }
    }
    result
}