//! Background worker for async database operations.
//!
//! Handles non-blocking operations to prevent database operations from blocking:
//! - HNSW index persistence (async saves after upserts)
//! - HNSW index loading (background load on startup)
//! - Embedding generation (batch processing)
//! - WAL flushes (periodic persistence)

use crate::types::Result;
use std::path::PathBuf;
use std::sync::Arc;
use std::time::Duration;
use tokio::sync::{mpsc, RwLock, Semaphore};
use tokio::task::JoinHandle;
use uuid::Uuid;

/// Background task types.
#[derive(Debug)]
pub enum Task {
    /// Save HNSW index to disk (non-blocking)
    SaveIndex {
        schema: String,
        index_path: PathBuf,
    },

    /// Load HNSW index from disk (background on startup)
    LoadIndex {
        schema: String,
        index_path: PathBuf,
    },

    /// Generate embeddings for batch of entities
    GenerateEmbeddings {
        entity_ids: Vec<Uuid>,
        texts: Vec<String>,
        schema: String,
    },

    /// Flush WAL to disk
    FlushWal,

    /// Compact RocksDB column family
    CompactCF {
        cf_name: String,
    },

    /// Shutdown worker gracefully
    Shutdown,
}

/// Task completion callback.
pub type TaskCallback = Box<dyn FnOnce(Result<TaskResult>) + Send + 'static>;

/// Task execution result.
#[derive(Debug, Clone)]
pub enum TaskResult {
    /// Index saved successfully
    IndexSaved { schema: String },

    /// Index loaded successfully
    IndexLoaded { schema: String, vector_count: usize },

    /// Embeddings generated
    EmbeddingsGenerated { count: usize },

    /// WAL flushed
    WalFlushed { entries: usize },

    /// CF compacted
    CfCompacted { cf_name: String },

    /// Worker stopped
    Shutdown,
}

/// Worker status.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum WorkerStatus {
    /// Worker not started
    Stopped,
    /// Worker running and idle
    Idle,
    /// Worker processing tasks
    Busy,
    /// Worker encountered error
    Error,
}

/// Background worker for async database operations.
///
/// # Architecture
///
/// - Single tokio task processing queue sequentially
/// - Thread-safe task submission via mpsc channel
/// - Graceful shutdown with pending task completion
/// - Status tracking for monitoring
///
/// # Performance
///
/// - Non-blocking inserts (index saves happen in background)
/// - Fast database startup (index loads asynchronously)
/// - Batched operations (embeddings, WAL flushes)
///
/// # Example
///
/// ```rust,ignore
/// let worker = BackgroundWorker::new();
/// worker.start();
///
/// // Submit task (non-blocking)
/// worker.submit(Task::SaveIndex {
///     schema: "articles".to_string(),
///     index_path: PathBuf::from("./data/indexes/articles.hnsw"),
/// }).await?;
///
/// // Wait for completion
/// worker.wait_idle(Duration::from_secs(5)).await;
///
/// // Shutdown gracefully
/// worker.shutdown(Duration::from_secs(10)).await?;
/// ```
pub struct BackgroundWorker {
    tx: mpsc::UnboundedSender<(Task, Option<TaskCallback>)>,
    status: Arc<RwLock<WorkerStatus>>,
    semaphore: Arc<Semaphore>,
    handle: Option<JoinHandle<()>>,
}

impl BackgroundWorker {
    /// Create new background worker.
    ///
    /// # Returns
    ///
    /// New `BackgroundWorker` instance (not started)
    pub fn new() -> Self {
        let (tx, _rx) = mpsc::unbounded_channel();
        Self {
            tx,
            status: Arc::new(RwLock::new(WorkerStatus::Stopped)),
            semaphore: Arc::new(Semaphore::new(0)),
            handle: None,
        }
    }

    /// Start background worker task.
    ///
    /// Spawns tokio task that processes queue until shutdown.
    pub fn start(&mut self) {
        todo!("Implement BackgroundWorker::start")
    }

    /// Submit task to worker queue.
    ///
    /// # Arguments
    ///
    /// * `task` - Task to execute
    ///
    /// # Returns
    ///
    /// Immediately (non-blocking)
    ///
    /// # Errors
    ///
    /// Returns error if worker is stopped or channel closed
    pub async fn submit(&self, task: Task) -> Result<()> {
        todo!("Implement BackgroundWorker::submit")
    }

    /// Submit task with completion callback.
    ///
    /// # Arguments
    ///
    /// * `task` - Task to execute
    /// * `callback` - Called when task completes
    ///
    /// # Errors
    ///
    /// Returns error if worker is stopped or channel closed
    pub async fn submit_with_callback<F>(&self, task: Task, callback: F) -> Result<()>
    where
        F: FnOnce(Result<TaskResult>) + Send + 'static,
    {
        todo!("Implement BackgroundWorker::submit_with_callback")
    }

    /// Wait for worker to become idle.
    ///
    /// # Arguments
    ///
    /// * `timeout` - Maximum wait time
    ///
    /// # Returns
    ///
    /// `true` if worker is idle, `false` if timeout
    pub async fn wait_idle(&self, timeout: Duration) -> bool {
        todo!("Implement BackgroundWorker::wait_idle")
    }

    /// Get current worker status.
    ///
    /// # Returns
    ///
    /// Current status
    pub async fn status(&self) -> WorkerStatus {
        *self.status.read().await
    }

    /// Get pending task count.
    ///
    /// # Returns
    ///
    /// Number of tasks in queue
    pub fn queue_size(&self) -> usize {
        todo!("Implement BackgroundWorker::queue_size")
    }

    /// Shutdown worker gracefully.
    ///
    /// # Arguments
    ///
    /// * `timeout` - Maximum wait time for pending tasks
    ///
    /// # Errors
    ///
    /// Returns error if shutdown fails or timeout exceeded
    ///
    /// # Behavior
    ///
    /// 1. Send shutdown task
    /// 2. Wait for pending tasks to complete (up to timeout)
    /// 3. Join worker task
    /// 4. Set status to Stopped
    pub async fn shutdown(&mut self, timeout: Duration) -> Result<()> {
        todo!("Implement BackgroundWorker::shutdown")
    }

    /// Process task loop (internal).
    ///
    /// # Arguments
    ///
    /// * `rx` - Task receiver channel
    /// * `status` - Shared status
    /// * `semaphore` - Idle/busy signaling
    async fn process_tasks(
        mut rx: mpsc::UnboundedReceiver<(Task, Option<TaskCallback>)>,
        status: Arc<RwLock<WorkerStatus>>,
        semaphore: Arc<Semaphore>,
    ) {
        todo!("Implement BackgroundWorker::process_tasks")
    }

    /// Execute single task (internal).
    ///
    /// # Arguments
    ///
    /// * `task` - Task to execute
    ///
    /// # Returns
    ///
    /// Task execution result
    async fn execute_task(task: Task) -> Result<TaskResult> {
        todo!("Implement BackgroundWorker::execute_task")
    }
}

impl Drop for BackgroundWorker {
    fn drop(&mut self) {
        // Attempt graceful shutdown on drop
        if self.handle.is_some() {
            // Note: Can't await in Drop, so this is best-effort
            let _ = self.tx.send((Task::Shutdown, None));
        }
    }
}

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

    #[tokio::test]
    async fn test_worker_lifecycle() {
        // TODO: Test worker start, submit, shutdown
    }

    #[tokio::test]
    async fn test_wait_idle() {
        // TODO: Test wait_idle with timeout
    }

    #[tokio::test]
    async fn test_shutdown_with_pending_tasks() {
        // TODO: Test graceful shutdown with pending work
    }
}
