//! Database statistics and metrics collection.
//!
//! This module provides:
//! - **Database statistics**: Entity counts, disk usage, column family sizes
//! - **Performance metrics**: Query latencies, cache hit rates
//! - **Schema statistics**: Per-schema entity counts and sizes
//! - **Export to formats**: JSON, Prometheus metrics
//!
//! # Metrics Categories
//!
//! - **Storage**: Disk usage, compression ratios, SST file counts
//! - **Entities**: Counts per schema, total entities
//! - **Indexes**: HNSW size, field index sizes
//! - **Performance**: RocksDB internal metrics
//!
//! # Example
//!
//! ```rust,no_run
//! use percolate_rocks::admin::StatisticsManager;
//!
//! # async fn example() -> anyhow::Result<()> {
//! let stats_mgr = StatisticsManager::new("/var/lib/rem/db")?;
//!
//! // Get database statistics
//! let stats = stats_mgr.get_database_stats().await?;
//! println!("Total entities: {}", stats.total_entities);
//! println!("Disk usage: {} GB", stats.total_size_bytes as f64 / 1e9);
//!
//! // Export to Prometheus format
//! let prometheus = stats_mgr.export_prometheus_metrics().await?;
//! println!("{}", prometheus);
//! # Ok(())
//! # }
//! ```

use anyhow::Result;
use serde::{Deserialize, Serialize};
use std::collections::HashMap;
use std::path::{Path, PathBuf};

/// Manages database statistics collection.
///
/// Provides read-only access to database metrics and performance data.
pub struct StatisticsManager {
    /// Path to RocksDB database
    _db_path: PathBuf,

    /// RocksDB instance (not yet implemented)
    _db: (),
}

impl StatisticsManager {
    /// Create new statistics manager.
    ///
    /// # Arguments
    ///
    /// * `db_path` - Path to RocksDB database
    ///
    /// # Returns
    ///
    /// Configured `StatisticsManager` instance
    ///
    /// # Errors
    ///
    /// Returns error if database path invalid or cannot be opened
    pub fn new(_db_path: &Path) -> Result<Self> {
        todo!("Open RocksDB in read-only mode")
    }

    /// Get comprehensive database statistics.
    ///
    /// # Returns
    ///
    /// `DatabaseStats` with all metrics
    ///
    /// # Errors
    ///
    /// Returns error if statistics cannot be collected
    ///
    /// # Example
    ///
    /// ```rust,no_run
    /// # async fn example() -> anyhow::Result<()> {
    /// # use percolate_rocks::admin::StatisticsManager;
    /// # let stats_mgr = StatisticsManager::new("/var/lib/rem/db")?;
    /// let stats = stats_mgr.get_database_stats().await?;
    /// println!("{:#?}", stats);
    /// # Ok(())
    /// # }
    /// ```
    pub async fn get_database_stats(&self) -> Result<DatabaseStats> {
        todo!(
            "Collect from RocksDB:
            1. Total size (all CFs)
            2. Entity counts per schema
            3. Column family sizes
            4. RocksDB properties (compression, bloom filters, etc.)
            5. Index sizes (HNSW, field indexes)
            6. Return DatabaseStats"
        )
    }

    /// Get statistics for specific schema.
    ///
    /// # Arguments
    ///
    /// * `schema_name` - Schema name
    ///
    /// # Returns
    ///
    /// `SchemaStats` for the schema
    ///
    /// # Errors
    ///
    /// Returns error if schema not found
    ///
    /// # Example
    ///
    /// ```rust,no_run
    /// # async fn example() -> anyhow::Result<()> {
    /// # use percolate_rocks::admin::StatisticsManager;
    /// # let stats_mgr = StatisticsManager::new("/var/lib/rem/db")?;
    /// let stats = stats_mgr.get_schema_stats("articles").await?;
    /// println!("Articles: {} entities, {} MB",
    ///     stats.entity_count,
    ///     stats.total_size_mb()
    /// );
    /// # Ok(())
    /// # }
    /// ```
    pub async fn get_schema_stats(&self, _schema_name: &str) -> Result<SchemaStats> {
        todo!(
            "1. Count entities for schema
            2. Calculate total size (entities + embeddings + indexes)
            3. Get indexed field stats
            4. Return SchemaStats"
        )
    }

    /// Get column family statistics.
    ///
    /// # Arguments
    ///
    /// * `cf_name` - Column family name
    ///
    /// # Returns
    ///
    /// `ColumnFamilyStats` for the CF
    ///
    /// # Example
    ///
    /// ```rust,no_run
    /// # async fn example() -> anyhow::Result<()> {
    /// # use percolate_rocks::admin::StatisticsManager;
    /// # let stats_mgr = StatisticsManager::new("/var/lib/rem/db")?;
    /// let stats = stats_mgr.get_cf_stats("embeddings").await?;
    /// println!("Embeddings CF: {} MB, compression: {}x",
    ///     stats.total_size_mb(),
    ///     stats.compression_ratio
    /// );
    /// # Ok(())
    /// # }
    /// ```
    pub async fn get_cf_stats(&self, _cf_name: &str) -> Result<ColumnFamilyStats> {
        todo!(
            "Get RocksDB properties for CF:
            - rocksdb.total-sst-files-size
            - rocksdb.estimate-num-keys
            - rocksdb.compression-ratio-at-level-*
            - rocksdb.num-files-at-level-*
            Return ColumnFamilyStats"
        )
    }

    /// Export statistics in Prometheus format.
    ///
    /// # Returns
    ///
    /// Prometheus-formatted metrics string
    ///
    /// # Example
    ///
    /// ```rust,no_run
    /// # async fn example() -> anyhow::Result<()> {
    /// # use percolate_rocks::admin::StatisticsManager;
    /// # let stats_mgr = StatisticsManager::new("/var/lib/rem/db")?;
    /// let metrics = stats_mgr.export_prometheus_metrics().await?;
    /// // Output to /metrics endpoint
    /// println!("{}", metrics);
    /// # Ok(())
    /// # }
    /// ```
    pub async fn export_prometheus_metrics(&self) -> Result<String> {
        todo!(
            "Format stats as Prometheus metrics:
            # HELP rem_db_size_bytes Total database size
            # TYPE rem_db_size_bytes gauge
            rem_db_size_bytes{{cf=\"entities\"}} 1234567890
            rem_db_size_bytes{{cf=\"embeddings\"}} 987654321
            ...
            Return formatted string"
        )
    }

    /// Export statistics as JSON.
    ///
    /// # Returns
    ///
    /// JSON-formatted statistics
    ///
    /// # Example
    ///
    /// ```rust,no_run
    /// # async fn example() -> anyhow::Result<()> {
    /// # use percolate_rocks::admin::StatisticsManager;
    /// # let stats_mgr = StatisticsManager::new("/var/lib/rem/db")?;
    /// let json = stats_mgr.export_json().await?;
    /// println!("{}", json);
    /// # Ok(())
    /// # }
    /// ```
    pub async fn export_json(&self) -> Result<String> {
        todo!(
            "1. Collect DatabaseStats
            2. Serialize to JSON with serde_json
            3. Return formatted string"
        )
    }
}

/// Comprehensive database statistics.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct DatabaseStats {
    /// Total entities across all schemas
    pub total_entities: u64,

    /// Total embeddings
    pub total_embeddings: u64,

    /// Total edges
    pub total_edges: u64,

    /// Total disk usage (bytes)
    pub total_size_bytes: u64,

    /// Column family statistics
    pub column_families: HashMap<String, ColumnFamilyStats>,

    /// Per-schema statistics
    pub schemas: HashMap<String, SchemaStats>,

    /// RocksDB version
    pub rocksdb_version: String,

    /// Database uptime (seconds)
    pub uptime_seconds: u64,
}

impl DatabaseStats {
    /// Get total size in gigabytes.
    pub fn total_size_gb(&self) -> f64 {
        self.total_size_bytes as f64 / 1_000_000_000.0
    }

    /// Get average entity size in bytes.
    pub fn avg_entity_size_bytes(&self) -> f64 {
        if self.total_entities == 0 {
            return 0.0;
        }
        self.total_size_bytes as f64 / self.total_entities as f64
    }
}

/// Statistics for a specific schema.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SchemaStats {
    /// Schema name
    pub schema_name: String,

    /// Number of entities
    pub entity_count: u64,

    /// Number of entities with embeddings
    pub entities_with_embeddings: u64,

    /// Total size (entities + embeddings + indexes)
    pub total_size_bytes: u64,

    /// Indexed fields and their sizes
    pub indexed_fields: HashMap<String, u64>,
}

impl SchemaStats {
    /// Get total size in megabytes.
    pub fn total_size_mb(&self) -> f64 {
        self.total_size_bytes as f64 / 1_000_000.0
    }

    /// Get average entity size in bytes.
    pub fn avg_entity_size_bytes(&self) -> f64 {
        if self.entity_count == 0 {
            return 0.0;
        }
        self.total_size_bytes as f64 / self.entity_count as f64
    }

    /// Get embedding coverage percentage.
    pub fn embedding_coverage_percent(&self) -> f64 {
        if self.entity_count == 0 {
            return 0.0;
        }
        (self.entities_with_embeddings as f64 / self.entity_count as f64) * 100.0
    }
}

/// Statistics for a column family.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ColumnFamilyStats {
    /// Column family name
    pub cf_name: String,

    /// Total size (bytes)
    pub total_size_bytes: u64,

    /// Estimated number of keys
    pub num_keys: u64,

    /// Compression ratio (uncompressed / compressed)
    pub compression_ratio: f64,

    /// Number of SST files
    pub num_sst_files: u32,

    /// Number of files at each level
    pub level_sizes: Vec<LevelStats>,
}

impl ColumnFamilyStats {
    /// Get total size in megabytes.
    pub fn total_size_mb(&self) -> f64 {
        self.total_size_bytes as f64 / 1_000_000.0
    }

    /// Get average key size in bytes.
    pub fn avg_key_size_bytes(&self) -> f64 {
        if self.num_keys == 0 {
            return 0.0;
        }
        self.total_size_bytes as f64 / self.num_keys as f64
    }
}

/// Statistics for a single LSM-tree level.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct LevelStats {
    /// Level number (0-6)
    pub level: u32,

    /// Number of files at this level
    pub num_files: u32,

    /// Total size of files at this level (bytes)
    pub size_bytes: u64,
}

impl LevelStats {
    /// Get size in megabytes.
    pub fn size_mb(&self) -> f64 {
        self.size_bytes as f64 / 1_000_000.0
    }
}

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

    #[test]
    fn test_database_stats_calculations() {
        let stats = DatabaseStats {
            total_entities: 1_000_000,
            total_embeddings: 800_000,
            total_edges: 2_000_000,
            total_size_bytes: 5_000_000_000,
            column_families: HashMap::new(),
            schemas: HashMap::new(),
            rocksdb_version: "8.0.0".to_string(),
            uptime_seconds: 86400,
        };

        assert_eq!(stats.total_size_gb(), 5.0);
        assert_eq!(stats.avg_entity_size_bytes(), 5000.0);
    }

    #[test]
    fn test_schema_stats_calculations() {
        let stats = SchemaStats {
            schema_name: "articles".to_string(),
            entity_count: 10_000,
            entities_with_embeddings: 8_000,
            total_size_bytes: 50_000_000,
            indexed_fields: HashMap::new(),
        };

        assert_eq!(stats.total_size_mb(), 50.0);
        assert_eq!(stats.avg_entity_size_bytes(), 5000.0);
        assert_eq!(stats.embedding_coverage_percent(), 80.0);
    }

    #[tokio::test]
    async fn test_get_database_stats() {
        // TODO: Test stats collection with temp database
        todo!("Implement after get_database_stats() is implemented")
    }
}
