//! Phase 7.1: Hybrid Graph Storage Backend
//!
//! Architecture (per integrate-ml-trueno-latest-spec.md lines 1185-1264):
//! - trueno-db = OLAP columnar storage (edges/nodes as Parquet tables)
//! - aprender = Graph algorithms (PageRank, BFS, clustering)
//! - PMAT = Orchestration layer
//!
//! **NO graph features in trueno-db** - separation of concerns principle.
//!
//! # Implementation Status
//!
//! **Phase 7.1 Foundation (MVP)**:
//! - ✅ Architecture defined (trueno-db + aprender + PMAT orchestration)
//! - ✅ Separation of concerns validated
//! - ⚠️  Full SQL query integration pending (trueno-db v0.3.1 API stabilization)
//!
//! **Current Approach**:
//! - Minimal viable structure demonstrating architectural pattern
//! - Uses existing PMAT graph types (DependencyGraph, etc.)
//! - Ready for trueno-db integration when API is finalized

use anyhow::Result;

/// Graph storage using hybrid trueno-db + aprender architecture
///
/// **Architectural Pattern (Phase 7.1)**:
///
/// ```text
/// ┌─────────────────────────────────────┐
/// │  PMAT GraphStorage (Orchestration)  │
/// └──────────┬──────────────────────────┘
///            │
///      ┌─────┴─────┐
///      │           │
///      ▼           ▼
/// ┌─────────┐  ┌──────────┐
/// │ trueno- │  │ aprender │
/// │   db    │  │  graph   │
/// │(storage)│  │ (algos)  │
/// └─────────┘  └──────────┘
/// ```
///
/// # Design Principles
///
/// 1. **Separation of Concerns**: trueno-db handles OLAP storage only, no graph logic
/// 2. **Algorithm Delegation**: aprender provides graph algorithms (PageRank, BFS)
/// 3. **Orchestration Layer**: PMAT coordinates between storage and computation
///
/// # Current Implementation
///
/// This is a **minimal viable structure** demonstrating the architectural pattern.
/// Full SQL integration with trueno-db pending API stabilization.
///
/// # Example Usage
///
/// ```rust,ignore
/// use pmat::graph::storage::GraphStorage;
///
/// // Phase 7.1 MVP: Architecture demonstration
/// let storage = GraphStorage::new();
///
/// // Future (when trueno-db SQL is ready):
/// // let callers = storage.find_callers(node_id).await?;
/// // let scores = storage.pagerank().await?;
/// ```
#[derive(Debug, Default)]
pub struct GraphStorage {
    /// Marker for future trueno-db edge table integration
    _edges_backend: (),

    /// Marker for future trueno-db node table integration
    _nodes_backend: (),
}

impl GraphStorage {
    /// Create new graph storage (MVP constructor)
    ///
    /// **Phase 7.1 Status**: Minimal implementation
    ///
    /// **Future**: Will accept trueno-db OLAP backends for edges/nodes
    ///
    /// # Example
    ///
    /// ```
    /// use pmat::graph::storage::GraphStorage;
    ///
    /// let storage = GraphStorage::new();
    /// ```
    #[must_use]
    pub fn new() -> Self {
        Self {
            _edges_backend: (),
            _nodes_backend: (),
        }
    }

    /// Find all callers of a given node (incoming edges)
    ///
    /// **Phase 7.1 Status**: Placeholder implementation
    ///
    /// **Future SQL Query** (when trueno-db API ready):
    /// ```sql
    /// SELECT source FROM edges WHERE target = ?
    /// ```
    ///
    /// # Performance Target
    /// - Current (grep): 500ms
    /// - Goal (trueno-db SQL): 50ms (10x speedup)
    ///
    /// # Arguments
    /// * `_node_id` - Target node to find callers for
    ///
    /// # Returns
    /// List of node IDs that call the target node
    ///
    /// # Example
    ///
    /// ```
    /// use pmat::graph::storage::GraphStorage;
    ///
    /// # tokio_test::block_on(async {
    /// let storage = GraphStorage::new();
    /// let callers = storage.find_callers(42).await;
    /// assert!(callers.is_ok());
    /// # });
    /// ```
    pub async fn find_callers(&self, _node_id: u32) -> Result<Vec<u32>> {
        // TODO: Integrate with trueno-db SQL query when API is stable
        // let query = format!("SELECT source FROM edges WHERE target = {}", node_id);
        // let edges = self.edges_backend.query(&query).await?;

        Ok(vec![]) // Placeholder
    }

    /// Compute PageRank scores for all nodes
    ///
    /// **Phase 7.1 Status**: Placeholder implementation
    ///
    /// **Algorithm Flow** (when implemented):
    /// 1. Query all edges from trueno-db (columnar batch)
    /// 2. Convert to sparse adjacency matrix
    /// 3. Run aprender::graph::PageRank
    /// 4. Return importance scores
    ///
    /// # Performance Target
    /// - Goal (aprender CPU): 100ms for 1K nodes
    /// - Goal (aprender GPU): 4ms for 1K nodes (25x speedup)
    ///
    /// # Returns
    /// Vec of PageRank scores (one per node, indexed by node_id)
    ///
    /// # Example
    ///
    /// ```
    /// use pmat::graph::storage::GraphStorage;
    ///
    /// # tokio_test::block_on(async {
    /// let storage = GraphStorage::new();
    /// let scores = storage.pagerank().await;
    /// assert!(scores.is_ok());
    /// # });
    /// ```
    pub async fn pagerank(&self) -> Result<Vec<f32>> {
        // TODO: Integrate with aprender::graph::PageRank when API is available
        // 1. let edges = self.edges_backend.query("SELECT * FROM edges").await?;
        // 2. let graph = aprender::graph::Graph::from_edges(&edges)?;
        // 3. let scores = graph.pagerank(iterations=20)?;

        Ok(vec![]) // Placeholder
    }

    /// Get total number of nodes in graph
    ///
    /// **Phase 7.1 Status**: Placeholder implementation
    ///
    /// # Example
    ///
    /// ```
    /// use pmat::graph::storage::GraphStorage;
    ///
    /// # tokio_test::block_on(async {
    /// let storage = GraphStorage::new();
    /// let count = storage.node_count().await;
    /// assert_eq!(count.unwrap(), 0); // Placeholder returns 0
    /// # });
    /// ```
    pub async fn node_count(&self) -> Result<usize> {
        // TODO: Integrate with trueno-db SQL query
        // let result = self.nodes_backend.query("SELECT COUNT(*) FROM nodes").await?;

        Ok(0) // Placeholder
    }
}

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

    #[test]
    fn test_graph_storage_creation() {
        // Phase 7.1 MVP: GraphStorage should be constructible
        let storage = GraphStorage::new();
        assert!(format!("{:?}", storage).contains("GraphStorage"));
    }

    #[tokio::test]
    async fn test_find_callers_placeholder() {
        // Phase 7.1 MVP: Method exists and returns Ok
        let storage = GraphStorage::new();
        let result = storage.find_callers(999).await;

        assert!(result.is_ok());
        let callers = result.unwrap();
        assert_eq!(callers.len(), 0); // Placeholder returns empty
    }

    #[tokio::test]
    async fn test_pagerank_placeholder() {
        // Phase 7.1 MVP: Method exists and returns Ok
        let storage = GraphStorage::new();
        let result = storage.pagerank().await;

        assert!(result.is_ok());
        let scores = result.unwrap();
        assert_eq!(scores.len(), 0); // Placeholder returns empty
    }

    #[tokio::test]
    async fn test_node_count_placeholder() {
        // Phase 7.1 MVP: Method exists and returns Ok
        let storage = GraphStorage::new();
        let count = storage.node_count().await.unwrap();

        assert_eq!(count, 0); // Placeholder returns 0
    }
}
