//! Memory pool management for efficient memory allocation
//!
//! This module provides advanced memory pool management strategies to reduce
//! fragmentation and improve allocation performance in pipeline operations.

use crate::error::Result;
use super::super::resource::allocation::SystemMetrics;
use serde::{Deserialize, Serialize};
use std::collections::HashMap;

/// Memory allocation and management strategy
#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum MemoryStrategy {
    /// Standard allocation with GC
    Standard,
    /// Memory pool allocation for reduced fragmentation
    MemoryPool { pool_size: usize },
    /// Memory mapping for large datasets
    MemoryMapped { chunk_size: usize },
    /// Streaming processing for advanced-large datasets
    Streaming { buffer_size: usize },
    /// Hybrid approach combining multiple strategies
    Hybrid {
        small_data_threshold: usize,
        memory_pool_size: usize,
        streaming_threshold: usize,
    },
}

/// Memory pool manager for efficient memory allocation
#[derive(Debug)]
pub struct MemoryPoolManager {
    pools: HashMap<usize, MemoryPool>,
    allocation_strategy: AllocationStrategy,
    fragmentation_monitor: FragmentationMonitor,
}

impl MemoryPoolManager {
    pub fn new() -> Self {
        Self {
            pools: HashMap::new(),
            allocation_strategy: AllocationStrategy::BestFit,
            fragmentation_monitor: FragmentationMonitor::new(),
        }
    }

    pub fn determine_optimal_strategy(
        &mut self,
        data_size: usize,
        system_metrics: &SystemMetrics,
    ) -> Result<MemoryStrategy> {
        let available_memory = system_metrics.memory_usage.available as usize;
        let memory_pressure = system_metrics.memory_usage.utilization;

        // Choose strategy based on data size and system state
        if data_size > available_memory / 2 {
            // Large dataset - use streaming or memory mapping
            if memory_pressure > 0.8 {
                Ok(MemoryStrategy::Streaming {
                    buffer_size: available_memory / 10,
                })
            } else {
                Ok(MemoryStrategy::MemoryMapped {
                    chunk_size: available_memory / 4,
                })
            }
        } else if data_size > 1024 * 1024 {
            // Medium dataset - use memory pool
            Ok(MemoryStrategy::MemoryPool {
                pool_size: data_size * 2,
            })
        } else {
            // Small dataset - use standard allocation
            Ok(MemoryStrategy::Standard)
        }
    }

    /// Allocate a memory pool of specified size
    pub fn allocate_pool(&mut self, pool_id: usize, pool_size: usize) -> Result<()> {
        if !self.pools.contains_key(&pool_id) {
            let pool = MemoryPool {
                pool_size,
                allocated: 0,
                free_blocks: vec![MemoryBlock {
                    address: 0, // Placeholder - would be actual memory address
                    size: pool_size,
                    is_free: true,
                }],
            };
            self.pools.insert(pool_id, pool);
        }
        Ok(())
    }

    /// Get memory pool statistics
    pub fn get_pool_stats(&self, pool_id: usize) -> Option<MemoryPoolStats> {
        self.pools.get(&pool_id).map(|pool| MemoryPoolStats {
            pool_size: pool.pool_size,
            allocated: pool.allocated,
            free_blocks: pool.free_blocks.len(),
            fragmentation: self.fragmentation_monitor.external_fragmentation,
        })
    }

    /// Perform memory compaction if fragmentation is too high
    pub fn compact_if_needed(&mut self) -> Result<bool> {
        if self.fragmentation_monitor.external_fragmentation > self.fragmentation_monitor.compaction_threshold {
            self.compact_memory()?;
            Ok(true)
        } else {
            Ok(false)
        }
    }

    fn compact_memory(&mut self) -> Result<()> {
        // Simplified compaction logic
        for pool in self.pools.values_mut() {
            pool.free_blocks.retain(|block| !block.is_free || block.size > 0);
            // In a real implementation, this would coalesce adjacent free blocks
        }
        self.fragmentation_monitor.external_fragmentation *= 0.5; // Reduce fragmentation
        Ok(())
    }
}

impl Default for MemoryPoolManager {
    fn default() -> Self {
        Self::new()
    }
}

#[derive(Debug)]
pub struct MemoryPool {
    pub pool_size: usize,
    pub allocated: usize,
    pub free_blocks: Vec<MemoryBlock>,
}

#[derive(Debug)]
pub struct MemoryBlock {
    pub address: usize,
    pub size: usize,
    pub is_free: bool,
}

#[derive(Debug)]
pub enum AllocationStrategy {
    FirstFit,
    BestFit,
    WorstFit,
    NextFit,
}

#[derive(Debug)]
pub struct FragmentationMonitor {
    pub internal_fragmentation: f64,
    pub external_fragmentation: f64,
    pub compaction_threshold: f64,
}

impl Default for FragmentationMonitor {
    fn default() -> Self {
        Self::new()
    }
}

impl FragmentationMonitor {
    pub fn new() -> Self {
        Self {
            internal_fragmentation: 0.0,
            external_fragmentation: 0.0,
            compaction_threshold: 0.3, // 30% fragmentation triggers compaction
        }
    }

    /// Update fragmentation metrics
    pub fn update_fragmentation(&mut self, internal: f64, external: f64) {
        self.internal_fragmentation = internal;
        self.external_fragmentation = external;
    }

    /// Check if compaction is needed
    pub fn needs_compaction(&self) -> bool {
        self.external_fragmentation > self.compaction_threshold
    }
}

#[derive(Debug, Clone)]
pub struct MemoryPoolStats {
    pub pool_size: usize,
    pub allocated: usize,
    pub free_blocks: usize,
    pub fragmentation: f64,
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::pipeline::optimization::resource::allocation::{SystemMetrics, MemoryUsage};

    #[test]
    fn test_memory_pool_manager_creation() {
        let manager = MemoryPoolManager::new();
        assert!(manager.pools.is_empty());
    }

    #[test]
    fn test_optimal_strategy_small_data() {
        let mut manager = MemoryPoolManager::new();
        let system_metrics = SystemMetrics {
            memory_usage: MemoryUsage {
                total: 8 * 1024 * 1024 * 1024,
                available: 4 * 1024 * 1024 * 1024,
                used: 4 * 1024 * 1024 * 1024,
                utilization: 0.5,
            },
            ..Default::default()
        };

        let strategy = manager.determine_optimal_strategy(1024, &system_metrics).expect("Operation failed");
        matches!(strategy, MemoryStrategy::Standard);
    }

    #[test]
    fn test_memory_pool_allocation() {
        let mut manager = MemoryPoolManager::new();
        let result = manager.allocate_pool(0, 1024 * 1024);
        assert!(result.is_ok());
        
        let stats = manager.get_pool_stats(0);
        assert!(stats.is_some());
        assert_eq!(stats.expect("Operation failed").pool_size, 1024 * 1024);
    }

    #[test]
    fn test_fragmentation_monitor() {
        let mut monitor = FragmentationMonitor::new();
        assert_eq!(monitor.compaction_threshold, 0.3);
        
        monitor.update_fragmentation(0.1, 0.4);
        assert!(monitor.needs_compaction());
    }
}