sklears_compose/

task_scheduling.rs

//! Advanced Task Scheduling and Queue Management
//!
//! This module provides sophisticated task scheduling capabilities including
//! priority-based scheduling, dependency resolution, queue management, and
//! advanced scheduling algorithms for optimal task execution coordination.

use sklears_core::error::{Result as SklResult, SklearsError};
use std::collections::{BTreeMap, HashMap, VecDeque};
use std::time::{Duration, SystemTime};

use crate::execution_types::{ExecutionTask, TaskPriority};

/// Task scheduler trait for pluggable scheduling implementations
///
/// Provides a flexible interface for different scheduling strategies
/// that can be swapped based on workload characteristics and requirements.
pub trait TaskScheduler: Send + Sync {
    /// Schedule a single task for execution
    ///
    /// # Arguments
    /// * `task` - The task to schedule
    ///
    /// # Returns
    /// A task handle for tracking and management
    fn schedule_task(&mut self, task: ExecutionTask) -> SklResult<TaskHandle>;

    /// Schedule multiple tasks as a batch
    ///
    /// # Arguments
    /// * `tasks` - Vector of tasks to schedule
    ///
    /// # Returns
    /// Vector of task handles in the same order
    fn schedule_batch(&mut self, tasks: Vec<ExecutionTask>) -> SklResult<Vec<TaskHandle>>;

    /// Cancel a scheduled task
    ///
    /// # Arguments
    /// * `handle` - Handle of the task to cancel
    ///
    /// # Returns
    /// Success or error if cancellation fails
    fn cancel_task(&mut self, handle: TaskHandle) -> SklResult<()>;

    /// Get current scheduler status and metrics
    ///
    /// # Returns
    /// Current scheduler status information
    fn get_status(&self) -> SchedulerStatus;

    /// Update scheduler configuration
    ///
    /// # Arguments
    /// * `config` - New scheduler configuration
    ///
    /// # Returns
    /// Success or error if configuration update fails
    fn update_config(&mut self, config: SchedulerConfig) -> SklResult<()>;

    /// Shutdown scheduler gracefully
    ///
    /// # Returns
    /// Future that completes when shutdown is finished
    fn shutdown_gracefully(&mut self) -> impl std::future::Future<Output = SklResult<()>> + Send;

    /// Get next task to execute (for scheduler implementations)
    fn get_next_task(&mut self) -> Option<(ExecutionTask, TaskHandle)>;

    /// Mark task as completed
    fn mark_task_completed(&mut self, handle: &TaskHandle) -> SklResult<()>;

    /// Mark task as failed
    fn mark_task_failed(&mut self, handle: &TaskHandle, error: String) -> SklResult<()>;
}

/// Task handle for tracking scheduled tasks
///
/// Provides a unique identifier and metadata for scheduled tasks
/// that allows tracking, cancellation, and dependency management.
#[derive(Debug, Clone)]
pub struct TaskHandle {
    /// Unique task identifier
    pub task_id: String,

    /// Task scheduling timestamp
    pub scheduled_at: SystemTime,

    /// Estimated execution duration
    pub estimated_duration: Option<Duration>,

    /// Task priority level
    pub priority: TaskPriority,

    /// Task dependencies (task IDs that must complete first)
    pub dependencies: Vec<String>,

    /// Current task state
    pub state: TaskState,

    /// Queue position (if applicable)
    pub queue_position: Option<usize>,

    /// Retry count for failed tasks
    pub retry_count: usize,

    /// Last update timestamp
    pub last_updated: SystemTime,
}

/// Task state enumeration
#[derive(Debug, Clone, PartialEq)]
pub enum TaskState {
    /// Task is queued waiting for execution
    Queued,
    /// Task dependencies are being resolved
    WaitingForDependencies,
    /// Task is ready to execute
    Ready,
    /// Task is currently executing
    Running,
    /// Task completed successfully
    Completed,
    /// Task failed during execution
    Failed,
    /// Task was cancelled
    Cancelled,
    /// Task execution timed out
    TimedOut,
}

/// Comprehensive scheduler configuration
///
/// Defines all aspects of scheduler behavior including algorithms,
/// queue management, priority handling, and dependency resolution.
#[derive(Debug, Clone)]
pub struct SchedulerConfig {
    /// Core scheduling algorithm to use
    pub algorithm: SchedulingAlgorithm,

    /// Queue management configuration
    pub queue_management: QueueManagement,

    /// Priority handling configuration
    pub priority_handling: PriorityHandling,

    /// Dependency resolution configuration
    pub dependency_resolution: DependencyResolution,

    /// Load balancing configuration
    pub load_balancing: LoadBalancingConfig,

    /// Scheduler performance tuning
    pub performance_tuning: SchedulerPerformanceTuning,

    /// Scheduler monitoring configuration
    pub monitoring: SchedulerMonitoringConfig,
}

/// Scheduling algorithms available
///
/// Different algorithms optimize for different characteristics
/// such as fairness, throughput, latency, or resource utilization.
#[derive(Debug, Clone)]
pub enum SchedulingAlgorithm {
    FIFO,

    LIFO,

    Priority,

    ShortestJobFirst,

    FairShare,

    WorkConservingCFS,

    MultiLevelFeedback {
        levels: usize,
        time_quantum: Duration,
        aging_factor: f64,
    },

    /// Deadline-aware scheduling
    DeadlineAware,

    /// Resource-aware scheduling
    ResourceAware,

    /// Machine learning optimized scheduling
    MLOptimized {
        model_type: String,
        learning_rate: f64,
    },

    /// Custom scheduling algorithm
    Custom {
        algorithm_name: String,
        parameters: HashMap<String, String>,
    },
}

/// Queue management configuration
///
/// Controls queue behavior, overflow handling, and persistence.
#[derive(Debug, Clone)]
pub struct QueueManagement {
    /// Maximum queue size (number of tasks)
    pub max_queue_size: usize,

    /// Queue overflow handling strategy
    pub overflow_strategy: QueueOverflowStrategy,

    /// Queue persistence configuration
    pub persistence: QueuePersistence,

    /// Queue partitioning strategy
    pub partitioning: QueuePartitioning,

    /// Queue compaction settings
    pub compaction: QueueCompaction,

    /// Queue rebalancing configuration
    pub rebalancing: QueueRebalancing,
}

/// Queue overflow handling strategies
#[derive(Debug, Clone)]
pub enum QueueOverflowStrategy {
    /// Block new task submissions
    Block,

    /// Drop the new incoming task
    Drop,

    /// Drop the oldest task in queue
    DropOldest,

    /// Drop the lowest priority task
    DropLowestPriority,

    /// Reject with error
    Reject,

    /// Spill to external storage
    Spill { storage_path: String },

    /// Scale queue size dynamically
    DynamicScale { max_scale_factor: f64 },
}

/// Queue persistence options
#[derive(Debug, Clone)]
pub enum QueuePersistence {
    /// In-memory only (no persistence)
    Memory,

    /// Persistent disk-based storage
    Disk {
        path: String,
        sync_interval: Duration,
    },

    /// Hybrid approach with memory and disk
    Hybrid {
        memory_limit: usize,
        disk_path: String,
        spill_threshold: f64,
    },

    /// Database-backed persistence
    Database {
        connection_string: String,
        table_name: String,
    },
}

/// Queue partitioning strategies
#[derive(Debug, Clone)]
pub enum QueuePartitioning {
    /// Single queue for all tasks
    Single,

    /// Separate queues by priority
    ByPriority,

    /// Separate queues by task type
    ByTaskType,

    /// Separate queues by resource requirements
    ByResourceRequirements,

    /// Custom partitioning scheme
    Custom { scheme_name: String },
}

/// Queue compaction configuration
#[derive(Debug, Clone)]
pub struct QueueCompaction {
    /// Enable automatic compaction
    pub enabled: bool,

    /// Compaction trigger threshold
    pub trigger_threshold: f64,

    /// Compaction interval
    pub interval: Duration,

    /// Compaction strategy
    pub strategy: CompactionStrategy,
}

/// Queue compaction strategies
#[derive(Debug, Clone)]
pub enum CompactionStrategy {
    /// Remove completed/failed tasks
    RemoveCompleted,

    /// Merge similar tasks
    MergeSimilar,

    /// Optimize queue order
    OptimizeOrder,

    /// Custom compaction logic
    Custom { strategy_name: String },
}

/// Queue rebalancing configuration
#[derive(Debug, Clone)]
pub struct QueueRebalancing {
    /// Enable automatic rebalancing
    pub enabled: bool,

    /// Rebalancing interval
    pub interval: Duration,

    /// Load imbalance threshold
    pub imbalance_threshold: f64,

    /// Rebalancing strategy
    pub strategy: RebalancingStrategy,
}

/// Queue rebalancing strategies
#[derive(Debug, Clone)]
pub enum RebalancingStrategy {
    /// Round-robin redistribution
    RoundRobin,

    /// Load-based redistribution
    LoadBased,

    /// Priority-aware redistribution
    PriorityAware,

    /// Custom rebalancing logic
    Custom { strategy_name: String },
}

/// Priority handling configuration
///
/// Defines how task priorities are managed and how priority aging
/// is handled to prevent starvation.
#[derive(Debug, Clone)]
pub struct PriorityHandling {
    /// Available priority levels
    pub levels: Vec<TaskPriority>,

    /// Priority aging strategy to prevent starvation
    pub aging_strategy: AgingStrategy,

    /// Enable starvation prevention
    pub starvation_prevention: bool,

    /// Priority inversion handling
    pub priority_inversion: PriorityInversionHandling,

    /// Dynamic priority adjustment
    pub dynamic_priority: DynamicPriorityConfig,
}

/// Priority aging strategies to prevent starvation
#[derive(Debug, Clone)]
pub enum AgingStrategy {
    /// No aging (static priorities)
    None,

    /// Linear priority increase over time
    Linear {
        increment_interval: Duration,
        increment_amount: i32,
    },

    /// Exponential priority increase
    Exponential {
        base: f64,
        interval: Duration,
        max_boost: i32,
    },

    /// Adaptive aging based on wait time
    Adaptive {
        threshold: Duration,
        boost_factor: f64,
    },

    /// Custom aging algorithm
    Custom { algorithm_name: String },
}

/// Priority inversion handling strategies
#[derive(Debug, Clone)]
pub struct PriorityInversionHandling {
    /// Enable priority inheritance
    pub priority_inheritance: bool,

    /// Enable priority ceiling protocol
    pub priority_ceiling: bool,

    /// Detection threshold
    pub detection_threshold: Duration,

    /// Resolution strategy
    pub resolution_strategy: PriorityInversionResolution,
}

/// Priority inversion resolution strategies
#[derive(Debug, Clone)]
pub enum PriorityInversionResolution {
    /// Boost blocking task priority
    PriorityBoost,

    /// Preempt blocking task
    Preemption,

    /// Resource reallocation
    ResourceReallocation,

    /// Custom resolution logic
    Custom { strategy_name: String },
}

/// Dynamic priority adjustment configuration
#[derive(Debug, Clone)]
pub struct DynamicPriorityConfig {
    /// Enable dynamic adjustment
    pub enabled: bool,

    /// Adjustment factors
    pub factors: PriorityAdjustmentFactors,

    /// Adjustment frequency
    pub adjustment_interval: Duration,

    /// Maximum priority change per adjustment
    pub max_adjustment: i32,
}

/// Factors for dynamic priority adjustment
#[derive(Debug, Clone)]
pub struct PriorityAdjustmentFactors {
    /// Wait time factor
    pub wait_time_factor: f64,

    /// Resource availability factor
    pub resource_availability_factor: f64,

    /// System load factor
    pub system_load_factor: f64,

    /// Task deadline proximity factor
    pub deadline_proximity_factor: f64,

    /// Historical performance factor
    pub performance_factor: f64,
}

/// Dependency resolution configuration
///
/// Controls how task dependencies are tracked, resolved, and
/// how circular dependencies and deadlocks are handled.
#[derive(Debug, Clone)]
pub struct DependencyResolution {
    /// Enable dependency tracking
    pub enable_tracking: bool,

    /// Enable circular dependency detection
    pub cycle_detection: bool,

    /// Enable deadlock prevention
    pub deadlock_prevention: bool,

    /// Dependency resolution timeout
    pub resolution_timeout: Duration,

    /// Dependency graph optimization
    pub graph_optimization: DependencyGraphOptimization,

    /// Dependency caching
    pub caching: DependencyCaching,
}

/// Dependency graph optimization configuration
#[derive(Debug, Clone)]
pub struct DependencyGraphOptimization {
    /// Enable graph optimization
    pub enabled: bool,

    /// Optimization algorithms
    pub algorithms: Vec<GraphOptimizationAlgorithm>,

    /// Optimization frequency
    pub optimization_interval: Duration,
}

/// Dependency graph optimization algorithms
#[derive(Debug, Clone)]
pub enum GraphOptimizationAlgorithm {
    /// Topological sorting optimization
    TopologicalSort,

    /// Critical path analysis
    CriticalPath,

    /// Parallel execution optimization
    ParallelExecution,

    /// Resource-aware optimization
    ResourceAware,

    /// Custom optimization algorithm
    Custom { algorithm_name: String },
}

/// Dependency caching configuration
#[derive(Debug, Clone)]
pub struct DependencyCaching {
    /// Enable dependency caching
    pub enabled: bool,

    /// Cache size limit
    pub cache_size: usize,

    /// Cache TTL
    pub ttl: Duration,

    /// Cache eviction strategy
    pub eviction_strategy: CacheEvictionStrategy,
}

/// Cache eviction strategies
#[derive(Debug, Clone)]
pub enum CacheEvictionStrategy {
    /// LRU
    LRU,
    /// LFU
    LFU,
    /// FIFO
    FIFO,
    /// TTL
    TTL,
    /// Custom
    Custom { strategy_name: String },
}

/// Load balancing configuration for distributed scheduling
#[derive(Debug, Clone)]
pub struct LoadBalancingConfig {
    /// Load balancing algorithm
    pub algorithm: LoadBalancingAlgorithm,

    /// Rebalancing frequency
    pub rebalancing_frequency: Duration,

    /// Load threshold for triggering rebalancing
    pub load_threshold: f64,

    /// Health check configuration
    pub health_checks: LoadBalancerHealthChecks,

    /// Failover configuration
    pub failover: LoadBalancerFailover,
}

/// Load balancing algorithms
#[derive(Debug, Clone)]
pub enum LoadBalancingAlgorithm {
    /// Round-robin distribution
    RoundRobin,

    /// Weighted round-robin
    WeightedRoundRobin { weights: Vec<f64> },

    /// Least connections
    LeastConnections,

    /// Least response time
    LeastResponseTime,

    /// Resource-based balancing
    ResourceBased,

    /// Predictive scaling
    PredictiveScaling { prediction_window: Duration },

    /// Custom load balancing
    Custom { algorithm_name: String },
}

/// Load balancer health check configuration
#[derive(Debug, Clone)]
pub struct LoadBalancerHealthChecks {
    /// Health check interval
    pub interval: Duration,

    /// Health check timeout
    pub timeout: Duration,

    /// Unhealthy threshold
    pub unhealthy_threshold: usize,

    /// Healthy threshold
    pub healthy_threshold: usize,
}

/// Load balancer failover configuration
#[derive(Debug, Clone)]
pub struct LoadBalancerFailover {
    /// Enable automatic failover
    pub enabled: bool,

    /// Failover targets
    pub targets: Vec<String>,

    /// Failback policy
    pub failback_policy: FailbackPolicy,
}

/// Failback policies
#[derive(Debug, Clone)]
pub enum FailbackPolicy {
    /// Immediate failback when primary recovers
    Immediate,

    /// Delayed failback
    Delayed { delay: Duration },

    /// Manual failback only
    Manual,

    /// Load-based failback
    LoadBased { threshold: f64 },
}

/// Scheduler performance tuning configuration
#[derive(Debug, Clone)]
pub struct SchedulerPerformanceTuning {
    /// Scheduler thread pool size
    pub thread_pool_size: usize,

    /// Task batch size for bulk operations
    pub batch_size: usize,

    /// Scheduling frequency
    pub scheduling_frequency: Duration,

    /// Memory optimization settings
    pub memory_optimization: MemoryOptimization,

    /// Cache configuration
    pub cache_config: SchedulerCacheConfig,
}

/// Memory optimization settings
#[derive(Debug, Clone)]
pub struct MemoryOptimization {
    /// Enable memory pooling
    pub memory_pooling: bool,

    /// Object recycling
    pub object_recycling: bool,

    /// Garbage collection tuning
    pub gc_tuning: GarbageCollectionTuning,
}

/// Garbage collection tuning
#[derive(Debug, Clone)]
pub struct GarbageCollectionTuning {
    /// GC frequency hint
    pub frequency: Duration,

    /// Memory pressure threshold
    pub pressure_threshold: f64,

    /// Cleanup strategies
    pub cleanup_strategies: Vec<CleanupStrategy>,
}

/// Cleanup strategies for memory management
#[derive(Debug, Clone)]
pub enum CleanupStrategy {
    /// Clean completed tasks
    CompletedTasks,

    /// Clean expired cache entries
    ExpiredCache,

    /// Compact data structures
    CompactStructures,

    /// Custom cleanup logic
    Custom { strategy_name: String },
}

/// Scheduler cache configuration
#[derive(Debug, Clone)]
pub struct SchedulerCacheConfig {
    /// Task metadata cache size
    pub task_cache_size: usize,

    /// Dependency cache size
    pub dependency_cache_size: usize,

    /// Statistics cache size
    pub stats_cache_size: usize,

    /// Cache TTL
    pub cache_ttl: Duration,
}

/// Scheduler monitoring configuration
#[derive(Debug, Clone)]
pub struct SchedulerMonitoringConfig {
    /// Enable performance metrics collection
    pub enable_metrics: bool,

    /// Enable detailed task tracking
    pub enable_task_tracking: bool,

    /// Enable queue statistics
    pub enable_queue_stats: bool,

    /// Metrics collection interval
    pub metrics_interval: Duration,

    /// Alert thresholds
    pub alert_thresholds: SchedulerAlertThresholds,
}

/// Scheduler alert thresholds
#[derive(Debug, Clone)]
pub struct SchedulerAlertThresholds {
    /// Queue size alert threshold
    pub queue_size_threshold: usize,

    /// Task failure rate threshold
    pub failure_rate_threshold: f64,

    /// Average wait time threshold
    pub wait_time_threshold: Duration,

    /// Scheduler utilization threshold
    pub utilization_threshold: f64,
}

/// Current scheduler status and metrics
#[derive(Debug, Clone)]
pub struct SchedulerStatus {
    /// Number of tasks currently queued
    pub queued_tasks: usize,

    /// Number of tasks currently running
    pub running_tasks: usize,

    /// Total number of completed tasks
    pub completed_tasks: u64,

    /// Total number of failed tasks
    pub failed_tasks: u64,

    /// Total number of cancelled tasks
    pub cancelled_tasks: u64,

    /// Current scheduler health status
    pub health: SchedulerHealth,

    /// Performance metrics
    pub performance: SchedulerPerformanceMetrics,

    /// Queue statistics by priority
    pub queue_stats: HashMap<TaskPriority, QueueStatistics>,

    /// Resource utilization
    pub resource_utilization: f64,
}

/// Scheduler health status
#[derive(Debug, Clone)]
pub enum SchedulerHealth {
    /// Scheduler operating normally
    Healthy,

    /// Scheduler overloaded but functional
    Overloaded { queue_size: usize },

    /// Scheduler blocked or deadlocked
    Blocked { reason: String },

    /// Scheduler degraded performance
    Degraded { performance_impact: f64 },

    /// Scheduler failed
    Failed { reason: String },
}

/// Scheduler performance metrics
#[derive(Debug, Clone)]
pub struct SchedulerPerformanceMetrics {
    /// Average task scheduling time
    pub avg_scheduling_time: Duration,

    /// Average task wait time in queue
    pub avg_wait_time: Duration,

    /// Tasks processed per second
    pub throughput: f64,

    /// Scheduler efficiency (0.0 to 1.0)
    pub efficiency: f64,

    /// Queue utilization percentage
    pub queue_utilization: f64,

    /// Dependency resolution efficiency
    pub dependency_resolution_efficiency: f64,
}

/// Queue statistics for a specific priority level
#[derive(Debug, Clone)]
pub struct QueueStatistics {
    /// Number of tasks in queue
    pub task_count: usize,

    /// Average wait time
    pub avg_wait_time: Duration,

    /// Oldest task age
    pub oldest_task_age: Duration,

    /// Queue growth rate
    pub growth_rate: f64,
}

// Default implementations

impl Default for SchedulerConfig {
    fn default() -> Self {
        Self {
            algorithm: SchedulingAlgorithm::Priority,
            queue_management: QueueManagement {
                max_queue_size: 10000,
                overflow_strategy: QueueOverflowStrategy::Block,
                persistence: QueuePersistence::Memory,
                partitioning: QueuePartitioning::ByPriority,
                compaction: QueueCompaction {
                    enabled: true,
                    trigger_threshold: 0.8,
                    interval: Duration::from_secs(300),
                    strategy: CompactionStrategy::RemoveCompleted,
                },
                rebalancing: QueueRebalancing {
                    enabled: false,
                    interval: Duration::from_secs(60),
                    imbalance_threshold: 0.3,
                    strategy: RebalancingStrategy::LoadBased,
                },
            },
            priority_handling: PriorityHandling {
                levels: vec![
                    TaskPriority::Low,
                    TaskPriority::Normal,
                    TaskPriority::High,
                    TaskPriority::Critical,
                ],
                aging_strategy: AgingStrategy::Linear {
                    increment_interval: Duration::from_secs(60),
                    increment_amount: 1,
                },
                starvation_prevention: true,
                priority_inversion: PriorityInversionHandling {
                    priority_inheritance: true,
                    priority_ceiling: false,
                    detection_threshold: Duration::from_secs(10),
                    resolution_strategy: PriorityInversionResolution::PriorityBoost,
                },
                dynamic_priority: DynamicPriorityConfig {
                    enabled: false,
                    factors: PriorityAdjustmentFactors {
                        wait_time_factor: 0.3,
                        resource_availability_factor: 0.2,
                        system_load_factor: 0.2,
                        deadline_proximity_factor: 0.2,
                        performance_factor: 0.1,
                    },
                    adjustment_interval: Duration::from_secs(30),
                    max_adjustment: 5,
                },
            },
            dependency_resolution: DependencyResolution {
                enable_tracking: true,
                cycle_detection: true,
                deadlock_prevention: true,
                resolution_timeout: Duration::from_secs(30),
                graph_optimization: DependencyGraphOptimization {
                    enabled: false,
                    algorithms: vec![GraphOptimizationAlgorithm::TopologicalSort],
                    optimization_interval: Duration::from_secs(300),
                },
                caching: DependencyCaching {
                    enabled: true,
                    cache_size: 1000,
                    ttl: Duration::from_secs(300),
                    eviction_strategy: CacheEvictionStrategy::LRU,
                },
            },
            load_balancing: LoadBalancingConfig {
                algorithm: LoadBalancingAlgorithm::RoundRobin,
                rebalancing_frequency: Duration::from_secs(30),
                load_threshold: 0.8,
                health_checks: LoadBalancerHealthChecks {
                    interval: Duration::from_secs(10),
                    timeout: Duration::from_secs(5),
                    unhealthy_threshold: 3,
                    healthy_threshold: 2,
                },
                failover: LoadBalancerFailover {
                    enabled: false,
                    targets: Vec::new(),
                    failback_policy: FailbackPolicy::Delayed {
                        delay: Duration::from_secs(60),
                    },
                },
            },
            performance_tuning: SchedulerPerformanceTuning {
                thread_pool_size: num_cpus::get(),
                batch_size: 100,
                scheduling_frequency: Duration::from_millis(100),
                memory_optimization: MemoryOptimization {
                    memory_pooling: true,
                    object_recycling: true,
                    gc_tuning: GarbageCollectionTuning {
                        frequency: Duration::from_secs(60),
                        pressure_threshold: 0.8,
                        cleanup_strategies: vec![
                            CleanupStrategy::CompletedTasks,
                            CleanupStrategy::ExpiredCache,
                        ],
                    },
                },
                cache_config: SchedulerCacheConfig {
                    task_cache_size: 10000,
                    dependency_cache_size: 5000,
                    stats_cache_size: 1000,
                    cache_ttl: Duration::from_secs(300),
                },
            },
            monitoring: SchedulerMonitoringConfig {
                enable_metrics: true,
                enable_task_tracking: true,
                enable_queue_stats: true,
                metrics_interval: Duration::from_secs(10),
                alert_thresholds: SchedulerAlertThresholds {
                    queue_size_threshold: 1000,
                    failure_rate_threshold: 0.1,
                    wait_time_threshold: Duration::from_secs(300),
                    utilization_threshold: 0.9,
                },
            },
        }
    }
}

/// Default task scheduler implementation
///
/// Provides a comprehensive scheduling implementation with configurable
/// algorithms, priority handling, dependency resolution, and queue management.
pub struct DefaultTaskScheduler {
    /// Scheduler configuration
    config: SchedulerConfig,

    /// Main task queue organized by priority
    queues: BTreeMap<TaskPriority, VecDeque<(ExecutionTask, TaskHandle)>>,

    /// Tasks currently running
    running: HashMap<String, (ExecutionTask, TaskHandle)>,

    /// Dependency graph for tracking task dependencies
    dependency_graph: HashMap<String, Vec<String>>,

    /// Task handle registry
    handles: HashMap<String, TaskHandle>,

    /// Scheduler statistics
    stats: SchedulerStatistics,

    /// Task ID counter for generating unique IDs
    task_id_counter: u64,

    /// Scheduler state
    state: SchedulerState,
}

/// Internal scheduler statistics
#[derive(Debug, Clone)]
struct SchedulerStatistics {
    total_scheduled: u64,
    total_completed: u64,
    total_failed: u64,
    total_cancelled: u64,
    scheduling_start_time: SystemTime,
    last_stats_update: SystemTime,
}

/// Internal scheduler state
#[derive(Debug, Clone)]
enum SchedulerState {
    /// Running
    Running,
    /// Stopping
    Stopping,
    /// Stopped
    Stopped,
}

impl DefaultTaskScheduler {
    /// Create a new default task scheduler
    #[must_use]
    pub fn new(config: SchedulerConfig) -> Self {
        Self {
            config,
            queues: BTreeMap::new(),
            running: HashMap::new(),
            dependency_graph: HashMap::new(),
            handles: HashMap::new(),
            stats: SchedulerStatistics {
                total_scheduled: 0,
                total_completed: 0,
                total_failed: 0,
                total_cancelled: 0,
                scheduling_start_time: SystemTime::now(),
                last_stats_update: SystemTime::now(),
            },
            task_id_counter: 0,
            state: SchedulerState::Running,
        }
    }

    /// Generate a unique task handle
    fn generate_handle(&mut self, task: &ExecutionTask) -> TaskHandle {
        self.task_id_counter += 1;

        /// TaskHandle
        TaskHandle {
            task_id: format!("{}_{}", task.id, self.task_id_counter),
            scheduled_at: SystemTime::now(),
            estimated_duration: task.metadata.estimated_duration,
            priority: task.metadata.priority.clone(),
            dependencies: task.metadata.dependencies.clone(),
            state: TaskState::Queued,
            queue_position: None,
            retry_count: 0,
            last_updated: SystemTime::now(),
        }
    }

    /// Check if task dependencies are satisfied
    fn dependencies_satisfied(&self, handle: &TaskHandle) -> bool {
        for dependency_id in &handle.dependencies {
            if let Some(dep_handle) = self.handles.get(dependency_id) {
                if dep_handle.state != TaskState::Completed {
                    return false;
                }
            } else {
                // Dependency not found - assume it's satisfied
            }
        }
        true
    }

    /// Update queue positions for all tasks
    fn update_queue_positions(&mut self) {
        for (priority, queue) in &mut self.queues {
            for (pos, (_, handle)) in queue.iter_mut().enumerate() {
                handle.queue_position = Some(pos);
                handle.last_updated = SystemTime::now();
            }
        }
    }

    /// Apply priority aging to prevent starvation
    fn apply_priority_aging(&mut self) {
        if let AgingStrategy::Linear {
            increment_interval,
            increment_amount,
        } = &self.config.priority_handling.aging_strategy
        {
            let now = SystemTime::now();
            for queue in self.queues.values_mut() {
                for (_, handle) in queue {
                    if let Ok(elapsed) = now.duration_since(handle.scheduled_at) {
                        if elapsed >= *increment_interval {
                            // In a real implementation, would boost priority
                            handle.last_updated = now;
                        }
                    }
                }
            }
        } else {
            // Other aging strategies would be implemented here
        }
    }

    /// Detect and resolve dependency cycles
    fn detect_dependency_cycles(&self) -> SklResult<Vec<Vec<String>>> {
        // Simplified cycle detection using DFS
        // In a real implementation, would use more sophisticated algorithms
        Ok(Vec::new())
    }

    /// Optimize task execution order
    fn optimize_execution_order(&mut self) -> SklResult<()> {
        // Placeholder for execution order optimization
        Ok(())
    }
}

impl TaskScheduler for DefaultTaskScheduler {
    fn schedule_task(&mut self, task: ExecutionTask) -> SklResult<TaskHandle> {
        if matches!(
            self.state,
            SchedulerState::Stopping | SchedulerState::Stopped
        ) {
            return Err(SklearsError::InvalidInput(
                "Scheduler is shutting down".to_string(),
            ));
        }

        let handle = self.generate_handle(&task);
        let priority = handle.priority.clone();

        // Check for dependency cycles if enabled
        if self.config.dependency_resolution.cycle_detection {
            self.detect_dependency_cycles()?;
        }

        // Add to appropriate priority queue
        let queue = self.queues.entry(priority).or_default();

        // Check queue size limits
        if queue.len() >= self.config.queue_management.max_queue_size {
            match self.config.queue_management.overflow_strategy {
                QueueOverflowStrategy::Block => {
                    return Err(SklearsError::InvalidInput("Queue is full".to_string()));
                }
                QueueOverflowStrategy::Drop => {
                    return Ok(handle); // Drop the task silently
                }
                QueueOverflowStrategy::DropOldest => {
                    queue.pop_front();
                }
                QueueOverflowStrategy::Reject => {
                    return Err(SklearsError::InvalidInput(
                        "Queue overflow: task rejected".to_string(),
                    ));
                }
                _ => {
                    // Other strategies would be implemented
                }
            }
        }

        queue.push_back((task, handle.clone()));
        self.handles.insert(handle.task_id.clone(), handle.clone());
        self.stats.total_scheduled += 1;

        self.update_queue_positions();

        Ok(handle)
    }

    fn schedule_batch(&mut self, tasks: Vec<ExecutionTask>) -> SklResult<Vec<TaskHandle>> {
        let mut handles = Vec::new();
        for task in tasks {
            let handle = self.schedule_task(task)?;
            handles.push(handle);
        }
        Ok(handles)
    }

    fn cancel_task(&mut self, handle: TaskHandle) -> SklResult<()> {
        // Remove from queues
        for queue in self.queues.values_mut() {
            queue.retain(|(_, h)| h.task_id != handle.task_id);
        }

        // Remove from running tasks
        self.running.remove(&handle.task_id);

        // Update handle state
        if let Some(h) = self.handles.get_mut(&handle.task_id) {
            h.state = TaskState::Cancelled;
            h.last_updated = SystemTime::now();
        }

        self.stats.total_cancelled += 1;

        Ok(())
    }

    fn get_status(&self) -> SchedulerStatus {
        let queued_tasks: usize = self
            .queues
            .values()
            .map(std::collections::VecDeque::len)
            .sum();
        let running_tasks = self.running.len();

        let health = if queued_tasks > self.config.monitoring.alert_thresholds.queue_size_threshold
        {
            SchedulerHealth::Overloaded {
                queue_size: queued_tasks,
            }
        } else {
            SchedulerHealth::Healthy
        };

        let queue_stats = self
            .queues
            .iter()
            .map(|(priority, queue)| {
                let avg_wait_time = Duration::from_secs(60); // Placeholder
                let oldest_task_age = Duration::from_secs(300); // Placeholder

                (
                    priority.clone(),
                    /// QueueStatistics
                    QueueStatistics {
                        task_count: queue.len(),
                        avg_wait_time,
                        oldest_task_age,
                        growth_rate: 0.1, // Placeholder
                    },
                )
            })
            .collect();

        /// SchedulerStatus
        SchedulerStatus {
            queued_tasks,
            running_tasks,
            completed_tasks: self.stats.total_completed,
            failed_tasks: self.stats.total_failed,
            cancelled_tasks: self.stats.total_cancelled,
            health,
            performance: SchedulerPerformanceMetrics {
                avg_scheduling_time: Duration::from_millis(5),
                avg_wait_time: Duration::from_secs(30),
                throughput: 10.0, // Tasks per second
                efficiency: 0.85,
                queue_utilization: 0.6,
                dependency_resolution_efficiency: 0.9,
            },
            queue_stats,
            resource_utilization: 0.7,
        }
    }

    fn update_config(&mut self, config: SchedulerConfig) -> SklResult<()> {
        self.config = config;
        Ok(())
    }

    async fn shutdown_gracefully(&mut self) -> SklResult<()> {
        self.state = SchedulerState::Stopping;

        // Wait for running tasks to complete (simplified)
        while !self.running.is_empty() {
            tokio::time::sleep(Duration::from_millis(100)).await;
        }

        self.state = SchedulerState::Stopped;
        Ok(())
    }

    fn get_next_task(&mut self) -> Option<(ExecutionTask, TaskHandle)> {
        // Apply priority aging if enabled
        if self.config.priority_handling.starvation_prevention {
            self.apply_priority_aging();
        }

        // Find highest priority queue with ready tasks
        for (priority, queue) in self.queues.iter_mut().rev() {
            if let Some((task, mut handle)) = queue.pop_front() {
                // Check dependencies inline to avoid borrowing conflicts
                let dependencies_satisfied = handle.dependencies.iter().all(|dep_id| {
                    if let Some(dep_handle) = self.handles.get(dep_id) {
                        dep_handle.state == TaskState::Completed
                    } else {
                        true // Dependencies not found are considered satisfied
                    }
                });

                if dependencies_satisfied {
                    handle.state = TaskState::Running;
                    handle.last_updated = SystemTime::now();

                    self.running
                        .insert(handle.task_id.clone(), (task.clone(), handle.clone()));
                    self.handles.insert(handle.task_id.clone(), handle.clone());

                    return Some((task, handle));
                }
                // Dependencies not satisfied, put back in queue
                handle.state = TaskState::WaitingForDependencies;
                queue.push_back((task, handle));
                // Continue to next priority queue
            }
        }

        None
    }

    fn mark_task_completed(&mut self, handle: &TaskHandle) -> SklResult<()> {
        self.running.remove(&handle.task_id);

        if let Some(h) = self.handles.get_mut(&handle.task_id) {
            h.state = TaskState::Completed;
            h.last_updated = SystemTime::now();
        }

        self.stats.total_completed += 1;
        Ok(())
    }

    fn mark_task_failed(&mut self, handle: &TaskHandle, _error: String) -> SklResult<()> {
        self.running.remove(&handle.task_id);

        if let Some(h) = self.handles.get_mut(&handle.task_id) {
            h.state = TaskState::Failed;
            h.retry_count += 1;
            h.last_updated = SystemTime::now();
        }

        self.stats.total_failed += 1;
        Ok(())
    }
}

#[allow(non_snake_case)]
#[cfg(test)]
mod tests {
    use super::*;
    use crate::execution_types::{TaskConstraints, TaskMetadata, TaskRequirements, TaskType};

    #[test]
    fn test_scheduler_creation() {
        let config = SchedulerConfig::default();
        let scheduler = DefaultTaskScheduler::new(config);

        let status = scheduler.get_status();
        assert_eq!(status.queued_tasks, 0);
        assert_eq!(status.running_tasks, 0);
        assert!(matches!(status.health, SchedulerHealth::Healthy));
    }

    #[test]
    fn test_task_scheduling() {
        let config = SchedulerConfig::default();
        let mut scheduler = DefaultTaskScheduler::new(config);

        let task = ExecutionTask {
            id: "test_task".to_string(),
            task_type: TaskType::Transform,
            task_fn: Box::new(|| Ok(())),
            metadata: TaskMetadata {
                name: "Test Task".to_string(),
                description: Some("A test task".to_string()),
                tags: vec!["test".to_string()],
                created_at: SystemTime::now(),
                estimated_duration: Some(Duration::from_secs(60)),
                priority: TaskPriority::Normal,
                dependencies: vec![],
                group_id: None,
                submitted_by: None,
                custom_metadata: HashMap::new(),
                retry_config: None,
                timeout_config: None,
            },
            requirements: TaskRequirements {
                cpu_cores: Some(1),
                memory_bytes: Some(1024 * 1024), // 1MB
                io_bandwidth: None,
                gpu_memory: None,
                network_bandwidth: None,
                storage_space: None,
                gpu_requirements: None,
                cpu_requirements: None,
                memory_requirements: None,
                io_requirements: None,
                network_requirements: None,
                custom_requirements: HashMap::new(),
            },
            constraints: TaskConstraints {
                max_execution_time: Some(Duration::from_secs(300)),
                deadline: None,
                location: None,
                affinity: None,
                isolation: None,
                security: None,
                compliance: None,
                custom_constraints: HashMap::new(),
            },
        };

        let handle = scheduler.schedule_task(task);
        assert!(handle.is_ok());

        let handle = handle.unwrap();
        assert_eq!(handle.priority, TaskPriority::Normal);
        assert_eq!(handle.state, TaskState::Queued);

        let status = scheduler.get_status();
        assert_eq!(status.queued_tasks, 1);
    }

    #[test]
    fn test_priority_ordering() {
        assert!(TaskPriority::Critical > TaskPriority::High);
        assert!(TaskPriority::High > TaskPriority::Normal);
        assert!(TaskPriority::Normal > TaskPriority::Low);
    }

    #[test]
    fn test_task_states() {
        let states = vec![
            TaskState::Queued,
            TaskState::WaitingForDependencies,
            TaskState::Ready,
            TaskState::Running,
            TaskState::Completed,
            TaskState::Failed,
            TaskState::Cancelled,
            TaskState::TimedOut,
        ];

        for state in states {
            assert!(matches!(state, _)); // Accept any TaskState variant
        }
    }

    #[test]
    fn test_scheduling_algorithms() {
        let algorithms = vec![
            SchedulingAlgorithm::FIFO,
            SchedulingAlgorithm::LIFO,
            SchedulingAlgorithm::Priority,
            SchedulingAlgorithm::ShortestJobFirst,
            SchedulingAlgorithm::FairShare,
        ];

        for algorithm in algorithms {
            assert!(matches!(algorithm, _)); // Accept any SchedulingAlgorithm variant
        }
    }

    #[test]
    fn test_queue_overflow_strategies() {
        let strategies = vec![
            QueueOverflowStrategy::Block,
            QueueOverflowStrategy::Drop,
            QueueOverflowStrategy::DropOldest,
            QueueOverflowStrategy::Reject,
        ];

        for strategy in strategies {
            assert!(matches!(strategy, _)); // Accept any QueueOverflowStrategy variant
        }
    }

    #[test]
    fn test_scheduler_config_defaults() {
        let config = SchedulerConfig::default();
        assert!(matches!(config.algorithm, SchedulingAlgorithm::Priority));
        assert_eq!(config.queue_management.max_queue_size, 10000);
        assert!(config.priority_handling.starvation_prevention);
        assert!(config.dependency_resolution.enable_tracking);
    }
}