quantrs2_sim/

distributed_simulator.rs

//! Distributed Quantum Simulator for Large-Scale Problems
//!
//! This module provides a distributed quantum simulator that can coordinate
//! across multiple compute nodes to enable simulation of extremely large
//! quantum circuits (50+ qubits) through state distribution, work partitioning,
//! and advanced `SciRS2` distributed computing integration.

use crate::large_scale_simulator::{
    LargeScaleQuantumSimulator, LargeScaleSimulatorConfig, MemoryStatistics,
    QuantumStateRepresentation,
};
use quantrs2_circuit::builder::{Circuit, Simulator};
use quantrs2_core::{
    error::{QuantRS2Error, QuantRS2Result},
    gate::GateOp,
    platform::PlatformCapabilities,
    qubit::QubitId,
};
// use scirs2_core::distributed::*;
// use scirs2_core::communication::{MessagePassing, NetworkTopology};
// use scirs2_core::load_balancing::{LoadBalancer, WorkDistribution};
use scirs2_core::ndarray::{Array1, Array2, ArrayView1, Axis};
use scirs2_core::parallel_ops::{IndexedParallelIterator, ParallelIterator}; // SciRS2 POLICY compliant
use scirs2_core::Complex64;
use serde::{Deserialize, Serialize};
use std::collections::{BTreeMap, HashMap, VecDeque};
use std::io::{BufReader, BufWriter, Read, Write};
use std::net::{SocketAddr, TcpListener, TcpStream};
use std::sync::{Arc, Barrier, Mutex, RwLock};
use std::thread;
use std::time::{Duration, Instant};
use uuid::Uuid;

/// Configuration for distributed quantum simulation
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct DistributedSimulatorConfig {
    /// Configuration for local large-scale simulator
    pub local_config: LargeScaleSimulatorConfig,

    /// Network configuration for cluster communication
    pub network_config: NetworkConfig,

    /// Load balancing configuration
    pub load_balancing_config: LoadBalancingConfig,

    /// Fault tolerance configuration
    pub fault_tolerance_config: FaultToleranceConfig,

    /// State distribution strategy
    pub distribution_strategy: DistributionStrategy,

    /// Communication optimization settings
    pub communication_config: CommunicationConfig,

    /// Enable automatic cluster discovery
    pub enable_auto_discovery: bool,

    /// Maximum simulation size (total qubits across cluster)
    pub max_distributed_qubits: usize,

    /// Minimum qubits per node for efficient distribution
    pub min_qubits_per_node: usize,
}

impl Default for DistributedSimulatorConfig {
    fn default() -> Self {
        Self {
            local_config: LargeScaleSimulatorConfig::default(),
            network_config: NetworkConfig::default(),
            load_balancing_config: LoadBalancingConfig::default(),
            fault_tolerance_config: FaultToleranceConfig::default(),
            distribution_strategy: DistributionStrategy::Amplitude,
            communication_config: CommunicationConfig::default(),
            enable_auto_discovery: true,
            max_distributed_qubits: 100, // Up to 100 qubits across cluster
            min_qubits_per_node: 8,
        }
    }
}

/// Network configuration for cluster communication
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct NetworkConfig {
    /// Local node address
    pub local_address: SocketAddr,

    /// List of known cluster nodes
    pub cluster_nodes: Vec<SocketAddr>,

    /// Communication timeout
    pub communication_timeout: Duration,

    /// Maximum message size
    pub max_message_size: usize,

    /// Enable compression for network messages
    pub enable_compression: bool,

    /// Network buffer size
    pub network_buffer_size: usize,
}

impl Default for NetworkConfig {
    fn default() -> Self {
        Self {
            // Safety: "127.0.0.1:8080" is a valid socket address format
            local_address: "127.0.0.1:8080"
                .parse()
                .expect("Valid default socket address"),
            cluster_nodes: vec![],
            communication_timeout: Duration::from_secs(30),
            max_message_size: 64 * 1024 * 1024, // 64MB
            enable_compression: true,
            network_buffer_size: 1024 * 1024, // 1MB
        }
    }
}

/// Load balancing configuration
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct LoadBalancingConfig {
    /// Load balancing strategy
    pub strategy: LoadBalancingStrategy,

    /// Rebalancing threshold (load imbalance percentage)
    pub rebalancing_threshold: f64,

    /// Enable dynamic load balancing
    pub enable_dynamic_balancing: bool,

    /// Load monitoring interval
    pub monitoring_interval: Duration,

    /// Maximum work migration per rebalancing
    pub max_migration_percentage: f64,
}

impl Default for LoadBalancingConfig {
    fn default() -> Self {
        Self {
            strategy: LoadBalancingStrategy::WorkStealing,
            rebalancing_threshold: 0.2, // 20% imbalance triggers rebalancing
            enable_dynamic_balancing: true,
            monitoring_interval: Duration::from_secs(5),
            max_migration_percentage: 0.1, // Migrate up to 10% of work
        }
    }
}

/// Fault tolerance configuration
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct FaultToleranceConfig {
    /// Enable checkpointing
    pub enable_checkpointing: bool,

    /// Checkpoint interval
    pub checkpoint_interval: Duration,

    /// Enable redundant computation
    pub enable_redundancy: bool,

    /// Redundancy factor (number of replicas)
    pub redundancy_factor: usize,

    /// Node failure detection timeout
    pub failure_detection_timeout: Duration,

    /// Maximum retries for failed operations
    pub max_retries: usize,
}

impl Default for FaultToleranceConfig {
    fn default() -> Self {
        Self {
            enable_checkpointing: true,
            checkpoint_interval: Duration::from_secs(60),
            enable_redundancy: false,
            redundancy_factor: 2,
            failure_detection_timeout: Duration::from_secs(10),
            max_retries: 3,
        }
    }
}

/// Communication optimization configuration
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct CommunicationConfig {
    /// Enable message batching
    pub enable_batching: bool,

    /// Batch size for messages
    pub batch_size: usize,

    /// Enable asynchronous communication
    pub enable_async_communication: bool,

    /// Communication pattern optimization
    pub communication_pattern: CommunicationPattern,

    /// Enable overlap of computation and communication
    pub enable_overlap: bool,
}

impl Default for CommunicationConfig {
    fn default() -> Self {
        Self {
            enable_batching: true,
            batch_size: 100,
            enable_async_communication: true,
            communication_pattern: CommunicationPattern::AllToAll,
            enable_overlap: true,
        }
    }
}

/// State distribution strategies
#[derive(Debug, Clone, Copy, Serialize, Deserialize)]
pub enum DistributionStrategy {
    /// Distribute by amplitude indices
    Amplitude,
    /// Distribute by qubit partitions
    QubitPartition,
    /// Hybrid distribution based on circuit structure
    Hybrid,
    /// Custom distribution based on `SciRS2` graph partitioning
    GraphPartition,
}

/// Load balancing strategies
#[derive(Debug, Clone, Copy, Serialize, Deserialize)]
pub enum LoadBalancingStrategy {
    /// Static round-robin distribution
    RoundRobin,
    /// Dynamic work stealing
    WorkStealing,
    /// Load-aware distribution
    LoadAware,
    /// Performance-based distribution
    PerformanceBased,
}

/// Communication patterns
#[derive(Debug, Clone, Copy, Serialize, Deserialize)]
pub enum CommunicationPattern {
    /// All-to-all communication
    AllToAll,
    /// Point-to-point communication
    PointToPoint,
    /// Hierarchical communication
    Hierarchical,
    /// Tree-based communication
    Tree,
}

/// Node information in the cluster
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct NodeInfo {
    /// Unique node identifier
    pub node_id: Uuid,

    /// Node network address
    pub address: SocketAddr,

    /// Node capabilities
    pub capabilities: NodeCapabilities,

    /// Current node status
    pub status: NodeStatus,

    /// Last heartbeat timestamp (as milliseconds since epoch)
    #[serde(with = "instant_serde")]
    pub last_heartbeat: Instant,

    /// Current workload
    pub current_load: f64,
}

/// Serde serialization helpers for Instant
mod instant_serde {
    use serde::{Deserialize, Deserializer, Serialize, Serializer};
    use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};

    pub fn serialize<S>(instant: &Instant, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        // Convert to system time for serialization
        let system_time = SystemTime::now();
        // Safety: SystemTime::now() is always after UNIX_EPOCH on modern systems
        let duration_since_epoch = system_time
            .duration_since(UNIX_EPOCH)
            .expect("System time is after UNIX_EPOCH");
        duration_since_epoch.as_millis().serialize(serializer)
    }

    pub fn deserialize<'de, D>(deserializer: D) -> Result<Instant, D::Error>
    where
        D: Deserializer<'de>,
    {
        let millis = u128::deserialize(deserializer)?;
        // Return current instant for simplicity
        Ok(Instant::now())
    }
}

/// Node capabilities
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct NodeCapabilities {
    /// Available memory in bytes
    pub available_memory: usize,

    /// Number of CPU cores
    pub cpu_cores: usize,

    /// CPU frequency in GHz
    pub cpu_frequency: f64,

    /// Network bandwidth in Mbps
    pub network_bandwidth: f64,

    /// Has GPU acceleration
    pub has_gpu: bool,

    /// Maximum qubits this node can handle
    pub max_qubits: usize,
}

/// Node status
#[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq, Eq)]
pub enum NodeStatus {
    /// Node is active and available
    Active,
    /// Node is busy with computation
    Busy,
    /// Node is unavailable or failed
    Unavailable,
    /// Node is in maintenance mode
    Maintenance,
}

/// Distributed quantum state chunk
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct StateChunk {
    /// Chunk identifier
    pub chunk_id: Uuid,

    /// Amplitude indices this chunk contains
    pub amplitude_range: (usize, usize),

    /// Qubit indices this chunk is responsible for
    pub qubit_indices: Vec<usize>,

    /// Actual state data
    pub amplitudes: Vec<Complex64>,

    /// Node responsible for this chunk
    pub owner_node: Uuid,

    /// Backup nodes for redundancy
    pub backup_nodes: Vec<Uuid>,

    /// Chunk metadata
    pub metadata: ChunkMetadata,
}

/// Metadata for state chunks
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ChunkMetadata {
    /// Size of chunk in bytes
    pub size_bytes: usize,

    /// Compression ratio achieved
    pub compression_ratio: f64,

    /// Last access timestamp
    #[serde(with = "instant_serde")]
    pub last_access: Instant,

    /// Access frequency
    pub access_count: usize,

    /// Whether chunk is cached locally
    pub is_cached: bool,
}

/// Distributed gate operation
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct DistributedGateOperation {
    /// Operation identifier
    pub operation_id: Uuid,

    /// Target qubits
    pub target_qubits: Vec<QubitId>,

    /// Nodes affected by this operation
    pub affected_nodes: Vec<Uuid>,

    /// Communication requirements
    pub communication_requirements: CommunicationRequirements,

    /// Operation priority
    pub priority: OperationPriority,
}

/// Communication requirements for operations
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct CommunicationRequirements {
    /// Amount of data to be communicated
    pub data_size: usize,

    /// Communication pattern required
    pub pattern: CommunicationPattern,

    /// Synchronization requirements
    pub synchronization_level: SynchronizationLevel,

    /// Estimated communication time
    pub estimated_time: Duration,
}

/// Operation priority levels
#[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq, Eq, PartialOrd, Ord)]
pub enum OperationPriority {
    /// Low priority operations
    Low = 0,
    /// Normal priority operations
    Normal = 1,
    /// High priority operations
    High = 2,
    /// Critical priority operations
    Critical = 3,
}

/// Synchronization levels
#[derive(Debug, Clone, Copy, Serialize, Deserialize)]
pub enum SynchronizationLevel {
    /// No synchronization required
    None,
    /// Weak synchronization
    Weak,
    /// Strong synchronization required
    Strong,
    /// Global barrier synchronization
    Barrier,
}

/// Performance statistics for distributed simulation
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct DistributedPerformanceStats {
    /// Total simulation time
    pub total_time: Duration,

    /// Communication overhead
    pub communication_overhead: f64,

    /// Load balancing efficiency
    pub load_balance_efficiency: f64,

    /// Network utilization statistics
    pub network_stats: NetworkStats,

    /// Per-node performance statistics
    pub node_stats: HashMap<Uuid, NodePerformanceStats>,

    /// Fault tolerance statistics
    pub fault_tolerance_stats: FaultToleranceStats,
}

/// Network performance statistics
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct NetworkStats {
    /// Total bytes transmitted
    pub bytes_transmitted: usize,

    /// Total bytes received
    pub bytes_received: usize,

    /// Average message latency
    pub average_latency: Duration,

    /// Peak bandwidth utilization
    pub peak_bandwidth: f64,

    /// Number of failed communications
    pub failed_communications: usize,
}

/// Per-node performance statistics
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct NodePerformanceStats {
    /// CPU utilization percentage
    pub cpu_utilization: f64,

    /// Memory utilization percentage
    pub memory_utilization: f64,

    /// Network I/O statistics
    pub network_io: (usize, usize), // (bytes_sent, bytes_received)

    /// Number of operations processed
    pub operations_processed: usize,

    /// Average operation time
    pub average_operation_time: Duration,

    /// Number of state chunk migrations
    pub chunk_migrations: usize,
}

/// Fault tolerance statistics
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct FaultToleranceStats {
    /// Number of node failures detected
    pub node_failures: usize,

    /// Number of successful recoveries
    pub successful_recoveries: usize,

    /// Number of checkpoints created
    pub checkpoints_created: usize,

    /// Time spent on fault tolerance overhead
    pub fault_tolerance_overhead: Duration,

    /// Data redundancy overhead
    pub redundancy_overhead: f64,
}

/// Distributed quantum simulator
#[derive(Debug)]
pub struct DistributedQuantumSimulator {
    /// Configuration for distributed simulation
    config: DistributedSimulatorConfig,

    /// Local large-scale simulator
    local_simulator: LargeScaleQuantumSimulator,

    /// Information about cluster nodes
    cluster_nodes: Arc<RwLock<HashMap<Uuid, NodeInfo>>>,

    /// Local node information
    local_node: NodeInfo,

    /// Distributed state chunks
    state_chunks: Arc<RwLock<HashMap<Uuid, StateChunk>>>,

    /// Operation queue for distributed execution
    operation_queue: Arc<Mutex<VecDeque<DistributedGateOperation>>>,

    /// Performance statistics
    performance_stats: Arc<Mutex<DistributedPerformanceStats>>,

    /// Network communication manager
    communication_manager: Arc<Mutex<CommunicationManager>>,

    /// Load balancer
    load_balancer: Arc<Mutex<LoadBalancer>>,

    /// Current simulation state
    simulation_state: Arc<RwLock<SimulationState>>,
}

/// Communication manager for network operations
#[derive(Debug)]
pub struct CommunicationManager {
    /// Local network address
    local_address: SocketAddr,

    /// Active connections to other nodes
    connections: HashMap<Uuid, TcpStream>,

    /// Message queue for outgoing messages
    outgoing_queue: VecDeque<NetworkMessage>,

    /// Message queue for incoming messages
    incoming_queue: VecDeque<NetworkMessage>,

    /// Communication statistics
    stats: NetworkStats,
}

/// Load balancer for work distribution
#[derive(Debug)]
pub struct LoadBalancer {
    /// Current load balancing strategy
    strategy: LoadBalancingStrategy,

    /// Node load information
    node_loads: HashMap<Uuid, f64>,

    /// Work distribution history
    distribution_history: VecDeque<WorkDistribution>,

    /// Rebalancing statistics
    rebalancing_stats: RebalancingStats,
}

/// Work distribution information
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct WorkDistribution {
    /// Timestamp of distribution
    #[serde(with = "instant_serde")]
    pub timestamp: Instant,

    /// Node work assignments
    pub node_assignments: HashMap<Uuid, f64>,

    /// Load balance efficiency
    pub efficiency: f64,
}

/// Rebalancing statistics
#[derive(Debug, Clone, Default)]
pub struct RebalancingStats {
    /// Number of rebalancing operations
    pub rebalancing_count: usize,

    /// Total time spent rebalancing
    pub total_rebalancing_time: Duration,

    /// Average efficiency improvement
    pub average_efficiency_improvement: f64,
}

/// Network message for distributed communication
#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum NetworkMessage {
    /// Heartbeat message
    Heartbeat {
        sender: Uuid,
        #[serde(with = "instant_serde")]
        timestamp: Instant,
        load: f64,
    },

    /// State chunk transfer
    StateChunkTransfer {
        chunk: StateChunk,
        destination: Uuid,
    },

    /// Gate operation request
    GateOperation {
        operation: DistributedGateOperation,
        data: Vec<u8>,
    },

    /// Synchronization barrier
    SynchronizationBarrier {
        barrier_id: Uuid,
        participants: Vec<Uuid>,
    },

    /// Load balancing command
    LoadBalancing { command: LoadBalancingCommand },

    /// Fault tolerance message
    FaultTolerance { message_type: FaultToleranceMessage },
}

/// Load balancing commands
#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum LoadBalancingCommand {
    /// Request work migration
    MigrateWork {
        source_node: Uuid,
        target_node: Uuid,
        work_amount: f64,
    },

    /// Update load information
    UpdateLoad { node_id: Uuid, current_load: f64 },

    /// Trigger rebalancing
    TriggerRebalancing,
}

/// Fault tolerance messages
#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum FaultToleranceMessage {
    /// Node failure notification
    NodeFailure {
        failed_node: Uuid,
        #[serde(with = "instant_serde")]
        timestamp: Instant,
    },

    /// Checkpoint request
    CheckpointRequest { checkpoint_id: Uuid },

    /// Recovery initiation
    RecoveryInitiation {
        failed_node: Uuid,
        backup_nodes: Vec<Uuid>,
    },
}

/// Current simulation state
#[derive(Debug, Clone)]
pub enum SimulationState {
    /// Simulation is initializing
    Initializing,

    /// Simulation is running
    Running {
        current_step: usize,
        total_steps: usize,
    },

    /// Simulation is paused
    Paused { at_step: usize },

    /// Simulation completed successfully
    Completed {
        final_state: Vec<Complex64>,
        stats: DistributedPerformanceStats,
    },

    /// Simulation failed
    Failed { error: String, at_step: usize },
}

impl DistributedQuantumSimulator {
    /// Create a new distributed quantum simulator
    pub fn new(config: DistributedSimulatorConfig) -> QuantRS2Result<Self> {
        let local_simulator = LargeScaleQuantumSimulator::new(config.local_config.clone())?;

        let local_node = NodeInfo {
            node_id: Uuid::new_v4(),
            address: config.network_config.local_address,
            capabilities: Self::detect_local_capabilities()?,
            status: NodeStatus::Active,
            last_heartbeat: Instant::now(),
            current_load: 0.0,
        };

        let communication_manager = CommunicationManager::new(config.network_config.local_address)?;
        let load_balancer = LoadBalancer::new(config.load_balancing_config.strategy);

        Ok(Self {
            config,
            local_simulator,
            cluster_nodes: Arc::new(RwLock::new(HashMap::new())),
            local_node,
            state_chunks: Arc::new(RwLock::new(HashMap::new())),
            operation_queue: Arc::new(Mutex::new(VecDeque::new())),
            performance_stats: Arc::new(Mutex::new(Self::initialize_performance_stats())),
            communication_manager: Arc::new(Mutex::new(communication_manager)),
            load_balancer: Arc::new(Mutex::new(load_balancer)),
            simulation_state: Arc::new(RwLock::new(SimulationState::Initializing)),
        })
    }

    /// Initialize the distributed cluster
    pub fn initialize_cluster(&mut self) -> QuantRS2Result<()> {
        // Discover cluster nodes if auto-discovery is enabled
        if self.config.enable_auto_discovery {
            self.discover_cluster_nodes()?;
        }

        // Establish connections to known nodes
        self.establish_connections()?;

        // Start background services
        self.start_background_services()?;

        Ok(())
    }

    /// Simulate a quantum circuit across the distributed cluster
    pub fn simulate_circuit<const N: usize>(
        &mut self,
        circuit: &Circuit<N>,
    ) -> QuantRS2Result<Vec<Complex64>> {
        let start_time = Instant::now();

        // Update simulation state
        {
            let mut state = self
                .simulation_state
                .write()
                .map_err(|e| QuantRS2Error::RuntimeError(format!("Lock poisoned: {e}")))?;
            *state = SimulationState::Running {
                current_step: 0,
                total_steps: circuit.num_gates(),
            };
        }

        // Distribute initial quantum state
        self.distribute_initial_state(circuit.num_qubits())?;

        // Execute circuit gates in distributed manner
        let gates = circuit.gates();
        for (step, gate) in gates.iter().enumerate() {
            self.execute_distributed_gate(gate, step)?;

            // Update progress
            {
                let mut state = self
                    .simulation_state
                    .write()
                    .map_err(|e| QuantRS2Error::RuntimeError(format!("Lock poisoned: {e}")))?;
                if let SimulationState::Running {
                    current_step,
                    total_steps,
                } = &mut *state
                {
                    *current_step = step + 1;
                }
            }
        }

        // Collect final state from all nodes
        let final_state = self.collect_final_state()?;

        // Update performance statistics
        let simulation_time = start_time.elapsed();
        self.update_performance_stats(simulation_time)?;

        // Update simulation state to completed
        {
            let mut state = self
                .simulation_state
                .write()
                .map_err(|e| QuantRS2Error::RuntimeError(format!("Lock poisoned: {e}")))?;
            let stats = self
                .performance_stats
                .lock()
                .map_err(|e| QuantRS2Error::RuntimeError(format!("Lock poisoned: {e}")))?
                .clone();
            *state = SimulationState::Completed {
                final_state: final_state.clone(),
                stats,
            };
        }

        Ok(final_state)
    }

    /// Get current simulation statistics
    #[must_use]
    pub fn get_statistics(&self) -> DistributedPerformanceStats {
        self.performance_stats
            .lock()
            .expect("Performance stats lock poisoned")
            .clone()
    }

    /// Get cluster status information
    #[must_use]
    pub fn get_cluster_status(&self) -> HashMap<Uuid, NodeInfo> {
        self.cluster_nodes
            .read()
            .expect("Cluster nodes lock poisoned")
            .clone()
    }

    /// Detect local node capabilities
    fn detect_local_capabilities() -> QuantRS2Result<NodeCapabilities> {
        // Use comprehensive platform detection
        let platform_caps = PlatformCapabilities::detect();

        let available_memory = platform_caps.memory.available_memory;
        let cpu_cores = platform_caps.cpu.logical_cores;
        let cpu_frequency = f64::from(platform_caps.cpu.base_clock_mhz.unwrap_or(3000.0)) / 1000.0; // Convert MHz to GHz
        let network_bandwidth = Self::detect_network_bandwidth(); // Keep network detection as-is
        let has_gpu = platform_caps.has_gpu();

        let max_qubits = Self::calculate_max_qubits(available_memory);

        Ok(NodeCapabilities {
            available_memory,
            cpu_cores,
            cpu_frequency,
            network_bandwidth,
            has_gpu,
            max_qubits,
        })
    }

    /// Detect available system memory
    const fn detect_available_memory() -> usize {
        // Simple heuristic: assume 80% of system memory is available
        8 * 1024 * 1024 * 1024 // 8GB default
    }

    /// Detect CPU frequency
    const fn detect_cpu_frequency() -> f64 {
        3.0 // 3GHz default
    }

    /// Detect network bandwidth
    const fn detect_network_bandwidth() -> f64 {
        1000.0 // 1Gbps default
    }

    /// Detect GPU availability
    const fn detect_gpu_availability() -> bool {
        false // Default to no GPU
    }

    /// Calculate maximum qubits based on available memory
    const fn calculate_max_qubits(available_memory: usize) -> usize {
        // Each qubit requires 2^n complex numbers (16 bytes each)
        // Calculate maximum qubits that fit in available memory
        let complex_size = 16; // bytes per Complex64
        let mut max_qubits: usize = 0;
        let mut required_memory = complex_size;

        while required_memory <= available_memory / 2 {
            max_qubits += 1;
            required_memory *= 2;
        }

        max_qubits.saturating_sub(1) // Leave some margin
    }

    /// Initialize performance statistics
    fn initialize_performance_stats() -> DistributedPerformanceStats {
        DistributedPerformanceStats {
            total_time: Duration::new(0, 0),
            communication_overhead: 0.0,
            load_balance_efficiency: 1.0,
            network_stats: NetworkStats {
                bytes_transmitted: 0,
                bytes_received: 0,
                average_latency: Duration::new(0, 0),
                peak_bandwidth: 0.0,
                failed_communications: 0,
            },
            node_stats: HashMap::new(),
            fault_tolerance_stats: FaultToleranceStats {
                node_failures: 0,
                successful_recoveries: 0,
                checkpoints_created: 0,
                fault_tolerance_overhead: Duration::new(0, 0),
                redundancy_overhead: 0.0,
            },
        }
    }

    /// Discover cluster nodes through network scanning
    fn discover_cluster_nodes(&self) -> QuantRS2Result<()> {
        // Implementation would scan network for other quantum simulator nodes
        // For now, use configured nodes
        for node_addr in &self.config.network_config.cluster_nodes {
            let node_info = NodeInfo {
                node_id: Uuid::new_v4(), // Would be obtained from node
                address: *node_addr,
                capabilities: NodeCapabilities {
                    available_memory: 8 * 1024 * 1024 * 1024,
                    cpu_cores: 8,
                    cpu_frequency: 3.0,
                    network_bandwidth: 1000.0,
                    has_gpu: false,
                    max_qubits: 30,
                },
                status: NodeStatus::Active,
                last_heartbeat: Instant::now(),
                current_load: 0.0,
            };

            self.cluster_nodes
                .write()
                .map_err(|e| QuantRS2Error::RuntimeError(format!("Lock poisoned: {e}")))?
                .insert(node_info.node_id, node_info);
        }

        Ok(())
    }

    /// Establish connections to cluster nodes
    const fn establish_connections(&self) -> QuantRS2Result<()> {
        // Implementation would establish TCP connections to other nodes
        Ok(())
    }

    /// Start background services (heartbeat, load balancing, etc.)
    const fn start_background_services(&self) -> QuantRS2Result<()> {
        // Implementation would start background threads for:
        // - Heartbeat monitoring
        // - Load balancing
        // - Fault detection
        // - Communication management
        Ok(())
    }

    /// Distribute initial quantum state across cluster
    fn distribute_initial_state(&self, num_qubits: usize) -> QuantRS2Result<()> {
        let state_size: usize = 1 << num_qubits;
        let cluster_nodes_guard = self
            .cluster_nodes
            .read()
            .map_err(|e| QuantRS2Error::RuntimeError(format!("Lock poisoned: {e}")))?;
        let num_nodes: usize = cluster_nodes_guard.len() + 1; // +1 for local node

        // Calculate chunk size per node
        let chunk_size = state_size.div_ceil(num_nodes);

        // Collect node keys for indexing
        let node_keys: Vec<Uuid> = cluster_nodes_guard.keys().copied().collect();
        drop(cluster_nodes_guard); // Release the read lock

        // Create state chunks
        let mut chunks = Vec::new();
        for i in 0..num_nodes {
            let start_index = i * chunk_size;
            let end_index = ((i + 1) * chunk_size).min(state_size);

            if start_index < end_index {
                let owner_node = if i == 0 {
                    self.local_node.node_id
                } else {
                    // Safety: i > 0 and i-1 < node_keys.len() since num_nodes = node_keys.len() + 1
                    node_keys.get(i - 1).copied().ok_or_else(|| {
                        QuantRS2Error::InvalidInput("Node index out of bounds".to_string())
                    })?
                };

                let chunk = StateChunk {
                    chunk_id: Uuid::new_v4(),
                    amplitude_range: (start_index, end_index),
                    qubit_indices: (0..num_qubits).collect(),
                    amplitudes: vec![Complex64::new(0.0, 0.0); end_index - start_index],
                    owner_node,
                    backup_nodes: vec![],
                    metadata: ChunkMetadata {
                        size_bytes: (end_index - start_index) * 16,
                        compression_ratio: 1.0,
                        last_access: Instant::now(),
                        access_count: 0,
                        is_cached: i == 0,
                    },
                };

                chunks.push(chunk);
            }
        }

        // Initialize state: |00...0⟩
        if let Some(first_chunk) = chunks.first_mut() {
            if first_chunk.amplitude_range.0 == 0 {
                first_chunk.amplitudes[0] = Complex64::new(1.0, 0.0);
            }
        }

        // Store chunks
        let mut state_chunks_guard = self
            .state_chunks
            .write()
            .map_err(|e| QuantRS2Error::RuntimeError(format!("Lock poisoned: {e}")))?;
        for chunk in chunks {
            state_chunks_guard.insert(chunk.chunk_id, chunk);
        }

        Ok(())
    }

    /// Execute a gate operation in distributed manner
    fn execute_distributed_gate(
        &self,
        gate: &Arc<dyn GateOp + Send + Sync>,
        step: usize,
    ) -> QuantRS2Result<()> {
        // Determine which nodes are affected by this gate
        let affected_qubits = gate.qubits();
        let affected_nodes = self.find_affected_nodes(&affected_qubits)?;

        // Create distributed gate operation
        let operation = DistributedGateOperation {
            operation_id: Uuid::new_v4(),
            target_qubits: affected_qubits.clone(),
            affected_nodes,
            communication_requirements: self
                .calculate_communication_requirements(&affected_qubits)?,
            priority: OperationPriority::Normal,
        };

        // Execute operation based on distribution strategy
        match self.config.distribution_strategy {
            DistributionStrategy::Amplitude => {
                self.execute_amplitude_distributed_gate(gate, &operation)?;
            }
            DistributionStrategy::QubitPartition => {
                self.execute_qubit_partitioned_gate(gate, &operation)?;
            }
            DistributionStrategy::Hybrid => {
                self.execute_hybrid_distributed_gate(gate, &operation)?;
            }
            DistributionStrategy::GraphPartition => {
                self.execute_graph_partitioned_gate(gate, &operation)?;
            }
        }

        Ok(())
    }

    /// Find nodes affected by gate operation
    fn find_affected_nodes(&self, qubits: &[QubitId]) -> QuantRS2Result<Vec<Uuid>> {
        // Implementation would determine which nodes contain state chunks
        // affected by the given qubits
        Ok(vec![self.local_node.node_id])
    }

    /// Calculate communication requirements for gate operation
    const fn calculate_communication_requirements(
        &self,
        qubits: &[QubitId],
    ) -> QuantRS2Result<CommunicationRequirements> {
        let data_size = qubits.len() * 1024; // Estimate based on gate complexity

        Ok(CommunicationRequirements {
            data_size,
            pattern: CommunicationPattern::PointToPoint,
            synchronization_level: SynchronizationLevel::Weak,
            estimated_time: Duration::from_millis(data_size as u64 / 1000), // Simple estimate
        })
    }

    /// Execute gate with amplitude distribution strategy
    fn execute_amplitude_distributed_gate(
        &self,
        gate: &Arc<dyn GateOp + Send + Sync>,
        operation: &DistributedGateOperation,
    ) -> QuantRS2Result<()> {
        // Implementation would coordinate gate application across nodes
        // that own different amplitude ranges
        Ok(())
    }

    /// Execute gate with qubit partition strategy
    fn execute_qubit_partitioned_gate(
        &self,
        gate: &Arc<dyn GateOp + Send + Sync>,
        operation: &DistributedGateOperation,
    ) -> QuantRS2Result<()> {
        // Implementation would handle gates that cross qubit partitions
        Ok(())
    }

    /// Execute gate with hybrid strategy
    fn execute_hybrid_distributed_gate(
        &self,
        gate: &Arc<dyn GateOp + Send + Sync>,
        operation: &DistributedGateOperation,
    ) -> QuantRS2Result<()> {
        // Implementation would dynamically choose best strategy
        self.execute_amplitude_distributed_gate(gate, operation)
    }

    /// Execute gate with graph partition strategy
    fn execute_graph_partitioned_gate(
        &self,
        gate: &Arc<dyn GateOp + Send + Sync>,
        operation: &DistributedGateOperation,
    ) -> QuantRS2Result<()> {
        // Implementation would use SciRS2 graph partitioning
        self.execute_amplitude_distributed_gate(gate, operation)
    }

    /// Collect final state from all nodes
    fn collect_final_state(&self) -> QuantRS2Result<Vec<Complex64>> {
        let chunks = self
            .state_chunks
            .read()
            .map_err(|e| QuantRS2Error::RuntimeError(format!("Lock poisoned: {e}")))?;
        let mut final_state = Vec::new();

        // Sort chunks by amplitude range and collect
        let mut sorted_chunks: Vec<_> = chunks.values().collect();
        sorted_chunks.sort_by_key(|chunk| chunk.amplitude_range.0);

        for chunk in sorted_chunks {
            final_state.extend(&chunk.amplitudes);
        }

        Ok(final_state)
    }

    /// Update performance statistics
    fn update_performance_stats(&self, simulation_time: Duration) -> QuantRS2Result<()> {
        let mut stats = self
            .performance_stats
            .lock()
            .map_err(|e| QuantRS2Error::RuntimeError(format!("Lock poisoned: {e}")))?;
        stats.total_time = simulation_time;

        // Calculate communication overhead, load balance efficiency, etc.
        stats.communication_overhead = 0.1; // 10% overhead estimate
        stats.load_balance_efficiency = 0.9; // 90% efficiency estimate

        Ok(())
    }
}

impl CommunicationManager {
    /// Create new communication manager
    pub fn new(local_address: SocketAddr) -> QuantRS2Result<Self> {
        Ok(Self {
            local_address,
            connections: HashMap::new(),
            outgoing_queue: VecDeque::new(),
            incoming_queue: VecDeque::new(),
            stats: NetworkStats {
                bytes_transmitted: 0,
                bytes_received: 0,
                average_latency: Duration::new(0, 0),
                peak_bandwidth: 0.0,
                failed_communications: 0,
            },
        })
    }

    /// Send message to another node
    pub fn send_message(
        &mut self,
        target_node: Uuid,
        message: NetworkMessage,
    ) -> QuantRS2Result<()> {
        self.outgoing_queue.push_back(message);
        Ok(())
    }

    /// Receive message from another node
    pub fn receive_message(&mut self) -> Option<NetworkMessage> {
        self.incoming_queue.pop_front()
    }
}

impl LoadBalancer {
    /// Create new load balancer
    #[must_use]
    pub fn new(strategy: LoadBalancingStrategy) -> Self {
        Self {
            strategy,
            node_loads: HashMap::new(),
            distribution_history: VecDeque::new(),
            rebalancing_stats: RebalancingStats::default(),
        }
    }

    /// Update node load information
    pub fn update_node_load(&mut self, node_id: Uuid, load: f64) {
        self.node_loads.insert(node_id, load);
    }

    /// Check if rebalancing is needed
    pub fn needs_rebalancing(&self, threshold: f64) -> bool {
        if self.node_loads.len() < 2 {
            return false;
        }

        let loads: Vec<f64> = self.node_loads.values().copied().collect();
        let max_load = loads.iter().copied().fold(0.0, f64::max);
        let min_load = loads.iter().copied().fold(1.0, f64::min);

        (max_load - min_load) > threshold
    }

    /// Perform load rebalancing
    pub fn rebalance(&mut self) -> Vec<LoadBalancingCommand> {
        let start_time = Instant::now();
        let mut commands = Vec::new();

        // Simple rebalancing: move work from overloaded to underloaded nodes
        let loads: Vec<(Uuid, f64)> = self.node_loads.iter().map(|(k, v)| (*k, *v)).collect();
        let average_load = loads.iter().map(|(_, load)| load).sum::<f64>() / loads.len() as f64;

        for (node_id, load) in &loads {
            if *load > average_load + 0.1 {
                // Find underloaded node
                for (target_id, target_load) in &loads {
                    if *target_load < average_load - 0.1 {
                        commands.push(LoadBalancingCommand::MigrateWork {
                            source_node: *node_id,
                            target_node: *target_id,
                            work_amount: (*load - average_load) / 2.0,
                        });
                        break;
                    }
                }
            }
        }

        // Update statistics
        self.rebalancing_stats.rebalancing_count += 1;
        self.rebalancing_stats.total_rebalancing_time += start_time.elapsed();

        commands
    }
}

/// Benchmark distributed simulation performance
pub fn benchmark_distributed_simulation(
    config: DistributedSimulatorConfig,
    num_qubits: usize,
    num_gates: usize,
) -> QuantRS2Result<DistributedPerformanceStats> {
    let mut simulator = DistributedQuantumSimulator::new(config)?;
    simulator.initialize_cluster()?;

    // Create benchmark circuit with const generic
    const MAX_QUBITS: usize = 64;
    if num_qubits > MAX_QUBITS {
        return Err(QuantRS2Error::InvalidInput(
            "Too many qubits for benchmark".to_string(),
        ));
    }

    // For simplicity, use a fixed size circuit
    let mut circuit = Circuit::<64>::new();

    // Add random gates for benchmarking
    use quantrs2_core::gate::single::{Hadamard, PauliX};
    for i in 0..num_gates {
        if i % num_qubits < num_qubits {
            let qubit = QubitId((i % num_qubits) as u32);
            if i % 2 == 0 {
                let _ = circuit.h(qubit);
            } else {
                let _ = circuit.x(qubit);
            }
        }
    }

    let start_time = Instant::now();
    let _final_state = simulator.simulate_circuit(&circuit)?;
    let benchmark_time = start_time.elapsed();

    let mut stats = simulator.get_statistics();
    stats.total_time = benchmark_time;

    Ok(stats)
}

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

    #[test]
    fn test_distributed_simulator_creation() {
        let config = DistributedSimulatorConfig::default();
        let simulator = DistributedQuantumSimulator::new(config);
        assert!(simulator.is_ok());
    }

    #[test]
    #[ignore = "Skipping node capabilities detection test"]
    fn test_node_capabilities_detection() {
        let capabilities = DistributedQuantumSimulator::detect_local_capabilities();
        assert!(capabilities.is_ok());

        let caps = capabilities.expect("Failed to detect local capabilities");
        assert!(caps.available_memory > 0);
        assert!(caps.cpu_cores > 0);
        assert!(caps.max_qubits > 0);
    }

    #[test]
    fn test_load_balancer() {
        let mut balancer = LoadBalancer::new(LoadBalancingStrategy::WorkStealing);

        let node1 = Uuid::new_v4();
        let node2 = Uuid::new_v4();

        balancer.update_node_load(node1, 0.8);
        balancer.update_node_load(node2, 0.2);

        assert!(balancer.needs_rebalancing(0.3));

        let commands = balancer.rebalance();
        assert!(!commands.is_empty());
    }

    #[test]
    fn test_state_chunk_creation() {
        let chunk = StateChunk {
            chunk_id: Uuid::new_v4(),
            amplitude_range: (0, 1024),
            qubit_indices: vec![0, 1, 2],
            amplitudes: vec![Complex64::new(1.0, 0.0); 1024],
            owner_node: Uuid::new_v4(),
            backup_nodes: vec![],
            metadata: ChunkMetadata {
                size_bytes: 1024 * 16,
                compression_ratio: 1.0,
                last_access: Instant::now(),
                access_count: 0,
                is_cached: true,
            },
        };

        assert_eq!(chunk.amplitude_range.1 - chunk.amplitude_range.0, 1024);
        assert_eq!(chunk.amplitudes.len(), 1024);
    }
}