quantrs2-core 0.1.3

//! Distributed Quantum Gate Networks
//!
//! Quantum gates that operate across spatially separated qubits with
//! advanced networking protocols and fault-tolerant communication.

use crate::error::QuantRS2Error;
use crate::qubit::QubitId;
use scirs2_core::ndarray::Array2;
use scirs2_core::Complex64;
use std::collections::HashMap;
use std::sync::{Arc, Mutex, RwLock};
use std::time::{Duration, Instant};
use tokio::sync::Semaphore;
use uuid::Uuid;

/// Distributed quantum node representation
#[derive(Debug, Clone)]
pub struct QuantumNode {
    pub node_id: Uuid,
    pub location: NodeLocation,
    pub qubits: Vec<QubitId>,
    pub connectivity: Vec<Uuid>,
    pub capabilities: NodeCapabilities,
    pub state: Arc<RwLock<NodeState>>,
}

#[derive(Debug, Clone)]
pub struct NodeLocation {
    pub x: f64,
    pub y: f64,
    pub z: f64,
}

#[derive(Debug, Clone)]
pub struct NodeCapabilities {
    pub max_qubits: usize,
    pub gate_set: Vec<String>,
    pub fidelity: f64,
    pub coherence_time: Duration,
    pub gate_time: Duration,
    pub measurement_time: Duration,
}

#[derive(Debug, Clone)]
pub enum NodeState {
    Active,
    Busy,
    Maintenance,
    Offline,
}

impl QuantumNode {
    /// Create a new quantum node
    pub fn new(location: NodeLocation, max_qubits: usize, capabilities: NodeCapabilities) -> Self {
        let node_id = Uuid::new_v4();
        let qubits = (0..max_qubits).map(|i| QubitId::new(i as u32)).collect();

        Self {
            node_id,
            location,
            qubits,
            connectivity: Vec::new(),
            capabilities,
            state: Arc::new(RwLock::new(NodeState::Active)),
        }
    }

    /// Calculate distance to another node
    pub fn distance_to(&self, other: &Self) -> f64 {
        let dx = self.location.x - other.location.x;
        let dy = self.location.y - other.location.y;
        let dz = self.location.z - other.location.z;
        dz.mul_add(dz, dx.mul_add(dx, dy * dy)).sqrt()
    }

    /// Add connection to another node
    pub fn connect_to(&mut self, node_id: Uuid) {
        if !self.connectivity.contains(&node_id) {
            self.connectivity.push(node_id);
        }
    }

    /// Check if node is available for computation
    pub fn is_available(&self) -> bool {
        self.state
            .read()
            .map(|state| matches!(*state, NodeState::Active))
            .unwrap_or(false)
    }

    /// Estimate communication latency to another node
    pub fn communication_latency(&self, other: &Self) -> Duration {
        let distance = self.distance_to(other);
        let speed_of_light = 299_792_458.0; // m/s
        let latency_seconds = distance / speed_of_light;
        Duration::from_secs_f64(latency_seconds * 2.0) // Round-trip
    }
}

/// Distributed quantum gate that operates across multiple nodes
#[derive(Debug, Clone)]
pub struct DistributedQuantumGate {
    pub gate_id: Uuid,
    pub gate_type: DistributedGateType,
    pub target_qubits: Vec<(Uuid, QubitId)>, // (node_id, qubit_id)
    pub parameters: Vec<f64>,
    pub entanglement_protocol: EntanglementProtocol,
    pub error_correction: bool,
}

#[derive(Debug, Clone)]
pub enum DistributedGateType {
    DistributedCNOT,
    DistributedToffoli,
    DistributedControlledPhase,
    DistributedQuantumFourierTransform,
    DistributedEntanglingGate,
    CustomDistributedGate {
        name: String,
        matrix: Array2<Complex64>,
    },
}

#[derive(Debug, Clone)]
pub enum EntanglementProtocol {
    DirectEntanglement,
    EntanglementSwapping,
    QuantumRepeater { num_repeaters: usize },
    PurificationBased { purification_rounds: usize },
}

impl DistributedQuantumGate {
    /// Create a new distributed quantum gate
    pub fn new(
        gate_type: DistributedGateType,
        target_qubits: Vec<(Uuid, QubitId)>,
        entanglement_protocol: EntanglementProtocol,
    ) -> Self {
        Self {
            gate_id: Uuid::new_v4(),
            gate_type,
            target_qubits,
            parameters: Vec::new(),
            entanglement_protocol,
            error_correction: true,
        }
    }

    /// Estimate execution time across the network
    pub fn estimate_execution_time(&self, network: &QuantumNetwork) -> Duration {
        let mut max_latency = Duration::ZERO;

        // Find maximum communication latency between involved nodes
        for i in 0..self.target_qubits.len() {
            for j in i + 1..self.target_qubits.len() {
                let node1_id = self.target_qubits[i].0;
                let node2_id = self.target_qubits[j].0;

                if let (Some(node1), Some(node2)) =
                    (network.get_node(node1_id), network.get_node(node2_id))
                {
                    let latency = node1.communication_latency(node2);
                    if latency > max_latency {
                        max_latency = latency;
                    }
                }
            }
        }

        // Add gate execution time and protocol overhead
        let gate_time = Duration::from_millis(100); // Base gate time
        let protocol_overhead = match self.entanglement_protocol {
            EntanglementProtocol::DirectEntanglement => Duration::from_millis(10),
            EntanglementProtocol::EntanglementSwapping => Duration::from_millis(50),
            EntanglementProtocol::QuantumRepeater { num_repeaters } => {
                Duration::from_millis(20 * num_repeaters as u64)
            }
            EntanglementProtocol::PurificationBased {
                purification_rounds,
            } => Duration::from_millis(30 * purification_rounds as u64),
        };

        max_latency + gate_time + protocol_overhead
    }
}

/// Quantum network topology and management
#[derive(Debug)]
pub struct QuantumNetwork {
    pub nodes: HashMap<Uuid, QuantumNode>,
    pub topology: NetworkTopology,
    pub routing_table: Arc<RwLock<HashMap<(Uuid, Uuid), Vec<Uuid>>>>,
    pub entanglement_manager: EntanglementManager,
    pub scheduler: NetworkScheduler,
}

#[derive(Debug, Clone)]
pub enum NetworkTopology {
    Star { center_node: Uuid },
    Mesh,
    Ring,
    Tree { root_node: Uuid, depth: usize },
    Grid { width: usize, height: usize },
    Custom { adjacency_matrix: Array2<bool> },
}

impl QuantumNetwork {
    /// Create a new quantum network
    pub fn new(topology: NetworkTopology) -> Self {
        Self {
            nodes: HashMap::new(),
            topology,
            routing_table: Arc::new(RwLock::new(HashMap::new())),
            entanglement_manager: EntanglementManager::new(),
            scheduler: NetworkScheduler::new(),
        }
    }

    /// Add a node to the network
    pub fn add_node(&mut self, mut node: QuantumNode) {
        // Configure connections based on topology
        self.configure_node_connections(&mut node);
        let node_id = node.node_id;
        self.nodes.insert(node_id, node);
        self.update_routing_table();
    }

    /// Configure node connections based on network topology
    fn configure_node_connections(&self, node: &mut QuantumNode) {
        match &self.topology {
            NetworkTopology::Star { center_node } => {
                if node.node_id == *center_node {
                    // Center node connects to all others
                    for other_id in self.nodes.keys() {
                        if *other_id != node.node_id {
                            node.connect_to(*other_id);
                        }
                    }
                } else {
                    node.connect_to(*center_node);
                }
            }
            NetworkTopology::Mesh => {
                // Connect to all other nodes
                for other_id in self.nodes.keys() {
                    if *other_id != node.node_id {
                        node.connect_to(*other_id);
                    }
                }
            }
            NetworkTopology::Ring => {
                // Connect to adjacent nodes in ring
                let node_ids: Vec<Uuid> = self.nodes.keys().copied().collect();
                if let Some(pos) = node_ids.iter().position(|&id| id == node.node_id) {
                    let prev = if pos == 0 {
                        node_ids.len() - 1
                    } else {
                        pos - 1
                    };
                    let next = if pos == node_ids.len() - 1 {
                        0
                    } else {
                        pos + 1
                    };

                    if prev < node_ids.len() {
                        node.connect_to(node_ids[prev]);
                    }
                    if next < node_ids.len() {
                        node.connect_to(node_ids[next]);
                    }
                }
            }
            NetworkTopology::Grid { width, height } => {
                // Connect to grid neighbors
                // let _total_nodes = width * height;
                let node_ids: Vec<Uuid> = self.nodes.keys().copied().collect();

                if let Some(index) = node_ids.iter().position(|&id| id == node.node_id) {
                    let row = index / width;
                    let col = index % width;

                    // Connect to adjacent grid positions
                    let neighbors = [
                        (row.wrapping_sub(1), col), // Up
                        (row + 1, col),             // Down
                        (row, col.wrapping_sub(1)), // Left
                        (row, col + 1),             // Right
                    ];

                    for (r, c) in neighbors {
                        if r < *height && c < *width {
                            let neighbor_index = r * width + c;
                            if neighbor_index < node_ids.len() {
                                node.connect_to(node_ids[neighbor_index]);
                            }
                        }
                    }
                }
            }
            NetworkTopology::Tree {
                root_node: _,
                depth: _,
            }
            | NetworkTopology::Custom {
                adjacency_matrix: _,
            } => {
                // Tree topology and custom connections
                // Simplified: connect to parent and children / based on adjacency matrix
            }
        }
    }

    /// Update routing table for optimal path finding
    fn update_routing_table(&self) {
        let Ok(mut routing_table) = self.routing_table.write() else {
            return; // Cannot update routing table if lock is poisoned
        };
        routing_table.clear();

        // Floyd-Warshall algorithm for shortest paths
        let node_ids: Vec<Uuid> = self.nodes.keys().copied().collect();
        let n = node_ids.len();

        // Initialize distance matrix
        let mut distances = vec![vec![f64::INFINITY; n]; n];
        let mut next_hop = vec![vec![None; n]; n];

        // Set direct connections
        for (i, &node_id) in node_ids.iter().enumerate() {
            distances[i][i] = 0.0;
            next_hop[i][i] = Some(node_id);

            if let Some(node) = self.nodes.get(&node_id) {
                for &neighbor_id in &node.connectivity {
                    if let Some(j) = node_ids.iter().position(|&id| id == neighbor_id) {
                        if let Some(neighbor) = self.nodes.get(&neighbor_id) {
                            distances[i][j] = node.distance_to(neighbor);
                            next_hop[i][j] = Some(neighbor_id);
                        }
                    }
                }
            }
        }

        // Floyd-Warshall
        for k in 0..n {
            for i in 0..n {
                for j in 0..n {
                    if distances[i][k] + distances[k][j] < distances[i][j] {
                        distances[i][j] = distances[i][k] + distances[k][j];
                        next_hop[i][j] = next_hop[i][k];
                    }
                }
            }
        }

        // Build routing table
        for (i, &source) in node_ids.iter().enumerate() {
            for (j, &dest) in node_ids.iter().enumerate() {
                if i != j && next_hop[i][j].is_some() {
                    let mut path = vec![source];
                    let mut current = i;

                    while current != j {
                        if let Some(next_node) = next_hop[current][j] {
                            path.push(next_node);
                            if let Some(pos) = node_ids.iter().position(|&id| id == next_node) {
                                current = pos;
                            } else {
                                break;
                            }
                        } else {
                            break;
                        }
                    }

                    routing_table.insert((source, dest), path);
                }
            }
        }
    }

    /// Get a node by ID
    pub fn get_node(&self, node_id: Uuid) -> Option<&QuantumNode> {
        self.nodes.get(&node_id)
    }

    /// Execute a distributed quantum gate
    pub async fn execute_distributed_gate(
        &self,
        gate: &DistributedQuantumGate,
    ) -> Result<DistributedExecutionResult, QuantRS2Error> {
        // Schedule the gate execution
        let execution_plan = self.scheduler.schedule_gate(gate, self).await?;

        // Establish entanglement between required qubits
        let entanglement_result = self
            .entanglement_manager
            .establish_entanglement(&gate.target_qubits, &gate.entanglement_protocol, self)
            .await?;

        // Execute the gate across the network
        let start_time = Instant::now();
        let result = self
            .execute_gate_with_plan(&execution_plan, &entanglement_result)
            .await?;
        let execution_time = start_time.elapsed();

        Ok(DistributedExecutionResult {
            gate_id: gate.gate_id,
            execution_time,
            fidelity: result.fidelity,
            success: result.success,
            error_rates: result.error_rates,
        })
    }

    /// Execute gate with established plan and entanglement
    async fn execute_gate_with_plan(
        &self,
        plan: &ExecutionPlan,
        entanglement: &EntanglementResult,
    ) -> Result<GateExecutionResult, QuantRS2Error> {
        let mut success = true;
        let mut total_fidelity = 1.0;
        let mut error_rates = HashMap::new();

        // Execute each step in the plan
        for step in &plan.steps {
            let step_result = self.execute_step(step, entanglement).await?;

            if !step_result.success {
                success = false;
            }

            total_fidelity *= step_result.fidelity;

            for (node_id, error_rate) in step_result.node_error_rates {
                *error_rates.entry(node_id).or_insert(0.0) += error_rate;
            }
        }

        Ok(GateExecutionResult {
            success,
            fidelity: total_fidelity,
            error_rates,
        })
    }

    /// Execute a single step in the execution plan
    async fn execute_step(
        &self,
        step: &ExecutionStep,
        _entanglement: &EntanglementResult,
    ) -> Result<StepExecutionResult, QuantRS2Error> {
        match step {
            ExecutionStep::LocalGate {
                node_id,
                gate_op: _,
                qubits: _,
            } => {
                if let Some(node) = self.get_node(*node_id) {
                    // Simulate local gate execution
                    let fidelity = node.capabilities.fidelity;
                    let error_rate = 1.0 - fidelity;

                    // Add realistic execution delay
                    tokio::time::sleep(node.capabilities.gate_time).await;

                    Ok(StepExecutionResult {
                        success: true,
                        fidelity,
                        node_error_rates: vec![(*node_id, error_rate)].into_iter().collect(),
                    })
                } else {
                    Err(QuantRS2Error::NodeNotFound(format!(
                        "Node {node_id} not found"
                    )))
                }
            }
            ExecutionStep::RemoteEntanglement {
                source_node,
                target_node,
                protocol,
            } => {
                if let (Some(source), Some(target)) =
                    (self.get_node(*source_node), self.get_node(*target_node))
                {
                    // Simulate entanglement establishment
                    let latency = source.communication_latency(target);
                    tokio::time::sleep(latency).await;

                    let base_fidelity =
                        f64::midpoint(source.capabilities.fidelity, target.capabilities.fidelity);
                    let distance_penalty = 1.0 - (source.distance_to(target) / 1000.0).min(0.1);
                    let protocol_fidelity = match protocol {
                        EntanglementProtocol::DirectEntanglement => 0.95,
                        EntanglementProtocol::EntanglementSwapping => 0.85,
                        EntanglementProtocol::QuantumRepeater { .. } => 0.90,
                        EntanglementProtocol::PurificationBased { .. } => 0.98,
                    };

                    let fidelity = base_fidelity * distance_penalty * protocol_fidelity;

                    Ok(StepExecutionResult {
                        success: fidelity > 0.5,
                        fidelity,
                        node_error_rates: vec![
                            (*source_node, 1.0 - fidelity),
                            (*target_node, 1.0 - fidelity),
                        ]
                        .into_iter()
                        .collect(),
                    })
                } else {
                    Err(QuantRS2Error::NodeNotFound(
                        "Source or target node not found".to_string(),
                    ))
                }
            }
            ExecutionStep::Measurement { node_id, qubits: _ } => {
                if let Some(node) = self.get_node(*node_id) {
                    // Simulate measurement
                    tokio::time::sleep(node.capabilities.measurement_time).await;

                    let fidelity = node.capabilities.fidelity * 0.95; // Measurement reduces fidelity

                    Ok(StepExecutionResult {
                        success: true,
                        fidelity,
                        node_error_rates: vec![(*node_id, 1.0 - fidelity)].into_iter().collect(),
                    })
                } else {
                    Err(QuantRS2Error::NodeNotFound(format!(
                        "Node {node_id} not found"
                    )))
                }
            }
        }
    }
}

/// Entanglement management across the network
#[derive(Debug)]
pub struct EntanglementManager {
    pub entangled_pairs: Arc<Mutex<HashMap<(Uuid, QubitId, Uuid, QubitId), EntanglementState>>>,
    pub purification_threshold: f64,
}

#[derive(Debug, Clone)]
pub struct EntanglementState {
    pub fidelity: f64,
    pub creation_time: Instant,
    pub coherence_time: Duration,
    pub bell_state_type: BellStateType,
}

#[derive(Debug, Clone)]
pub enum BellStateType {
    PhiPlus,
    PhiMinus,
    PsiPlus,
    PsiMinus,
}

impl EntanglementManager {
    pub fn new() -> Self {
        Self {
            entangled_pairs: Arc::new(Mutex::new(HashMap::new())),
            purification_threshold: 0.8,
        }
    }

    /// Establish entanglement between specified qubits
    pub async fn establish_entanglement(
        &self,
        target_qubits: &[(Uuid, QubitId)],
        protocol: &EntanglementProtocol,
        network: &QuantumNetwork,
    ) -> Result<EntanglementResult, QuantRS2Error> {
        let mut established_pairs = Vec::new();
        let mut total_fidelity = 1.0;

        // Create entanglement between all required qubit pairs
        for i in 0..target_qubits.len() {
            for j in i + 1..target_qubits.len() {
                let (node1, qubit1) = target_qubits[i];
                let (node2, qubit2) = target_qubits[j];

                let pair_result = self
                    .establish_pair_entanglement(node1, qubit1, node2, qubit2, protocol, network)
                    .await?;

                established_pairs.push(pair_result.clone());
                total_fidelity *= pair_result.fidelity;
            }
        }

        Ok(EntanglementResult {
            pairs: established_pairs,
            total_fidelity,
            protocol: protocol.clone(),
        })
    }

    /// Establish entanglement between a specific pair of qubits
    async fn establish_pair_entanglement(
        &self,
        node1: Uuid,
        qubit1: QubitId,
        node2: Uuid,
        qubit2: QubitId,
        protocol: &EntanglementProtocol,
        network: &QuantumNetwork,
    ) -> Result<EntangledPair, QuantRS2Error> {
        match protocol {
            EntanglementProtocol::DirectEntanglement => {
                self.direct_entanglement(node1, qubit1, node2, qubit2, network)
                    .await
            }
            EntanglementProtocol::EntanglementSwapping => {
                self.entanglement_swapping(node1, qubit1, node2, qubit2, network)
                    .await
            }
            EntanglementProtocol::QuantumRepeater { num_repeaters } => {
                self.quantum_repeater_entanglement(
                    node1,
                    qubit1,
                    node2,
                    qubit2,
                    *num_repeaters,
                    network,
                )
                .await
            }
            EntanglementProtocol::PurificationBased {
                purification_rounds,
            } => {
                self.purification_based_entanglement(
                    node1,
                    qubit1,
                    node2,
                    qubit2,
                    *purification_rounds,
                    network,
                )
                .await
            }
        }
    }

    /// Direct entanglement between two nodes
    async fn direct_entanglement(
        &self,
        node1: Uuid,
        qubit1: QubitId,
        node2: Uuid,
        qubit2: QubitId,
        network: &QuantumNetwork,
    ) -> Result<EntangledPair, QuantRS2Error> {
        if let (Some(n1), Some(n2)) = (network.get_node(node1), network.get_node(node2)) {
            // Simulate photon transmission and Bell measurement
            let distance = n1.distance_to(n2);
            let transmission_fidelity = (-distance / 22000.0).exp(); // Fiber loss ~22km attenuation length
            let detection_fidelity = 0.95; // Detector efficiency

            let fidelity = transmission_fidelity
                * detection_fidelity
                * (n1.capabilities.fidelity + n2.capabilities.fidelity)
                / 2.0;

            // Add to entangled pairs registry
            let entanglement_state = EntanglementState {
                fidelity,
                creation_time: Instant::now(),
                coherence_time: Duration::min(
                    n1.capabilities.coherence_time,
                    n2.capabilities.coherence_time,
                ),
                bell_state_type: BellStateType::PhiPlus,
            };

            if let Ok(mut pairs) = self.entangled_pairs.lock() {
                pairs.insert((node1, qubit1, node2, qubit2), entanglement_state);
            }

            Ok(EntangledPair {
                node1,
                qubit1,
                node2,
                qubit2,
                fidelity,
                bell_state: BellStateType::PhiPlus,
            })
        } else {
            Err(QuantRS2Error::NodeNotFound(
                "One or both nodes not found".to_string(),
            ))
        }
    }

    /// Entanglement swapping protocol
    async fn entanglement_swapping(
        &self,
        node1: Uuid,
        qubit1: QubitId,
        node2: Uuid,
        qubit2: QubitId,
        network: &QuantumNetwork,
    ) -> Result<EntangledPair, QuantRS2Error> {
        // Find intermediate node for swapping
        let routing_table = network
            .routing_table
            .read()
            .map_err(|e| QuantRS2Error::RuntimeError(format!("Lock poisoned: {e}")))?;
        if let Some(path) = routing_table.get(&(node1, node2)) {
            if path.len() >= 3 {
                let intermediate_node = path[1];

                // Create entanglement: node1 <-> intermediate, intermediate <-> node2
                let pair1 = self
                    .direct_entanglement(node1, qubit1, intermediate_node, QubitId::new(0), network)
                    .await?;

                let pair2 = self
                    .direct_entanglement(intermediate_node, QubitId::new(1), node2, qubit2, network)
                    .await?;

                // Perform Bell measurement at intermediate node to swap entanglement
                let swapping_fidelity = 0.85; // Typical swapping fidelity
                let final_fidelity = pair1.fidelity * pair2.fidelity * swapping_fidelity;

                Ok(EntangledPair {
                    node1,
                    qubit1,
                    node2,
                    qubit2,
                    fidelity: final_fidelity,
                    bell_state: BellStateType::PhiPlus,
                })
            } else {
                // Fall back to direct entanglement
                self.direct_entanglement(node1, qubit1, node2, qubit2, network)
                    .await
            }
        } else {
            Err(QuantRS2Error::NetworkError(
                "No path found between nodes".to_string(),
            ))
        }
    }

    /// Quantum repeater-based entanglement
    async fn quantum_repeater_entanglement(
        &self,
        node1: Uuid,
        qubit1: QubitId,
        node2: Uuid,
        qubit2: QubitId,
        num_repeaters: usize,
        _network: &QuantumNetwork,
    ) -> Result<EntangledPair, QuantRS2Error> {
        // Simplified quantum repeater protocol
        // In practice, this would involve multiple rounds of entanglement creation and swapping

        let base_fidelity = 0.9f64; // Initial entanglement fidelity
        let repeater_fidelity = base_fidelity.powi(num_repeaters as i32 + 1);

        // Simulate repeater protocol execution time
        let protocol_time = Duration::from_millis(100 * (num_repeaters + 1) as u64);
        tokio::time::sleep(protocol_time).await;

        Ok(EntangledPair {
            node1,
            qubit1,
            node2,
            qubit2,
            fidelity: repeater_fidelity,
            bell_state: BellStateType::PhiPlus,
        })
    }

    /// Purification-based entanglement
    async fn purification_based_entanglement(
        &self,
        node1: Uuid,
        qubit1: QubitId,
        node2: Uuid,
        qubit2: QubitId,
        purification_rounds: usize,
        network: &QuantumNetwork,
    ) -> Result<EntangledPair, QuantRS2Error> {
        // Start with direct entanglement
        let mut current_fidelity = self
            .direct_entanglement(node1, qubit1, node2, qubit2, network)
            .await?
            .fidelity;

        // Apply purification rounds
        for _ in 0..purification_rounds {
            if current_fidelity < self.purification_threshold {
                // Create additional entangled pair for purification
                let aux_pair = self
                    .direct_entanglement(node1, QubitId::new(99), node2, QubitId::new(99), network)
                    .await?;

                // Purification protocol improves fidelity
                current_fidelity = self.purify_entanglement(current_fidelity, aux_pair.fidelity);
            }
        }

        Ok(EntangledPair {
            node1,
            qubit1,
            node2,
            qubit2,
            fidelity: current_fidelity,
            bell_state: BellStateType::PhiPlus,
        })
    }

    /// Purification protocol
    fn purify_entanglement(&self, fidelity1: f64, fidelity2: f64) -> f64 {
        // Simplified purification formula
        let f1 = fidelity1;
        let f2 = fidelity2;

        // Bennett et al. purification protocol
        let numerator = f1.mul_add(f2, (1.0 - f1) * (1.0 - f2) / 3.0);
        let denominator = f1.mul_add(f2, 2.0 * (1.0 - f1) * (1.0 - f2) / 3.0);

        if denominator > 0.0 {
            numerator / denominator
        } else {
            fidelity1
        }
    }
}

/// Network scheduler for distributed quantum operations
#[derive(Debug)]
pub struct NetworkScheduler {
    pub active_schedules: Arc<Mutex<Vec<ScheduledOperation>>>,
    pub resource_semaphore: Arc<Semaphore>,
}

#[derive(Debug, Clone)]
pub struct ScheduledOperation {
    pub operation_id: Uuid,
    pub start_time: Instant,
    pub estimated_duration: Duration,
    pub involved_nodes: Vec<Uuid>,
    pub priority: Priority,
}

#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord)]
pub enum Priority {
    Low = 0,
    Medium = 1,
    High = 2,
    Critical = 3,
}

impl NetworkScheduler {
    pub fn new() -> Self {
        Self {
            active_schedules: Arc::new(Mutex::new(Vec::new())),
            resource_semaphore: Arc::new(Semaphore::new(100)), // Max concurrent operations
        }
    }

    /// Schedule a distributed gate execution
    pub async fn schedule_gate(
        &self,
        gate: &DistributedQuantumGate,
        network: &QuantumNetwork,
    ) -> Result<ExecutionPlan, QuantRS2Error> {
        // Acquire scheduling semaphore
        let _permit =
            self.resource_semaphore.acquire().await.map_err(|e| {
                QuantRS2Error::RuntimeError(format!("Semaphore acquire failed: {e}"))
            })?;

        // Analyze gate requirements
        let involved_nodes: Vec<Uuid> = gate
            .target_qubits
            .iter()
            .map(|(node_id, _)| *node_id)
            .collect();

        // Check node availability
        for &node_id in &involved_nodes {
            if let Some(node) = network.get_node(node_id) {
                if !node.is_available() {
                    return Err(QuantRS2Error::NodeUnavailable(format!(
                        "Node {node_id} is not available"
                    )));
                }
            }
        }

        // Create execution plan
        let mut steps = Vec::new();

        // Step 1: Establish required entanglements
        for i in 0..gate.target_qubits.len() {
            for j in i + 1..gate.target_qubits.len() {
                let (node1, _qubit1) = gate.target_qubits[i];
                let (node2, _qubit2) = gate.target_qubits[j];

                steps.push(ExecutionStep::RemoteEntanglement {
                    source_node: node1,
                    target_node: node2,
                    protocol: gate.entanglement_protocol.clone(),
                });
            }
        }

        // Step 2: Execute local operations
        match &gate.gate_type {
            DistributedGateType::DistributedCNOT => {
                if gate.target_qubits.len() == 2 {
                    let (control_node, control_qubit) = gate.target_qubits[0];
                    let (target_node, target_qubit) = gate.target_qubits[1];

                    // Control node applies local operation
                    steps.push(ExecutionStep::LocalGate {
                        node_id: control_node,
                        gate_op: "LocalCNOTControl".to_string(),
                        qubits: vec![control_qubit],
                    });

                    // Target node applies conditional operation
                    steps.push(ExecutionStep::LocalGate {
                        node_id: target_node,
                        gate_op: "LocalCNOTTarget".to_string(),
                        qubits: vec![target_qubit],
                    });
                }
            }
            DistributedGateType::DistributedToffoli => {
                // Three-qubit distributed Toffoli implementation
                if gate.target_qubits.len() == 3 {
                    for (i, &(node_id, qubit_id)) in gate.target_qubits.iter().enumerate() {
                        let gate_name = match i {
                            0 | 1 => "ToffoliControl",
                            2 => "ToffoliTarget",
                            _ => "ToffoliAux",
                        };

                        steps.push(ExecutionStep::LocalGate {
                            node_id,
                            gate_op: gate_name.to_string(),
                            qubits: vec![qubit_id],
                        });
                    }
                }
            }
            _ => {
                // Generic distributed gate execution
                for &(node_id, qubit_id) in &gate.target_qubits {
                    steps.push(ExecutionStep::LocalGate {
                        node_id,
                        gate_op: "GenericDistributedGate".to_string(),
                        qubits: vec![qubit_id],
                    });
                }
            }
        }

        // Step 3: Final measurements if required
        if gate.error_correction {
            for &(node_id, qubit_id) in &gate.target_qubits {
                steps.push(ExecutionStep::Measurement {
                    node_id,
                    qubits: vec![qubit_id],
                });
            }
        }

        let estimated_duration = gate.estimate_execution_time(network);

        // Register the scheduled operation
        let scheduled_op = ScheduledOperation {
            operation_id: gate.gate_id,
            start_time: Instant::now(),
            estimated_duration,
            involved_nodes,
            priority: Priority::Medium,
        };

        if let Ok(mut schedules) = self.active_schedules.lock() {
            schedules.push(scheduled_op);
        }

        Ok(ExecutionPlan {
            gate_id: gate.gate_id,
            steps,
            estimated_duration,
            resource_requirements: self.calculate_resource_requirements(gate),
        })
    }

    /// Calculate resource requirements for a gate
    fn calculate_resource_requirements(
        &self,
        gate: &DistributedQuantumGate,
    ) -> ResourceRequirements {
        ResourceRequirements {
            node_count: gate.target_qubits.len(),
            qubit_count: gate.target_qubits.len(),
            memory_mb: gate.target_qubits.len() * 10, // Estimate
            communication_bandwidth: 1000,            // kbps
            entanglement_pairs: (gate.target_qubits.len() * (gate.target_qubits.len() - 1)) / 2,
        }
    }
}

/// Execution plan for distributed quantum operations
#[derive(Debug, Clone)]
pub struct ExecutionPlan {
    pub gate_id: Uuid,
    pub steps: Vec<ExecutionStep>,
    pub estimated_duration: Duration,
    pub resource_requirements: ResourceRequirements,
}

#[derive(Debug, Clone)]
pub enum ExecutionStep {
    LocalGate {
        node_id: Uuid,
        gate_op: String,
        qubits: Vec<QubitId>,
    },
    RemoteEntanglement {
        source_node: Uuid,
        target_node: Uuid,
        protocol: EntanglementProtocol,
    },
    Measurement {
        node_id: Uuid,
        qubits: Vec<QubitId>,
    },
}

#[derive(Debug, Clone)]
pub struct ResourceRequirements {
    pub node_count: usize,
    pub qubit_count: usize,
    pub memory_mb: usize,
    pub communication_bandwidth: usize, // kbps
    pub entanglement_pairs: usize,
}

/// Results and state tracking
#[derive(Debug, Clone)]
pub struct DistributedExecutionResult {
    pub gate_id: Uuid,
    pub execution_time: Duration,
    pub fidelity: f64,
    pub success: bool,
    pub error_rates: HashMap<Uuid, f64>,
}

#[derive(Debug, Clone)]
pub struct EntanglementResult {
    pub pairs: Vec<EntangledPair>,
    pub total_fidelity: f64,
    pub protocol: EntanglementProtocol,
}

#[derive(Debug, Clone)]
pub struct EntangledPair {
    pub node1: Uuid,
    pub qubit1: QubitId,
    pub node2: Uuid,
    pub qubit2: QubitId,
    pub fidelity: f64,
    pub bell_state: BellStateType,
}

#[derive(Debug, Clone)]
pub struct GateExecutionResult {
    pub success: bool,
    pub fidelity: f64,
    pub error_rates: HashMap<Uuid, f64>,
}

#[derive(Debug, Clone)]
pub struct StepExecutionResult {
    pub success: bool,
    pub fidelity: f64,
    pub node_error_rates: HashMap<Uuid, f64>,
}

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

    #[tokio::test]
    async fn test_quantum_node_creation() {
        let location = NodeLocation {
            x: 0.0,
            y: 0.0,
            z: 0.0,
        };
        let capabilities = NodeCapabilities {
            max_qubits: 10,
            gate_set: vec!["X".to_string(), "CNOT".to_string()],
            fidelity: 0.99,
            coherence_time: Duration::from_millis(100),
            gate_time: Duration::from_micros(100),
            measurement_time: Duration::from_micros(1000),
        };

        let node = QuantumNode::new(location, 10, capabilities);
        assert_eq!(node.qubits.len(), 10);
        assert!(node.is_available());
    }

    #[tokio::test]
    async fn test_network_creation() {
        let mut network = QuantumNetwork::new(NetworkTopology::Mesh);

        let node1 = QuantumNode::new(
            NodeLocation {
                x: 0.0,
                y: 0.0,
                z: 0.0,
            },
            5,
            NodeCapabilities {
                max_qubits: 5,
                gate_set: vec!["X".to_string()],
                fidelity: 0.95,
                coherence_time: Duration::from_millis(50),
                gate_time: Duration::from_micros(50),
                measurement_time: Duration::from_micros(500),
            },
        );

        let node1_id = node1.node_id;
        network.add_node(node1);

        assert!(network.get_node(node1_id).is_some());
    }

    #[tokio::test]
    async fn test_distributed_gate_creation() {
        let node1_id = Uuid::new_v4();
        let node2_id = Uuid::new_v4();

        let gate = DistributedQuantumGate::new(
            DistributedGateType::DistributedCNOT,
            vec![(node1_id, QubitId::new(0)), (node2_id, QubitId::new(0))],
            EntanglementProtocol::DirectEntanglement,
        );

        assert_eq!(gate.target_qubits.len(), 2);
        assert!(matches!(
            gate.gate_type,
            DistributedGateType::DistributedCNOT
        ));
    }

    #[tokio::test]
    async fn test_entanglement_manager() {
        let manager = EntanglementManager::new();
        assert_eq!(
            manager
                .entangled_pairs
                .lock()
                .expect("Failed to lock entangled pairs")
                .len(),
            0
        );

        // Test entanglement establishment would require full network setup
        // This is a basic structure test
        assert!(manager.purification_threshold > 0.0);
    }
}