quantrs2_circuit/

fault_tolerant.rs

//! Fault-tolerant quantum circuit compilation
//!
//! This module provides tools for compiling logical quantum circuits into
//! fault-tolerant implementations using quantum error correction codes,
//! magic state distillation, and syndrome extraction.

use crate::builder::Circuit;
use quantrs2_core::{
    error::{QuantRS2Error, QuantRS2Result},
    gate::GateOp,
    qubit::QubitId,
};
use serde::{Deserialize, Serialize};
use std::collections::{HashMap, HashSet, VecDeque};
use std::sync::Arc;

/// Quantum error correction codes
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub enum QECCode {
    /// Surface code with distance d
    SurfaceCode { distance: usize },
    /// Color code
    ColorCode { distance: usize },
    /// Repetition code
    RepetitionCode { distance: usize },
    /// Steane code (7-qubit CSS code)
    SteaneCode,
    /// Shor code (9-qubit code)
    ShorCode,
    /// Bacon-Shor code
    BaconShorCode { m: usize, n: usize },
    /// Concatenated code
    ConcatenatedCode {
        inner_code: Box<Self>,
        outer_code: Box<Self>,
        levels: usize,
    },
}

impl QECCode {
    /// Get the number of physical qubits required
    #[must_use]
    pub fn physical_qubits(&self) -> usize {
        match self {
            Self::SurfaceCode { distance } => distance * distance,
            Self::ColorCode { distance } => 2 * distance * distance - 2 * distance + 1,
            Self::RepetitionCode { distance } => *distance,
            Self::SteaneCode => 7,
            Self::ShorCode => 9,
            Self::BaconShorCode { m, n } => m * n,
            Self::ConcatenatedCode {
                inner_code, levels, ..
            } => {
                let base_qubits = inner_code.physical_qubits();
                (0..*levels).fold(1, |acc, _| acc * base_qubits)
            }
        }
    }

    /// Get the code distance
    #[must_use]
    pub fn distance(&self) -> usize {
        match self {
            Self::SurfaceCode { distance }
            | Self::ColorCode { distance }
            | Self::RepetitionCode { distance } => *distance,
            Self::SteaneCode | Self::ShorCode => 3,
            Self::BaconShorCode { m, n } => (*m).min(*n),
            Self::ConcatenatedCode {
                inner_code, levels, ..
            } => {
                let base_distance = inner_code.distance();
                (0..*levels).fold(1, |acc, _| acc * base_distance)
            }
        }
    }

    /// Check if the code can correct t errors
    #[must_use]
    pub fn can_correct(&self, t: usize) -> bool {
        self.distance() > 2 * t
    }
}

/// Logical qubit representation
#[derive(Debug, Clone)]
pub struct LogicalQubit {
    /// Logical qubit ID
    pub id: usize,
    /// Physical qubits used for encoding
    pub physical_qubits: Vec<usize>,
    /// Error correction code
    pub code: QECCode,
    /// Current error syndrome
    pub syndrome: Option<Vec<u8>>,
    /// Error tracking
    pub error_count: usize,
}

impl LogicalQubit {
    /// Create a new logical qubit
    #[must_use]
    pub fn new(id: usize, code: QECCode) -> Self {
        let num_physical = code.physical_qubits();
        let physical_qubits = (id * num_physical..(id + 1) * num_physical).collect();

        Self {
            id,
            physical_qubits,
            code,
            syndrome: None,
            error_count: 0,
        }
    }

    /// Get data qubits (excluding ancilla qubits)
    #[must_use]
    pub fn data_qubits(&self) -> Vec<usize> {
        match &self.code {
            QECCode::SurfaceCode { distance } => {
                // For surface code, data qubits are at specific positions
                let mut data_qubits = Vec::new();
                for i in 0..*distance {
                    for j in 0..*distance {
                        if (i + j) % 2 == 0 {
                            // Data qubits on even positions
                            data_qubits.push(self.physical_qubits[i * distance + j]);
                        }
                    }
                }
                data_qubits
            }
            QECCode::SteaneCode => {
                // First 4 qubits are data qubits in Steane code
                self.physical_qubits[0..4].to_vec()
            }
            QECCode::ShorCode => {
                // Specific data qubit positions for Shor code
                vec![
                    self.physical_qubits[0],
                    self.physical_qubits[3],
                    self.physical_qubits[6],
                ]
            }
            _ => {
                // Default: first half are data qubits
                let half = self.physical_qubits.len() / 2;
                self.physical_qubits[0..half].to_vec()
            }
        }
    }

    /// Get ancilla qubits for syndrome measurement
    #[must_use]
    pub fn ancilla_qubits(&self) -> Vec<usize> {
        let data_qubits = self.data_qubits();
        self.physical_qubits
            .iter()
            .filter(|&&q| !data_qubits.contains(&q))
            .copied()
            .collect()
    }
}

/// Fault-tolerant gate implementation
#[derive(Debug, Clone)]
pub struct FaultTolerantGate {
    /// Gate name
    pub name: String,
    /// Logical qubits involved
    pub logical_qubits: Vec<usize>,
    /// Physical gate sequence
    pub physical_gates: Vec<PhysicalGate>,
    /// Syndrome measurements required
    pub syndrome_measurements: Vec<SyndromeMeasurement>,
    /// Magic states consumed
    pub magic_states: usize,
    /// Error correction overhead
    pub correction_overhead: f64,
}

/// Physical gate in fault-tolerant implementation
#[derive(Debug, Clone)]
pub struct PhysicalGate {
    /// Gate type
    pub gate_type: String,
    /// Physical qubits
    pub qubits: Vec<usize>,
    /// Gate parameters
    pub parameters: Vec<f64>,
    /// Execution time
    pub time: Option<f64>,
}

/// Syndrome measurement for error detection
#[derive(Debug, Clone)]
pub struct SyndromeMeasurement {
    /// Measurement type
    pub measurement_type: SyndromeType,
    /// Qubits involved in measurement
    pub qubits: Vec<usize>,
    /// Ancilla qubit for measurement
    pub ancilla: usize,
    /// Expected syndrome value
    pub expected_value: u8,
}

/// Types of syndrome measurements
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum SyndromeType {
    /// X-type stabilizer measurement
    XStabilizer,
    /// Z-type stabilizer measurement
    ZStabilizer,
    /// Joint XZ measurement
    XZStabilizer,
}

/// Magic state type for universal computation
#[derive(Debug, Clone, PartialEq)]
pub enum MagicState {
    /// T state |T⟩ = (|0⟩ + e^(iπ/4)|1⟩)/√2
    TState,
    /// Y state |Y⟩ = (|0⟩ + i|1⟩)/√2
    YState,
    /// CCZ state for multi-controlled operations
    CCZState,
    /// Custom magic state
    Custom { name: String, fidelity: f64 },
}

impl MagicState {
    /// Get the fidelity threshold required for distillation
    #[must_use]
    pub const fn fidelity_threshold(&self) -> f64 {
        match self {
            Self::TState | Self::YState => 0.95,
            Self::CCZState => 0.99,
            Self::Custom { fidelity, .. } => *fidelity,
        }
    }

    /// Get distillation overhead
    #[must_use]
    pub const fn distillation_overhead(&self) -> f64 {
        match self {
            Self::TState => 15.0, // Approximate overhead for T state distillation
            Self::YState => 10.0,
            Self::CCZState => 50.0,
            Self::Custom { .. } => 20.0,
        }
    }
}

/// Fault-tolerant circuit compiler
pub struct FaultTolerantCompiler {
    /// Default error correction code
    default_code: QECCode,
    /// Magic state factory configuration
    magic_state_factory: MagicStateFactory,
    /// Error threshold
    error_threshold: f64,
    /// Compilation options
    options: CompilationOptions,
}

/// Magic state factory for producing high-fidelity magic states
#[derive(Debug, Clone)]
pub struct MagicStateFactory {
    /// Types of magic states produced
    supported_states: Vec<MagicState>,
    /// Production rate (states per second)
    production_rate: HashMap<MagicState, f64>,
    /// Distillation protocols
    distillation_protocols: HashMap<MagicState, DistillationProtocol>,
}

/// Distillation protocol for magic states
#[derive(Debug, Clone)]
pub struct DistillationProtocol {
    /// Protocol name
    pub name: String,
    /// Input state fidelity requirement
    pub input_fidelity: f64,
    /// Output state fidelity
    pub output_fidelity: f64,
    /// Success probability
    pub success_probability: f64,
    /// Physical qubits required
    pub qubits_required: usize,
    /// Time overhead
    pub time_overhead: f64,
}

/// Compilation options for fault-tolerant circuits
#[derive(Debug, Clone)]
pub struct CompilationOptions {
    /// Optimize for space (fewer qubits)
    pub optimize_space: bool,
    /// Optimize for time (fewer operations)
    pub optimize_time: bool,
    /// Enable magic state recycling
    pub recycle_magic_states: bool,
    /// Syndrome extraction frequency
    pub syndrome_frequency: usize,
    /// Error correction strategy
    pub correction_strategy: CorrectionStrategy,
}

/// Error correction strategies
#[derive(Debug, Clone, PartialEq)]
pub enum CorrectionStrategy {
    /// Correct errors immediately when detected
    Immediate,
    /// Defer correction until end of computation
    Deferred,
    /// Adaptive strategy based on error rates
    Adaptive { threshold: f64 },
}

impl Default for CompilationOptions {
    fn default() -> Self {
        Self {
            optimize_space: false,
            optimize_time: true,
            recycle_magic_states: true,
            syndrome_frequency: 1,
            correction_strategy: CorrectionStrategy::Immediate,
        }
    }
}

impl FaultTolerantCompiler {
    /// Create a new fault-tolerant compiler
    #[must_use]
    pub fn new(code: QECCode) -> Self {
        let magic_state_factory = MagicStateFactory {
            supported_states: vec![MagicState::TState, MagicState::YState],
            production_rate: HashMap::new(),
            distillation_protocols: HashMap::new(),
        };

        Self {
            default_code: code,
            magic_state_factory,
            error_threshold: 1e-3,
            options: CompilationOptions::default(),
        }
    }

    /// Compile a logical circuit to fault-tolerant implementation
    pub fn compile<const N: usize>(
        &self,
        logical_circuit: &Circuit<N>,
    ) -> QuantRS2Result<FaultTolerantCircuit> {
        // Create logical qubits
        let logical_qubits = self.create_logical_qubits(N)?;

        // Compile each gate
        let mut ft_gates = Vec::new();
        let mut magic_state_count = 0;

        for gate in logical_circuit.gates() {
            let ft_gate = self.compile_gate(gate.as_ref(), &logical_qubits)?;
            magic_state_count += ft_gate.magic_states;
            ft_gates.push(ft_gate);
        }

        // Generate syndrome extraction circuits
        let syndrome_circuits = self.generate_syndrome_circuits(&logical_qubits)?;

        // Calculate resource requirements
        let total_physical_qubits: usize = logical_qubits
            .iter()
            .map(|lq| lq.physical_qubits.len())
            .sum();

        let ancilla_qubits = syndrome_circuits
            .iter()
            .map(|sc| sc.ancilla_qubits.len())
            .sum::<usize>();

        Ok(FaultTolerantCircuit {
            logical_qubits,
            ft_gates,
            syndrome_circuits,
            magic_state_requirements: magic_state_count,
            physical_qubit_count: total_physical_qubits + ancilla_qubits,
            error_threshold: self.error_threshold,
            code: self.default_code.clone(),
        })
    }

    /// Create logical qubits for the circuit
    fn create_logical_qubits(&self, num_logical: usize) -> QuantRS2Result<Vec<LogicalQubit>> {
        let mut logical_qubits = Vec::new();

        for i in 0..num_logical {
            let logical_qubit = LogicalQubit::new(i, self.default_code.clone());
            logical_qubits.push(logical_qubit);
        }

        Ok(logical_qubits)
    }

    /// Compile a single logical gate to fault-tolerant implementation
    fn compile_gate(
        &self,
        gate: &dyn GateOp,
        logical_qubits: &[LogicalQubit],
    ) -> QuantRS2Result<FaultTolerantGate> {
        let gate_name = gate.name();
        let logical_targets: Vec<_> = gate.qubits().iter().map(|q| q.id() as usize).collect();

        match gate_name {
            "H" => self.compile_hadamard_gate(&logical_targets, logical_qubits),
            "CNOT" => self.compile_cnot_gate(&logical_targets, logical_qubits),
            "T" => self.compile_t_gate(&logical_targets, logical_qubits),
            "S" => self.compile_s_gate(&logical_targets, logical_qubits),
            "X" | "Y" | "Z" => self.compile_pauli_gate(gate_name, &logical_targets, logical_qubits),
            _ => Err(QuantRS2Error::InvalidInput(format!(
                "Gate {gate_name} not supported in fault-tolerant compilation"
            ))),
        }
    }

    /// Compile Hadamard gate
    fn compile_hadamard_gate(
        &self,
        targets: &[usize],
        logical_qubits: &[LogicalQubit],
    ) -> QuantRS2Result<FaultTolerantGate> {
        if targets.len() != 1 {
            return Err(QuantRS2Error::InvalidInput(
                "Hadamard gate requires exactly one target".to_string(),
            ));
        }

        let target_lq = &logical_qubits[targets[0]];
        let mut physical_gates = Vec::new();

        // For surface code, Hadamard is transversal
        match &target_lq.code {
            QECCode::SurfaceCode { .. } => {
                // Apply Hadamard to all data qubits
                for &data_qubit in &target_lq.data_qubits() {
                    physical_gates.push(PhysicalGate {
                        gate_type: "H".to_string(),
                        qubits: vec![data_qubit],
                        parameters: vec![],
                        time: Some(1.0),
                    });
                }
            }
            _ => {
                // For other codes, might need magic state injection
                physical_gates.push(PhysicalGate {
                    gate_type: "H".to_string(),
                    qubits: target_lq.data_qubits(),
                    parameters: vec![],
                    time: Some(1.0),
                });
            }
        }

        Ok(FaultTolerantGate {
            name: "H".to_string(),
            logical_qubits: targets.to_vec(),
            physical_gates,
            syndrome_measurements: vec![],
            magic_states: 0,
            correction_overhead: 1.0,
        })
    }

    /// Compile CNOT gate
    fn compile_cnot_gate(
        &self,
        targets: &[usize],
        logical_qubits: &[LogicalQubit],
    ) -> QuantRS2Result<FaultTolerantGate> {
        if targets.len() != 2 {
            return Err(QuantRS2Error::InvalidInput(
                "CNOT gate requires exactly two targets".to_string(),
            ));
        }

        let control_lq = &logical_qubits[targets[0]];
        let target_lq = &logical_qubits[targets[1]];
        let mut physical_gates = Vec::new();

        // For surface code, CNOT requires lattice surgery or braiding
        if let (QECCode::SurfaceCode { .. }, QECCode::SurfaceCode { .. }) =
            (&control_lq.code, &target_lq.code)
        {
            // Implement lattice surgery for CNOT
            let control_data = control_lq.data_qubits();
            let target_data = target_lq.data_qubits();

            // This is a simplified implementation
            // Real lattice surgery requires careful boundary management
            for (i, (&c_qubit, &t_qubit)) in control_data.iter().zip(target_data.iter()).enumerate()
            {
                physical_gates.push(PhysicalGate {
                    gate_type: "CNOT".to_string(),
                    qubits: vec![c_qubit, t_qubit],
                    parameters: vec![],
                    time: Some(10.0), // Lattice surgery takes longer
                });
            }
        } else {
            // Simplified implementation for other codes
            let control_data = control_lq.data_qubits();
            let target_data = target_lq.data_qubits();

            for (&c_qubit, &t_qubit) in control_data.iter().zip(target_data.iter()) {
                physical_gates.push(PhysicalGate {
                    gate_type: "CNOT".to_string(),
                    qubits: vec![c_qubit, t_qubit],
                    parameters: vec![],
                    time: Some(2.0),
                });
            }
        }

        Ok(FaultTolerantGate {
            name: "CNOT".to_string(),
            logical_qubits: targets.to_vec(),
            physical_gates,
            syndrome_measurements: vec![],
            magic_states: 0,
            correction_overhead: 2.0,
        })
    }

    /// Compile T gate (requires magic state)
    fn compile_t_gate(
        &self,
        targets: &[usize],
        logical_qubits: &[LogicalQubit],
    ) -> QuantRS2Result<FaultTolerantGate> {
        if targets.len() != 1 {
            return Err(QuantRS2Error::InvalidInput(
                "T gate requires exactly one target".to_string(),
            ));
        }

        let target_lq = &logical_qubits[targets[0]];
        let mut physical_gates = Vec::new();
        let mut syndrome_measurements = Vec::new();

        // T gate requires magic state injection
        let data_qubits = target_lq.data_qubits();
        let ancilla_qubits = target_lq.ancilla_qubits();

        // Simplified magic state injection
        for (&data_qubit, &ancilla_qubit) in data_qubits.iter().zip(ancilla_qubits.iter()) {
            // CNOT from magic state to data qubit
            physical_gates.push(PhysicalGate {
                gate_type: "CNOT".to_string(),
                qubits: vec![ancilla_qubit, data_qubit], // Magic state as control
                parameters: vec![],
                time: Some(1.0),
            });

            // Measure ancilla in X basis
            syndrome_measurements.push(SyndromeMeasurement {
                measurement_type: SyndromeType::XStabilizer,
                qubits: vec![ancilla_qubit],
                ancilla: ancilla_qubit,
                expected_value: 0,
            });
        }

        Ok(FaultTolerantGate {
            name: "T".to_string(),
            logical_qubits: targets.to_vec(),
            physical_gates,
            syndrome_measurements,
            magic_states: 1,
            correction_overhead: 15.0, // High overhead due to magic state distillation
        })
    }

    /// Compile S gate
    fn compile_s_gate(
        &self,
        targets: &[usize],
        logical_qubits: &[LogicalQubit],
    ) -> QuantRS2Result<FaultTolerantGate> {
        if targets.len() != 1 {
            return Err(QuantRS2Error::InvalidInput(
                "S gate requires exactly one target".to_string(),
            ));
        }

        let target_lq = &logical_qubits[targets[0]];
        let mut physical_gates = Vec::new();

        // S gate is transversal for many codes
        for &data_qubit in &target_lq.data_qubits() {
            physical_gates.push(PhysicalGate {
                gate_type: "S".to_string(),
                qubits: vec![data_qubit],
                parameters: vec![],
                time: Some(1.0),
            });
        }

        Ok(FaultTolerantGate {
            name: "S".to_string(),
            logical_qubits: targets.to_vec(),
            physical_gates,
            syndrome_measurements: vec![],
            magic_states: 0,
            correction_overhead: 1.0,
        })
    }

    /// Compile Pauli gates (X, Y, Z)
    fn compile_pauli_gate(
        &self,
        gate_name: &str,
        targets: &[usize],
        logical_qubits: &[LogicalQubit],
    ) -> QuantRS2Result<FaultTolerantGate> {
        if targets.len() != 1 {
            return Err(QuantRS2Error::InvalidInput(
                "Pauli gate requires exactly one target".to_string(),
            ));
        }

        let target_lq = &logical_qubits[targets[0]];
        let mut physical_gates = Vec::new();

        // Pauli gates are transversal
        for &data_qubit in &target_lq.data_qubits() {
            physical_gates.push(PhysicalGate {
                gate_type: gate_name.to_string(),
                qubits: vec![data_qubit],
                parameters: vec![],
                time: Some(1.0),
            });
        }

        Ok(FaultTolerantGate {
            name: gate_name.to_string(),
            logical_qubits: targets.to_vec(),
            physical_gates,
            syndrome_measurements: vec![],
            magic_states: 0,
            correction_overhead: 1.0,
        })
    }

    /// Generate syndrome extraction circuits
    fn generate_syndrome_circuits(
        &self,
        logical_qubits: &[LogicalQubit],
    ) -> QuantRS2Result<Vec<SyndromeCircuit>> {
        let mut circuits = Vec::new();

        for logical_qubit in logical_qubits {
            match &logical_qubit.code {
                QECCode::SurfaceCode { distance } => {
                    circuits.push(self.generate_surface_code_syndrome(logical_qubit, *distance)?);
                }
                QECCode::SteaneCode => {
                    circuits.push(self.generate_steane_syndrome(logical_qubit)?);
                }
                _ => {
                    // Generic syndrome circuit
                    circuits.push(self.generate_generic_syndrome(logical_qubit)?);
                }
            }
        }

        Ok(circuits)
    }

    /// Generate syndrome circuit for surface code
    fn generate_surface_code_syndrome(
        &self,
        logical_qubit: &LogicalQubit,
        distance: usize,
    ) -> QuantRS2Result<SyndromeCircuit> {
        let mut measurements = Vec::new();
        let mut ancilla_qubits = Vec::new();

        // X-type stabilizers (star operators)
        for i in 0..distance - 1 {
            for j in 0..distance {
                if (i + j) % 2 == 1 {
                    // X-stabilizers on odd positions
                    let ancilla = logical_qubit.physical_qubits.len() + ancilla_qubits.len();
                    ancilla_qubits.push(ancilla);

                    // Qubits involved in this stabilizer
                    let involved_qubits = vec![
                        logical_qubit.physical_qubits[i * distance + j],
                        logical_qubit.physical_qubits[(i + 1) * distance + j],
                    ];

                    measurements.push(SyndromeMeasurement {
                        measurement_type: SyndromeType::XStabilizer,
                        qubits: involved_qubits,
                        ancilla,
                        expected_value: 0,
                    });
                }
            }
        }

        // Z-type stabilizers (face operators)
        for i in 0..distance {
            for j in 0..distance - 1 {
                if (i + j) % 2 == 0 {
                    // Z-stabilizers on even positions
                    let ancilla = logical_qubit.physical_qubits.len() + ancilla_qubits.len();
                    ancilla_qubits.push(ancilla);

                    let involved_qubits = vec![
                        logical_qubit.physical_qubits[i * distance + j],
                        logical_qubit.physical_qubits[i * distance + j + 1],
                    ];

                    measurements.push(SyndromeMeasurement {
                        measurement_type: SyndromeType::ZStabilizer,
                        qubits: involved_qubits,
                        ancilla,
                        expected_value: 0,
                    });
                }
            }
        }

        Ok(SyndromeCircuit {
            logical_qubit_id: logical_qubit.id,
            measurements: measurements.clone(),
            ancilla_qubits,
            syndrome_length: measurements.len(),
        })
    }

    /// Generate syndrome circuit for Steane code
    fn generate_steane_syndrome(
        &self,
        logical_qubit: &LogicalQubit,
    ) -> QuantRS2Result<SyndromeCircuit> {
        let mut measurements = Vec::new();
        let ancilla_qubits = vec![7, 8, 9, 10, 11, 12]; // 6 ancilla qubits for Steane code

        // X-type stabilizers
        let x_stabilizers = [
            [0, 1, 2, 3], // First X stabilizer
            [1, 2, 5, 6], // Second X stabilizer
            [0, 3, 4, 5], // Third X stabilizer
        ];

        for (i, stabilizer) in x_stabilizers.iter().enumerate() {
            measurements.push(SyndromeMeasurement {
                measurement_type: SyndromeType::XStabilizer,
                qubits: stabilizer
                    .iter()
                    .map(|&q| logical_qubit.physical_qubits[q])
                    .collect(),
                ancilla: ancilla_qubits[i],
                expected_value: 0,
            });
        }

        // Z-type stabilizers
        let z_stabilizers = [
            [0, 1, 4, 6], // First Z stabilizer
            [1, 3, 4, 5], // Second Z stabilizer
            [0, 2, 3, 6], // Third Z stabilizer
        ];

        for (i, stabilizer) in z_stabilizers.iter().enumerate() {
            measurements.push(SyndromeMeasurement {
                measurement_type: SyndromeType::ZStabilizer,
                qubits: stabilizer
                    .iter()
                    .map(|&q| logical_qubit.physical_qubits[q])
                    .collect(),
                ancilla: ancilla_qubits[i + 3],
                expected_value: 0,
            });
        }

        Ok(SyndromeCircuit {
            logical_qubit_id: logical_qubit.id,
            measurements,
            ancilla_qubits,
            syndrome_length: 6,
        })
    }

    /// Generate generic syndrome circuit
    fn generate_generic_syndrome(
        &self,
        logical_qubit: &LogicalQubit,
    ) -> QuantRS2Result<SyndromeCircuit> {
        let measurements = vec![SyndromeMeasurement {
            measurement_type: SyndromeType::ZStabilizer,
            qubits: logical_qubit.data_qubits(),
            ancilla: logical_qubit.ancilla_qubits()[0],
            expected_value: 0,
        }];

        Ok(SyndromeCircuit {
            logical_qubit_id: logical_qubit.id,
            measurements: measurements.clone(),
            ancilla_qubits: logical_qubit.ancilla_qubits(),
            syndrome_length: measurements.len(),
        })
    }
}

/// Syndrome extraction circuit
#[derive(Debug, Clone)]
pub struct SyndromeCircuit {
    /// Logical qubit this circuit applies to
    pub logical_qubit_id: usize,
    /// Syndrome measurements
    pub measurements: Vec<SyndromeMeasurement>,
    /// Ancilla qubits used
    pub ancilla_qubits: Vec<usize>,
    /// Length of syndrome bit string
    pub syndrome_length: usize,
}

/// Fault-tolerant circuit representation
#[derive(Debug, Clone)]
pub struct FaultTolerantCircuit {
    /// Logical qubits
    pub logical_qubits: Vec<LogicalQubit>,
    /// Fault-tolerant gates
    pub ft_gates: Vec<FaultTolerantGate>,
    /// Syndrome extraction circuits
    pub syndrome_circuits: Vec<SyndromeCircuit>,
    /// Total magic states required
    pub magic_state_requirements: usize,
    /// Total physical qubits needed
    pub physical_qubit_count: usize,
    /// Error threshold
    pub error_threshold: f64,
    /// Error correction code used
    pub code: QECCode,
}

impl FaultTolerantCircuit {
    /// Calculate total execution time
    #[must_use]
    pub fn execution_time(&self) -> f64 {
        let gate_time: f64 = self
            .ft_gates
            .iter()
            .flat_map(|gate| &gate.physical_gates)
            .filter_map(|pg| pg.time)
            .sum();

        let syndrome_time = self.syndrome_circuits.len() as f64 * 10.0; // Assume 10 units per syndrome round

        gate_time + syndrome_time
    }

    /// Estimate resource overhead compared to logical circuit
    #[must_use]
    pub fn resource_overhead(&self, logical_gates: usize) -> ResourceOverhead {
        let space_overhead = self.physical_qubit_count as f64 / self.logical_qubits.len() as f64;

        let physical_gates: usize = self
            .ft_gates
            .iter()
            .map(|gate| gate.physical_gates.len())
            .sum();
        let time_overhead = physical_gates as f64 / logical_gates as f64;

        ResourceOverhead {
            space_overhead,
            time_overhead,
            magic_state_overhead: self.magic_state_requirements as f64,
        }
    }
}

/// Resource overhead analysis
#[derive(Debug, Clone)]
pub struct ResourceOverhead {
    /// Physical qubits per logical qubit
    pub space_overhead: f64,
    /// Physical gates per logical gate
    pub time_overhead: f64,
    /// Magic states required
    pub magic_state_overhead: f64,
}

#[cfg(test)]
mod tests {
    use super::*;
    use quantrs2_core::gate::multi::CNOT;
    use quantrs2_core::gate::single::{Hadamard, PauliX};

    #[test]
    fn test_qec_code_properties() {
        let surface_code = QECCode::SurfaceCode { distance: 3 };
        assert_eq!(surface_code.physical_qubits(), 9);
        assert_eq!(surface_code.distance(), 3);
        assert!(surface_code.can_correct(1));

        let steane_code = QECCode::SteaneCode;
        assert_eq!(steane_code.physical_qubits(), 7);
        assert_eq!(steane_code.distance(), 3);
    }

    #[test]
    fn test_logical_qubit_creation() {
        let code = QECCode::SurfaceCode { distance: 3 };
        let logical_qubit = LogicalQubit::new(0, code);

        assert_eq!(logical_qubit.id, 0);
        assert_eq!(logical_qubit.physical_qubits.len(), 9);
        assert!(!logical_qubit.data_qubits().is_empty());
    }

    #[test]
    fn test_ft_compiler_creation() {
        let code = QECCode::SteaneCode;
        let compiler = FaultTolerantCompiler::new(code);

        assert!(matches!(compiler.default_code, QECCode::SteaneCode));
        assert!(compiler.error_threshold > 0.0);
    }

    #[test]
    fn test_magic_state_properties() {
        let t_state = MagicState::TState;
        assert!(t_state.fidelity_threshold() > 0.9);
        assert!(t_state.distillation_overhead() > 1.0);

        let custom_state = MagicState::Custom {
            name: "Test".to_string(),
            fidelity: 0.98,
        };
        assert_eq!(custom_state.fidelity_threshold(), 0.98);
    }

    #[test]
    fn test_syndrome_measurement() {
        let measurement = SyndromeMeasurement {
            measurement_type: SyndromeType::XStabilizer,
            qubits: vec![0, 1, 2],
            ancilla: 3,
            expected_value: 0,
        };

        assert_eq!(measurement.qubits.len(), 3);
        assert_eq!(measurement.measurement_type, SyndromeType::XStabilizer);
    }

    #[test]
    fn test_ft_circuit_properties() {
        let code = QECCode::SteaneCode;
        let logical_qubits = vec![LogicalQubit::new(0, code.clone())];

        let circuit = FaultTolerantCircuit {
            logical_qubits,
            ft_gates: vec![],
            syndrome_circuits: vec![],
            magic_state_requirements: 5,
            physical_qubit_count: 20,
            error_threshold: 1e-3,
            code,
        };

        assert_eq!(circuit.magic_state_requirements, 5);
        assert_eq!(circuit.physical_qubit_count, 20);

        let overhead = circuit.resource_overhead(10);
        assert!(overhead.space_overhead > 1.0);
    }

    #[test]
    fn test_compilation_options() {
        let options = CompilationOptions::default();
        assert!(options.optimize_time);
        assert!(options.recycle_magic_states);
        assert_eq!(options.syndrome_frequency, 1);
    }
}