quantrs2_device/topological/

topological_codes.rs

//! Topological quantum error correcting codes
//!
//! This module implements various topological quantum error correcting codes
//! including surface codes, color codes, and other topological stabilizer codes.

use super::{NonAbelianAnyonType, TopologicalCharge, TopologicalError, TopologicalResult};
use serde::{Deserialize, Serialize};
use std::collections::{HashMap, HashSet};

/// Types of topological quantum error correcting codes
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub enum TopologicalCodeType {
    /// Surface code (toric code)
    SurfaceCode,
    /// Planar surface code
    PlanarSurfaceCode,
    /// Color code (triangular lattice)
    ColorCode,
    /// Honeycomb code
    HoneycombCode,
    /// Fibonacci code
    FibonacciCode,
    /// Ising anyon code
    IsingCode,
}

/// Stabilizer for topological codes
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct TopologicalStabilizer {
    /// Stabilizer ID
    pub stabilizer_id: usize,
    /// Type of stabilizer (X-type, Z-type, or mixed)
    pub stabilizer_type: StabilizerType,
    /// Qubits involved in the stabilizer
    pub qubits: Vec<usize>,
    /// Pauli operators on each qubit
    pub operators: Vec<PauliOperator>,
    /// Geometric location (for surface codes)
    pub location: Option<(i32, i32)>,
}

/// Types of stabilizers
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub enum StabilizerType {
    /// X-type stabilizer (star operator)
    XType,
    /// Z-type stabilizer (plaquette operator)
    ZType,
    /// Mixed stabilizer
    Mixed,
}

/// Pauli operators
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub enum PauliOperator {
    I, // Identity
    X, // Pauli-X
    Y, // Pauli-Y
    Z, // Pauli-Z
}

/// Logical operator for encoded qubits
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct LogicalOperator {
    /// Operator ID
    pub operator_id: usize,
    /// Type of logical operator
    pub operator_type: LogicalOperatorType,
    /// Physical qubits involved
    pub qubits: Vec<usize>,
    /// Pauli operators
    pub operators: Vec<PauliOperator>,
    /// Weight of the operator
    pub weight: usize,
}

/// Types of logical operators
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub enum LogicalOperatorType {
    /// Logical X operator
    LogicalX,
    /// Logical Z operator
    LogicalZ,
    /// Logical Y operator
    LogicalY,
}

/// Syndrome measurement result
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SyndromeMeasurement {
    /// Stabilizer ID
    pub stabilizer_id: usize,
    /// Measurement outcome (+1 or -1)
    pub outcome: i8,
    /// Timestamp of measurement
    pub timestamp: f64,
    /// Measurement fidelity
    pub fidelity: f64,
}

/// Error correction decoder
pub trait TopologicalDecoder {
    /// Decode syndrome measurements to find error correction
    fn decode_syndrome(
        &self,
        syndrome: &[SyndromeMeasurement],
        code_distance: usize,
    ) -> TopologicalResult<Vec<ErrorCorrection>>;

    /// Calculate error probability for a given syndrome
    fn calculate_error_probability(&self, syndrome: &[SyndromeMeasurement]) -> f64;
}

/// Error correction operation
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ErrorCorrection {
    /// Qubits to apply correction to
    pub qubits: Vec<usize>,
    /// Correction operators
    pub corrections: Vec<PauliOperator>,
    /// Confidence in this correction
    pub confidence: f64,
}

/// Surface code implementation
pub struct SurfaceCode {
    /// Code distance
    pub distance: usize,
    /// Lattice dimensions
    pub lattice_size: (usize, usize),
    /// Physical qubits
    pub physical_qubits: HashMap<(i32, i32), usize>,
    /// X-type stabilizers
    pub x_stabilizers: Vec<TopologicalStabilizer>,
    /// Z-type stabilizers
    pub z_stabilizers: Vec<TopologicalStabilizer>,
    /// Logical X operators
    pub logical_x: Vec<LogicalOperator>,
    /// Logical Z operators
    pub logical_z: Vec<LogicalOperator>,
}

impl SurfaceCode {
    /// Create a new surface code
    pub fn new(distance: usize) -> TopologicalResult<Self> {
        if distance < 3 || distance % 2 == 0 {
            return Err(TopologicalError::InvalidInput(
                "Surface code distance must be odd and >= 3".to_string(),
            ));
        }

        let lattice_size = (2 * distance - 1, 2 * distance - 1);
        let mut physical_qubits = HashMap::new();
        let mut qubit_id = 0;

        // Create physical qubits on lattice sites
        for i in 0..(lattice_size.0 as i32) {
            for j in 0..(lattice_size.1 as i32) {
                if (i + j) % 2 == 1 {
                    // Qubits on edges
                    physical_qubits.insert((i, j), qubit_id);
                    qubit_id += 1;
                }
            }
        }

        let mut surface_code = Self {
            distance,
            lattice_size,
            physical_qubits,
            x_stabilizers: Vec::new(),
            z_stabilizers: Vec::new(),
            logical_x: Vec::new(),
            logical_z: Vec::new(),
        };

        surface_code.build_stabilizers()?;
        surface_code.build_logical_operators()?;

        Ok(surface_code)
    }

    /// Build stabilizer generators
    fn build_stabilizers(&mut self) -> TopologicalResult<()> {
        let mut stabilizer_id = 0;

        // X-type stabilizers (star operators)
        for i in (0..(self.lattice_size.0 as i32)).step_by(2) {
            for j in (0..(self.lattice_size.1 as i32)).step_by(2) {
                let mut qubits = Vec::new();
                let mut operators = Vec::new();

                // Check neighboring edges
                for (di, dj) in &[(0, 1), (1, 0), (0, -1), (-1, 0)] {
                    let ni = i + di;
                    let nj = j + dj;

                    if let Some(&qubit_id) = self.physical_qubits.get(&(ni, nj)) {
                        qubits.push(qubit_id);
                        operators.push(PauliOperator::X);
                    }
                }

                if !qubits.is_empty() {
                    self.x_stabilizers.push(TopologicalStabilizer {
                        stabilizer_id,
                        stabilizer_type: StabilizerType::XType,
                        qubits,
                        operators,
                        location: Some((i, j)),
                    });
                    stabilizer_id += 1;
                }
            }
        }

        // Z-type stabilizers (plaquette operators)
        for i in (1..(self.lattice_size.0 as i32)).step_by(2) {
            for j in (1..(self.lattice_size.1 as i32)).step_by(2) {
                let mut qubits = Vec::new();
                let mut operators = Vec::new();

                // Check neighboring edges
                for (di, dj) in &[(0, 1), (1, 0), (0, -1), (-1, 0)] {
                    let ni = i + di;
                    let nj = j + dj;

                    if let Some(&qubit_id) = self.physical_qubits.get(&(ni, nj)) {
                        qubits.push(qubit_id);
                        operators.push(PauliOperator::Z);
                    }
                }

                if !qubits.is_empty() {
                    self.z_stabilizers.push(TopologicalStabilizer {
                        stabilizer_id,
                        stabilizer_type: StabilizerType::ZType,
                        qubits,
                        operators,
                        location: Some((i, j)),
                    });
                    stabilizer_id += 1;
                }
            }
        }

        Ok(())
    }

    /// Build logical operators
    fn build_logical_operators(&mut self) -> TopologicalResult<()> {
        // Logical X: horizontal string
        let mut logical_x_qubits = Vec::new();
        let middle_row = (self.lattice_size.1 / 2) as i32;

        for i in (1..(self.lattice_size.0 as i32)).step_by(2) {
            if let Some(&qubit_id) = self.physical_qubits.get(&(i, middle_row)) {
                logical_x_qubits.push(qubit_id);
            }
        }

        self.logical_x.push(LogicalOperator {
            operator_id: 0,
            operator_type: LogicalOperatorType::LogicalX,
            qubits: logical_x_qubits.clone(),
            operators: vec![PauliOperator::X; logical_x_qubits.len()],
            weight: logical_x_qubits.len(),
        });

        // Logical Z: vertical string
        let mut logical_z_qubits = Vec::new();
        let middle_col = (self.lattice_size.0 / 2) as i32;

        for j in (1..(self.lattice_size.1 as i32)).step_by(2) {
            if let Some(&qubit_id) = self.physical_qubits.get(&(middle_col, j)) {
                logical_z_qubits.push(qubit_id);
            }
        }

        self.logical_z.push(LogicalOperator {
            operator_id: 0,
            operator_type: LogicalOperatorType::LogicalZ,
            qubits: logical_z_qubits.clone(),
            operators: vec![PauliOperator::Z; logical_z_qubits.len()],
            weight: logical_z_qubits.len(),
        });

        Ok(())
    }

    /// Get number of physical qubits
    pub fn physical_qubit_count(&self) -> usize {
        self.physical_qubits.len()
    }

    /// Get number of logical qubits
    pub const fn logical_qubit_count(&self) -> usize {
        1 // Surface code encodes 1 logical qubit
    }

    /// Get all stabilizers
    pub fn get_all_stabilizers(&self) -> Vec<&TopologicalStabilizer> {
        let mut stabilizers = Vec::new();
        stabilizers.extend(self.x_stabilizers.iter());
        stabilizers.extend(self.z_stabilizers.iter());
        stabilizers
    }
}

/// Minimum weight perfect matching decoder for surface codes
pub struct MWPMDecoder {
    code_distance: usize,
    error_probability: f64,
}

impl MWPMDecoder {
    /// Create a new MWPM decoder
    pub const fn new(code_distance: usize, error_probability: f64) -> Self {
        Self {
            code_distance,
            error_probability,
        }
    }

    /// Find minimum weight matching for syndrome
    fn find_minimum_weight_matching(
        &self,
        defects: &[(i32, i32)],
    ) -> TopologicalResult<Vec<((i32, i32), (i32, i32))>> {
        // Simplified implementation - would use proper MWPM algorithm
        let mut matching = Vec::new();
        let mut unmatched = defects.to_vec();

        while unmatched.len() >= 2 {
            // Safe: loop condition guarantees at least 2 elements
            let defect1 = unmatched
                .pop()
                .expect("Should have at least 2 elements in unmatched");
            let defect2 = unmatched
                .pop()
                .expect("Should have at least 1 element in unmatched");
            matching.push((defect1, defect2));
        }

        Ok(matching)
    }

    /// Convert matching to error correction
    fn matching_to_correction(
        &self,
        matching: &[((i32, i32), (i32, i32))],
        surface_code: &SurfaceCode,
    ) -> TopologicalResult<Vec<ErrorCorrection>> {
        let mut corrections = Vec::new();

        for &((x1, y1), (x2, y2)) in matching {
            // Find path between defects and apply correction
            let path = self.find_path((x1, y1), (x2, y2));
            let mut qubits = Vec::new();
            let mut operators = Vec::new();

            for (x, y) in path {
                if let Some(&qubit_id) = surface_code.physical_qubits.get(&(x, y)) {
                    qubits.push(qubit_id);
                    operators.push(PauliOperator::X); // Simplified
                }
            }

            if !qubits.is_empty() {
                corrections.push(ErrorCorrection {
                    qubits,
                    corrections: operators,
                    confidence: 0.95, // Would be calculated properly
                });
            }
        }

        Ok(corrections)
    }

    /// Find path between two points (simplified)
    fn find_path(&self, start: (i32, i32), end: (i32, i32)) -> Vec<(i32, i32)> {
        let mut path = Vec::new();
        let (mut x, mut y) = start;
        let (target_x, target_y) = end;

        // Simple path finding - move horizontally then vertically
        while x != target_x {
            if x < target_x {
                x += 1;
            } else {
                x -= 1;
            }
            path.push((x, y));
        }

        while y != target_y {
            if y < target_y {
                y += 1;
            } else {
                y -= 1;
            }
            path.push((x, y));
        }

        path
    }
}

impl TopologicalDecoder for MWPMDecoder {
    fn decode_syndrome(
        &self,
        syndrome: &[SyndromeMeasurement],
        _code_distance: usize,
    ) -> TopologicalResult<Vec<ErrorCorrection>> {
        // Find defects (syndrome violations)
        let mut defects = Vec::new();

        for measurement in syndrome {
            if measurement.outcome == -1 {
                // This is a simplified implementation
                // Would map stabilizer_id to lattice coordinates
                defects.push((measurement.stabilizer_id as i32, 0));
            }
        }

        // Find minimum weight matching
        let matching = self.find_minimum_weight_matching(&defects)?;

        // Convert to error correction
        // This is simplified - would need actual surface code instance
        let corrections = vec![ErrorCorrection {
            qubits: vec![0],
            corrections: vec![PauliOperator::X],
            confidence: 0.95,
        }];

        Ok(corrections)
    }

    fn calculate_error_probability(&self, syndrome: &[SyndromeMeasurement]) -> f64 {
        // Simplified probability calculation
        let defect_count = syndrome.iter().filter(|m| m.outcome == -1).count();

        self.error_probability.powi(defect_count as i32)
    }
}

/// Color code implementation (simplified)
pub struct ColorCode {
    /// Code distance
    pub distance: usize,
    /// Triangle lattice qubits
    pub qubits: HashMap<(i32, i32), usize>,
    /// Color stabilizers (red, green, blue)
    pub stabilizers: Vec<TopologicalStabilizer>,
    /// Logical operators
    pub logical_operators: Vec<LogicalOperator>,
}

impl ColorCode {
    /// Create a new color code
    pub fn new(distance: usize) -> TopologicalResult<Self> {
        let mut color_code = Self {
            distance,
            qubits: HashMap::new(),
            stabilizers: Vec::new(),
            logical_operators: Vec::new(),
        };

        color_code.build_triangular_lattice()?;
        color_code.build_color_stabilizers()?;

        Ok(color_code)
    }

    /// Build triangular lattice
    fn build_triangular_lattice(&mut self) -> TopologicalResult<()> {
        let mut qubit_id = 0;

        for i in 0..(2 * self.distance) {
            for j in 0..(2 * self.distance) {
                // Triangular lattice placement
                self.qubits.insert((i as i32, j as i32), qubit_id);
                qubit_id += 1;
            }
        }

        Ok(())
    }

    /// Build color-coded stabilizers
    fn build_color_stabilizers(&mut self) -> TopologicalResult<()> {
        // This is a simplified implementation
        // Would build proper color code stabilizers for each color

        let mut stabilizer_id = 0;

        // Red stabilizers (simplified)
        for i in (0..(self.distance * 2)).step_by(3) {
            for j in (0..(self.distance * 2)).step_by(3) {
                let mut qubits = Vec::new();
                let mut operators = Vec::new();

                // Collect qubits in red plaquette
                for di in 0..3 {
                    for dj in 0..3 {
                        if let Some(&qubit_id) =
                            self.qubits.get(&((i + di) as i32, (j + dj) as i32))
                        {
                            qubits.push(qubit_id);
                            operators.push(PauliOperator::X);
                        }
                    }
                }

                if !qubits.is_empty() {
                    self.stabilizers.push(TopologicalStabilizer {
                        stabilizer_id,
                        stabilizer_type: StabilizerType::XType,
                        qubits,
                        operators,
                        location: Some((i as i32, j as i32)),
                    });
                    stabilizer_id += 1;
                }
            }
        }

        Ok(())
    }

    /// Get number of physical qubits
    pub fn physical_qubit_count(&self) -> usize {
        self.qubits.len()
    }
}

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

    #[test]
    fn test_surface_code_creation() {
        let surface_code = SurfaceCode::new(3).expect("Surface code creation should succeed");
        assert_eq!(surface_code.distance, 3);
        assert!(surface_code.physical_qubit_count() > 0);
        assert_eq!(surface_code.logical_qubit_count(), 1);
    }

    #[test]
    fn test_surface_code_stabilizers() {
        let surface_code = SurfaceCode::new(3).expect("Surface code creation should succeed");
        let stabilizers = surface_code.get_all_stabilizers();
        assert!(!stabilizers.is_empty());

        // Check that we have both X and Z stabilizers
        let x_count = stabilizers
            .iter()
            .filter(|s| s.stabilizer_type == StabilizerType::XType)
            .count();
        let z_count = stabilizers
            .iter()
            .filter(|s| s.stabilizer_type == StabilizerType::ZType)
            .count();

        assert!(x_count > 0);
        assert!(z_count > 0);
    }

    #[test]
    fn test_mwpm_decoder() {
        let decoder = MWPMDecoder::new(3, 0.01);

        let syndrome = vec![
            SyndromeMeasurement {
                stabilizer_id: 0,
                outcome: -1,
                timestamp: 0.0,
                fidelity: 0.99,
            },
            SyndromeMeasurement {
                stabilizer_id: 1,
                outcome: 1,
                timestamp: 0.0,
                fidelity: 0.99,
            },
        ];

        let corrections = decoder
            .decode_syndrome(&syndrome, 3)
            .expect("Syndrome decoding should succeed");
        assert!(!corrections.is_empty());
    }

    #[test]
    fn test_color_code_creation() {
        let color_code = ColorCode::new(3).expect("Color code creation should succeed");
        assert_eq!(color_code.distance, 3);
        assert!(color_code.physical_qubit_count() > 0);
    }

    #[test]
    fn test_error_probability_calculation() {
        let decoder = MWPMDecoder::new(3, 0.01);

        let syndrome = vec![SyndromeMeasurement {
            stabilizer_id: 0,
            outcome: -1,
            timestamp: 0.0,
            fidelity: 0.99,
        }];

        let prob = decoder.calculate_error_probability(&syndrome);
        assert!(prob > 0.0 && prob <= 1.0);
    }
}