quantrs2-core 0.1.3

Core types and traits for the QuantRS2 quantum computing framework
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//! Concatenated quantum error correction codes

use super::pauli::{Pauli, PauliString};
use super::stabilizer::StabilizerCode;
use crate::error::{QuantRS2Error, QuantRS2Result};
use scirs2_core::Complex64;

/// Concatenated quantum error correction codes
#[derive(Debug, Clone)]
pub struct ConcatenatedCode {
    /// Inner code (applied first)
    pub inner_code: StabilizerCode,
    /// Outer code (applied to logical qubits of inner code)
    pub outer_code: StabilizerCode,
}

impl ConcatenatedCode {
    /// Create a new concatenated code
    pub const fn new(inner_code: StabilizerCode, outer_code: StabilizerCode) -> Self {
        Self {
            inner_code,
            outer_code,
        }
    }

    /// Get total number of physical qubits
    pub const fn total_qubits(&self) -> usize {
        self.inner_code.n * self.outer_code.n
    }

    /// Get number of logical qubits
    pub const fn logical_qubits(&self) -> usize {
        self.inner_code.k * self.outer_code.k
    }

    /// Get effective distance
    pub const fn distance(&self) -> usize {
        self.inner_code.d * self.outer_code.d
    }

    /// Encode a logical state
    pub fn encode(&self, logical_state: &[Complex64]) -> QuantRS2Result<Vec<Complex64>> {
        if logical_state.len() != 1 << self.logical_qubits() {
            return Err(QuantRS2Error::InvalidInput(
                "Logical state dimension mismatch".to_string(),
            ));
        }

        // First encode with outer code
        let outer_encoded = self.encode_with_code(logical_state, &self.outer_code)?;

        // Then encode each logical qubit of outer code with inner code
        let mut final_encoded = vec![Complex64::new(0.0, 0.0); 1 << self.total_qubits()];

        // This is a simplified encoding - proper implementation would require
        // tensor product operations and proper state manipulation
        for (i, &amplitude) in outer_encoded.iter().enumerate() {
            if amplitude.norm() > 1e-10 {
                final_encoded[i * (1 << self.inner_code.n)] = amplitude;
            }
        }

        Ok(final_encoded)
    }

    /// Correct errors using concatenated decoding
    pub fn correct_error(
        &self,
        encoded_state: &[Complex64],
        error: &PauliString,
    ) -> QuantRS2Result<Vec<Complex64>> {
        if error.paulis.len() != self.total_qubits() {
            return Err(QuantRS2Error::InvalidInput(
                "Error must act on all physical qubits".to_string(),
            ));
        }

        // Simplified error correction - apply error and return corrected state
        // In practice, would implement syndrome extraction and decoding
        let mut corrected = encoded_state.to_vec();

        // Apply error (simplified)
        for (i, &pauli) in error.paulis.iter().enumerate() {
            if pauli != Pauli::I && i < corrected.len() {
                // Simplified error application
                corrected[i] *= -1.0;
            }
        }

        Ok(corrected)
    }

    /// Encode with a specific code
    fn encode_with_code(
        &self,
        state: &[Complex64],
        code: &StabilizerCode,
    ) -> QuantRS2Result<Vec<Complex64>> {
        // Simplified encoding - proper implementation would use stabilizer formalism
        let mut encoded = vec![Complex64::new(0.0, 0.0); 1 << code.n];

        for (i, &amplitude) in state.iter().enumerate() {
            if i < encoded.len() {
                encoded[i * (1 << (code.n - code.k))] = amplitude;
            }
        }

        Ok(encoded)
    }
}