quantrs2_sim/

mps_basic.rs

//! Basic MPS simulator implementation without external linear algebra dependencies
//!
//! This provides a simplified MPS implementation that doesn't require ndarray-linalg

use ndarray::{array, s, Array2, Array3};
use num_complex::Complex64;
use quantrs2_circuit::builder::{Circuit, Simulator};
use quantrs2_core::{
    error::{QuantRS2Error, QuantRS2Result},
    gate::GateOp,
    register::Register,
};
use rand::{thread_rng, Rng};
use std::f64::consts::SQRT_2;

/// Configuration for basic MPS simulator
#[derive(Debug, Clone)]
pub struct BasicMPSConfig {
    /// Maximum allowed bond dimension
    pub max_bond_dim: usize,
    /// SVD truncation threshold
    pub svd_threshold: f64,
}

impl Default for BasicMPSConfig {
    fn default() -> Self {
        BasicMPSConfig {
            max_bond_dim: 64,
            svd_threshold: 1e-10,
        }
    }
}

/// MPS tensor for a single qubit
#[derive(Debug, Clone)]
struct MPSTensor {
    /// The tensor data: left_bond x physical x right_bond
    data: Array3<Complex64>,
}

impl MPSTensor {
    /// Create initial tensor for |0> state
    fn zero_state(position: usize, num_qubits: usize) -> Self {
        let is_first = position == 0;
        let is_last = position == num_qubits - 1;

        let data = if is_first && is_last {
            // Single qubit: 1x2x1 tensor
            let mut tensor = Array3::zeros((1, 2, 1));
            tensor[[0, 0, 0]] = Complex64::new(1.0, 0.0);
            tensor
        } else if is_first {
            // First qubit: 1x2x2 tensor
            let mut tensor = Array3::zeros((1, 2, 2));
            tensor[[0, 0, 0]] = Complex64::new(1.0, 0.0);
            tensor
        } else if is_last {
            // Last qubit: 2x2x1 tensor
            let mut tensor = Array3::zeros((2, 2, 1));
            tensor[[0, 0, 0]] = Complex64::new(1.0, 0.0);
            tensor
        } else {
            // Middle qubit: 2x2x2 tensor
            let mut tensor = Array3::zeros((2, 2, 2));
            tensor[[0, 0, 0]] = Complex64::new(1.0, 0.0);
            tensor
        };
        Self { data }
    }
}

/// Basic Matrix Product State representation
pub struct BasicMPS {
    /// MPS tensors for each qubit
    tensors: Vec<MPSTensor>,
    /// Number of qubits
    num_qubits: usize,
    /// Configuration
    config: BasicMPSConfig,
}

impl BasicMPS {
    /// Create a new MPS in the |0...0> state
    pub fn new(num_qubits: usize, config: BasicMPSConfig) -> Self {
        let tensors = (0..num_qubits)
            .map(|i| MPSTensor::zero_state(i, num_qubits))
            .collect();

        Self {
            tensors,
            num_qubits,
            config,
        }
    }

    /// Apply a single-qubit gate
    pub fn apply_single_qubit_gate(
        &mut self,
        gate_matrix: &Array2<Complex64>,
        qubit: usize,
    ) -> QuantRS2Result<()> {
        if qubit >= self.num_qubits {
            return Err(QuantRS2Error::InvalidQubitId(qubit as u32));
        }

        let tensor = &mut self.tensors[qubit];
        let shape = tensor.data.shape();
        let (left_dim, _, right_dim) = (shape[0], shape[1], shape[2]);

        let mut new_data = Array3::zeros((left_dim, 2, right_dim));

        // Apply gate to physical index
        for l in 0..left_dim {
            for r in 0..right_dim {
                for new_phys in 0..2 {
                    for old_phys in 0..2 {
                        new_data[[l, new_phys, r]] +=
                            gate_matrix[[new_phys, old_phys]] * tensor.data[[l, old_phys, r]];
                    }
                }
            }
        }

        tensor.data = new_data;
        Ok(())
    }

    /// Apply a two-qubit gate to adjacent qubits
    pub fn apply_two_qubit_gate(
        &mut self,
        gate_matrix: &Array2<Complex64>,
        qubit1: usize,
        qubit2: usize,
    ) -> QuantRS2Result<()> {
        if (qubit1 as i32 - qubit2 as i32).abs() != 1 {
            return Err(QuantRS2Error::InvalidInput(
                "MPS requires adjacent qubits for two-qubit gates".to_string(),
            ));
        }

        let (left_q, right_q) = if qubit1 < qubit2 {
            (qubit1, qubit2)
        } else {
            (qubit2, qubit1)
        };

        // Simple implementation: contract and re-decompose
        // This is not optimal but works for demonstration

        let left_shape = self.tensors[left_q].data.shape().to_vec();
        let right_shape = self.tensors[right_q].data.shape().to_vec();

        // Contract the two tensors
        let mut combined = Array3::<Complex64>::zeros((left_shape[0], 4, right_shape[2]));

        for l in 0..left_shape[0] {
            for r in 0..right_shape[2] {
                for i in 0..2 {
                    for j in 0..2 {
                        for m in 0..left_shape[2] {
                            combined[[l, i * 2 + j, r]] += self.tensors[left_q].data[[l, i, m]]
                                * self.tensors[right_q].data[[m, j, r]];
                        }
                    }
                }
            }
        }

        // Apply gate
        let mut result = Array3::<Complex64>::zeros((left_shape[0], 4, right_shape[2]));
        for l in 0..left_shape[0] {
            for r in 0..right_shape[2] {
                for out_idx in 0..4 {
                    for in_idx in 0..4 {
                        result[[l, out_idx, r]] +=
                            gate_matrix[[out_idx, in_idx]] * combined[[l, in_idx, r]];
                    }
                }
            }
        }

        // Simple decomposition (not optimal, doesn't use SVD)
        // Just reshape back - this doesn't preserve optimal MPS form
        let new_bond = 2.min(self.config.max_bond_dim);

        let mut left_new = Array3::zeros((left_shape[0], 2, new_bond));
        let mut right_new = Array3::zeros((new_bond, 2, right_shape[2]));

        // Copy data (simplified - proper implementation would use SVD)
        for l in 0..left_shape[0] {
            for r in 0..right_shape[2] {
                for i in 0..2 {
                    for j in 0..2 {
                        let bond_idx = (i + j) % new_bond;
                        left_new[[l, i, bond_idx]] = result[[l, i * 2 + j, r]];
                        right_new[[bond_idx, j, r]] = Complex64::new(1.0, 0.0);
                    }
                }
            }
        }

        self.tensors[left_q].data = left_new;
        self.tensors[right_q].data = right_new;

        Ok(())
    }

    /// Get amplitude of a computational basis state
    pub fn get_amplitude(&self, bitstring: &[bool]) -> QuantRS2Result<Complex64> {
        if bitstring.len() != self.num_qubits {
            return Err(QuantRS2Error::InvalidInput(format!(
                "Bitstring length {} doesn't match qubit count {}",
                bitstring.len(),
                self.num_qubits
            )));
        }

        // Contract MPS from left to right
        let mut result = Array2::from_elem((1, 1), Complex64::new(1.0, 0.0));

        for (i, &bit) in bitstring.iter().enumerate() {
            let tensor = &self.tensors[i];
            let physical_idx = if bit { 1 } else { 0 };

            // Extract matrix for this physical index
            let matrix = tensor.data.slice(s![.., physical_idx, ..]);

            // Contract with accumulated result
            result = result.dot(&matrix);
        }

        Ok(result[[0, 0]])
    }

    /// Sample a measurement outcome
    pub fn sample(&self) -> Vec<bool> {
        let mut rng = thread_rng();
        let mut result = vec![false; self.num_qubits];
        let mut accumulated = Array2::from_elem((1, 1), Complex64::new(1.0, 0.0));

        for i in 0..self.num_qubits {
            let tensor = &self.tensors[i];

            // Compute probabilities for this qubit
            let matrix0 = tensor.data.slice(s![.., 0, ..]);
            let matrix1 = tensor.data.slice(s![.., 1, ..]);

            let branch0: Array2<Complex64> = accumulated.dot(&matrix0);
            let branch1: Array2<Complex64> = accumulated.dot(&matrix1);

            // Compute norms (simplified - doesn't contract remaining qubits)
            let norm0_sq: f64 = branch0.iter().map(|x| x.norm_sqr()).sum();
            let norm1_sq: f64 = branch1.iter().map(|x| x.norm_sqr()).sum();

            let total = norm0_sq + norm1_sq;
            let prob0 = norm0_sq / total;

            if rng.gen::<f64>() < prob0 {
                result[i] = false;
                accumulated = branch0;
            } else {
                result[i] = true;
                accumulated = branch1;
            }

            // Renormalize
            let norm_sq: f64 = accumulated.iter().map(|x| x.norm_sqr()).sum();
            if norm_sq > 0.0 {
                accumulated /= Complex64::new(norm_sq.sqrt(), 0.0);
            }
        }

        result
    }
}

/// Basic MPS quantum simulator
pub struct BasicMPSSimulator {
    config: BasicMPSConfig,
}

impl BasicMPSSimulator {
    /// Create a new basic MPS simulator
    pub fn new(config: BasicMPSConfig) -> Self {
        Self { config }
    }

    /// Create with default configuration
    pub fn default() -> Self {
        Self::new(BasicMPSConfig::default())
    }
}

impl<const N: usize> Simulator<N> for BasicMPSSimulator {
    fn run(&self, circuit: &Circuit<N>) -> QuantRS2Result<Register<N>> {
        // Create initial MPS state
        let mut mps = BasicMPS::new(N, self.config.clone());

        // Apply gates from circuit
        for gate in circuit.gates() {
            match gate.name().as_ref() {
                "H" => {
                    let h_matrix = {
                        let h = 1.0 / SQRT_2;
                        array![
                            [Complex64::new(h, 0.), Complex64::new(h, 0.)],
                            [Complex64::new(h, 0.), Complex64::new(-h, 0.)]
                        ]
                    };
                    if let Some(&qubit) = gate.qubits().first() {
                        mps.apply_single_qubit_gate(&h_matrix, qubit.id() as usize)?;
                    }
                }
                "X" => {
                    let x_matrix = array![
                        [Complex64::new(0., 0.), Complex64::new(1., 0.)],
                        [Complex64::new(1., 0.), Complex64::new(0., 0.)]
                    ];
                    if let Some(&qubit) = gate.qubits().first() {
                        mps.apply_single_qubit_gate(&x_matrix, qubit.id() as usize)?;
                    }
                }
                "CNOT" | "CX" => {
                    let cnot_matrix = array![
                        [
                            Complex64::new(1., 0.),
                            Complex64::new(0., 0.),
                            Complex64::new(0., 0.),
                            Complex64::new(0., 0.)
                        ],
                        [
                            Complex64::new(0., 0.),
                            Complex64::new(1., 0.),
                            Complex64::new(0., 0.),
                            Complex64::new(0., 0.)
                        ],
                        [
                            Complex64::new(0., 0.),
                            Complex64::new(0., 0.),
                            Complex64::new(0., 0.),
                            Complex64::new(1., 0.)
                        ],
                        [
                            Complex64::new(0., 0.),
                            Complex64::new(0., 0.),
                            Complex64::new(1., 0.),
                            Complex64::new(0., 0.)
                        ],
                    ];
                    let qubits = gate.qubits();
                    if qubits.len() == 2 {
                        mps.apply_two_qubit_gate(
                            &cnot_matrix,
                            qubits[0].id() as usize,
                            qubits[1].id() as usize,
                        )?;
                    }
                }
                _ => {
                    // Gate not supported in basic implementation
                }
            }
        }

        // Create register from final state
        // For now, just return empty register
        Ok(Register::new())
    }
}

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

    #[test]
    fn test_basic_mps_initialization() {
        let mps = BasicMPS::new(4, BasicMPSConfig::default());

        // Check |0000> state
        let amp = mps.get_amplitude(&[false, false, false, false]).unwrap();
        assert!((amp.norm() - 1.0).abs() < 1e-10);

        let amp = mps.get_amplitude(&[true, false, false, false]).unwrap();
        assert!(amp.norm() < 1e-10);
    }

    #[test]
    fn test_single_qubit_gate() {
        let mut mps = BasicMPS::new(3, BasicMPSConfig::default());

        // Apply X to first qubit
        let x_matrix = array![
            [Complex64::new(0., 0.), Complex64::new(1., 0.)],
            [Complex64::new(1., 0.), Complex64::new(0., 0.)]
        ];
        mps.apply_single_qubit_gate(&x_matrix, 0).unwrap();

        // Check |100> state
        let amp = mps.get_amplitude(&[true, false, false]).unwrap();
        assert!((amp.norm() - 1.0).abs() < 1e-10);
    }
}