oxicuda-quantum 0.2.0

use crate::embedding::zz_feature::zz_feature_map;
use crate::error::{QuantumError, QuantumResult};

/// Overlap kernel k(x, y) = |⟨ψ(x)|ψ(y)⟩|².
///
/// Both x and y are encoded via the ZZ feature map with 1 repetition.
pub fn overlap_kernel(x: &[f32], y: &[f32]) -> QuantumResult<f32> {
    if x.len() != y.len() {
        return Err(QuantumError::DimensionMismatch {
            expected: x.len(),
            got: y.len(),
        });
    }
    if x.is_empty() {
        return Err(QuantumError::EmptyInput);
    }

    let psi_x = zz_feature_map(x, 1)?;
    let psi_y = zz_feature_map(y, 1)?;

    let ip = psi_x.inner_product(&psi_y)?;
    Ok(ip.norm_sqr())
}

/// Compute the full kernel matrix K\[i,j\] = k(xs\[i\], xs\[j\]).
pub fn kernel_matrix(xs: &[Vec<f32>]) -> QuantumResult<Vec<Vec<f32>>> {
    if xs.is_empty() {
        return Err(QuantumError::EmptyInput);
    }
    let n = xs.len();
    let mut mat = vec![vec![0.0_f32; n]; n];

    for i in 0..n {
        for j in i..n {
            let k = overlap_kernel(&xs[i], &xs[j])?;
            mat[i][j] = k;
            mat[j][i] = k;
        }
    }

    Ok(mat)
}

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

    #[test]
    fn self_kernel_is_one() {
        let x = vec![0.5_f32, 1.0];
        let k = overlap_kernel(&x, &x)
            .expect("self-overlap of a non-empty, same-length vector pair cannot fail");
        assert!((k - 1.0).abs() < 1e-4, "k={k}");
    }

    #[test]
    fn kernel_matrix_diagonal_is_one() {
        let xs = vec![vec![0.3_f32, 0.7], vec![1.0_f32, -0.5]];
        let mat = kernel_matrix(&xs)
            .expect("kernel matrix of non-empty, equal-length vectors cannot fail");
        assert!((mat[0][0] - 1.0).abs() < 1e-4);
        assert!((mat[1][1] - 1.0).abs() < 1e-4);
    }
}