quadratic_rep.hpp

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/*
 * Copyright © 2025-2026 Akreon LLC
 * SPDX-License-Identifier: Apache-2.0
 */
 
#pragma once
 
#include <functional>
 
#include "akreon/linalg/linalg_util.hpp"
 
namespace linalg {
 
template <SquareMatrix JxJ, typename Circ>
struct QuadraticRep {
  using T = matrix_scalar_t<JxJ>;
  static constexpr std::size_t N = mat_rows_v<JxJ>;
  using J = Vector<T, N>;
 
  constexpr explicit QuadraticRep(const J& x_in) : x(x_in) {}
 
  constexpr auto apply(const J& y) const {
    const Circ circ;
    return T(2) * circ(x, circ(x, y)) - circ(circ(x, x), y);
  }
 
  template <MatrixContainer MatX>
    requires(N == mat_rows_v<MatX>)
  constexpr auto apply(const MatX& Y) const {
    constexpr std::size_t Cols = mat_cols_v<MatX>;
    Matrix<T, N, Cols> result{};
    for (std::size_t j = 0; j < Cols; ++j) {
      const auto y_col = get_col(Y, j);
      const auto p_y_col = apply(y_col);
      for (std::size_t i = 0; i < N; ++i) result(i, j) = p_y_col(i);
    }
    return result;
  }
 
  J x; 
};
 
} // namespace linalg