hessian.hpp

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/*
 * Copyright © 2025-2026 Akreon LLC
 * SPDX-License-Identifier: Apache-2.0
 */
 
#pragma once
 
#include "akreon/linalg/linalg_util.hpp"
 
namespace linalg {
 
template <SquareMatrix Mat,
          DifferentialType Type = DifferentialType::PUSHFORWARD>
struct Hessian : Endomorphism<Mat, Hessian<Mat, Type>> {
  using OperatorBase = Endomorphism<Mat, Hessian<Mat, Type>>;
  using typename OperatorBase::T;
 
  using OperatorBase::N;
 
  using Vec = linalg::Vector<T, N>;
 
  constexpr explicit Hessian(const Mat& A) : H(A) {}
 
  constexpr auto operator()(std::size_t i, std::size_t j) const {
    return H(i, j);
  }
 
  template <MatrixContainer MatX>
    requires(N == mat_rows_v<MatX>)
  constexpr auto apply(const MatX& X) const {
    return H * X;
  }
 
  constexpr auto apply(const Vec& x) const { return H * x; }
 
  constexpr auto apply(const T& a) const { return a * H; }
 
  constexpr bool is_pullback() const {
    return Type == DifferentialType::PULLBACK;
  }
 
  Mat H; 
};
 
} // namespace linalg