weighted_sum.hpp

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/*
 * Copyright © 2025-2026 Akreon LLC
 * SPDX-License-Identifier: Apache-2.0
 */
 
#pragma once
 
#include "akreon/linalg/linalg_util.hpp"
#include "akreon/convex/sum.hpp"
 
namespace convex {
 
template <linalg::RnSpace Space>
struct WeightedSum {
  using T = linalg::space_type_t<Space>;
  static constexpr std::size_t N = linalg::space_dim_v<Space>;
 
  using Vec = linalg::Vector<T, N>;
 
  template <AtomLike Var>
  constexpr auto operator()(const Vec& q, const Var& x) {
    static_assert(is_var_type_v<Var>,
                  "Weighted sum must operate on a var type");
    static_assert(
        std::same_as<dom_space_t<meta_codom_t<atom_meta_t<Var>>>, Space>,
        "Invalid q^T x operator mismatch on vector types");
 
    const auto W = linalg::make_diag(q);
    const auto weighted_x = x.mul(W);
    const auto vars = weighted_x.split();
    return std::apply([](const auto&... cs) { return Sum(cs...); }, vars);
  }
};
 
template <AtomLike Var>
constexpr auto make_weighted_sum(
    const linalg::Vector<dom_type_t<meta_codom_t<atom_meta_t<Var>>>,
                         dom_size_v<meta_codom_t<atom_meta_t<Var>>>>& q,
    const Var& x) {
  using Space = dom_space_t<meta_codom_t<atom_meta_t<Var>>>;
  return WeightedSum<Space>()(q, x);
}
 
} // namespace convex