placeholder.hpp

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// Copyright (c) 2024--present, The Simons Foundation
// This file is part of TRIQS/nda and is licensed under the Apache License, Version 2.0.
// SPDX-License-Identifier: Apache-2.0
// See LICENSE in the root of this distribution for details.

 
#pragma once
 
#include "./expression.hpp"
#include "./utils.hpp"
 
#include <type_traits>
#include <utility>
 
namespace nda::clef {


  // Forward declarations.
  template <int N, typename T>
  struct pair;

  template <int N>
  struct placeholder {
    static_assert(N >= 0 && N < 64, "Placeholder index must be in {0, 1, ..., 63}");

    static constexpr int index = N;

    template <typename RHS>
    pair<N, RHS> operator=(RHS &&rhs) const { // NOLINT (we want to return a pair)
      return {std::forward<RHS>(rhs)};
    }

    template <typename... Args>
    auto operator()(Args &&...args) const {
      return expr<tags::function, placeholder, expr_storage_t<Args>...>{tags::function{}, *this, std::forward<Args>(args)...};
    }

    template <typename T>
    auto operator[](T &&t) const {
      return expr<tags::subscript, placeholder, expr_storage_t<T>>{tags::subscript{}, *this, std::forward<T>(t)};
    }
  };

  template <int N, typename T>
  struct pair {
    T rhs;

    static constexpr int p = N;

    using value_type = std::decay_t<T>;
  };
 
  namespace detail {
 
    // Specialization of force_copy_in_expr_impl for nda::clef::placeholder types (always true).
    template <int N>
    constexpr bool force_copy_in_expr_impl<placeholder<N>> = true;
 
    // Specialization of ph_set for nda::clef::placeholder types.
    template <int N>
    struct ph_set<placeholder<N>> {
      static constexpr ull_t value = 1ull << N;
    };
 
    // Specialization of ph_set for nda::clef::pair types.
    template <int N, typename T>
    struct ph_set<pair<N, T>> : ph_set<placeholder<N>> {};
 
    // Specialization of is_lazy_impl for nda::clef::placeholder types.
    template <int N>
    constexpr bool is_lazy_impl<placeholder<N>> = true;
 
  } // namespace detail

 
} // namespace nda::clef