include/ceres/internal/array_selector.h

// Ceres Solver - A fast non-linear least squares minimizer
// Copyright 2023 Google Inc. All rights reserved.
// http://ceres-solver.org/
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// Author: darius.rueckert@fau.de (Darius Rueckert)
//

#ifndef CERES_PUBLIC_INTERNAL_ARRAY_SELECTOR_H_
#define CERES_PUBLIC_INTERNAL_ARRAY_SELECTOR_H_

#include <array>
#include <vector>

#include "absl/container/fixed_array.h"
#include "absl/log/check.h"
#include "ceres/types.h"

namespace ceres::internal {

// StaticFixedArray selects the best array implementation based on template
// arguments. If the size is not known at compile-time, pass
// ceres::DYNAMIC as a size-template argument.
//
// Three different containers are selected in different scenarios:
//
//   num_elements == DYNAMIC:
//      -> absl::FixedArray<T, max_stack_size>(size)

//   num_elements != DYNAMIC  &&  num_elements <= max_stack_size
//      -> std::array<T,num_elements>

//   num_elements != DYNAMIC  &&  num_elements >  max_stack_size
//      -> std::vector<T>(num_elements)
//
template <typename T,
          int num_elements,
          int max_num_elements_on_stack,
          bool dynamic = (num_elements == DYNAMIC),
          bool fits_on_stack = (num_elements <= max_num_elements_on_stack)>
struct ArraySelector {};

template <typename T,
          int num_elements,
          int max_num_elements_on_stack,
          bool fits_on_stack>
struct ArraySelector<T,
                     num_elements,
                     max_num_elements_on_stack,
                     true,
                     fits_on_stack>
    : absl::FixedArray<T, max_num_elements_on_stack> {
  explicit ArraySelector(int s)
      : absl::FixedArray<T, max_num_elements_on_stack>(s) {}
};

template <typename T, int num_elements, int max_num_elements_on_stack>
struct ArraySelector<T, num_elements, max_num_elements_on_stack, false, true>
    : std::array<T, num_elements> {
  explicit ArraySelector(int s) { CHECK_EQ(s, num_elements); }
};

template <typename T, int num_elements, int max_num_elements_on_stack>
struct ArraySelector<T, num_elements, max_num_elements_on_stack, false, false>
    : std::vector<T> {
  explicit ArraySelector(int s) : std::vector<T>(s) {
    CHECK_EQ(s, num_elements);
  }
};

}  // namespace ceres::internal

#endif  // CERES_PUBLIC_INTERNAL_ARRAY_SELECTOR_H_