include/ceres/internal/variadic_evaluate.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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// modification, are permitted provided that the following conditions are met:
//
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//   this list of conditions and the following disclaimer.
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//   this list of conditions and the following disclaimer in the documentation
//   and/or other materials provided with the distribution.
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//   used to endorse or promote products derived from this software without
//   specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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// Author: sameeragarwal@google.com (Sameer Agarwal)
//         mierle@gmail.com (Keir Mierle)
//         jodebo_beck@gmx.de (Johannes Beck)

#ifndef CERES_PUBLIC_INTERNAL_VARIADIC_EVALUATE_H_
#define CERES_PUBLIC_INTERNAL_VARIADIC_EVALUATE_H_

#include <algorithm>
#include <array>
#include <cstddef>
#include <functional>
#include <tuple>
#include <type_traits>
#include <utility>

#include "ceres/cost_function.h"
#include "ceres/internal/parameter_dims.h"

namespace ceres::internal {

// VariadicEvaluate is a helper function to evaluate Ceres cost functions or
// functors using an input, output and the parameter dimensions. There are
// several possibilities:
// 1) If the passed functor is a 'ceres::CostFunction', its Evaluate method is
// called.
// 2) If the functor is not a 'ceres::CostFunction' and the specified parameter
// dimensions are dynamic, the functor must have the following signature:
// 'bool(T const* const* input, T* output)'.
// 3) If the functor is not a 'ceres::CostFunction' and the specified parameter
// dimensions are not dynamic, the input is expanded by using the number of
// parameter blocks. The signature of the functor must have the following
// signature:
// 'bool()(const T* i_1, const T* i_2, ... const T* i_n, T* output)'.
template <typename ParameterDims, typename Functor, typename T>
inline bool VariadicEvaluate(const Functor& functor,
                             T const* const* input,
                             T* output) {
  if constexpr (std::is_base_of_v<CostFunction, Functor>) {
    return functor.Evaluate(input, output, nullptr);
  } else {
    if constexpr (ParameterDims::kIsDynamic) {
      return functor(input, output);
    } else {
      constexpr int kNumParameterBlocks = ParameterDims::kNumParameterBlocks;
      static_assert(kNumParameterBlocks > 0,
                    "Invalid number of parameter blocks. At least one "
                    "parameter block must be specified.");

      std::array<const T*, kNumParameterBlocks> parameter_blocks;
      std::copy_n(input, kNumParameterBlocks, parameter_blocks.begin());
      return std::apply(
          [&functor, output](auto const*... blocks) {
            static_assert(
                std::is_invocable_v<const Functor&, decltype(blocks)..., T*>,
                "The provided Functor's operator() signature does not match "
                "the expected number of parameter blocks defined in "
                "ParameterDims.");
            return std::invoke(functor, blocks..., output);
          },
          parameter_blocks);
    }
  }
}

// When differentiating dynamically sized CostFunctions, VariadicEvaluate
// expects a functor with the signature:
//
// bool operator()(double const* const* parameters, double* cost) const
//
// However for NumericDiffFirstOrderFunction, the functor has the signature
//
// bool operator()(double const* parameters, double* cost) const
//
// This thin wrapper adapts the latter to the former.
template <typename Functor>
class FirstOrderFunctorAdapter {
 public:
  explicit FirstOrderFunctorAdapter(const Functor& functor)
      : functor_(functor) {}
  bool operator()(double const* const* parameters, double* cost) const {
    return functor_(*parameters, cost);
  }

 private:
  const Functor& functor_;
};

}  // namespace ceres::internal

#endif  // CERES_PUBLIC_INTERNAL_VARIADIC_EVALUATE_H_