// Ceres Solver - A fast non-linear least squares minimizer
// Copyright 2024 Google Inc. All rights reserved.
// http://ceres-solver.org/
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// Author: mierle@gmail.com (Keir Mierle)
//         sameeragarwal@google.com (Sameer Agarwal)
//         thadh@gmail.com (Thad Hughes)
//         tbennun@gmail.com (Tal Ben-Nun)

#ifndef CERES_PUBLIC_DYNAMIC_NUMERIC_DIFF_COST_FUNCTION_H_
#define CERES_PUBLIC_DYNAMIC_NUMERIC_DIFF_COST_FUNCTION_H_

#include <cmath>
#include <memory>
#include <numeric>
#include <type_traits>
#include <vector>

#include "absl/container/fixed_array.h"
#include "absl/log/check.h"
#include "ceres/dynamic_cost_function.h"
#include "ceres/internal/eigen.h"
#include "ceres/internal/numeric_diff.h"
#include "ceres/internal/parameter_dims.h"
#include "ceres/numeric_diff_options.h"
#include "ceres/types.h"

namespace ceres {

// This numeric diff implementation differs from the one found in
// numeric_diff_cost_function.h by supporting numeric diff on cost
// functions with variable numbers of parameters with variable
// sizes. With the other implementation, all the sizes (both the
// number of parameter blocks and the size of each block) must be
// fixed at compile time.
//
// The functor API differs slightly from the API for fixed size
// numeric diff; the expected interface for the cost functors is:
//
//   struct MyCostFunctor {
//     bool operator()(double const*
//                     const* parameters,
//                     double* residuals) const {
//       // Use parameters[i] to access the i'th parameter block.
//     }
//   };
//
// Since the sizing of the parameters is done at runtime, you must
// also specify the sizes after creating the
// DynamicNumericDiffCostFunction. For example:
//
//   DynamicNumericDiffCostFunction<MyCostFunctor, CENTRAL> cost_function;
//   cost_function.AddParameterBlock(5);
//   cost_function.AddParameterBlock(10);
//   cost_function.SetNumResiduals(21);
template <typename CostFunctor, NumericDiffMethodType kMethod = CENTRAL>
class DynamicNumericDiffCostFunction final : public DynamicCostFunction {
 public:
  // Takes ownership of functor by default.
  explicit DynamicNumericDiffCostFunction(
      std::unique_ptr<const CostFunctor> functor,
      const NumericDiffOptions& options = NumericDiffOptions())
      : DynamicNumericDiffCostFunction(
            std::move(functor), TAKE_OWNERSHIP, options) {}

  // Constructs the CostFunctor on the heap and takes ownership.
  template <class... Args,
            typename = std::enable_if_t<std::is_constructible_v<CostFunctor,
                                                               Args&&...>>>
  explicit DynamicNumericDiffCostFunction(Args&&... args)
      // NOTE We explicitly use direct initialization using parentheses instead
      // of uniform initialization using braces to avoid narrowing conversion
      // warnings.
      : DynamicNumericDiffCostFunction(
            std::make_unique<const CostFunctor>(std::forward<Args>(args)...),
            TAKE_OWNERSHIP,
            NumericDiffOptions()) {}

  explicit DynamicNumericDiffCostFunction(
      const CostFunctor* functor,
      Ownership ownership = TAKE_OWNERSHIP,
      const NumericDiffOptions& options = NumericDiffOptions())
      : DynamicNumericDiffCostFunction(
            std::unique_ptr<const CostFunctor>(functor), ownership, options) {}

  DynamicNumericDiffCostFunction(const DynamicNumericDiffCostFunction&) =
      delete;
  DynamicNumericDiffCostFunction& operator=(
      const DynamicNumericDiffCostFunction&) = delete;
  DynamicNumericDiffCostFunction(
      DynamicNumericDiffCostFunction&& other) noexcept = default;
  DynamicNumericDiffCostFunction& operator=(
      DynamicNumericDiffCostFunction&& other) noexcept = default;

  ~DynamicNumericDiffCostFunction() override {
    if (ownership_ == DO_NOT_TAKE_OWNERSHIP) {
      functor_.release();
    }
  }

  bool Evaluate(double const* const* parameters,
                double* residuals,
                double** jacobians) const override {
    using internal::NumericDiff;
    CHECK_GT(num_residuals(), 0)
        << "You must call DynamicNumericDiffCostFunction::SetNumResiduals() "
        << "before DynamicNumericDiffCostFunction::Evaluate().";

    const std::vector<int32_t>& block_sizes = parameter_block_sizes();
    CHECK(!block_sizes.empty())
        << "You must call DynamicNumericDiffCostFunction::AddParameterBlock() "
        << "before DynamicNumericDiffCostFunction::Evaluate().";

    const bool status =
        internal::VariadicEvaluate<internal::DynamicParameterDims>(
            *functor_.get(), parameters, residuals);
    if (jacobians == nullptr || !status) {
      return status;
    }

    // Create local space for a copy of the parameters which will get mutated.
    int parameters_size =
        std::accumulate(block_sizes.begin(), block_sizes.end(), 0);
    absl::FixedArray<double> parameters_copy(parameters_size);
    absl::FixedArray<double*> parameters_references_copy(block_sizes.size());
    parameters_references_copy[0] = parameters_copy.data();
    for (size_t block = 1; block < block_sizes.size(); ++block) {
      parameters_references_copy[block] =
          parameters_references_copy[block - 1] + block_sizes[block - 1];
    }

    // Copy the parameters into the local temp space.
    for (size_t block = 0; block < block_sizes.size(); ++block) {
      std::copy_n(parameters[block],
                  block_sizes[block],
                  parameters_references_copy[block]);
    }

    for (size_t block = 0; block < block_sizes.size(); ++block) {
      if (jacobians[block] != nullptr &&
          !NumericDiff<CostFunctor,
                       kMethod,
                       ceres::DYNAMIC,
                       internal::DynamicParameterDims,
                       ceres::DYNAMIC,
                       ceres::DYNAMIC>::
              EvaluateJacobianForParameterBlock(
                  functor_.get(),
                  residuals,
                  options_,
                  this->num_residuals(),
                  block,
                  block_sizes[block],
                  parameters_references_copy.data(),
                  jacobians[block])) {
        return false;
      }
    }
    return true;
  }

 private:
  explicit DynamicNumericDiffCostFunction(
      std::unique_ptr<const CostFunctor> functor,
      Ownership ownership,
      const NumericDiffOptions& options)
      : functor_(std::move(functor)),
        ownership_(ownership),
        options_(options) {}

  std::unique_ptr<const CostFunctor> functor_;
  Ownership ownership_;
  NumericDiffOptions options_;
};

// Deduction guide that allows the user to avoid explicitly specifying the
// template parameter of DynamicNumericDiffCostFunction. The class can instead
// be instantiated as follows:
//
//   new DynamicNumericDiffCostFunction{new MyCostFunctor{}};
//   new DynamicNumericDiffCostFunction{std::make_unique<MyCostFunctor>()};
//
template <typename CostFunctor>
DynamicNumericDiffCostFunction(CostFunctor* functor)
    -> DynamicNumericDiffCostFunction<CostFunctor>;
template <typename CostFunctor>
DynamicNumericDiffCostFunction(CostFunctor* functor, Ownership ownership)
    -> DynamicNumericDiffCostFunction<CostFunctor>;
template <typename CostFunctor>
DynamicNumericDiffCostFunction(CostFunctor* functor,
                               Ownership ownership,
                               const NumericDiffOptions& options)
    -> DynamicNumericDiffCostFunction<CostFunctor>;
template <typename CostFunctor>
DynamicNumericDiffCostFunction(std::unique_ptr<CostFunctor> functor)
    -> DynamicNumericDiffCostFunction<CostFunctor>;
template <typename CostFunctor>
DynamicNumericDiffCostFunction(std::unique_ptr<CostFunctor> functor,
                               const NumericDiffOptions& options)
    -> DynamicNumericDiffCostFunction<CostFunctor>;

}  // namespace ceres

#endif  // CERES_PUBLIC_DYNAMIC_NUMERIC_DIFF_COST_FUNCTION_H_