// Ceres Solver - A fast non-linear least squares minimizer
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// http://ceres-solver.org/
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// Author: sameeragarwal@google.com (Sameer Agarwal)

#include "ceres/numeric_diff_first_order_function.h"

#include <memory>

#include "ceres/array_utils.h"
#include "ceres/first_order_function.h"
#include "gtest/gtest.h"

namespace ceres {
namespace internal {

class QuadraticCostFunctor {
 public:
  explicit QuadraticCostFunctor(double a) : a_(a) {}
  bool operator()(const double* const x, double* cost) const {
    cost[0] = x[0] * x[1] + x[2] * x[3] - a_;
    return true;
  }

 private:
  double a_;
};

TEST(NumericDiffFirstOrderFunction, BilinearDifferentiationTest) {
  std::unique_ptr<FirstOrderFunction> function(
      new NumericDiffFirstOrderFunction<QuadraticCostFunctor, CENTRAL, 4>(
          new QuadraticCostFunctor(1.0)));

  double parameters[4] = {1.0, 2.0, 3.0, 4.0};
  double gradient[4];
  double cost;

  function->Evaluate(parameters, &cost, nullptr);
  EXPECT_EQ(cost, 13.0);

  cost = -1.0;
  function->Evaluate(parameters, &cost, gradient);

  EXPECT_EQ(cost, 13.0);

  const double kTolerance = 1e-9;
  EXPECT_NEAR(gradient[0], parameters[1], kTolerance);
  EXPECT_NEAR(gradient[1], parameters[0], kTolerance);
  EXPECT_NEAR(gradient[2], parameters[3], kTolerance);
  EXPECT_NEAR(gradient[3], parameters[2], kTolerance);
}

}  // namespace internal
}  // namespace ceres