Derivative.h

/*
 * ########################################################################
 * The contents of this file is free and unencumbered software released into the
 * public domain. For more information, please refer to <http://unlicense.org/>
 * ########################################################################
 */
 
#ifndef BSPLINE_OPERATORS_DERIVATIVE_H
#define BSPLINE_OPERATORS_DERIVATIVE_H
 
#include <bspline/internal/misc.h>
#include <bspline/operators/GenericOperators.h>
 
namespace bspline::operators {
namespace internal = bspline::internal;
 
template <size_t n>
class Derivative final : public Operator {
 public:
  static constexpr size_t outputOrder(size_t inputOrder) {
    return std::max(n, inputOrder) - n;
  }
 
  template <typename T, size_t size>
  std::array<T, outputOrder(size - 1) + 1> transform(
      const std::array<T, size> &input,
      [[maybe_unused]] const support::Grid<T> &grid,
      [[maybe_unused]] size_t intervalIndex) const {
    static_assert(size >= 1, "Arrays of size zero not supported.");
    // The order of the input spline.
    constexpr size_t SPLINE_ORDER = size - 1;
    // The size of the output array.
    constexpr size_t OUTPUT_SIZE = outputOrder(SPLINE_ORDER) + 1;
 
    if constexpr (n > SPLINE_ORDER) {
      return {static_cast<T>(0)};
    } else {
      std::array<T, OUTPUT_SIZE> retVal;
      for (size_t i = 0; i < OUTPUT_SIZE; i++) {
        retVal[i] = internal::facultyRatio<T>(i + n, i) * input[i + n];
      }
      return retVal;
    }
  }
};
 
template <size_t n>
using Dx = Derivative<n>;
 
}  // namespace bspline::operators
#endif  // BSPLINE_OPERATORS_DERIVATIVE_H