File heavisidestepfunction.h

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#ifndef TEMPLAT_LATTICE_ALGEBRA_OPERATORS_HEAVISIDESTEPFUNCTION_H
#define TEMPLAT_LATTICE_ALGEBRA_OPERATORS_HEAVISIDESTEPFUNCTION_H

/* This file is part of TempLat, available at https://cosmolattice.github.io/templat .
   Copyright 2021-2026 The TempLat authors, see AUTHORS.md.
   Released under the MIT license, see LICENSE.md. */

// File info: Main contributor(s): Wessel Valkenburg, Year: 2019

#include "TempLat/lattice/algebra/conditional/conditionalunarygetter.h"
#include "TempLat/lattice/algebra/helpers/getderiv.h"
#include "TempLat/lattice/algebra/operators/diracdeltafunction.h"
#include "TempLat/lattice/algebra/operators/unaryoperator.h"

#include "TempLat/lattice/algebra/constants/onetype.h"
#include "TempLat/lattice/algebra/constants/zerotype.h"

namespace TempLat
{
  template <typename R> class HeavisideStepFunction : public UnaryOperator<R>
  {
  public:
    using UnaryOperator<R>::mR;

    HeavisideStepFunction(const R &pR) : UnaryOperator<R>(pR) {}

    template <typename... IDX>
      requires requires(std::decay_t<R> r, IDX... idx) {
        requires IsVariadicIndex<IDX...>;
        DoEval::eval(r, idx...);
      }
    DEVICE_INLINE_FUNCTION auto eval(const IDX &...idx) const
    {
      return (DoEval::eval(mR, idx...) >= 0 ? 1 : 0);
    }

    template <typename U> auto d(const U &other) { return GetDeriv::get(mR, other) * DiracDelta(mR); }

    virtual std::string operatorString() const override { return "Heaviside"; }
  };

  template <typename R>
    requires ConditionalUnaryGetter<R>
  auto heaviside(const R &r)
  {
    return HeavisideStepFunction<R>(r);
  }

  constexpr inline OneType heaviside(ZeroType a) { return {}; }

  constexpr inline OneType heaviside(OneType a) { return {}; }
} // namespace TempLat

#endif