grass_forage.h

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// SPDX-FileCopyrightText: 2020 W. Traylor <wolfgang.traylor@senckenberg.de>
//
// SPDX-License-Identifier: LGPL-3.0-or-later
 
#ifndef FAUNA_GRASS_FORAGE_H
#define FAUNA_GRASS_FORAGE_H
 
#include <cassert>
#include "Fauna/forage_base.h"
 
namespace Fauna {
class GrassForage : public ForageBase {
 private:
  double fpc;
 
  static constexpr double MAX_SWARD_DENSITY = 2e7;  // 20 kgDM/m²
 public:
  GrassForage() : ForageBase(), fpc(0.0) {}
 
 
  double get_sward_density() const {
    if (get_fpc() == 0) return 0.0;
    const double sd = get_mass() / get_fpc();
    assert(sd >= 0.0 && sd >= get_mass());
    // Only throw an error about preposterously high sward density if there is
    // actually substantial grass in the habitat. Otherwise there might be
    // spurious erros if both FPC and total grass density are extremely low due
    // to model artifacts.
    // The value of 50 gDM/m² (= 5e4 kgDM/km²) is chosen arbitrarily.
    if (sd > MAX_SWARD_DENSITY && get_mass() > 5e4) {
      throw std::logic_error(
          "Fauna::GrassForage::get_sward_density() "
          "The grass sward density is unreasonably high (" +
          std::to_string(sd / 1e4) +
          " gDM/m²). "
          "This might be because the vegetation model gave an unrealistically "
          "low FPC value (fraction of habitat covered by grass).\n"
          "\tFPC = " +
          std::to_string(get_fpc()) +
          "\n"
          "\twhole-habitat grass density = " +
          std::to_string(get_mass() / 1e4) +
          " gDM/m²\n"
          "Please check the vegetation model. Consider hard-setting the FPC "
          "value to a constant (e.g. 0.8 in grassland) or a minimum (e.g. "
          "0.1).");
    }
    return sd;
  }
 
 
  double get_fpc() const {
    if (get_mass() == 0.0 && fpc != 0.0)
      throw std::logic_error(
          "Fauna::GrassForage::get_fpc() "
          "Mass is zero, but FPC is not zero.");
    if (get_mass() != 0.0 && fpc == 0.0)
      throw std::logic_error(
          "Fauna::GrassForage::get_fpc() "
          "Mass is not zero, but FPC is zero.");
    return fpc;
  }
 
  GrassForage& merge(const GrassForage& other, const double this_weight,
                     const double other_weight);
 
  void set_fpc(const double f) {
    if (!(f >= 0.0 && f <= 1.0))
      throw std::invalid_argument(
          "Fauna::GrassForage::set_fpc() "
          "FPC out of valid range (0.0–1.0).");
    if (get_mass() == 0.0 && f > 0.0)
      throw std::logic_error(
          "Fauna::GrassForage::set_fpc() "
          "FPC must be zero if mass is zero.");
    if (get_mass() > 0.0 && f == 0.0)
      throw std::logic_error(
          "Fauna::GrassForage::set_fpc() "
          "FPC cannot be zero if there is grass mass.");
    fpc = f;
  }
};
}  // namespace Fauna
#endif  // FAUNA_GRASS_FORAGE_H