changes_only_state.hpp

#ifndef LIVE_CELLS_CHANGES_ONLY_STATE_HPP
#define LIVE_CELLS_CHANGES_ONLY_STATE_HPP
 
#include <optional>
 
#include "compute_state.hpp"
 
namespace live_cells {
 
    template <typename Base>
    class changes_only_cell_state : public Base {
    public:
        using Base::Base;
 
        void will_update(const key_ref &k) override {
            Base::handle_will_update([this] { pre_update(); },
                                     [this] { Base::notify_will_update(); });
        }
 
        void update(const key_ref &k, bool changed) override {
            Base::handle_update(changed,
                                [this] { return did_change(); },
                                [this] (bool changed) { Base::notify_update(changed); },
                                [this] { post_update(); });
        }
 
    private:
        std::optional<typename Base::value_type> old;
 
        bool did_change() {
            try {
                return !old.has_value() || Base::value() != old.value();
            }
            catch (...) {
                old.reset();
            }
 
            return true;
        }
 
        void pre_update() {
            try {
                old = Base::value();
            }
            catch (...) {
                old.reset();
            }
        }
 
        void post_update() {
            old.reset();
        }
    };
 
    struct changes_only_option {};
 
    static constexpr changes_only_option changes_only{};
 
}  // live_cells
 
#endif /* LIVE_CELLS_CHANGES_ONLY_STATE_HPP */