gregorian.hpp

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/*
 
Copyright 2025 Matthew Tolman
 
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
 
   http://www.apache.org/licenses/LICENSE-2.0
 
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
 
*/
 
#ifndef MTCORE_CALENDARS_GREGORIAN_HPP
#define MTCORE_CALENDARS_GREGORIAN_HPP
 
#include "mtcore_calendars/epochs.hpp"
#include "mtcore_calendars/fixed.hpp"
#include "mtcore_calendars/traits.hpp"
#include <string_view>
 
namespace mtcore::calendars::systems {
  struct Gregorian {
    static constexpr auto EPOCH            = GREGORIAN_EPOCH;
    static constexpr std::string_view name = "Gregorian";
 
    enum class Month {
      January = 1,
      February,
      March,
      April,
      May,
      June,
      July,
      August,
      September,
      October,
      November,
      December
    };
 
    AstronomicalYear year = {};
    Month month           = Month::January;
    u8 day                = 1;
 
    void init(i32 year, Month month, u8 day) {
      this->year  = AstronomicalYear{year};
      this->month = month;
      this->day   = day;
    }
 
    [[nodiscard]] constexpr std::strong_ordering operator<=>(const Gregorian &g) const {
      if (const auto c = year <=> g.year; c != std::strong_ordering::equivalent) {
        return c;
      }
      if (const auto c = month <=> g.month; c != std::strong_ordering::equivalent) {
        return c;
      }
      return day <=> g.day;
    }
 
    [[nodiscard]] static constexpr bool is_leap_year(const AstronomicalYear year) {
      const auto y          = static_cast<YearBase>(year);
      const auto yearMod400 = math::mod(y, 400);
      return math::mod(y, 4) == 0 && yearMod400 != 100 && yearMod400 != 200 && yearMod400 != 300;
    }
 
    [[nodiscard]] constexpr bool leap_year() const { return is_leap_year(this->year); }
 
    [[nodiscard]] static constexpr Gregorian year_start_for(const AstronomicalYear year) {
      return {.year = year, .month = Month::January, .day = 1};
    }
 
    [[nodiscard]] constexpr Gregorian year_start() const { return year_start_for(this->year); }
 
    [[nodiscard]] static constexpr Gregorian year_end_for(const AstronomicalYear year) {
      return {.year = year, .month = Month::December, .day = 31};
    }
 
    [[nodiscard]] constexpr Gregorian year_end() const { return year_start_for(this->year); }
 
    [[nodiscard]] constexpr Fixed to_fixed() const {
      return Fixed{.day = EPOCH - 1 + 365 * (static_cast<YearBase>(year) - 1) +
                          math::floor<double, i32>(static_cast<double>(static_cast<YearBase>(year) - 1) / 4.0) -
                          math::floor<double, i32>(static_cast<double>(static_cast<YearBase>(year) - 1) / 100.0) +
                          math::floor<double, i32>(static_cast<double>(static_cast<YearBase>(year) - 1) / 400.0) +
                          math::floor<double, i32>((367. * static_cast<double>(month) - 362.) / 12.0) +
                          (static_cast<i32>(month) <= 2 ? 0
                           : is_leap_year(year)         ? -1
                                                        : -2) +
                          static_cast<i32>(day)};
    }
 
    [[nodiscard]] static constexpr Gregorian from_fixed(const Fixed &f) {
      const auto year             = year_from_fixed(f);
      const auto yearStart        = year_start_for(year);
      const auto yearStartRdDate  = yearStart.to_fixed();
      const auto priorDays        = f.day_difference(yearStartRdDate);
      const auto marchFirst       = Gregorian{year, Month::March, 1};
      const auto marchFirstRdDate = marchFirst.to_fixed();
      const auto correction = (f <=> marchFirstRdDate) == std::strong_ordering::less ? 0 : is_leap_year(year) ? 1 : 2;
      const auto month = math::floor<double, Month>(static_cast<double>(12 * (priorDays + correction) + 373) / 367.0);
      const auto firstOfMonth       = Gregorian{year, month, 1};
      const auto firstOfMonthRdDate = firstOfMonth.to_fixed();
      return Gregorian{year, month, static_cast<u8>(f.day_difference(firstOfMonthRdDate) + 1)};
    }
 
    [[nodiscard]] constexpr u16 days_in_year() const { return days_in_year_for(year); }
 
    [[nodiscard]] static constexpr u16 days_in_year_for(const AstronomicalYear year) {
      if (is_leap_year(year)) {
        return 366;
      }
      return 365;
    }
 
    [[nodiscard]] constexpr u8 days_in_month() const { return days_in_month_for(year, month); }
 
    [[nodiscard]] static constexpr u8 days_in_month_for(const AstronomicalYear year, const Month month) {
      switch (month) {
        case Month::January:
        case Month::March:
        case Month::May:
        case Month::July:
        case Month::August:
        case Month::October:
        case Month::December:
          return 31;
        case Month::February:
          return is_leap_year(year) ? 29 : 28;
        case Month::April:
        case Month::June:
        case Month::September:
        case Month::November:
          return 30;
      }
      unreachable("BAD MONTH");
    }
 
  private:
    [[nodiscard]] static constexpr AstronomicalYear year_from_fixed(const Fixed &f) {
      const auto d0 = f.sub_days(EPOCH).day;
      // 146097 = number of days in 400 years (adjusted for leap years)
      const auto n400 = math::floor<double, int64_t>(static_cast<double>(d0) / 146097.0);
      const auto d1   = math::mod(d0, 146097);
      // 36524 = number of days in 100 years (adjusted for leap years)
      const auto n100 = math::floor<double, int64_t>(static_cast<double>(d1) / 36524.0);
      const auto d2   = math::mod(d1, 36524);
      // 1461 = number of days in 4 years (adjusted for leap years)
      const auto n4   = math::floor<double, int64_t>(static_cast<double>(d2) / 1461.0);
      const auto d3   = math::mod(d2, 1461);
      const auto n1   = math::floor<double, int64_t>(static_cast<double>(d3) / 365.0);
      const auto year = 400 * n400 + 100 * n100 + 4 * n4 + n1 + 1 -
                        static_cast<int64_t>(static_cast<unsigned>(n100 == 4) | static_cast<unsigned>(n1 == 4));
      return year;
    }
  };
 
  static_assert(IsDayCalendarSystem<Gregorian>);
}  // namespace mtcore::calendars::systems
 
#endif  // MTCORE_CALENDARS_GREGORIAN_HPP