EpochConverterHinnant.h

/*
 * MIT License
 * Copyright (c) 2022 Brian T. Park
 */
 
#ifndef ACE_TIME_EPOCH_CONVERTER_HINNANT_H
#define ACE_TIME_EPOCH_CONVERTER_HINNANT_H
 
#include <stdint.h>
 
namespace ace_time {
 
class EpochConverterHinnant {
  public:
    static const int16_t kInternalEpochYear = 2000;
 
    static const int32_t kDaysToInternalEpochFromUnixEpoch = 10957;
 
    static int32_t toEpochDays(int16_t year, uint8_t month, uint8_t day) {
      uint16_t yearPrime = year - ((month <= 2) ? 1 : 0);
      uint16_t era = yearPrime / 400; // [0,24]
      uint16_t yearOfEra = yearPrime - 400 * era; // [0,399]
 
      uint8_t monthPrime = (month <= 2) ? month + 9 : month - 3; // [0,11]
      uint16_t daysUntilMonthPrime = toDaysUntilMonthPrime(monthPrime);
      uint16_t dayOfYearPrime = daysUntilMonthPrime + day - 1; // [0,365]
      uint32_t dayOfEra = (uint32_t) 365 * yearOfEra + (yearOfEra / 4)
          - (yearOfEra / 100) + dayOfYearPrime; // [0, 146096]
 
      int32_t dayOfEpochPrime = dayOfEra + 146097 * era;
      return dayOfEpochPrime
          - (kInternalEpochYear / 400) * 146097 /*relative to 2000-03-01*/
          + 60 /*relative to 2000-01-01, 2000 is a leap year*/;
    }
 
    static void fromEpochDays(int32_t epochDays,
        int16_t& year, uint8_t& month, uint8_t& day) {
 
      int32_t dayOfEpochPrime = epochDays
          + (kInternalEpochYear / 400) * 146097 - 60;
      uint16_t era = (uint32_t) dayOfEpochPrime / 146097; // [0,24]
      uint32_t dayOfEra = dayOfEpochPrime - 146097 * era; // [0,146096]
      uint16_t yearOfEra = (dayOfEra - dayOfEra / 1460 + dayOfEra / 36524
          - dayOfEra / 146096) / 365; // [0,399]
      uint16_t yearPrime = yearOfEra + 400 * era; // [0,9999]
      uint16_t dayOfYearPrime = dayOfEra - (365 * yearOfEra + yearOfEra/4
          - yearOfEra/100);
      uint8_t monthPrime = (5 * dayOfYearPrime + 2) / 153;
      uint16_t daysUntilMonthPrime = toDaysUntilMonthPrime(monthPrime);
 
      day = dayOfYearPrime - daysUntilMonthPrime + 1; // [1,31]
      month = (monthPrime < 10) ? monthPrime + 3 : monthPrime - 9; // [1,12]
      year = yearPrime + ((month <= 2) ? 1 : 0); // [1,9999]
    }
 
    static uint16_t toDaysUntilMonthPrime(uint8_t monthPrime) {
      return (153 * monthPrime + 2) / 5;
    }
};
 
}
 
#endif