8bitマイコンで比較的楽に曜日を何とかする方法(0-9999のみの対応)

calc_day_of_week.c

#include <stdint.h>

uint8_t calc_day_of_week(uint8_t cent, uint8_t year, uint8_t month, uint8_t day) {
  const uint8_t non_leap[] = {6, 2, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
  const uint8_t leap[] = {6, 2, 3, 6, 1, 4, 6, 2, 5, 0, 3, 5, 1};
  uint8_t c = 7 - (cent * 2 - cent / 4) % 7;
  uint8_t is_leap = 0;

  if ((year == 0) && ((cent & 0b11) == 0)) {
    is_leap = 1;
  } else if (year == 0) {
    is_leap = 0;
  } else if ((year & 0b11) == 0) {
    is_leap = 1;
  }

  if (is_leap) {
    return (year + year / 4 + leap[month - 1] + day + c) % 7;
  } else {
    return (year + year / 4 + non_leap[month - 1] + day + c) % 7;
  }
}