8bitマイコンで比較的楽に曜日を何とかする方法(2000-2099のみの対応)

calc_day_of_week.c

#include <stdint.h>

uint8_t calc_day_of_week(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[] = {5, 1, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4, 0};
  if (year & 0b11) {
    return (year + year / 4 + non_leap[month - 1] + day) % 7;
  } else {
    return (year + year / 4 + leap[month - 1] + day) % 7;
  }
}