53 lines
1.5 KiB
C
53 lines
1.5 KiB
C
#include <stdio.h>
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#include <stdint.h>
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#include <stdbool.h>
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typedef struct {
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uint16_t year;
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uint8_t month;
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uint8_t day;
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} Date;
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bool leap(uint16_t year) {
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return year%4==0 && (year%100!=0 || year%400==0);
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}
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const char *weekday(Date date) {
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static const uint8_t leapdoom[] = {4,1,7,4,2,6,4,1,5,3,7,5};
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static const uint8_t normdoom[] = {3,7,7,4,2,6,4,1,5,3,7,5};
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static const char *days[] = {
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"Sunday", "Monday", "Tuesday", "Wednesday",
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"Thursday", "Friday", "Saturday"
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};
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unsigned c = date.year/100, r = date.year%100;
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unsigned s = r/12, t = r%12;
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unsigned c_anchor = (5 * (c%4) + 2) % 7;
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unsigned doom = (s + t + (t/4) + c_anchor) % 7;
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unsigned anchor = (leap(date.year) ? leapdoom : normdoom)[date.month-1];
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return days[(doom+date.day-anchor+7)%7];
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}
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int main(void) {
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const char *past = "was", *future = "will be";
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const char *months[] = { "",
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"January", "February", "March", "April", "May", "June",
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"July", "August", "September", "October", "November", "December"
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};
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const Date dates[] = {
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{1800,1,6}, {1875,3,29}, {1915,12,7}, {1970,12,23}, {2043,5,14},
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{2077,2,12}, {2101,4,2}
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};
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int i;
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for (i=0; i < sizeof(dates)/sizeof(Date); i++) {
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printf("%s %d, %d %s on a %s.\n",
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months[dates[i].month], dates[i].day, dates[i].year,
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dates[i].year > 2021 ? future : past,
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weekday(dates[i]));
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}
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return 0;
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}
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