Initial data commit

Task/Doomsday-rule/C/doomsday-rule.c

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