Data update

Task/Doomsday-rule/C-sharp/doomsday-rule.cs

@@ -0,0 +1,57 @@
+using System;
+
+class Doom {
+    public static void Main(string[] args) {
+        Date[] dates = {
+            new Date(1800,1,6),
+            new Date(1875,3,29),
+            new Date(1915,12,7),
+            new Date(1970,12,23),
+            new Date(2043,5,14),
+            new Date(2077,2,12),
+            new Date(2101,4,2)
+        };
+
+        foreach (Date d in dates)
+            Console.WriteLine($"{d.Format()}: {d.Weekday()}");
+    }
+}
+
+class Date {
+    private int year, month, day;
+
+    private static readonly int[] leapDoom = {4,1,7,4,2,6,4,1,5,3,7,5};
+    private static readonly int[] normDoom = {3,7,7,4,2,6,4,1,5,3,7,5};
+    public static readonly string[] weekdays = {
+        "Sunday", "Monday", "Tuesday", "Wednesday",
+        "Thursday", "Friday", "Saturday"
+    };
+
+    public Date(int year, int month, int day) {
+        this.year = year;
+        this.month = month;
+        this.day = day;
+    }
+
+    public bool IsLeapYear() {
+        return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
+    }
+
+    public string Format() {
+        return $"{month:00}/{day:00}/{year:0000}";
+    }
+
+    public string Weekday() {
+        int c = year / 100;
+        int r = year % 100;
+        int s = r / 12;
+        int t = r % 12;
+
+        int cAnchor = (5 * (c % 4) + 2) % 7;
+        int doom = (s + t + t / 4 + cAnchor) % 7;
+        int anchor =
+            IsLeapYear() ? leapDoom[month - 1] : normDoom[month - 1];
+
+        return weekdays[(doom + day - anchor + 7) % 7];
+    }
+}