Initial data commit

Task/Doomsday-rule/Java/doomsday-rule.java

@@ -0,0 +1,56 @@
+class Doom {
+    public static void main(String[] args) {
+        final 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)
+        };
+
+        for (Date d : dates)
+            System.out.println(
+                String.format("%s: %s", d.format(), d.weekday()));
+    }
+}
+
+class Date {
+    private int year, month, day;
+
+    private static final int[] leapdoom = {4,1,7,4,2,6,4,1,5,3,7,5};
+    private static final int[] normdoom = {3,7,7,4,2,6,4,1,5,3,7,5};
+    public static final 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 boolean isLeapYear() {
+        return year%4 == 0 && (year%100 != 0 || year%400 == 0);
+    }
+
+    public String format() {
+        return String.format("%02d/%02d/%04d", month, day, year);
+    }
+
+    public String weekday() {
+        final int c = year/100;
+        final int r = year%100;
+        final int s = r/12;
+        final int t = r%12;
+
+        final int c_anchor = (5 * (c%4) + 2) % 7;
+        final int doom = (s + t + t/4 + c_anchor) % 7;
+        final int anchor =
+            isLeapYear() ? leapdoom[month-1] : normdoom[month-1];
+
+        return weekdays[(doom + day - anchor + 7) % 7];
+    }
+}