Data update

Task/Doomsday-rule/JavaScript/doomsday-rule.js

@@ -0,0 +1,52 @@
+// doom.js
+const LEAP_DOOM = [4, 1, 7, 4, 2, 6, 4, 1, 5, 3, 7, 5];
+const NORM_DOOM = [3, 7, 7, 4, 2, 6, 4, 1, 5, 3, 7, 5];
+const WEEKDAYS = [
+  'Sunday', 'Monday', 'Tuesday', 'Wednesday',
+  'Thursday', 'Friday', 'Saturday'
+];
+
+class Date {
+  constructor(year, month, day) {
+    this.year = year;
+    this.month = month;   // 1-based
+    this.day = day;
+  }
+
+  isLeapYear() {
+    const y = this.year;
+    return y % 4 === 0 && (y % 100 !== 0 || y % 400 === 0);
+  }
+
+  format() {
+    const { month, day, year } = this;
+    return `${String(month).padStart(2, '0')}/${String(day).padStart(2, '0')}/${year}`;
+  }
+
+  weekday() {
+    const { year, month, day } = this;
+    const c = Math.floor(year / 100);
+    const r = year % 100;
+    const s = Math.floor(r / 12);
+    const t = r % 12;
+
+    const cAnchor = (5 * (c % 4) + 2) % 7;
+    const doom = (s + t + Math.floor(t / 4) + cAnchor) % 7;
+    const anchor = (this.isLeapYear() ? LEAP_DOOM : NORM_DOOM)[month - 1];
+
+    return WEEKDAYS[(doom + day - anchor + 7) % 7];
+  }
+}
+
+/* ---------- demo ---------- */
+const dates = [
+  new Date(1800, 1, 6),
+  new Date(1875, 3, 29),
+  new Date(1915, 12, 7),
+  new Date(1970, 12, 23),
+  new Date(2043, 5, 14),
+  new Date(2077, 2, 12),
+  new Date(2101, 4, 2)
+];
+
+dates.forEach(d => console.log(`${d.format()}: ${d.weekday()}`));