Data commit

Task/Palindrome-dates/JavaScript/palindrome-dates-1.js

@@ -0,0 +1,44 @@
+/**
+ * Adds zeros for 1 digit days/months
+ * @param date: string
+ */
+const addMissingZeros = date => (/^\d$/.test(date) ? `0${date}` : date);
+
+/**
+ * Formats a Date to a string. If readable is false,
+ * string is only numbers (used for comparison), else
+ * is a human readable date.
+ * @param date: Date
+ * @param readable: boolean
+ */
+const formatter = (date, readable) => {
+  const year = date.getFullYear();
+  const month = addMissingZeros(date.getMonth() + 1);
+  const day = addMissingZeros(date.getDate());
+
+  return readable ? `${year}-${month}-${day}` : `${year}${month}${day}`;
+};
+
+/**
+ * Returns n (palindromesToShow) palindrome dates
+ * since start (or 2020-02-02)
+ * @param start: Date
+ * @param palindromesToShow: number
+ */
+function getPalindromeDates(start, palindromesToShow = 15) {
+  let date = start || new Date(2020, 3, 2);
+
+  for (
+    let i = 0;
+    i < palindromesToShow;
+    date = new Date(date.setDate(date.getDate() + 1))
+  ) {
+    const formattedDate = formatter(date);
+    if (formattedDate === formattedDate.split("").reverse().join("")) {
+      i++;
+      console.log(formatter(date, true));
+    }
+  }
+}
+
+getPalindromeDates();

Task/Palindrome-dates/JavaScript/palindrome-dates-2.js

@@ -0,0 +1,57 @@
+(() => {
+    'use strict';
+
+    // ----------------- PALINDROME DATES ------------------
+
+    // palindromeDate :: Int -> [String]
+    const palindromeDate = year => {
+        // Either an empty list, if no palindromic date
+        // can be derived from this year, or a list
+        // containing a palindromic IS0 8601 date.
+        const
+            s = year.toString(),
+            r = reverse(s),
+            iso = [
+                s,
+                r.slice(0, 2),
+                r.slice(2, 4)
+            ].join('-');
+        return isNaN(new Date(iso)) ? (
+            []
+        ) : [iso];
+    };
+
+    // ----------------------- TEST ------------------------
+    const main = () => {
+        const
+            xs = enumFromTo(2021)(9999).flatMap(
+                palindromeDate
+            );
+        return [
+            `Count of palindromic dates [2021..9999]: ${
+                xs.length
+            }`,
+            '',
+            `First 15: ${'\n' + xs.slice(0, 15).join('\n')}`,
+            '',
+            `Last 15: ${'\n' + xs.slice(-15).join('\n')}`
+        ].join('\n');
+    };
+
+    // ----------------- GENERIC FUNCTIONS -----------------
+
+    // enumFromTo :: Int -> Int -> [Int]
+    const enumFromTo = m =>
+        n => Array.from({
+            length: 1 + n - m
+        }, (_, i) => m + i);
+
+    // reverse :: [a] -> [a]
+    const reverse = xs =>
+        'string' !== typeof xs ? (
+            xs.slice(0).reverse()
+        ) : xs.split('').reverse().join('');
+
+    // MAIN ---
+    return main();
+})();