2016 Update

Task/Happy-numbers/JavaScript/happy-numbers-4.js

@@ -0,0 +1,35 @@
+(() => {
+    'use strict';
+
+    // isHappy :: Int -> Bool
+    let isHappy = n => {
+        let f = n => n.toString()
+            .split('')
+            .reduce((a, x) => a + Math.pow(parseInt(x, 10), 2), 0),
+            p = (s, n) => n === 1 ? true : (
+                s.has(n) ? false : p(s.add(n), f(n))
+            );
+        return p(new Set(), n);
+    },
+
+    // until :: (a -> Bool) -> (a -> a) -> a -> a
+    until = (p, f, x) => {
+        let v = x;
+        while (!p(v)) v = f(v);
+        return v;
+    };
+
+    return until(
+        m => m.xs.length === 8,
+        m => {
+            let n = m.n;
+            return {
+                n: n + 1,
+                xs: isHappy(n) ? m.xs.concat(n) : m.xs
+            };
+        }, {
+            n: 1,
+            xs: []
+        }
+    ).xs;
+})();