September 2017 Update

Task/Harshad-or-Niven-series/JavaScript/harshad-or-niven-series-2.js

@@ -0,0 +1,69 @@
+(() => {
+    'use strict';
+
+    // HARSHADS ---------------------------------------------------------------
+
+    // nHarshads :: Int -> [Int]
+    const nHarshads = n => {
+
+        // isHarshad :: Int -> Bool
+        const isHarshad = n => 0 === n % sum(digitList(n));
+
+        return until(
+                dct => dct.nth === n,
+                dct => {
+                    const
+                        next = succ(dct.i),
+                        blnHarshad = isHarshad(next);
+                    return {
+                        i: next,
+                        hs: blnHarshad ? dct.hs.concat(next) : dct.hs,
+                        nth: dct.nth + (blnHarshad ? 1 : 0)
+                    };
+                }, {
+                    i: 0,
+                    hs: [],
+                    nth: 0
+                }
+            )
+            .hs;
+    };
+
+    // GENERIC FUNCTIONS ------------------------------------------------------
+
+    // digitList :: Int -> [Int]
+    const digitList = n =>
+        n > 0 ? [n % 10].concat(digitList(Math.floor(n / 10))) : [];
+
+    // dropWhile :: (a -> Bool) -> [a] -> [a]
+    const dropWhile = (p, xs) => {
+        let i = 0;
+        for (let lng = xs.length;
+            (i < lng) && p(xs[i]); i++) {}
+        return xs.slice(i);
+    };
+
+    // head :: [a] -> a
+    const head = xs => xs.length ? xs[0] : undefined;
+
+    // a -> String
+    const show = x => JSON.stringify(x, null, 2);
+
+    // succ :: Int -> Int
+    const succ = x => x + 1
+
+    // sum :: (Num a) => [a] -> a
+    const sum = xs => xs.reduce((a, x) => a + x, 0);
+
+    // until :: (a -> Bool) -> (a -> a) -> a -> a
+    const until = (p, f, x) => {
+        const go = x => p(x) ? x : go(f(x));
+        return go(x);
+    };
+
+    // TEST -------------------------------------------------------------------
+    return show({
+        firstTwenty: nHarshads(20),
+        firstOver1000: head(dropWhile(x => x <= 1000, nHarshads(1000)))
+    });
+})();