September 2017 Update

Task/Casting-out-nines/JavaScript/casting-out-nines-2.js

@@ -0,0 +1,46 @@
+(() => {
+    'use strict';
+
+    // co9 :: Int -> Int
+    const co9 = n =>
+        n <= 8 ? n : co9(
+            digits(10, n)
+            .reduce((a, x) => x !== 9 ? a + x : a, 0)
+        );
+
+    // GENERIC FUNCTIONS
+
+    // digits :: Int -> Int -> [Int]
+    const digits = (base, n) => {
+        if (n < base) return [n];
+        const [q, r] = quotRem(n, base);
+        return [r].concat(digits(base, q));
+    };
+
+    // quotRem :: Integral a => a -> a -> (a, a)
+    const quotRem = (m, n) => [Math.floor(m / n), m % n];
+
+    // range :: Int -> Int -> [Int]
+    const range = (m, n) =>
+        Array.from({
+            length: Math.floor(n - m) + 1
+        }, (_, i) => m + i);
+
+    // squared :: Num a => a -> a
+    const squared = n => Math.pow(n, 2);
+
+    // show :: a -> String
+    const show = x => JSON.stringify(x, null, 2);
+
+    // TESTS
+    return show({
+        test1: co9(232345), //-> 1
+        test2: co9(34234234), //-> 7
+        test3: co9(232345 + 34234234) === co9(232345) + co9(34234234), //-> true
+        test4: co9(232345 * 34234234) === co9(232345) * co9(34234234), //-> true,
+        task2: range(1, 100)
+            .filter(n => co9(n) === co9(squared(n))),
+        task3: (k => range(1, 100)
+            .filter(n => (n % k) === (squared(n) % k)))(16)
+    });
+})();