September 2017 Update

Task/Forward-difference/JavaScript/forward-difference.js

@@ -0,0 +1,57 @@
+(() => {
+    'use strict';
+
+    // forwardDifference :: [Int] -> [Int]
+    const forwardDifference = (n, xs) => {
+        const fd = xs => zipWith((a, b) => a - b, tail(xs), xs);
+        return until(
+                m => m.index < 1,
+                m => ({
+                    index: m.index - 1,
+                    list: fd(m.list)
+                }), {
+                    index: n,
+                    list: xs
+                }
+            )
+            .list;
+    };
+
+
+    // GENERIC FUNCTIONS ---------------------------------------
+
+    // zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
+    const zipWith = (f, xs, ys) => {
+        const ny = ys.length;
+        return (xs.length <= ny ? xs : xs.slice(0, ny))
+            .map((x, i) => f(x, ys[i]));
+    };
+
+    // until :: (a -> Bool) -> (a -> a) -> a -> a
+    const until = (p, f, x) => {
+        const go = x => p(x) ? x : go(f(x));
+        return go(x);
+    };
+
+    // tail :: [a] -> [a]
+    const tail = xs => xs.length ? xs.slice(1) : undefined;
+
+
+    // TEST ----------------------------------------------------
+
+    // range :: Int -> Int -> [Int]
+    const range = (m, n) =>
+        Array.from({
+            length: Math.floor(n - m) + 1
+        }, (_, i) => m + i);
+
+    // show :: a -> String
+    const show = x => JSON.stringify(x);
+
+    // Sample
+    const test = [90, 47, 58, 29, 22, 32, 55, 5, 55, 73];
+
+    return range(1, 9)
+        .map(x => `${x}    ${show(forwardDifference(x, test))}`)
+        .join('\n');
+})();