September 2017 Update

Task/Combinations-with-repetitions/JavaScript/combinations-with-repetitions-4.js

@@ -0,0 +1,62 @@
+(() => {
+    'use strict';
+
+    // COMBINATIONS WITH REPETITIONS -------------------------------------------
+
+    // combsWithRep :: Int -> [a] -> [[a]]
+    const combsWithRep = (k, xs) => {
+        const comb = (n, ys) => {
+            if (0 === n) return ys;
+            if (isNull(ys)) return comb(n - 1, map(pure, xs));
+
+            return comb(n - 1, concatMap(zs => {
+                const h = head(zs);
+                return map(x => [x].concat(zs), dropWhile(x => x !== h, xs));
+            }, ys));
+        };
+        return comb(k, []);
+    };
+
+    // GENERIC FUNCTIONS ------------------------------------------------------
+
+    // concatMap :: (a -> [b]) -> [a] -> [b]
+    const concatMap = (f, xs) => [].concat.apply([], xs.map(f));
+
+    // 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);
+    };
+
+    // enumFromTo :: Int -> Int -> [Int]
+    const enumFromTo = (m, n) =>
+        Array.from({
+            length: Math.floor(n - m) + 1
+        }, (_, i) => m + i);
+
+    // head :: [a] -> Maybe a
+    const head = xs => xs.length ? xs[0] : undefined;
+
+    // isNull :: [a] -> Bool
+    const isNull = xs => (xs instanceof Array) ? xs.length < 1 : undefined;
+
+    // length :: [a] -> Int
+    const length = xs => xs.length;
+
+    // map :: (a -> b) -> [a] -> [b]
+    const map = (f, xs) => xs.map(f);
+
+    // pure :: a -> [a]
+    const pure = x => [x];
+
+    // show :: a -> String
+    const show = x => JSON.stringify(x, null, 2);
+
+    // TEST -------------------------------------------------------------------
+    return show({
+        twoFromThree: combsWithRep(2, ['iced', 'jam', 'plain']),
+        threeFromTen: length(combsWithRep(3, enumFromTo(0, 9)))
+    });
+})();