September 2017 Update

Task/Combinations/JavaScript/combinations-7.js

@@ -0,0 +1,68 @@
+(() => {
+    'use strict';
+
+    // COMBINATIONS -----------------------------------------------------------
+
+    // comb :: Int -> Int -> [[Int]]
+    const comb = (m, n) => combinations(m, enumFromTo(0, n - 1));
+
+    // combinations :: Int -> [a] -> [[a]]
+    const combinations = (k, xs) =>
+        sort(filter(xs => k === xs.length, subsequences(xs)));
+
+
+    // GENERIC FUNCTIONS -----------------------------------------------------
+
+    // cons :: a -> [a] -> [a]
+    const cons = (x, xs) => [x].concat(xs);
+
+    // enumFromTo :: Int -> Int -> [Int]
+    const enumFromTo = (m, n) =>
+        Array.from({
+            length: Math.floor(n - m) + 1
+        }, (_, i) => m + i);
+
+    // filter :: (a -> Bool) -> [a] -> [a]
+    const filter = (f, xs) => xs.filter(f);
+
+    // foldr (a -> b -> b) -> b -> [a] -> b
+    const foldr = (f, a, xs) => xs.reduceRight(f, a);
+
+    // isNull :: [a] -> Bool
+    const isNull = xs => (xs instanceof Array) ? xs.length < 1 : undefined;
+
+    // show :: a -> String
+    const show = x => JSON.stringify(x) //, null, 2);
+
+    // sort :: Ord a => [a] -> [a]
+    const sort = xs => xs.sort();
+
+    // stringChars :: String -> [Char]
+    const stringChars = s => s.split('');
+
+    // subsequences :: [a] -> [[a]]
+    const subsequences = xs => {
+
+        // nonEmptySubsequences :: [a] -> [[a]]
+        const nonEmptySubsequences = xxs => {
+            if (isNull(xxs)) return [];
+            const [x, xs] = uncons(xxs);
+            const f = (r, ys) => cons(ys, cons(cons(x, ys), r));
+
+            return cons([x], foldr(f, [], nonEmptySubsequences(xs)));
+        };
+
+        return nonEmptySubsequences(
+            (typeof xs === 'string' ? stringChars(xs) : xs)
+        );
+    };
+
+    // uncons :: [a] -> Maybe (a, [a])
+    const uncons = xs => xs.length ? [xs[0], xs.slice(1)] : undefined;
+
+
+    // TEST -------------------------------------------------------------------
+    return show(
+        comb(3, 5)
+    );
+})();