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 (() => {
    '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)
    // );
    
    console.log(show(comb(3,15)));
})();  

 // combinations :: Int -> [a] -> [[a]]
const combinations = (m) => (ns) => (ns.length == 0 || m == 0)
  ? []
  : m == 1
    ? ns
    : combinations (m - 1) (ns .slice(1)) .map(xs => [ns[0]] .concat(xs))
        .concat (combinations (m) (ns.slice(1) ) )

// combinations (3) (['a', 'b', 'c', 'd', 'e'])
//   .map(ls => ls .join('') )
     //=> ["abc", "abd", "abe", "acd", "ace", "ade", "bcd", "bce", "bde", "cde"]

// range :: Int -> Int -> [Int]
const range = (lo) => (hi) => [...Array(hi - lo   1)].map((_, i) => i   lo)

// comb :: Int -> Int -> [[Int]]
const comb = (m, n) => combinations (m) (range (0) (n - 1))

console.clear()
const now = new Date();
console.log(comb(3, 20).length);
console.log(`time: ${new Date() - now} ms`)
// ~> 1140
// ~> time 2 ms