function k_combinations(set, k) {
    var i, j, combs, head, tailcombs;

    if (k > set.length || k <= 0) {
        return [];
    }

    if (k == set.length) {
        return [set];
    }

    if (k == 1) {
        combs = [];
        for (i = 0; i < set.length; i++) {
            combs.push([set[i]]);
        }
        return combs;
    }

    combs = [];
    for (i = 0; i < set.length - k + 1; i++) {
        // head is a list that includes only our current element.
        head = set.slice(i, i + 1);
        // We take smaller combinations from the subsequent elements
        tailcombs = k_combinations(set.slice(i + 1), k - 1);
        // For each (k-1)-combination we join it with the current
        // and store it to the set of k-combinations.
        for (j = 0; j < tailcombs.length; j++) {
            combs.push(head.concat(tailcombs[j]));
        }
    }
    return combs;
}

function stringPermutations(str) {
    let letters = str.split('')
        , results = [[letters.shift()]]
    while (letters.length) {
        const currLetter = letters.shift()
        let tmpResults = []
        results.forEach(result => {
            let rIdx = 0
            while (rIdx <= result.length) {
                const tmp = [...result]
                tmp.splice(rIdx, 0, currLetter)
                tmpResults.push(tmp)
                rIdx++
            }
        })
        results = tmpResults
    }
    return results
        .map(letterArray => letterArray.join(''))
        .filter((el, idx, self) => (self.indexOf(el) === idx))
        .sort()
}

export function calculateCombination(text, n) {
    return k_combinations(text.split(''), n).map(it => stringPermutations(it.join(''))).flat().filter((value, index, self) => {
        return self.indexOf(value) === index;
    });
}