presto-main/src/main/java/io/prestosql/operator/scalar/ArrayCombinationsFunction.java/ArrayCombinationsFunction/combinationCount
class ArrayCombinationsFunction:    @VisibleForTesting
    static int combinationCount(int arrayLength, int combinationLength)
    {
        try {
            /*
             * Then combinationCount(n, k) = combinationCount(n-1, k-1) * n/k (https://en.wikipedia.org/wiki/Combination#Number_of_k-combinations)
             * The formula is recursive. Here, instead of starting with k=combinationCount, n=arrayLength and recursing,
             * we start with k=0 n=(arrayLength-combinationLength) and proceed "bottom up".
             */
            int combinations = 1;
            for (int i = 1; i <= combinationLength; i++) {
                combinations = multiplyExact(combinations, arrayLength - combinationLength + i) / i;
            }
            return combinations;
        }
        catch (ArithmeticException e) {
            throw new PrestoException(INVALID_FUNCTION_ARGUMENT, format("Number of combinations too large for array of size %s and combination length %s", arrayLength, combinationLength));
        }
    }