presto-main/src/main/java/io/prestosql/operator/scalar/BitwiseFunctions.java/BitwiseFunctions/bitCount
class BitwiseFunctions:    @Description("count number of set bits in 2's complement representation")
    @ScalarFunction
    @SqlType(StandardTypes.BIGINT)
    public static long bitCount(@SqlType(StandardTypes.BIGINT) long num, @SqlType(StandardTypes.BIGINT) long bits)
    {
        if (bits == 64) {
            return Long.bitCount(num);
        }
        if (bits <= 1 || bits > 64) {
            throw new PrestoException(INVALID_FUNCTION_ARGUMENT, "Bits specified in bit_count must be between 2 and 64, got " + bits);
        }
        long lowBitsMask = (1L << (bits - 1)) - 1; // set the least (bits - 1) bits
        if (num > lowBitsMask || num < ~lowBitsMask) {
            throw new PrestoException(INVALID_FUNCTION_ARGUMENT, "Number must be representable with the bits specified. " + num + " can not be represented with " + bits + " bits");
        }
        long mask = (1L << bits) - 1;
        return Long.bitCount(num & mask);
    }