presto-main/src/main/java/io/prestosql/operator/scalar/StringFunctions.java/StringFunctions/splitPart
class StringFunctions:    @SqlNullable
    @Description("splits a string by a delimiter and returns the specified field (counting from one)")
    @ScalarFunction
    @LiteralParameters({"x", "y"})
    @SqlType("varchar(x)")
    public static Slice splitPart(@SqlType("varchar(x)") Slice string, @SqlType("varchar(y)") Slice delimiter, @SqlType(StandardTypes.BIGINT) long index)
    {
        checkCondition(index > 0, INVALID_FUNCTION_ARGUMENT, "Index must be greater than zero");
        // Empty delimiter? Then every character will be a split
        if (delimiter.length() == 0) {
            int startCodePoint = toIntExact(index);

            int indexStart = offsetOfCodePoint(string, startCodePoint - 1);
            if (indexStart < 0) {
                // index too big
                return null;
            }
            int length = lengthOfCodePoint(string, indexStart);
            if (indexStart + length > string.length()) {
                throw new PrestoException(INVALID_FUNCTION_ARGUMENT, "Invalid UTF-8 encoding");
            }
            return string.slice(indexStart, length);
        }

        int matchCount = 0;

        int previousIndex = 0;
        while (previousIndex < string.length()) {
            int matchIndex = string.indexOf(delimiter, previousIndex);
            // No match
            if (matchIndex < 0) {
                break;
            }
            // Reached the requested part?
            if (++matchCount == index) {
                return string.slice(previousIndex, matchIndex - previousIndex);
            }
            // Continue searching after the delimiter
            previousIndex = matchIndex + delimiter.length();
        }

        if (matchCount == index - 1) {
            // returns last section of the split
            return string.slice(previousIndex, string.length() - previousIndex);
        }

        // index is too big, null is returned
        return null;
    }