Custom string interception function

CREATE OR REPLACE FUNCTION SBUSTR(STR_INPUT IN VARCHAR2, LENGTH_INPUT IN NUMBER)
    RETURN VARCHAR2 IS
    I          NUMBER;
    SUB_STR    VARCHAR2(2000);
    SUB_LENGTH NUMBER := 0;
    STR_COUNT  NUMBER := 0;
    TEMP       NUMBER := 0;
    RETURN_STR VARCHAR2(2000) := '';
  BEGIN
    SELECT LENGTH(STR_INPUT) INTO SUB_LENGTH FROM DUAL;
    FOR I IN 1 .. SUB_LENGTH LOOP
      SELECT SUBSTR(STR_INPUT, I, 1) INTO SUB_STR FROM DUAL;
      SELECT LENGTHB(SUB_STR) INTO SUB_LENGTH FROM DUAL;
      IF (SUB_LENGTH = 1) THEN
        TEMP := 1;
      ELSE
        TEMP := 2;
      END IF;
      IF (STR_COUNT + TEMP <= LENGTH_INPUT) THEN
        IF (SUB_LENGTH = 1) THEN
          STR_COUNT := STR_COUNT + 1;
        ELSE
          STR_COUNT := STR_COUNT + 2;
        END IF;
        RETURN_STR := RETURN_STR || SUB_STR;
      ELSE
        EXIT;
      END IF;
    END LOOP;
    RETURN RETURN_STR;
  END SBUSTR;
/