/* FPAE_Scanner.java
** CMPS 144L
** Authors: R. McCloskey and < students' names >
** Known flaws: ...
**
** An instance of this class is intended to "scan" a given FPAE 
** (fully-parenthesized arithmetic expression) for the purpose of 
** identifying the tokens contained therein, one after the other.
**
** The categories of tokens expected to appear in an FPAE are left and right
** parentheses ('(' and ')', respectively), operator symbols (any of '+', '-',
** '*', '/'), and unsigned integer literals (e.g., "3783", "2").  Consecutive
** tokens need not be separated from each other by spaces, but they could be.
**
** As an example, the tokens in "((167* 3 ) -15 )  " are, in order, "(", "(",
** "167", "*", "3", ")", "-", "15", and ")".
*/

public class FPAE_Scanner {

   // instance variables
   // ------------------

   private String expr;   // the string containing the expression to be scanned
   private int pos;       // the position, within expr, where the next token
                          // begins (or expr.length() if there are no more)

   // constructor
   // -----------

   public FPAE_Scanner(String s) {
      expr = s;
      pos = 0;
      advanceToNextToken();  // sets 'pos' to the position at which the 
   }                         // first token begins

   // observer
   // --------

   /* Reports whether or not there are any more tokens in the expression
   ** being scanned.
   */
   public boolean hasNext() { return pos != expr.length(); }


   // observer/mutator
   // ----------------

   /* Returns that substring of expr corresponding to the "next" token
   ** of the expression being scanned (i.e., the token following the one
   ** returned by the previous call to this method, or, in the case that
   ** this method had not been called previously, the first token)
   ** pre: hasNext() &&
   **      every character in 'expr' is either a parenthesis, 
   **      an operator symbol, a digit, or a space &&
   **      the next token (i.e., the one to be returned) begins at
   **      the position of 'expr' indicated by the value of 'pos'. 
   */
   public String next() {
      STUB!!
   }


   // private
   // -------

   /* Increases the value of instance variable 'pos' until either it is 
   ** equal to expr.length() or expr.charAt(pos) is a non-space.
   */
   public void advanceToNextToken() {
      final int N = expr.length();
      while (pos != N  &&  expr.charAt(pos) == ' ') { 
         pos = pos + 1;
      }
   }

}