/** class Expr, our internal representation of an expression
 * in the L0 language.
 * See http://www.radford.edu/itec380/2008fall/Hw06/hw06.html
 * 
 * @author Ian Barland
 * @version 2008.Nov.30
 */
public abstract class Expr {
  /** Return (our internal representation of) the expression s.
   * @param s The source code for exactly one Expr.  Must by syntactically correct.
   * @return (our internal representation of) the expression s.
   */
  public static Expr parse(String s) { return Expr.parse(new java.util.Scanner(s)); }
  
  public static final String PUNCTUATION = "(){}<>!@$%^&*~+=;:'\"?.,#" 
                                         + java.util.regex.Pattern.quote("[]");
                                         // I am not sure why the [] needs to be quoted,
                                         // but nothing can be quoted.  It's a bug in UtilIan. --Ian

    /** Return (our internal representation of) the expression s.
   * @param s A scanner reading the source code for exactly one Expr.
   *          Must by syntactically correct.
   * @return (our internal representation of) the expression s.
   */
  public static Expr parse(java.util.Scanner s) {
    if (UtilIan.hasNextDoubleSplittingBy(s,PUNCTUATION)) {
      return new Num( UtilIan.nextDoubleSplittingBy(s,PUNCTUATION) );
      }
    else if (UtilIan.hasNextChar(s,'(') ) {// a sum or product
      UtilIan.nextChar(s,'(');  // Consume the opening "(" and continue.
      Expr subExpr1 = parse(s);
      String operator = UtilIan.nextSplittingBy(s,PUNCTUATION);
      Expr subExpr2 = parse(s);
      assert UtilIan.hasNextChar(s,')') : "Expected a ')' to finish the Expr " + operator.toString();
      UtilIan.nextChar(s,')');  // Consume the trailing '}'.
      if (operator.equals(Sum.TOKEN)) {
          return new Sum( subExpr1, subExpr2 );
          }
      else if (operator.equals(Prod.TOKEN)) {
          return new Prod( subExpr1, subExpr2 );
          }
      else { /* Unknown syntax! */
        String tokens = "";
        while (s.hasNext()) { tokens += s.next(); }
        throw new IllegalArgumentException( "Couldn't parse " + tokens );
        }
      }
    else if (UtilIan.hasNextSplittingBy(s,IfZero.TOKEN, PUNCTUATION)) {
        UtilIan.nextSplittingBy(s, PUNCTUATION);  // consume the "ifZero"
        assert UtilIan.hasNextChar(s,'(') : "'(' must follow " + IfZero.TOKEN;
        UtilIan.nextChar(s,'(');  // Consume the open-paren
        Expr subExpr1 = parse(s);
        assert UtilIan.hasNextChar(s,',') : "',' separate " + IfZero.TOKEN + " args.";
        UtilIan.nextChar(s,',');  // Consume the open-paren
        Expr subExpr2 = parse(s);
        assert UtilIan.hasNextChar(s,',') : "',' separate " + IfZero.TOKEN + " args.";
        UtilIan.nextChar(s,',');  // Consume the open-paren
        Expr subExpr3 = parse(s);
        assert UtilIan.hasNextChar(s,')') : "')' must close " + IfZero.TOKEN;
        UtilIan.nextChar(s,')');  // Consume the open-paren
        return new IfZero(subExpr1,subExpr2,subExpr3);
        }
    else { /* Unknown syntax! */
      String tokens = "";
      while (s.hasNext()) { tokens += s.next(); }
      throw new IllegalArgumentException( "Couldn't parse " + tokens );
      }
        
    
    }
  
  
  /** Evaluate a given Expr.
   * @return the Value this Expr evaluates to.
   *  (In L0, all values are numbers (doubles), but
   *   in L3 that will change, which is why we have
   *   pre-emptively made the return type 'Value'.)
   */
  abstract public Value eval();


  /** Return a String representation of this Expr.
   *  The result will be something which can be
   *  passed into 'parse(String)' to get the same
   *  Expr back.  That is, toString and parse are
   *  inverses of each other.
   *  @return a String representation of this Expr.
   */
  abstract public String toString();
  
  }