/** class Expr, our internal representation of an expression
 * in the O0 language.
 * See http://www.radford.edu/itec380/2013fall/Homeworks/hw06.html
 * 
 * @author Ian Barland
 * @version 2008.Nov.30
 */
public abstract class Expr {
  
  /** Evaluate a given Expr.
   * @return the Value this Expr evaluates to.
   *  (In O0, all values are numbers (doubles), but
   *   in O3 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();
  
  
  /** 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.hasNextSplittingBy(s,Is0Expr.TOKEN, PUNCTUATION)) {
      UtilIan.nextSplittingBy(s, PUNCTUATION);  // consume the "if"
      Expr subExpr1 = parse(s);
      UtilIan.nextSplittingBy(s, PUNCTUATION);  // Consume the `is0`
      UtilIan.nextSplittingBy(s, PUNCTUATION);  // Consume the `then`
      Expr subExpr2 = parse(s);
      UtilIan.nextSplittingBy(s, PUNCTUATION);  // Consume the `else`
      Expr subExpr3 = parse(s);
      assert UtilIan.hasNextChar(s,';') : "`;` must close " + Is0Expr.TOKEN;
      UtilIan.nextChar(s,';');  // Consume the closing semicolon
      return new Is0Expr(subExpr1,subExpr2,subExpr3);
      }
    else if (UtilIan.hasNextChar(s,'(') ) { // a BinExpr or a ParenExpr
      UtilIan.nextChar(s,'(');  // Consume the opening "(" and continue.
      Expr subExpr1 = parse(s);
      if (UtilIan.hasNextChar(s,')')) {
        UtilIan.nextChar(s,')');  // consume the ')'
        return new ParenExpr(subExpr1);
        }
      else { // a BinExpr
        String operator = UtilIan.nextSplittingBy(s,PUNCTUATION);
        Expr subExpr2 = parse(s);
        UtilIan.nextChar(s,')');  // consume the ')'
        return new BinExpr(subExpr1, operator, subExpr2);
        }
      }
    else { /* Unknown syntax! */
      String tokens = "";
      while (s.hasNext()) { tokens += s.next(); }
      throw new IllegalArgumentException( "Syntax error: Couldn't parse " + tokens );
      }
        
    
    }
    
    /*  Note that I removed some sanity-checks from 'parse', so
        that students can concentrate on the code/algorithm. E.g.
    
          assert UtilIan.hasNextSplittingBy(s,"is0",PUNCTUATION)
                : "Expected `is0` after `" + subExpr1.toString() 
                + "`, got `" + UtilIan.nextSplittingBy(s,PUNCTUATION) + "`";
     */
    
 
  }