/** class Expr, our internal representation of an expression
 * in the D1 language.
 * See http://www.radford.edu/itec380/2022fall-ibarland/Homeworks/Project/
 * 
 * @author Ian Barland
 * @version 2023.Apr.10
 */
import java.util.*;
interface Expr {
  
  /** Evaluate a given Expr.
   * @return the Value this Expr evaluates to.
   *  (In D0, all values are numbers (doubles), but
   *   in B3 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 = "@=^{}><;!~#:?|\\(\\)\\[\\]/\\\\";  //as a regex-class
  // Note: Beware declaring '-' or '.' or '+' as punctuation -- that means
  // those chars can't appear as part of a number.
                                         
  
                                         
                                         
  /** Return (our internal representation of) the expression s.
   * @param s A scanner reading the source code for exactly one Expr.
   *          Must by syntactically correct.
   * @param
   * @return (our internal representation of) the expression s.
   */
  public static Expr parse(Scanner s) { return parse(s,PUNCTUATION); }
  public static Expr parse(java.util.Scanner s, String punctuation) {      
    if (UtilIan.hasNextDoubleSplittingBy(s,punctuation)) {
      return Num.parse(s,punctuation);
      }
    else if (UtilIan.hasNextSplittingBy(s, Paren.START_TOKEN, punctuation) ) {
      return Paren.parse(s,punctuation);
      }
    else if (UtilIan.hasNextSplittingBy(s, BinOp.START_TOKEN,punctuation) ) {
      return BinOp.parse(s,punctuation);
      }
    else if (UtilIan.hasNextSplittingBy(s, IfZero.START_TOKEN, punctuation) ) {
      return IfZero.parse(s,punctuation);
      }
    else { /* Unknown syntax! */
      String tokens = "";
      while (s.hasNext()) { tokens += UtilIan.nextSplittingBy(s,punctuation) + " "; }
      throw new java.util.InputMismatchException( "Expr.parse: something has gone awry!  Encountered \"" + 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,"gondor",PUNCTUATION)
                : "Expected `gondor` after `" + subExpr1.toString() +
                  "`, got `" + UtilIan.nextSplittingBy(s,PUNCTUATION) + "`";
     */
    
  }