/** Our internal representation of a IfZero expr
 * in the E0 language.
 * See http://www.radford.edu/itec380/2022fall-ibarland/Project
 * 
 * @author Ian Barland
 * @version 2023.Apr.10
 */
public record IfZero(Expr test, Expr thenAns, Expr elseAns) implements Expr {
  static final String START_TOKEN = "if";
  static final String CONT_TOKEN1a = "!";
  static final String CONT_TOKEN1b = "=";
  static final String CONT_TOKEN2 = "?";
  static final String CONT_TOKEN3 = ":";
  
  
  @Override
  public String toString() {
    return START_TOKEN + " "
         + this.test.toString() + " "
         + CONT_TOKEN1a + CONT_TOKEN1b + "0" + " " + CONT_TOKEN2 + " "
         + this.thenAns.toString() + " " + CONT_TOKEN3 + " "
         + this.elseAns.toString()
         ;
    }


  /** “@Override” {@link Expr#parse} */
  public static IfZero parse( java.util.Scanner s, String punctuation ) {
      UtilIan.verifyToken( UtilIan.nextSplittingBy(s,punctuation), START_TOKEN ); // consume&verify the "if"

      // NOTE: recur with `Expr.parse` -- not `parse` = `IfZero.parse` which is NOT what we want!
      Expr theTest       = Expr.parse(s,punctuation);
      UtilIan.verifyToken( UtilIan.nextSplittingBy(s,punctuation), CONT_TOKEN1a ); // consume&verify the "!" of "!=0 ?"
      UtilIan.verifyToken( UtilIan.nextSplittingBy(s,punctuation), CONT_TOKEN1b ); // consume&verify the "="
      double cont_token1c = Num.parse(s,punctuation).doubleValue();                // consume&verify the "0", via Num.parse
      assert cont_token1c == 0.0;
      UtilIan.verifyToken( UtilIan.nextSplittingBy(s,punctuation), CONT_TOKEN2  ); // consume&verify the "?"
      Expr theThenAnswer = Expr.parse(s,punctuation);
      UtilIan.verifyToken( UtilIan.nextSplittingBy(s,punctuation), CONT_TOKEN3  ); // consume&verify the ":"
      Expr theElseAnswer = Expr.parse(s,punctuation);
      return new IfZero(theTest,theThenAnswer,theElseAnswer);
      }
    
    
  static final double EPSILON = 1e-10;
  @Override
  public Value eval() {
    if ( Math.abs(((Num)(this.test.eval())).doubleValue()) < EPSILON ) {
      return this.thenAns.eval();
      }
   else {
      return this.elseAns.eval();
      }
    }
    
  }