// Program: 4033
//  Author: Manuel J. A. Eugster ( e0126312@student.tuwien.ac.at )
//  FileId: $Id: Parser.java,v 1.1 2002/01/14 17:28:09 heder Exp $



package Wider;



import java.lang.*;



// CLASS:
//    Implements contextfree grammar.
//
//    <expression> : <term> | <term> + <expression> | <term> / <expression>
//          <term> : (<expression>) | v
//
public class Parser implements Token
{
  private Scanner scanner;  // Scanner object with the resistor network description

  
  
  // METHOD:
  //    Constructor.
  //
  public Parser( Scanner scanner )
  {
    this.scanner = scanner;
  }
  
  
  
  // METHOD:
  //    Starts the parsing run.
  //  
  public float start() throws Exception
  {
    return( expression() );
  }
  
  
  
  // METHOD:
  //    Implements first rule of the contextfree grammar.
  //    Recursive, and returns the result.
  //
  private float expression() throws Exception
  {
    float result = term();
    
    
    if ( scanner.valueTok == T_SER )
    {
      scanner.nextToken();
      result = CalcResistor.ser( result, expression() );
    }
    else if ( scanner.valueTok == T_PAR )
    {
      scanner.nextToken();
      result = CalcResistor.par( result, term() );
    }


    return( result );
  }  

  

  // METHOD:
  //    Implements second rule of the contextfree grammar.
  //    Recursive, and returns the result.
  //  
  private float term() throws Exception
  {
    float result = 0f;
    
    
    if ( scanner.valueTok == T_BRO )
    {
      scanner.nextToken();
      result = expression();
      
      if ( scanner.valueTok != T_BRC )
      {
        throw new ParserException( "Bracket closed" );
      }
      
      scanner.nextToken();
      
      
      return( result );
    }
    else if ( scanner.valueTok == T_NUM )
    {
      result = scanner.valueNum;
      scanner.nextToken();
      
      
      return( result );
    }
    
    
    throw new ParserException( "Missing term" );
  }
}