// Parser implementation
// Parser is a class to represent a recursive descent parser for the PL/0
// programming language, described in Algorithms + Data Structures = programs
// by Niklaus Wirth, Prentice-Hall, 1976.

class SyntaxAnalyzer {

  protected PL0Lexer lexer;	// lexical analyzer
  protected Token token;	// current token

  public SyntaxAnalyzer () throws java.io.IOException {
    lexer = new PL0Lexer (System . in);
    getToken ();
  }

  private void getToken () throws java.io.IOException { 
    token = lexer . nextToken (); 
  }

  public void constDeclaration () throws java.io.IOException {
    if (token . symbol () == TokenClass . ID) {
      getToken ();
      if (token . symbol () == TokenClass . EQ) {
        getToken ();
        if (token . symbol () == TokenClass . INTEGER)
          getToken ();
        else 
          ErrorMessage . print (lexer . position (), "Integer expected");
      }
      else ErrorMessage . print (lexer . position (), "= expected");
    }
    else ErrorMessage . print (lexer . position (), "id expected");
  }

  public void varDeclaration () throws java.io.IOException {
    if (token . symbol () == TokenClass . ID)
      getToken ();
    else 
      ErrorMessage . print (lexer . position (), "ID EXPECTED");
  }

  public void factor () throws java.io.IOException {
    if (token . symbol () == TokenClass . ID) 
      getToken ();
    else if (token . symbol () == TokenClass . INTEGER) 
      getToken ();
    else if (token . symbol () == TokenClass . LPAREN) {
      getToken ();
      expression ();
      if (token . symbol () == TokenClass . RPAREN) 
        getToken ();
      else 
        ErrorMessage . print (lexer . position (), "MISSING )");
    }
    else ErrorMessage . print (lexer . position (), "UNRECOGNIZABLE SYMBOL");
  }

  public void term () throws java.io.IOException {
    factor ();
    while (token . symbol () == TokenClass . TIMES || 
	token . symbol () == TokenClass . SLASH) { 
      getToken ();
      factor (); 
    }
  }

  public void expression () throws java.io.IOException {
    if (token . symbol () == TokenClass . PLUS || 
	token . symbol () == TokenClass . MINUS) { 
      getToken ();
      term (); 
    } 
    else 
      term ();
    while (token . symbol () == TokenClass . PLUS || 
	token . symbol () == TokenClass . MINUS) {
      getToken ();
      term (); 
    }
  }

  public void condition () throws java.io.IOException {
    if (token . symbol () == TokenClass . ODD) { 
      getToken ();
      expression (); 
    }
    else {
      expression ();
      switch (token . symbol ()) {
        case EQ : 
	case LT : 
	case GT : 
	case NE : 
	case LE : 
	case GE :
          getToken ();
	  expression (); 
	  break;
        default : 
	  ErrorMessage . print 
	    (lexer . position (), "RELATIONAL OPERATOR EXPECTED");
      }
    }
  }

  public void statement () throws java.io.IOException {
    switch (token . symbol ()) {
      case ID :
        getToken ();
        if (token . symbol () == TokenClass . ASSIGN) 
          getToken ();
        else 
          ErrorMessage . print (lexer . position (), ":= EXPECTED");
        expression ();
        break;
      case CALL :
        getToken ();
        if (token . symbol () != TokenClass . ID) 
	  ErrorMessage . print (lexer . position (), "ID EXPECTED");
        else {
          getToken ();
          break;
        }
      case IF :
        getToken ();
        condition ();
        if (token . symbol () == TokenClass . THEN) 
          getToken ();
        else 
          ErrorMessage . print (lexer . position (), "THEN EXPECTED");
        statement ();
        break;
      case BEGIN :
        getToken ();
        statement ();
        while (token . symbol () == TokenClass . SEMICOLON) { 
	  getToken ();
	  statement (); 
	}
        if (token . symbol () == TokenClass . END) 
          getToken ();
        else 
          ErrorMessage . print (lexer . position (), "END OR ; EXPECTED");
        break;
      case WHILE :
        getToken ();
        condition ();
        if (token . symbol () == TokenClass . DO) 
          getToken ();
        else 
          ErrorMessage . print (lexer . position (), "DO EXPECTED");
        statement ();
        break;
    }
  }

  public void block () throws java.io.IOException {
    if (token . symbol () == TokenClass . CONST) {
      getToken ();
      constDeclaration ();
      while (token . symbol () == TokenClass . COMMA) { 
	getToken ();
	constDeclaration (); 
      }
      if (token . symbol () == TokenClass . SEMICOLON) 
        getToken ();
      else 
        ErrorMessage . print (lexer . position (), ", OR ; EXPECTED");
    }
    if (token . symbol () == TokenClass . VAR) {
      getToken ();
      varDeclaration ();
      while (token . symbol () == TokenClass . COMMA) { 
	getToken ();
	varDeclaration (); 
      }
      if (token . symbol () == TokenClass . SEMICOLON) 
        getToken ();
      else 
        ErrorMessage . print (lexer . position (), ", OR ; EXPECTED");
    }
    while (token . symbol () == TokenClass . PROC) {
      getToken ();
      if (token . symbol () == TokenClass . ID)
        getToken ();
      else 
        ErrorMessage . print (lexer . position (), "ID EXPECTED");
      if (token . symbol () == TokenClass . SEMICOLON) 
        getToken ();
      else 
        ErrorMessage . print (lexer . position (), "; EXPECTED");
      block ();
      if (token . symbol () == TokenClass . SEMICOLON) 
        getToken ();
      else 
        ErrorMessage . print (lexer . position (), "; EXPECTED");
    }
    statement ();
  }

  public void program () throws java.io.IOException {
    block ();
    if (token . symbol () != TokenClass . PERIOD) 
      ErrorMessage . print (lexer . position (), ". EXPECTED");
  }

}