/* Arith0.java: simple parser -- no output
 * grammar:
 *   P ---> E '#'
 *   E ---> T {('+'|'-') T}
 *   T ---> S {('*'|'/') S}
 *   S ---> F '^' S | F
 *   F ---> char | '(' E ')'
 */
class Arith0 {
   private int level = 0;
   private GetChar getChar = new GetChar();
   private char next;

   private void P() {
      scan();
      E();
      if (next != '#') error(1);
      else System.out.println("Successful parse");
   }

   private void E() {
      enter('E');
      T();
      while (next == '+' || next == '-') {
         scan();
         T();
      }
      leave('E');
   }

   private void T() {
      enter('T');
      S();
      while (next == '*' || next == '/') {
         scan();
         S();
      }
      leave('T');
   }

   private void S() {
      enter('S');
      F();
      if (next == '^') {
         scan();
         S();
      }
      leave('S');
   }

   private void F() {
      enter('F');
      if (Character.isLetter(next)) {
         scan();
      }
      else if (next == '(') {
         scan();
         E();
         if (next == ')') scan();
         else error(2);
      }
      else {
         error(3);
      }
      leave('F');
   }

   private void scan() {
      while (Character.isWhitespace(next = getChar.getNextChar()))
         ;
   }

   private void error(int n) {
      System.out.println("*** ERROR: " + n);
      System.exit(1);
   }

   private void enter(char name) {
      spaces(level++);
      System.out.print("+-" + name + ": Enter, \t");
      System.out.println("Next == " + next);
   }

   private void leave(char name) {
      spaces(--level);
      System.out.print("+-" + name + ": Leave, \t");
      System.out.println("Next == " + next);
   }

   private void spaces(int local_level) {
      while (local_level-- > 0)
         System.out.print("| ");
   }

   public static void main(String[] args) {
      Arith0 arith0 = new Arith0();
      arith0.P();
   }
}

// GetChar: fetch next char 
import java.io.*;
public class GetChar { 
   private Reader in; // internal file name for input stream
  
   // GetChar: constructor  
   public GetChar () { 
      in = new InputStreamReader(System.in);
   }

   // getNextChar: fetches next char
   public char getNextChar() {
      char ch = ' '; // = ' ' to keep compiler happy
      try {
         ch = (char)in.read();
      } catch (IOException e) {
         System.out.println("Exception reading character");
      }
      return ch;
   }
}

ten42% !javac
javac Arith0.java
ten42% java Arith0
a + b + c #
+-E: Enter,     Next == a
| +-T: Enter,   Next == a
| | +-S: Enter,         Next == a
| | | +-F: Enter,       Next == a
| | | +-F: Leave,       Next == +
| | +-S: Leave,         Next == +
| +-T: Leave,   Next == +
| +-T: Enter,   Next == b
| | +-S: Enter,         Next == b
| | | +-F: Enter,       Next == b
| | | +-F: Leave,       Next == +
| | +-S: Leave,         Next == +
| +-T: Leave,   Next == +
| +-T: Enter,   Next == c
| | +-S: Enter,         Next == c
| | | +-F: Enter,       Next == c
| | | +-F: Leave,       Next == #
| | +-S: Leave,         Next == #
| +-T: Leave,   Next == #
+-E: Leave,     Next == #
Successful parse
ten42% !ja
java Arith0
a ^ b ^ c #
+-E: Enter,     Next == a
| +-T: Enter,   Next == a
| | +-S: Enter,         Next == a
| | | +-F: Enter,       Next == a
| | | +-F: Leave,       Next == ^
| | | +-S: Enter,       Next == b
| | | | +-F: Enter,     Next == b
| | | | +-F: Leave,     Next == ^
| | | | +-S: Enter,     Next == c
| | | | | +-F: Enter,   Next == c
| | | | | +-F: Leave,   Next == #
| | | | +-S: Leave,     Next == #
| | | +-S: Leave,       Next == #
| | +-S: Leave,         Next == #
| +-T: Leave,   Next == #
+-E: Leave,     Next == #
Successful parse
ten42% !ja      
java Arith0
( a * b #
+-E: Enter,     Next == (
| +-T: Enter,   Next == (
| | +-S: Enter,         Next == (
| | | +-F: Enter,       Next == (
| | | | +-E: Enter,     Next == a
| | | | | +-T: Enter,   Next == a
| | | | | | +-S: Enter,         Next == a
| | | | | | | +-F: Enter,       Next == a
| | | | | | | +-F: Leave,       Next == *
| | | | | | +-S: Leave,         Next == *
| | | | | | +-S: Enter,         Next == b
| | | | | | | +-F: Enter,       Next == b
| | | | | | | +-F: Leave,       Next == #
| | | | | | +-S: Leave,         Next == #
| | | | | +-T: Leave,   Next == #
| | | | +-E: Leave,     Next == #
*** ERROR: 2
ten42% !ja
java Arith0
(a + b) * c #
+-E: Enter,     Next == (
| +-T: Enter,   Next == (
| | +-S: Enter,         Next == (
| | | +-F: Enter,       Next == (
| | | | +-E: Enter,     Next == a
| | | | | +-T: Enter,   Next == a
| | | | | | +-S: Enter,         Next == a
| | | | | | | +-F: Enter,       Next == a
| | | | | | | +-F: Leave,       Next == +
| | | | | | +-S: Leave,         Next == +
| | | | | +-T: Leave,   Next == +
| | | | | +-T: Enter,   Next == b
| | | | | | +-S: Enter,         Next == b
| | | | | | | +-F: Enter,       Next == b
| | | | | | | +-F: Leave,       Next == )
| | | | | | +-S: Leave,         Next == )
| | | | | +-T: Leave,   Next == )
| | | | +-E: Leave,     Next == )
| | | +-F: Leave,       Next == *
| | +-S: Leave,         Next == *
| | +-S: Enter,         Next == c
| | | +-F: Enter,       Next == c
| | | +-F: Leave,       Next == #
| | +-S: Leave,         Next == #
| +-T: Leave,   Next == #
+-E: Leave,     Next == #
Successful parse
ten42%