import java.util.List;
import java.util.ArrayList;

/*
 * Lexical analyzer for arithmetic expressions which include variable names 
 * and positive integer literals
 * e.g. (sum + 47) / total
 */
public class Lexer {

  List<Token> tokens;
  int index = 0;

  Lexer(String s) {
    tokens = tokenize(s);
    index = 0;
  }

  public List<Token> tokenize(String input) {
    List<Token> result = new ArrayList<Token>();
    for(int i = 0; i < input.length(); ) {
      switch(input.charAt(i)) {
        case '(':
          result.add(new Token(TokenTypes.LPAREN, "("));
          i++;
          break;
        case ')':
          result.add(new Token(TokenTypes.RPAREN, ")"));
          i++;
          break;
        case '+':
          result.add(new Token(TokenTypes.ADD_OP, "+"));
          i++;
          break;
        case '-':
          result.add(new Token(TokenTypes.SUB_OP, "-"));
          i++;
          break;
        case '*':
          result.add(new Token(TokenTypes.MULT_OP, "*"));
          i++;
          break;
        case '/':
          result.add(new Token(TokenTypes.DIV_OP, "/"));
          i++;
          break;
        default:
          if(Character.isWhitespace(input.charAt(i)))
            i++;
          else if (Character.isDigit(input.charAt(i))) {
            int j = i;
            while ((j < input.length()) && Character.isDigit(input.charAt(j)))
              j++;
            result.add(new Token(TokenTypes.INT_LIT, input.substring(i,j)));
            i = j;
          }
          else if (Character.isLetter(input.charAt(i))){
            int j = i;
            while ((j < input.length()) && (Character.isDigit(input.charAt(j)) ||
                                            Character.isLetter(input.charAt(j))))
              j++;
            result.add(new Token(TokenTypes.IDENT, input.substring(i,j)));
            i = j;
          }
          else {
            System.out.println("UNEXPECTED CHARACTER ENCOUNTERED");
            System.exit(-1);
          }
          break;
      }
    }
    result.add(new Token(TokenTypes.EOF, "-1"));
    return result;
  }

  public Token lex() {
    Token t = null;
    if (index < tokens.size()) {
      t = tokens.get(index);
      index++;
    }
    System.out.println("Next Token is: "+t.getTokenID()+", Next lexeme is "+t);
    return t;  
  }

}