/* front.c - a lexical analyzer system for simple
             arithmetic expressions */
#include <stdio.h>
#include <ctype.h>
  /* Global declarations */
  /* Variables */
int charClass;
char lexeme[100];
char nextChar;
int lexLen;
int token;
int nextToken;
FILE *in_fp, *fopen();
/* Function declarations */
void addChar();
void getChar();
void getNonBlank();
int lex();
/* Character classes */

#define LETTER 0
#define DIGIT 1
#define UNKNOWN 99
/* Token codes */

#define INT_LIT 10
#define IDENT 11
#define ASSIGN_OP 20
#define ADD_OP 21
#define SUB_OP 22
#define MULT_OP 23
#define DIV_OP 24
#define LEFT_PAREN 25
#define RIGHT_PAREN 26
/******************************************************/
/* main driver */

int main() {
  /* Open the input data file and process its contents */
  if ((in_fp = fopen("front.in", "r")) == NULL)
    printf("ERROR - cannot open front.in \n");
  else {
    getChar();
    do {
      lex();
    }
    while (nextToken != EOF);
  }
}
/*****************************************************/
/* lookup - a function to lookup operators and parentheses
            and return the token */
int lookup(char ch) {
  switch (ch) {
      case '(':
        addChar();
        nextToken = LEFT_PAREN;
        break;
      case ')':
        addChar();
        nextToken = RIGHT_PAREN;
        break;
      case '+':
        addChar();
        nextToken = ADD_OP;
        break;
      case '-':
        addChar();
        nextToken = SUB_OP;
        break;
      case '*':
        addChar();
        nextToken = MULT_OP;
        break;

      case '/':
        addChar();
        nextToken = DIV_OP;
        break;
      default:
        addChar();
        nextToken = EOF;
        break;
  }
  return nextToken;
}
/*****************************************************/
/* addChar - a function to add nextChar to lexeme */
void addChar() {
  if (lexLen <= 98) {
    lexeme[lexLen++] = nextChar;
    lexeme[lexLen] = 0;
  } else
    printf("Error - lexeme is too long \n");
}

/*****************************************************/
/* getChar - a function to get the next character of
             input and determine its character class */
void getChar() {
  if ((nextChar = getc(in_fp)) != EOF) {
    if (isalpha(nextChar)) { charClass = LETTER; }
    else if (isdigit(nextChar)) { charClass = DIGIT; }
    else { charClass = UNKNOWN; }

  } else { charClass = EOF; }
}
/*****************************************************/
/* getNonBlank - a function to call getChar until it
                 returns a non-whitespace character */
void getNonBlank() {
  while (isspace(nextChar)) { getChar(); }
}

/*****************************************************/
/* lex - a simple lexical analyzer for arithmetic
       expressions */
int lex() {
  lexLen = 0;
  getNonBlank();
  switch (charClass) {
    /* Parse identifiers */
  case
  LETTER:
    addChar();
    getChar();
    while (charClass == LETTER || charClass == DIGIT) {
      addChar();
      getChar();
    }
    nextToken = IDENT;
    break;
    /* Parse integer literals */
  case
  DIGIT:
    addChar();
    getChar();
    while (charClass == DIGIT) {
      addChar();
      getChar();
    }
    nextToken = INT_LIT;
    break;
    /* Parentheses and operators */
  case
  UNKNOWN:
    lookup(nextChar);
    getChar();
    break;
    /* EOF */
  case
  EOF:
    nextToken = EOF;
    lexeme[0] = 'E';
    lexeme[1] = 'O';
    lexeme[2] = 'F';
    lexeme[3] = 0;
    break;
  } /* End of switch */
  printf("Next token is: %d, Next lexeme is %s\n",
    nextToken, lexeme);
  return nextToken;
} /* End of function lex */