// A simple scanner for a language with identifiers, integers,
 // parenthesis, and plus and minus symbols.
 #include <stdio.h>
 #include <string.h>
 #include <ctype.h>
 enum {Ident_max = 16};
 typedef char Ident[Ident_max+1];
 typedef enum {
     null, times, plus, rparen, lparen, number, writesym, eofsym
 } Symbol;
 class Scanner {
 public:
     bool init(const char fn[]);
     Symbol getsym();
     int getval() { return val; }
     const char *getid() { return id; }
 private:
     void read_ch();
     Symbol get_ident();
     Symbol get_number();
     void enter_kw(Symbol sym, Ident name);
     enum {KW=25};
     int val;
     Ident id;
     int ch;
     int nkw;
     char *keyTab[KW+1];
     Symbol keySym[KW+1];
     FILE *f;
 };
 void Scanner::read_ch() {
     ch = fgetc(f);
     if (ch != EOF)
         putchar((char)ch);
 }
 Symbol Scanner::get_ident() {
     int i;
     i = 0;
     do {
         if (i < Ident_max) {
             id[i] = (char)ch;
             i++;
         }
         read_ch();
     } while (ch != EOF && isalnum(ch));
     id[i] = '\0';
     for (i = 0; i < nkw && strcmp(id, keyTab[i]) != 0; i++)
         ;
     return keySym[i];
 }
 Symbol Scanner::get_number() {
     val = 0;
     do {
         val = 10 * val + (ch - '0');
         read_ch();
     } while (ch != EOF && isdigit(ch));
     return number;
 }
 Symbol Scanner::getsym() {
     while (ch != EOF && ch <= ' ')
         read_ch();
     switch (ch) {
         case EOF: return eofsym;
         case '+': read_ch(); return plus;
         case '*': read_ch(); return times;
         case '(': read_ch(); return lparen;
         case ')': read_ch(); return rparen;
         default:
             if (isalpha(ch))
                 return get_ident();
             if (isdigit(ch))
                 return get_number();
             read_ch();
             return null;
     }
 }
 void Scanner::enter_kw(Symbol sym, Ident name) {
     keyTab[nkw] = name;
     keySym[nkw] = sym;
     nkw++;
 }
 bool Scanner::init(const char fn[]) {
     if ((f = fopen(fn, "r")) == NULL)
         return false;
     ch = ' ';
     read_ch();
     nkw = 0;
     enter_kw(writesym,  "write");
     enter_kw(null,      "");
     return true;
 }
 int main(int argc, char *argv[]) {
     Scanner s;
     Symbol sym;
     if (argc < 2) {
         printf("Need a filename\n");
         return 2;
     }
     if (!s.init(argv[1])) {
         printf("Can't open %s\n", argv[1]);
         return 2;
     }
     do {
         sym = s.getsym();
     } while (sym != eofsym);
     return 0;
 }