#include <stdio.h>
#include "recdesc.h"

/* global variables */

int tok;

extern FILE *yyin;

void start (void) {
  yyin = fopen("recdesc_in.txt", "r");
  tok = yylex();
  printf("Recognized %d\n", tok);
}

void advance(void) {
  tok = yylex();
  printf("Recognized %d\n", tok);
}

void stop (void) {
  fclose(yyin);
}

void error(void) {
  printf("Error in token %4d\n", tok);
}

void eat(enum token t) {
  /*  printf("Reading token %4d, expecting %4d\n", tok, t); */
  if (tok == t) advance(); else error();
}

/* Implementation of the grammar */

void E (void) { 
  eat(NUM); eat(EQ); eat(NUM); 
  printf("Recognized rule E\n"); 
}

/* The original grammar is ambiguous because of
- associativity. 
  Solution: introduced right-associativity
- construct 'if e then c1 else c2; c3'
  Solution: Introduced 'begin .. end'

Thus,
S -> print NUM | if E then S else S | begin S end | S; S

is transformed to:
S -> print NUM | print NUM ; S
     if E then S else S | if E then S else S ; S
     begin S end
omitting a rule of the form S -> begin S end; S
*/


void S (void) { 
  switch (tok) {
  case PRINT: 
    eat(PRINT); eat(NUM); 
    switch (tok) {
    case SEMI: 
      eat(SEMI); S();
      printf("Recognized rule S 1 semi\n"); 
      break;
    default:
      printf("Recognized rule S 1\n"); 
    }
    break;
  case IF: 
    eat(IF); E(); eat(THEN); S(); eat(ELSE); S();
    switch (tok) {
    case SEMI: 
      eat(SEMI); S();
      printf("Recognized rule S 2 semi\n"); 
      break;
    default:
      printf("Recognized rule S 2\n"); 
    }
    break;
  case BEG: 
    eat(BEG); S(); eat(END);
    printf("Recognized rule S 3\n"); 
    break;
  default: error();
  }
}

/* Main function */

int main (void) {

  start();
  S();
  stop();

  return 0;
}