#include <stdio.h>
#include <stdlib.h>

#include "grammar.h"

// forward declarations
void program(void);
void stmt_list(void);
void stmt(void);
void expr(void);
void term_tail(void);
void term(void);
void factor_tail(void);
void factor(void);
void add_op(void);
void mult_op(void);

void match(int x);
void epsilon(void);
void fail(void)
  __attribute__ ((noreturn))
;

void get_nexttoken();

extern int nextToken;

void program()
{
  stmt_list();
  match(0);
}

void stmt_list()
{
  if(nextToken == ID || nextToken == READ || nextToken == WRITE)
    {
      stmt();
      stmt_list();
    }
  else
    {
      epsilon();
    }
}

void stmt()
{
  if(nextToken == ID)
    {
      match(ID);
      match(ASSIGN);
      expr();
    }
  else if(nextToken == READ)
    {
      match(READ);
      match(ID);
    }
  else if(nextToken == WRITE)
    {
      match(WRITE);
      expr();
    }
  else
    {
      fail();
    }
}

void expr()
{
  term();
  term_tail();
}

void term_tail()
{
  if(nextToken == PLUS || nextToken == MINUS)
    {
      add_op();
      term();
      term_tail();
    }
  else
    {
      epsilon();
    }
}


void term()
{
  factor();
  factor_tail();
}

void factor_tail()
{
  if(nextToken == DIVIDE || nextToken == MULTIPLY)
    {
      mult_op();
      factor();
      factor_tail();
    }
  else
    {
      epsilon();
    }
}


void factor()
{
  if(nextToken == LPAREN)
    {
      match(LPAREN);
      expr();
      match(RPAREN);
    }
  else if(nextToken == ID)
    {
      match(ID);
    }
  else if(nextToken == NUMBER)
    {
      match(NUMBER);
    }
  else
    {
      fail();
    }
}

void add_op()
{
  if(nextToken == PLUS)
    {
      match(PLUS);
    }
  else if(nextToken == MINUS)
    {
      match(MINUS);
    }
  else
    {
      fail();
    }
}

void mult_op()
{
  if(nextToken == MULTIPLY)
    {
      match(MULTIPLY);
    }
  else if(nextToken == DIVIDE)
    {
      match(DIVIDE);
    }
  else
    {
      fail();
    }
}

void epsilon()
{
}

void match(int x)
{
  if(nextToken == x)
    {
      get_nexttoken();
    }
  else
    {
      fail();
    }
}


void fail()
{
  fprintf(stderr,"Parsing failed!\n");
  exit(99);
}