/*
 *      eval --- evaluate expression
 *
 *      an expression is constructed like this:
 *
 *      expr ::=  expr + term |
 *                expr - term ;
 *                expr * term ;
 *                expr / term ;
 *                expr | term ;
 *                expr & term ;
 *                expr % term ;
 *                expr ^ term ;
 *
 *      term ::=  symbol |
 *                * |
 *                constant ;
 *
 *      symbol ::=  string of alphanumerics with non-initial digit
 *
 *      constant ::= hex constant |
 *                   binary constant |
 *                   octal constant |
 *                   decimal constant |
 *                   ascii constant;
 *
 *      hex constant ::= '$' {hex digits};
 *
 *      octal constant ::= '@' {octal digits};
 *
 *      binary constant ::= '%' { 1 | 0 };
 *
 *      decimal constant ::= {decimal digits};
 *
 *      ascii constant ::= ''' any printing char;
 *
 */
eval()
{
 int     left,right;     /* left and right terms for expression */
 char    o;              /* operator character */
 
#ifdef DEBUG
 printf("Evaluating %s\n",Optr);
#endif
 Force_byte = NO;
 Force_word = NO;
 if(*Optr=='<'){
  Force_byte++;
  Optr++;
  }
 else if(*Optr=='>'){
  Force_word++;
  Optr++;
  }
 left = get_term();         /* pickup first part of expression */
 
 while( is_op(*Optr)){
  o = *Optr++; /* pickup connector and skip */
  right = get_term();     /* pickup current rightmost side */
  switch(o){
   case '+': left += right; break;
   case '-': left -= right; break;
   case '*': left *= right; break;
   case '/': left /= right; break;
   case '|': left |= right; break;
   case '&': left &= right; break;
   case '%': left %= right; break;
   case '^': left = left^right; break;
   }
  }
 
 Result= left;
#ifdef DEBUG
 printf("Result=%x\n",Result);
 printf("Force_byte=%d  Force_word=%d\n",Force_byte,Force_word);
#endif
 return(YES);
}
 
/*
 *      is_op --- is character an expression operator?
 */
is_op(c)
char c;
{
 if( any(c,"+-*/&%|^"))
  return(YES);
 return(NO);
}
 
 
/*
 *      get_term --- evaluate a single item in an expression
 */
get_term()
{
 char    hold[MAXBUF];
 char    *tmp;
 int     val = 0;        /* local value being built */
 int     minus;          /* unary minus flag */
 struct nlist *lookup();
 
 if( *Optr == '-' ){
  Optr++;
  minus =YES;
  }
 else
  minus = NO;
 
 while( *Optr == '#' ) Optr++;
 
 /* look at rest of expression */
 
 if(*Optr=='%'){ /* binary constant */
  Optr++;
  while( any(*Optr,"01"))
   val = (val * 2) + ( (*Optr++)-'0');
  }
 else if(*Optr=='@'){ /* octal constant */
  Optr++;
  while( any(*Optr,"01234567"))
   val = (val * 8) + ((*Optr++)-'0');
  }
 else if(*Optr=='$'){ /* hex constant */
  Optr++;
  while( any(*Optr,"0123456789abcdefABCDEF"))
   if( *Optr > '9' )
    val = (val * 16) + 10 + (mapdn(*Optr++)-'a');
   else
    val = (val * 16) + ((*Optr++)-'0');
  }
 else if( any(*Optr,"0123456789")){ /* decimal constant */
  while(*Optr >= '0' && *Optr <= '9')
   val = (val * 10) + ( (*Optr++)-'0');
  }
 else if(*Optr=='*'){    /* current location counter */
  Optr++;
  val = Old_pc;
  }
 else if(*Optr=='\''){   /* character literal */
  Optr++;
  if(*Optr == EOS)
   val = 0;
  else
   val = *Optr++;
  }
 else if( alpha(*Optr) ){ /* a symbol */
  tmp = hold;     /* collect symbol name */
  while(alphan(*Optr))
   *tmp++ = *Optr++;
  *tmp = EOS;
  if(lookup(hold) != NULL)
   val = Last_sym;
  else{
   if(Pass==1){    /* forward ref here */
    fwdmark();
    if( !Force_byte )
     Force_word++;
    val = 0;
    }
   }
  if(Pass==2 && Line_num==F_ref && Cfn==Ffn){
   if( !Force_byte  )
    Force_word++;
   fwdnext();
   }
  }
 else
  /* none of the above */
  val = 0;
 
 if(minus)
  return(-val);
 else
  return(val);
}