/*
 *      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 ;
 *                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;
			case '>': left >>= right; break;
			case '<': 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);
}