/*****************************************************************************
 * RRDtool 1.2.23  Copyright by Tobi Oetiker, 1997-2007
 * This code is stolen from rateup (mrtg-2.x) by Dave Rand
 *****************************************************************************
 * diff calculate the difference between two very long integers available as
 *      strings
 *****************************************************************************
 * $Id: rrd_diff.c 1050 2007-05-02 18:06:05Z oetiker $
 * $Log$
 * Revision 1.4  2003/03/10 00:30:34  oetiker
 * handle cases with two negative numbers
 * --  Sasha Mikheev <sasha@avalon-net.co.il>
 *
 * Revision 1.3  2002/04/01 18:31:22  oetiker
 * "!" takes a higher preference than "||" this means rrd_update N:: would
 * segfault -- Oliver Cook <ollie@uk.clara.net>
 *
 * Revision 1.2  2002/02/01 20:34:49  oetiker
 * fixed version number and date/time
 *
 * Revision 1.1.1.1  2001/02/25 22:25:05  oetiker
 * checkin
 *
 * Revision 1.1  1998/10/08 18:21:45  oetiker
 * Initial revision
 *
 * Revision 1.3  1998/02/06 21:10:52  oetiker
 * removed max define .. it is now in rrd_tool.h
 *
 * Revision 1.2  1997/12/07 20:38:03  oetiker
 * ansified
 *
 * Revision 1.1  1997/11/28 23:31:59  oetiker
 * Initial revision
 *
 *****************************************************************************/

#include "rrd_tool.h"

double
rrd_diff(char *a, char *b)
{
    char res[LAST_DS_LEN+1], *a1, *b1, *r1, *fix;
    int c,x,m;
    char a_neg=0, b_neg=0;
    double result;
   
    while (!(isdigit((int)*a) || *a==0)) {
        if(*a=='-') 
	    a_neg = 1;
        a++;
    }
    fix=a;
    while (isdigit((int)*fix)) 
	fix++;
    *fix = 0; /* maybe there is some non digit data in the string */ 
    while (!(isdigit((int)*b) || *b==0)) {
	if(*b=='-') 
	    b_neg = 1;  
        b++;
    }
    fix=b;
    while (isdigit((int)*fix)) 
	fix++;
    *fix = 0; /* maybe there is some non digit data in the string */ 
    if(!isdigit((int)*a) || !isdigit((int)*b))
	return DNAN;
    if(a_neg+b_neg == 1) /* can not handle numbers with different signs yet */
	return DNAN;
    a1 = &a[strlen(a)-1];
    m = max(strlen(a),strlen(b));
    if (m > LAST_DS_LEN) return DNAN; /* result string too short */

    r1 = &res[m+1];
    for (b1 = res;b1 <= r1; b1++) *b1 = ' ';
    b1 = &b[strlen(b)-1];
    r1[1] = 0;  /* Null terminate result */
    c = 0;
    for (x=0; x<m; x++) {
        if (a1 >= a && b1 >= b) {
            *r1 = ((*a1 - c) - *b1) + '0';
        } else if (a1 >= a) {
            *r1 = (*a1 - c);
        } else {
            *r1 = ('0' - *b1 - c) + '0';
        }
        if (*r1 < '0') {
            *r1 += 10;
            c=1;
        } else
	  if (*r1 > '9') { /* 0 - 10 */
	    *r1 -= 10;
	    c=1;	    
	  } else {
            c=0;
        }
        a1--;b1--;r1--;
    }
    if (c) {
        r1 = &res[m+1];
        for (x=0; isdigit((int)*r1) && x<m; x++,r1--)  {
            *r1 = ('9' - *r1 + c) + '0';
            if (*r1 > '9') {
                *r1 -= 10;
                c=1;
            } else {
                c=0;
            }
        }
        result = -atof(res);
    } else
        result = atof(res);

    if(a_neg+b_neg==2) /* both are negatives, reverse sign */
        result = -result;
    
    return result;
}