/*

 * Copyright (C) 2004-2007, 2011, 2012  Internet Systems Consortium, Inc. ("ISC")

 * Copyright (C) 1999-2001, 2003  Internet Software Consortium.

 *

 * Permission to use, copy, modify, and/or distribute this software for any

 * purpose with or without fee is hereby granted, provided that the above

 * copyright notice and this permission notice appear in all copies.

 *

 * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH

 * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY

 * AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT,

 * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM

 * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE

 * OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR

 * PERFORMANCE OF THIS SOFTWARE.

 */

/* $Id$ */

/*! \file */

#include <config.h>

#include <limits.h>

#include <isc/string.h>

/* Making a portable memcmp that has no internal branches and loops always

 * once for every byte without early-out shortcut has a few challenges.

 *

 * Inspired by 'timingsafe_memcmp()' from the BSD system and

 * https://github.com/libressl-portable/openbsd/blob/master/src/lib/libc/string/timingsafe_memcmp.c

 *

 * Sadly, that one is not portable C: It makes assumptions on the representation

 * of negative integers and assumes sign-preserving right-shift of negative

 * signed values. This is a rewrite from scratch that should not suffer from

 * such issues.

 *

 * 2015-12-12, J. Perlinger (perlinger-at-ntp-dot-org)

 */

int

isc_tsmemcmp(const void *p1, const void *p2, size_t nb) {

  const unsigned char *ucp1 = p1;

  const unsigned char *ucp2 = p2;

  unsigned int isLT = 0u;

  unsigned int isGT = 0u;

  volatile unsigned int mask = (1u << CHAR_BIT);

  for (/*NOP*/; 0 != nb; --nb, ++ucp1, ++ucp2) {

    isLT |= mask &

        ((unsigned int)*ucp1 - (unsigned int)*ucp2);

    isGT |= mask &

        ((unsigned int)*ucp2 - (unsigned int)*ucp1);

    mask &= ~(isLT | isGT);

  }

  return (int)(isGT >> CHAR_BIT) - (int)(isLT >> CHAR_BIT);

}