/* memxor3.c

   Copyright (C) 2010, 2014 Niels Möller

   This file is part of GNU Nettle.

   GNU Nettle is free software: you can redistribute it and/or

   modify it under the terms of either:

     * the GNU Lesser General Public License as published by the Free

       Software Foundation; either version 3 of the License, or (at your

       option) any later version.

   or

     * the GNU General Public License as published by the Free

       Software Foundation; either version 2 of the License, or (at your

       option) any later version.

   or both in parallel, as here.

   GNU Nettle is distributed in the hope that it will be useful,

   but WITHOUT ANY WARRANTY; without even the implied warranty of

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

   General Public License for more details.

   You should have received copies of the GNU General Public License and

   the GNU Lesser General Public License along with this program.  If

   not, see http://www.gnu.org/licenses/.

*/

/* Implementation inspired by memcmp in glibc, contributed to the FSF

   by Torbjorn Granlund.

 */

#if HAVE_CONFIG_H

# include "config.h"

#endif

#include <assert.h>

#include <limits.h>

#include "memxor.h"

#include "memxor-internal.h"

#define WORD_T_THRESH 16

/* For fat builds */

#if HAVE_NATIVE_memxor3

void *

_nettle_memxor3_c(void *dst_in, const void *a_in, const void *b_in, size_t n);

# define nettle_memxor3 _nettle_memxor3_c

#endif

/* XOR word-aligned areas. n is the number of words, not bytes. */

static void

memxor3_common_alignment (word_t *dst,

        const word_t *a, const word_t *b, size_t n)

{

  /* FIXME: Require n > 0? */

  if (n & 1)

    {

      n--;

      dst[n] = a[n] ^ b[n];

    }

  while (n > 0)

    {

      n -= 2;

      dst[n+1] = a[n+1] ^ b[n+1];

      dst[n] = a[n] ^ b[n];

    }

}

static void

memxor3_different_alignment_b (word_t *dst,

             const word_t *a, const unsigned char *b,

             unsigned offset, size_t n)

{

  int shl, shr;

  const word_t *b_word;

  word_t s0, s1;

  assert (n > 0);

  shl = CHAR_BIT * offset;

  shr = CHAR_BIT * (sizeof(word_t) - offset);

  b_word = (const word_t *) ((uintptr_t) b & -sizeof(word_t));

  /* Read top offset bytes, in native byte order. */

  READ_PARTIAL (s0, (unsigned char *) &b_word[n], offset);

#ifdef WORDS_BIGENDIAN

  s0 <<= shr;

#endif

  if (n & 1)

    s1 = s0;

  else

    {

      n--;

      s1 = b_word[n];

      dst[n] = a[n] ^ MERGE (s1, shl, s0, shr);

    }

  while (n > 2)

    {

      n -= 2;

      s0 = b_word[n+1];

      dst[n+1] = a[n+1] ^ MERGE(s0, shl, s1, shr);

      s1 = b_word[n];

      dst[n] = a[n] ^ MERGE(s1, shl, s0, shr);

    }

  assert (n == 1);

  /* Read low wordsize - offset bytes */

  READ_PARTIAL (s0, b, sizeof(word_t) - offset);

#ifndef WORDS_BIGENDIAN

  s0 <<= shl;

#endif /* !WORDS_BIGENDIAN */

  dst[0] = a[0] ^ MERGE(s0, shl, s1, shr);

}

static void

memxor3_different_alignment_ab (word_t *dst,

        const unsigned char *a, const unsigned char *b,

        unsigned offset, size_t n)

{

  int shl, shr;

  const word_t *a_word;

  const word_t *b_word;

  word_t s0, s1, t;

  assert (n > 0);

  shl = CHAR_BIT * offset;

  shr = CHAR_BIT * (sizeof(word_t) - offset);

  a_word = (const word_t *) ((uintptr_t) a & -sizeof(word_t));

  b_word = (const word_t *) ((uintptr_t) b & -sizeof(word_t));

  /* Read top offset bytes, in native byte order. */

  READ_PARTIAL (s0, (unsigned char *) &a_word[n], offset);

  READ_PARTIAL (t,  (unsigned char *) &b_word[n], offset);

  s0 ^= t;

#ifdef WORDS_BIGENDIAN

  s0 <<= shr;

#endif

  if (n & 1)

    s1 = s0;

  else

    {

      n--;

      s1 = a_word[n] ^ b_word[n];

      dst[n] = MERGE (s1, shl, s0, shr);

    }

  while (n > 2)

    {

      n -= 2;

      s0 = a_word[n+1] ^ b_word[n+1];

      dst[n+1] = MERGE(s0, shl, s1, shr);

      s1 = a_word[n] ^ b_word[n];

      dst[n] = MERGE(s1, shl, s0, shr);

    }

  assert (n == 1);

  /* Read low wordsize - offset bytes */

  READ_PARTIAL (s0, a, sizeof(word_t) - offset);

  READ_PARTIAL (t,  b, sizeof(word_t) - offset);

  s0 ^= t;

#ifndef WORDS_BIGENDIAN

  s0 <<= shl;

#endif /* !WORDS_BIGENDIAN */

  dst[0] = MERGE(s0, shl, s1, shr);

}

static void

memxor3_different_alignment_all (word_t *dst,

         const unsigned char *a, const unsigned char *b,

         unsigned a_offset, unsigned b_offset,

         size_t n)

{

  int al, ar, bl, br;

  const word_t *a_word;

  const word_t *b_word;

  word_t a0, a1, b0, b1;

  al = CHAR_BIT * a_offset;

  ar = CHAR_BIT * (sizeof(word_t) - a_offset);

  bl = CHAR_BIT * b_offset;

  br = CHAR_BIT * (sizeof(word_t) - b_offset);

  a_word = (const word_t *) ((uintptr_t) a & -sizeof(word_t));

  b_word = (const word_t *) ((uintptr_t) b & -sizeof(word_t));

  /* Read top offset bytes, in native byte order. */

  READ_PARTIAL (a0, (unsigned char *) &a_word[n], a_offset);

  READ_PARTIAL (b0, (unsigned char *) &b_word[n], b_offset);

#ifdef WORDS_BIGENDIAN

  a0 <<= ar;

  b0 <<= br;

#endif

  if (n & 1)

    {

      a1 = a0; b1 = b0;

    }

  else

    {

      n--;

      a1 = a_word[n];

      b1 = b_word[n];

      dst[n] = MERGE (a1, al, a0, ar) ^ MERGE (b1, bl, b0, br);

    }

  while (n > 2)

    {

      n -= 2;

      a0 = a_word[n+1]; b0 = b_word[n+1];

      dst[n+1] = MERGE(a0, al, a1, ar) ^ MERGE(b0, bl, b1, br);

      a1 = a_word[n]; b1 = b_word[n];

      dst[n] = MERGE(a1, al, a0, ar) ^ MERGE(b1, bl, b0, br);

    }

  assert (n == 1);

  /* Read low wordsize - offset bytes */

  READ_PARTIAL (a0, a, sizeof(word_t) - a_offset);

  READ_PARTIAL (b0, b, sizeof(word_t) - b_offset);

#ifndef WORDS_BIGENDIAN

  a0 <<= al;

  b0 <<= bl;

#endif /* !WORDS_BIGENDIAN */

  dst[0] = MERGE(a0, al, a1, ar) ^ MERGE(b0, bl, b1, br);

}

/* Current implementation processes data in descending order, to

   support overlapping operation with one of the sources overlapping

   the start of the destination area. This feature is used only

   internally by cbc decrypt, and it is not advertised or documented

   to nettle users. */

void *

nettle_memxor3(void *dst_in, const void *a_in, 

         const void *b_in, size_t n)

{

  unsigned char *dst = dst_in;

  const unsigned char *a = a_in;

  const unsigned char *b = b_in;

  if (n >= WORD_T_THRESH)

    {

      unsigned i;

      unsigned a_offset;

      unsigned b_offset;

      size_t nwords;

      for (i = ALIGN_OFFSET(dst + n); i > 0; i--)

  {

    n--;

    dst[n] = a[n] ^ b[n];

  }

      a_offset = ALIGN_OFFSET(a + n);

      b_offset = ALIGN_OFFSET(b + n);

      nwords = n / sizeof (word_t);

      n %= sizeof (word_t);

      if (a_offset == b_offset)

  {

    if (!a_offset)

      memxor3_common_alignment((word_t *) (dst + n),

             (const word_t *) (a + n),

             (const word_t *) (b + n), nwords);

    else

      memxor3_different_alignment_ab((word_t *) (dst + n),

             a + n, b + n, a_offset,

             nwords);

  }

      else if (!a_offset)

  memxor3_different_alignment_b((word_t *) (dst + n),

              (const word_t *) (a + n), b + n,

              b_offset, nwords);

      else if (!b_offset)

  memxor3_different_alignment_b((word_t *) (dst + n),

              (const word_t *) (b + n), a + n,

              a_offset, nwords);

      else

  memxor3_different_alignment_all((word_t *) (dst + n), a + n, b + n,

          a_offset, b_offset, nwords);

    }

  while (n-- > 0)

    dst[n] = a[n] ^ b[n];

  return dst;

}