Thread: hex increment

Code:

for ( i = 15 ; i >= 0 ; i-- ) {
  if ( ++key[i] ) break;
  // continues if a value rolls over from 0xff to 0x00
}

Originally Posted by Salem

Code:

for ( i = 15 ; i >= 0 ; i-- ) {
  if ( ++key[i] ) break;
  // continues if a value rolls over from 0xff to 0x00
}

Maybe, like this?

Code:

#include <stdio.h>

void incr( unsigned char * );
void show( char *, unsigned char * );

int main( void )
{
  static unsigned char keys[16] = { [13] = 0x30, 0x30, 0xff };

  show( "before", keys );
  incr( keys );
  show( "after ", keys );
}

/* OBS: Salem's code is better than this one! */
void incr( unsigned char *p )
{
  unsigned int i, t;

  /* using 'unsigned int' because has more bits than 'unsigned char'.
     Anything bigger then 255 will be used to increment the previous item
     in the array (carry). */
  t = p[15];
  p[15] = ++t;

  /* Here, add carry-outs, if any. */
  for ( i = 14; i >= 0; i-- )
  {
    if ( t < 256 )
      break;
    t = p[i];
    p[i] = ++t;
  }
}

void show( char *s, unsigned char *p )
{
  int i;

  printf( "%s: {", s );
  for ( i = 0; i < 15; i++ )
    printf( "0x%02x, ", p[i] );
  printf( "0x%02x }\n", p[i] );
}

If you want to use SSSE3 (on x86):

Code:

void incr2( unsigned char *p )
{
  __m128i *a = (__m128i *)p;
  __m128i b = _mm_set_epi8( 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 );

  // the array is backwards (big endian)...
  *a = _mm_shuffle_epi8( *a, b );

  (*a)++;

  *a = _mm_shuffle_epi8( *a, b );
}