release/src/router/mysql/mysys/my_vle.c

   1 /* Copyright (c) 2005-2007 MySQL AB
   2
   3    This program is free software; you can redistribute it and/or
   4    modify it under the terms of the GNU General Public License as
   5    published by the Free Software Foundation; version 2 of the License.
   6
   7    This program is distributed in the hope that it will be useful, but
   8    WITHOUT ANY WARRANTY; without even the implied warranty of
   9    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  10    General Public License for more details.
  11
  12    You should have received a copy of the GNU General Public License
  13    along with this program; if not, write to the Free Software
  14    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA */
  15
  16 /*
  17   Variable length encoding.
  18
  19   A method to store an arbitrary-size non-negative integer. We let the
  20   most significant bit of the number indicate that the next byte
  21   should be contatenated to form the real number.
  22 */
  23
  24 #include "my_vle.h"
  25
  26 /*
  27   Function to encode an unsigned long as VLE. The bytes for the VLE
  28   will be written to the location pointed to by 'out'.  The maximum
  29   number of bytes written will be 'max'.
  30
  31   PARAMETERS
  32
  33     out  Pointer to beginning of where to store VLE bytes.
  34     max  Maximum number of bytes to write.
  35     n    Number to encode.
  36
  37   RETURN VALUE
  38     On success, one past the end of the array containing the VLE
  39     bytes.  On failure, the 'out' pointer is returned.
  40 */
  41
  42 uchar*
  43 my_vle_encode(uchar* out, size_t max, ulong n)
  44 {
  45   uchar buf[my_vle_sizeof(n)];
  46   uchar *ptr= buf;
  47   size_t len;
  48
  49   do
  50   {
  51     *ptr++= (uchar) (n & 0x7F);
  52     n>>= 7;
  53   }
  54   while (n > 0);
  55
  56   len= ptr - buf;
  57
  58   if (len <= max)
  59   {
  60     /*
  61       The bytes are stored in reverse order in 'buf'. Let's write them
  62       in correct order to the output buffer and set the MSB at the
  63       same time.
  64     */
  65     while (ptr-- > buf)
  66     {
  67       uchar v= *ptr;
  68       if (ptr > buf)
  69         v|= 0x80;
  70       *out++= v;
  71     }
  72   }
  73
  74   return out;
  75 }
  76
  77 /*
  78   Function to decode a VLE representation of an integral value.
  79
  80
  81   PARAMETERS
  82
  83     result_ptr  Pointer to an unsigned long where the value will be written.
  84     vle         Pointer to the VLE bytes.
  85
  86   RETURN VALUE
  87
  88     One-past the end of the VLE bytes. The routine will never read
  89     more than sizeof(*result_ptr) + 1 bytes.
  90 */
  91
  92 uchar const*
  93 my_vle_decode(ulong *result_ptr, uchar const *vle)
  94 {
  95   ulong result= 0;
  96   size_t cnt= 1;
  97
  98   do
  99   {
 100     result<<= 7;
 101     result|= (*vle & 0x7F);
 102   }
 103   while ((*vle++ & 0x80) && ++cnt <= sizeof(*result_ptr) + 1);
 104
 105   if (cnt <= sizeof(*result_ptr) + 1)
 106     *result_ptr= result;
 107
 108   return vle;
 109 }