libavcodec/lagarithrac.c

   1 /*
   2  * Lagarith range decoder
   3  * Copyright (c) 2009 Nathan Caldwell <saintdev (at) gmail.com>
   4  * Copyright (c) 2009 David Conrad
   5  *
   6  * This file is part of Libav.
   7  *
   8  * Libav is free software; you can redistribute it and/or
   9  * modify it under the terms of the GNU Lesser General Public
  10  * License as published by the Free Software Foundation; either
  11  * version 2.1 of the License, or (at your option) any later version.
  12  *
  13  * Libav is distributed in the hope that it will be useful,
  14  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  16  * Lesser General Public License for more details.
  17  *
  18  * You should have received a copy of the GNU Lesser General Public
  19  * License along with Libav; if not, write to the Free Software
  20  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  21  */
  22
  23 /**
  24  * @file
  25  * Lagarith range decoder
  26  * @author Nathan Caldwell
  27  * @author David Conrad
  28  */
  29
  30 #include "bitstream.h"
  31 #include "lagarithrac.h"
  32
  33 void ff_lag_rac_init(lag_rac *l, BitstreamContext *bc, int length)
  34 {
  35     int i, j, left;
  36
  37     /* According to reference decoder "1st byte is garbage",
  38      * however, it gets skipped by the call to bitstream_align()
  39      */
  40     bitstream_align(bc);
  41     left                = bitstream_bits_left(bc) >> 3;
  42     l->bytestream_start =
  43     l->bytestream       = bc->buffer + bitstream_tell(bc) / 8;
  44     l->bytestream_end   = l->bytestream_start + FFMIN(length, left);
  45
  46     l->range        = 0x80;
  47     l->low          = *l->bytestream >> 1;
  48     l->hash_shift   = FFMAX(l->scale, 8) - 8;
  49
  50     for (i = j = 0; i < 256; i++) {
  51         unsigned r = i << l->hash_shift;
  52         while (l->prob[j + 1] <= r)
  53             j++;
  54         l->range_hash[i] = j;
  55     }
  56
  57     /* Add conversion factor to hash_shift so we don't have to in lag_get_rac. */
  58     l->hash_shift += 23;
  59 }