libavcodec/rangecoder.c

   1 /*
   2  * Range coder
   3  * Copyright (c) 2004 Michael Niedermayer <michaelni@gmx.at>
   4  *
   5  * This file is part of FFmpeg.
   6  *
   7  * FFmpeg is free software; you can redistribute it and/or
   8  * modify it under the terms of the GNU Lesser General Public
   9  * License as published by the Free Software Foundation; either
  10  * version 2.1 of the License, or (at your option) any later version.
  11  *
  12  * FFmpeg is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15  * Lesser General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU Lesser General Public
  18  * License along with FFmpeg; if not, write to the Free Software
  19  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  20  */
  21
  22 /**
  23  * @file
  24  * Range coder.
  25  * based upon
  26  *    "Range encoding: an algorithm for removing redundancy from a digitised
  27  *                     message.
  28  *     G. N. N. Martin                  Presented in March 1979 to the Video &
  29  *                                      Data Recording Conference,
  30  *     IBM UK Scientific Center         held in Southampton July 24-27 1979."
  31  */
  32
  33 #include <string.h>
  34
  35 #include "libavutil/attributes.h"
  36 #include "libavutil/avassert.h"
  37 #include "libavutil/bswap.h"
  38 #include "libavutil/intreadwrite.h"
  39
  40 #include "rangecoder.h"
  41
  42 av_cold void ff_init_range_encoder(RangeCoder *c, uint8_t *buf, int buf_size)
  43 {
  44     c->bytestream_start  =
  45     c->bytestream        = buf;
  46     c->bytestream_end    = buf + buf_size;
  47     c->low               = 0;
  48     c->range             = 0xFF00;
  49     c->outstanding_count = 0;
  50     c->outstanding_byte  = -1;
  51 }
  52
  53 av_cold void ff_init_range_decoder(RangeCoder *c, const uint8_t *buf,
  54                                    int buf_size)
  55 {
  56     /* cast to avoid compiler warning */
  57     ff_init_range_encoder(c, (uint8_t *)buf, buf_size);
  58
  59     c->low         = AV_RB16(c->bytestream);
  60     c->bytestream += 2;
  61     c->overread    = 0;
  62     if (c->low >= 0xFF00) {
  63         c->low = 0xFF00;
  64         c->bytestream_end = c->bytestream;
  65     }
  66 }
  67
  68 void ff_build_rac_states(RangeCoder *c, int factor, int max_p)
  69 {
  70     const int64_t one = 1LL << 32;
  71     int64_t p;
  72     int last_p8, p8, i;
  73
  74     memset(c->zero_state, 0, sizeof(c->zero_state));
  75     memset(c->one_state, 0, sizeof(c->one_state));
  76
  77     last_p8 = 0;
  78     p       = one / 2;
  79     for (i = 0; i < 128; i++) {
  80         p8 = (256 * p + one / 2) >> 32; // FIXME: try without the one
  81         if (p8 <= last_p8)
  82             p8 = last_p8 + 1;
  83         if (last_p8 && last_p8 < 256 && p8 <= max_p)
  84             c->one_state[last_p8] = p8;
  85
  86         p      += ((one - p) * factor + one / 2) >> 32;
  87         last_p8 = p8;
  88     }
  89
  90     for (i = 256 - max_p; i <= max_p; i++) {
  91         if (c->one_state[i])
  92             continue;
  93
  94         p  = (i * one + 128) >> 8;
  95         p += ((one - p) * factor + one / 2) >> 32;
  96         p8 = (256 * p + one / 2) >> 32; // FIXME: try without the one
  97         if (p8 <= i)
  98             p8 = i + 1;
  99         if (p8 > max_p)
 100             p8 = max_p;
 101         c->one_state[i] = p8;
 102     }
 103
 104     for (i = 1; i < 255; i++)
 105         c->zero_state[i] = 256 - c->one_state[256 - i];
 106 }
 107
 108 /* Return the number of bytes written. */
 109 int ff_rac_terminate(RangeCoder *c, int version)
 110 {
 111     if (version == 1)
 112         put_rac(c, (uint8_t[]) { 129 }, 0);
 113     c->range = 0xFF;
 114     c->low  += 0xFF;
 115     renorm_encoder(c);
 116     c->range = 0xFF;
 117     renorm_encoder(c);
 118
 119     av_assert1(c->low   == 0);
 120     av_assert1(c->range >= 0x100);
 121
 122     return c->bytestream - c->bytestream_start;
 123 }