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libavcodec/lagarithrac.c

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00001 /*
00002  * Lagarith range decoder
00003  * Copyright (c) 2009 Nathan Caldwell <saintdev (at) gmail.com>
00004  * Copyright (c) 2009 David Conrad
00005  *
00006  * This file is part of FFmpeg.
00007  *
00008  * FFmpeg is free software; you can redistribute it and/or
00009  * modify it under the terms of the GNU Lesser General Public
00010  * License as published by the Free Software Foundation; either
00011  * version 2.1 of the License, or (at your option) any later version.
00012  *
00013  * FFmpeg is distributed in the hope that it will be useful,
00014  * but WITHOUT ANY WARRANTY; without even the implied warranty of
00015  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
00016  * Lesser General Public License for more details.
00017  *
00018  * You should have received a copy of the GNU Lesser General Public
00019  * License along with FFmpeg; if not, write to the Free Software
00020  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
00021  */
00022 
00030 #include "get_bits.h"
00031 #include "lagarithrac.h"
00032 
00033 void lag_rac_init(lag_rac *l, GetBitContext *gb, int length)
00034 {
00035     int i, j;
00036 
00037     /* According to reference decoder "1st byte is garbage",
00038      * however, it gets skipped by the call to align_get_bits()
00039      */
00040     align_get_bits(gb);
00041     l->bytestream_start =
00042     l->bytestream       = gb->buffer + get_bits_count(gb) / 8;
00043     l->bytestream_end   = l->bytestream_start + length;
00044 
00045     l->range        = 0x80;
00046     l->low          = *l->bytestream >> 1;
00047     l->hash_shift   = FFMAX(l->scale - 8, 0);
00048 
00049     for (i = j = 0; i < 256; i++) {
00050         unsigned r = i << l->hash_shift;
00051         while (l->prob[j + 1] <= r)
00052             j++;
00053         l->range_hash[i] = j;
00054     }
00055 
00056     /* Add conversion factor to hash_shift so we don't have to in lag_get_rac. */
00057     l->hash_shift += 23;
00058 }

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