mirror of https://git.ffmpeg.org/ffmpeg.git
117 lines
3.7 KiB
C
117 lines
3.7 KiB
C
/*
|
|
* Lagarith range decoder
|
|
* Copyright (c) 2009 Nathan Caldwell <saintdev (at) gmail.com>
|
|
* Copyright (c) 2009 David Conrad
|
|
*
|
|
* This file is part of Libav.
|
|
*
|
|
* Libav is free software; you can redistribute it and/or
|
|
* modify it under the terms of the GNU Lesser General Public
|
|
* License as published by the Free Software Foundation; either
|
|
* version 2.1 of the License, or (at your option) any later version.
|
|
*
|
|
* Libav is distributed in the hope that it will be useful,
|
|
* but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
|
|
* Lesser General Public License for more details.
|
|
*
|
|
* You should have received a copy of the GNU Lesser General Public
|
|
* License along with Libav; if not, write to the Free Software
|
|
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
|
|
*/
|
|
|
|
/**
|
|
* @file
|
|
* Lagarith range decoder
|
|
* @author Nathan Caldwell
|
|
* @author David Conrad
|
|
*/
|
|
|
|
#ifndef AVCODEC_LAGARITHRAC_H
|
|
#define AVCODEC_LAGARITHRAC_H
|
|
|
|
#include <stdint.h>
|
|
#include "libavutil/common.h"
|
|
#include "libavutil/intreadwrite.h"
|
|
#include "avcodec.h"
|
|
#include "get_bits.h"
|
|
|
|
typedef struct lag_rac {
|
|
AVCodecContext *avctx;
|
|
unsigned low;
|
|
unsigned range;
|
|
unsigned scale; /*!< Number of bits of precision in range. */
|
|
unsigned hash_shift; /*!< Number of bits to shift to calculate hash for radix search. */
|
|
|
|
const uint8_t *bytestream_start; /*!< Start of input bytestream. */
|
|
const uint8_t *bytestream; /*!< Current position in input bytestream. */
|
|
const uint8_t *bytestream_end; /*!< End position of input bytestream. */
|
|
|
|
uint32_t prob[258]; /*!< Table of cumulative probability for each symbol. */
|
|
uint8_t range_hash[256]; /*!< Hash table mapping upper byte to approximate symbol. */
|
|
} lag_rac;
|
|
|
|
void lag_rac_init(lag_rac *l, GetBitContext *gb, int length);
|
|
|
|
/* TODO: Optimize */
|
|
static inline void lag_rac_refill(lag_rac *l)
|
|
{
|
|
while (l->range <= 0x800000) {
|
|
l->low <<= 8;
|
|
l->range <<= 8;
|
|
l->low |= 0xff & (AV_RB16(l->bytestream) >> 1);
|
|
if (l->bytestream < l->bytestream_end)
|
|
l->bytestream++;
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Decode a single byte from the compressed plane described by *l.
|
|
* @param l pointer to lag_rac for the current plane
|
|
* @return next byte of decoded data
|
|
*/
|
|
static inline uint8_t lag_get_rac(lag_rac *l)
|
|
{
|
|
unsigned range_scaled, low_scaled, div;
|
|
int val;
|
|
uint8_t shift;
|
|
|
|
lag_rac_refill(l);
|
|
|
|
range_scaled = l->range >> l->scale;
|
|
|
|
if (l->low < range_scaled * l->prob[255]) {
|
|
/* val = 0 is frequent enough to deserve a shortcut */
|
|
if (l->low < range_scaled * l->prob[1]) {
|
|
val = 0;
|
|
} else {
|
|
/* FIXME __builtin_clz is ~20% faster here, but not allowed in generic code. */
|
|
shift = 30 - av_log2(range_scaled);
|
|
div = ((range_scaled << shift) + (1 << 23) - 1) >> 23;
|
|
/* low>>24 ensures that any cases too big for exact FASTDIV are
|
|
* under- rather than over-estimated
|
|
*/
|
|
low_scaled = FASTDIV(l->low - (l->low >> 24), div);
|
|
shift -= l->hash_shift;
|
|
shift &= 31;
|
|
low_scaled = (low_scaled << shift) | (low_scaled >> (32 - shift));
|
|
/* low_scaled is now a lower bound of low/range_scaled */
|
|
val = l->range_hash[(uint8_t) low_scaled];
|
|
while (l->low >= range_scaled * l->prob[val + 1])
|
|
val++;
|
|
}
|
|
|
|
l->range = range_scaled * (l->prob[val + 1] - l->prob[val]);
|
|
} else {
|
|
val = 255;
|
|
l->range -= range_scaled * l->prob[255];
|
|
}
|
|
|
|
l->low -= range_scaled * l->prob[val];
|
|
|
|
return val;
|
|
}
|
|
|
|
|
|
#endif /* AVCODEC_LAGARITHRAC_H */
|