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/*
* Lagarith range decoder
* Copyright (c) 2009 Nathan Caldwell <saintdev (at) gmail.com>
* Copyright (c) 2009 David Conrad
*
* This file is part of FFmpeg.
*
* FFmpeg 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.
*
* FFmpeg 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 FFmpeg; 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 */
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