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