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116 lines
3.7 KiB
116 lines
3.7 KiB
/* |
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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 FFmpeg. |
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* |
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* FFmpeg 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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* FFmpeg 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 FFmpeg; 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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