/* * 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 libavcodec/lagarithrac.h * 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 */