yading@10: /*
yading@10:  * Lagarith range decoder
yading@10:  * Copyright (c) 2009 Nathan Caldwell <saintdev (at) gmail.com>
yading@10:  * Copyright (c) 2009 David Conrad
yading@10:  *
yading@10:  * This file is part of FFmpeg.
yading@10:  *
yading@10:  * FFmpeg is free software; you can redistribute it and/or
yading@10:  * modify it under the terms of the GNU Lesser General Public
yading@10:  * License as published by the Free Software Foundation; either
yading@10:  * version 2.1 of the License, or (at your option) any later version.
yading@10:  *
yading@10:  * FFmpeg is distributed in the hope that it will be useful,
yading@10:  * but WITHOUT ANY WARRANTY; without even the implied warranty of
yading@10:  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
yading@10:  * Lesser General Public License for more details.
yading@10:  *
yading@10:  * You should have received a copy of the GNU Lesser General Public
yading@10:  * License along with FFmpeg; if not, write to the Free Software
yading@10:  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
yading@10:  */
yading@10: 
yading@10: /**
yading@10:  * @file
yading@10:  * Lagarith range decoder
yading@10:  * @author Nathan Caldwell
yading@10:  * @author David Conrad
yading@10:  */
yading@10: 
yading@10: #ifndef AVCODEC_LAGARITHRAC_H
yading@10: #define AVCODEC_LAGARITHRAC_H
yading@10: 
yading@10: #include <stdint.h>
yading@10: #include "libavutil/common.h"
yading@10: #include "libavutil/intreadwrite.h"
yading@10: #include "avcodec.h"
yading@10: #include "get_bits.h"
yading@10: 
yading@10: typedef struct lag_rac {
yading@10:     AVCodecContext *avctx;
yading@10:     unsigned low;
yading@10:     unsigned range;
yading@10:     unsigned scale;             /**< Number of bits of precision in range. */
yading@10:     unsigned hash_shift;        /**< Number of bits to shift to calculate hash for radix search. */
yading@10: 
yading@10:     const uint8_t *bytestream_start;  /**< Start of input bytestream. */
yading@10:     const uint8_t *bytestream;        /**< Current position in input bytestream. */
yading@10:     const uint8_t *bytestream_end;    /**< End position of input bytestream. */
yading@10: 
yading@10:     uint32_t prob[258];         /**< Table of cumulative probability for each symbol. */
yading@10:     uint8_t  range_hash[256];   /**< Hash table mapping upper byte to approximate symbol. */
yading@10: } lag_rac;
yading@10: 
yading@10: void ff_lag_rac_init(lag_rac *l, GetBitContext *gb, int length);
yading@10: 
yading@10: /* TODO: Optimize */
yading@10: static inline void lag_rac_refill(lag_rac *l)
yading@10: {
yading@10:     while (l->range <= 0x800000) {
yading@10:         l->low   <<= 8;
yading@10:         l->range <<= 8;
yading@10:         l->low |= 0xff & (AV_RB16(l->bytestream) >> 1);
yading@10:         if (l->bytestream < l->bytestream_end)
yading@10:             l->bytestream++;
yading@10:     }
yading@10: }
yading@10: 
yading@10: /**
yading@10:  * Decode a single byte from the compressed plane described by *l.
yading@10:  * @param l pointer to lag_rac for the current plane
yading@10:  * @return next byte of decoded data
yading@10:  */
yading@10: static inline uint8_t lag_get_rac(lag_rac *l)
yading@10: {
yading@10:     unsigned range_scaled, low_scaled, div;
yading@10:     int val;
yading@10:     uint8_t shift;
yading@10: 
yading@10:     lag_rac_refill(l);
yading@10: 
yading@10:     range_scaled = l->range >> l->scale;
yading@10: 
yading@10:     if (l->low < range_scaled * l->prob[255]) {
yading@10:         /* val = 0 is frequent enough to deserve a shortcut */
yading@10:         if (l->low < range_scaled * l->prob[1]) {
yading@10:             val = 0;
yading@10:         } else {
yading@10:             /* FIXME __builtin_clz is ~20% faster here, but not allowed in generic code. */
yading@10:             shift = 30 - av_log2(range_scaled);
yading@10:             div = ((range_scaled << shift) + (1 << 23) - 1) >> 23;
yading@10:             /* low>>24 ensures that any cases too big for exact FASTDIV are
yading@10:              * under- rather than over-estimated
yading@10:              */
yading@10:             low_scaled = FASTDIV(l->low - (l->low >> 24), div);
yading@10:             shift -= l->hash_shift;
yading@10:             shift &= 31;
yading@10:             low_scaled = (low_scaled << shift) | (low_scaled >> (32 - shift));
yading@10:             /* low_scaled is now a lower bound of low/range_scaled */
yading@10:             val = l->range_hash[(uint8_t) low_scaled];
yading@10:             while (l->low >= range_scaled * l->prob[val + 1])
yading@10:                 val++;
yading@10:         }
yading@10: 
yading@10:         l->range = range_scaled * (l->prob[val + 1] - l->prob[val]);
yading@10:     } else {
yading@10:         val = 255;
yading@10:         l->range -= range_scaled * l->prob[255];
yading@10:     }
yading@10: 
yading@10:     if (!l->range)
yading@10:         l->range = 0x80;
yading@10: 
yading@10:     l->low -= range_scaled * l->prob[val];
yading@10: 
yading@10:     return val;
yading@10: }
yading@10: 
yading@10: 
yading@10: #endif /* AVCODEC_LAGARITHRAC_H */