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1 | /* |
2 | * Lagarith range decoder | |
3 | * Copyright (c) 2009 Nathan Caldwell <saintdev (at) gmail.com> | |
4 | * Copyright (c) 2009 David Conrad | |
5 | * | |
6 | * This file is part of FFmpeg. | |
7 | * | |
8 | * FFmpeg is free software; you can redistribute it and/or | |
9 | * modify it under the terms of the GNU Lesser General Public | |
10 | * License as published by the Free Software Foundation; either | |
11 | * version 2.1 of the License, or (at your option) any later version. | |
12 | * | |
13 | * FFmpeg is distributed in the hope that it will be useful, | |
14 | * but WITHOUT ANY WARRANTY; without even the implied warranty of | |
15 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
16 | * Lesser General Public License for more details. | |
17 | * | |
18 | * You should have received a copy of the GNU Lesser General Public | |
19 | * License along with FFmpeg; if not, write to the Free Software | |
20 | * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA | |
21 | */ | |
22 | ||
23 | /** | |
24 | * @file | |
25 | * Lagarith range decoder | |
26 | * @author Nathan Caldwell | |
27 | * @author David Conrad | |
28 | */ | |
29 | ||
30 | #ifndef AVCODEC_LAGARITHRAC_H | |
31 | #define AVCODEC_LAGARITHRAC_H | |
32 | ||
33 | #include <stdint.h> | |
34 | #include "libavutil/common.h" | |
35 | #include "libavutil/intreadwrite.h" | |
36 | #include "avcodec.h" | |
37 | #include "get_bits.h" | |
38 | ||
39 | typedef struct lag_rac { | |
40 | AVCodecContext *avctx; | |
41 | unsigned low; | |
42 | unsigned range; | |
43 | unsigned scale; /**< Number of bits of precision in range. */ | |
44 | unsigned hash_shift; /**< Number of bits to shift to calculate hash for radix search. */ | |
45 | ||
46 | const uint8_t *bytestream_start; /**< Start of input bytestream. */ | |
47 | const uint8_t *bytestream; /**< Current position in input bytestream. */ | |
48 | const uint8_t *bytestream_end; /**< End position of input bytestream. */ | |
49 | ||
50 | uint32_t prob[258]; /**< Table of cumulative probability for each symbol. */ | |
51 | uint8_t range_hash[1024]; /**< Hash table mapping upper byte to approximate symbol. */ | |
52 | } lag_rac; | |
53 | ||
54 | void ff_lag_rac_init(lag_rac *l, GetBitContext *gb, int length); | |
55 | ||
56 | /* TODO: Optimize */ | |
57 | static inline void lag_rac_refill(lag_rac *l) | |
58 | { | |
59 | while (l->range <= 0x800000) { | |
60 | l->low <<= 8; | |
61 | l->range <<= 8; | |
62 | l->low |= 0xff & (AV_RB16(l->bytestream) >> 1); | |
63 | if (l->bytestream < l->bytestream_end) | |
64 | l->bytestream++; | |
65 | } | |
66 | } | |
67 | ||
68 | /** | |
69 | * Decode a single byte from the compressed plane described by *l. | |
70 | * @param l pointer to lag_rac for the current plane | |
71 | * @return next byte of decoded data | |
72 | */ | |
73 | static inline uint8_t lag_get_rac(lag_rac *l) | |
74 | { | |
75 | unsigned range_scaled, low_scaled; | |
76 | int val; | |
77 | ||
78 | lag_rac_refill(l); | |
79 | ||
80 | range_scaled = l->range >> l->scale; | |
81 | ||
82 | if (l->low < range_scaled * l->prob[255]) { | |
83 | /* val = 0 is frequent enough to deserve a shortcut */ | |
84 | if (l->low < range_scaled * l->prob[1]) { | |
85 | val = 0; | |
86 | } else { | |
87 | low_scaled = l->low / (range_scaled<<(l->hash_shift)); | |
88 | ||
89 | val = l->range_hash[low_scaled]; | |
90 | while (l->low >= range_scaled * l->prob[val + 1]) | |
91 | val++; | |
92 | } | |
93 | ||
94 | l->range = range_scaled * (l->prob[val + 1] - l->prob[val]); | |
95 | } else { | |
96 | val = 255; | |
97 | l->range -= range_scaled * l->prob[255]; | |
98 | } | |
99 | ||
100 | if (!l->range) | |
101 | l->range = 0x80; | |
102 | ||
103 | l->low -= range_scaled * l->prob[val]; | |
104 | ||
105 | return val; | |
106 | } | |
107 | ||
108 | ||
109 | #endif /* AVCODEC_LAGARITHRAC_H */ |