1 /*
2  *  Copyright (c) 2010 The WebM project authors. All Rights Reserved.
3  *
4  *  Use of this source code is governed by a BSD-style license
5  *  that can be found in the LICENSE file in the root of the source
6  *  tree. An additional intellectual property rights grant can be found
7  *  in the file PATENTS.  All contributing project authors may
8  *  be found in the AUTHORS file in the root of the source tree.
9  */
10 
11 #ifndef VPX_VP8_DECODER_DBOOLHUFF_H_
12 #define VPX_VP8_DECODER_DBOOLHUFF_H_
13 
14 #include <stddef.h>
15 #include <limits.h>
16 
17 #include "./vpx_config.h"
18 #include "vpx_ports/mem.h"
19 #include "vpx/vp8dx.h"
20 #include "vpx/vpx_integer.h"
21 
22 #ifdef __cplusplus
23 extern "C" {
24 #endif
25 
26 typedef size_t VP8_BD_VALUE;
27 
28 #define VP8_BD_VALUE_SIZE ((int)sizeof(VP8_BD_VALUE) * CHAR_BIT)
29 
30 /*This is meant to be a large, positive constant that can still be efficiently
31    loaded as an immediate (on platforms like ARM, for example).
32   Even relatively modest values like 100 would work fine.*/
33 #define VP8_LOTS_OF_BITS (0x40000000)
34 
35 typedef struct {
36   const unsigned char *user_buffer_end;
37   const unsigned char *user_buffer;
38   VP8_BD_VALUE value;
39   int count;
40   unsigned int range;
41   vpx_decrypt_cb decrypt_cb;
42   void *decrypt_state;
43 } BOOL_DECODER;
44 
45 DECLARE_ALIGNED(16, extern const unsigned char, vp8_norm[256]);
46 
47 int vp8dx_start_decode(BOOL_DECODER *br, const unsigned char *source,
48                        unsigned int source_sz, vpx_decrypt_cb decrypt_cb,
49                        void *decrypt_state);
50 
51 void vp8dx_bool_decoder_fill(BOOL_DECODER *br);
52 
vp8dx_decode_bool(BOOL_DECODER * br,int probability)53 static int vp8dx_decode_bool(BOOL_DECODER *br, int probability) {
54   unsigned int bit = 0;
55   VP8_BD_VALUE value;
56   unsigned int split;
57   VP8_BD_VALUE bigsplit;
58   int count;
59   unsigned int range;
60 
61   split = 1 + (((br->range - 1) * probability) >> 8);
62 
63   if (br->count < 0) vp8dx_bool_decoder_fill(br);
64 
65   value = br->value;
66   count = br->count;
67 
68   bigsplit = (VP8_BD_VALUE)split << (VP8_BD_VALUE_SIZE - 8);
69 
70   range = split;
71 
72   if (value >= bigsplit) {
73     range = br->range - split;
74     value = value - bigsplit;
75     bit = 1;
76   }
77 
78   {
79     const unsigned char shift = vp8_norm[(unsigned char)range];
80     range <<= shift;
81     value <<= shift;
82     count -= shift;
83   }
84   br->value = value;
85   br->count = count;
86   br->range = range;
87 
88   return bit;
89 }
90 
vp8_decode_value(BOOL_DECODER * br,int bits)91 static INLINE int vp8_decode_value(BOOL_DECODER *br, int bits) {
92   int z = 0;
93   int bit;
94 
95   for (bit = bits - 1; bit >= 0; bit--) {
96     z |= (vp8dx_decode_bool(br, 0x80) << bit);
97   }
98 
99   return z;
100 }
101 
vp8dx_bool_error(BOOL_DECODER * br)102 static INLINE int vp8dx_bool_error(BOOL_DECODER *br) {
103   /* Check if we have reached the end of the buffer.
104    *
105    * Variable 'count' stores the number of bits in the 'value' buffer, minus
106    * 8. The top byte is part of the algorithm, and the remainder is buffered
107    * to be shifted into it. So if count == 8, the top 16 bits of 'value' are
108    * occupied, 8 for the algorithm and 8 in the buffer.
109    *
110    * When reading a byte from the user's buffer, count is filled with 8 and
111    * one byte is filled into the value buffer. When we reach the end of the
112    * data, count is additionally filled with VP8_LOTS_OF_BITS. So when
113    * count == VP8_LOTS_OF_BITS - 1, the user's data has been exhausted.
114    */
115   if ((br->count > VP8_BD_VALUE_SIZE) && (br->count < VP8_LOTS_OF_BITS)) {
116     /* We have tried to decode bits after the end of
117      * stream was encountered.
118      */
119     return 1;
120   }
121 
122   /* No error. */
123   return 0;
124 }
125 
126 #ifdef __cplusplus
127 }  // extern "C"
128 #endif
129 
130 #endif  // VPX_VP8_DECODER_DBOOLHUFF_H_
131