1 /* license:BSD-3-Clause
2 * copyright-holders:Aaron Giles
3 ***************************************************************************
4
5 bitstream.c
6
7 Helper classes for reading/writing at the bit level.
8
9 ***************************************************************************/
10
11 #include "bitstream.h"
12 #include <stdlib.h>
13
14 /***************************************************************************
15 * INLINE FUNCTIONS
16 ***************************************************************************
17 */
18
bitstream_overflow(struct bitstream * bitstream)19 int bitstream_overflow(struct bitstream* bitstream) { return ((bitstream->doffset - bitstream->bits / 8) > bitstream->dlength); }
20
21 /*-------------------------------------------------
22 * create_bitstream - constructor
23 *-------------------------------------------------
24 */
25
create_bitstream(const void * src,uint32_t srclength)26 struct bitstream* create_bitstream(const void *src, uint32_t srclength)
27 {
28 struct bitstream* bitstream = (struct bitstream*)malloc(sizeof(struct bitstream));
29 bitstream->buffer = 0;
30 bitstream->bits = 0;
31 bitstream->read = (const uint8_t*)src;
32 bitstream->doffset = 0;
33 bitstream->dlength = srclength;
34 return bitstream;
35 }
36
37
38 /*-----------------------------------------------------
39 * bitstream_peek - fetch the requested number of bits
40 * but don't advance the input pointer
41 *-----------------------------------------------------
42 */
43
bitstream_peek(struct bitstream * bitstream,int numbits)44 uint32_t bitstream_peek(struct bitstream* bitstream, int numbits)
45 {
46 if (numbits == 0)
47 return 0;
48
49 /* fetch data if we need more */
50 if (numbits > bitstream->bits)
51 {
52 while (bitstream->bits <= 24)
53 {
54 if (bitstream->doffset < bitstream->dlength)
55 bitstream->buffer |= bitstream->read[bitstream->doffset] << (24 - bitstream->bits);
56 bitstream->doffset++;
57 bitstream->bits += 8;
58 }
59 }
60
61 /* return the data */
62 return bitstream->buffer >> (32 - numbits);
63 }
64
65
66 /*-----------------------------------------------------
67 * bitstream_remove - advance the input pointer by the
68 * specified number of bits
69 *-----------------------------------------------------
70 */
71
bitstream_remove(struct bitstream * bitstream,int numbits)72 void bitstream_remove(struct bitstream* bitstream, int numbits)
73 {
74 bitstream->buffer <<= numbits;
75 bitstream->bits -= numbits;
76 }
77
78
79 /*-----------------------------------------------------
80 * bitstream_read - fetch the requested number of bits
81 *-----------------------------------------------------
82 */
83
bitstream_read(struct bitstream * bitstream,int numbits)84 uint32_t bitstream_read(struct bitstream* bitstream, int numbits)
85 {
86 uint32_t result = bitstream_peek(bitstream, numbits);
87 bitstream_remove(bitstream, numbits);
88 return result;
89 }
90
91
92 /*-------------------------------------------------
93 * read_offset - return the current read offset
94 *-------------------------------------------------
95 */
96
bitstream_read_offset(struct bitstream * bitstream)97 uint32_t bitstream_read_offset(struct bitstream* bitstream)
98 {
99 uint32_t result = bitstream->doffset;
100 int bits = bitstream->bits;
101 while (bits >= 8)
102 {
103 result--;
104 bits -= 8;
105 }
106 return result;
107 }
108
109
110 /*-------------------------------------------------
111 * flush - flush to the nearest byte
112 *-------------------------------------------------
113 */
114
bitstream_flush(struct bitstream * bitstream)115 uint32_t bitstream_flush(struct bitstream* bitstream)
116 {
117 while (bitstream->bits >= 8)
118 {
119 bitstream->doffset--;
120 bitstream->bits -= 8;
121 }
122 bitstream->bits = bitstream->buffer = 0;
123 return bitstream->doffset;
124 }
125
126