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
bitstream_overflow(struct bitstream * bitstream)18 int bitstream_overflow(struct bitstream* bitstream) { return ((bitstream->doffset - bitstream->bits / 8) > bitstream->dlength); }
19
20 //-------------------------------------------------
21 // create_bitstream - constructor
22 //-------------------------------------------------
23
create_bitstream(const void * src,uint32_t srclength)24 struct bitstream* create_bitstream(const void *src, uint32_t srclength)
25 {
26 struct bitstream* bitstream = (struct bitstream*)malloc(sizeof(struct bitstream));
27 bitstream->buffer = 0;
28 bitstream->bits = 0;
29 bitstream->read = (const uint8_t*)src;
30 bitstream->doffset = 0;
31 bitstream->dlength = srclength;
32 return bitstream;
33 }
34
35
36 //-----------------------------------------------------
37 // bitstream_peek - fetch the requested number of bits
38 // but don't advance the input pointer
39 //-----------------------------------------------------
40
bitstream_peek(struct bitstream * bitstream,int numbits)41 uint32_t bitstream_peek(struct bitstream* bitstream, int numbits)
42 {
43 if (numbits == 0)
44 return 0;
45
46 // fetch data if we need more
47 if (numbits > bitstream->bits)
48 {
49 while (bitstream->bits <= 24)
50 {
51 if (bitstream->doffset < bitstream->dlength)
52 bitstream->buffer |= bitstream->read[bitstream->doffset] << (24 - bitstream->bits);
53 bitstream->doffset++;
54 bitstream->bits += 8;
55 }
56 }
57
58 // return the data
59 return bitstream->buffer >> (32 - numbits);
60 }
61
62
63 //-----------------------------------------------------
64 // bitstream_remove - advance the input pointer by the
65 // specified number of bits
66 //-----------------------------------------------------
67
bitstream_remove(struct bitstream * bitstream,int numbits)68 void bitstream_remove(struct bitstream* bitstream, int numbits)
69 {
70 bitstream->buffer <<= numbits;
71 bitstream->bits -= numbits;
72 }
73
74
75 //-----------------------------------------------------
76 // bitstream_read - fetch the requested number of bits
77 //-----------------------------------------------------
78
bitstream_read(struct bitstream * bitstream,int numbits)79 uint32_t bitstream_read(struct bitstream* bitstream, int numbits)
80 {
81 uint32_t result = bitstream_peek(bitstream, numbits);
82 bitstream_remove(bitstream, numbits);
83 return result;
84 }
85
86
87 //-------------------------------------------------
88 // read_offset - return the current read offset
89 //-------------------------------------------------
90
bitstream_read_offset(struct bitstream * bitstream)91 uint32_t bitstream_read_offset(struct bitstream* bitstream)
92 {
93 uint32_t result = bitstream->doffset;
94 int bits = bitstream->bits;
95 while (bits >= 8)
96 {
97 result--;
98 bits -= 8;
99 }
100 return result;
101 }
102
103
104 //-------------------------------------------------
105 // flush - flush to the nearest byte
106 //-------------------------------------------------
107
bitstream_flush(struct bitstream * bitstream)108 uint32_t bitstream_flush(struct bitstream* bitstream)
109 {
110 while (bitstream->bits >= 8)
111 {
112 bitstream->doffset--;
113 bitstream->bits -= 8;
114 }
115 bitstream->bits = bitstream->buffer = 0;
116 return bitstream->doffset;
117 }
118
119