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1 | #[derive(Debug)] | |
2 | pub enum BitReaderMode { | |
3 | BE, | |
4 | LE, | |
5 | LE16MSB, | |
6 | LE32MSB, | |
7 | } | |
8 | ||
9 | #[derive(Debug)] | |
10 | pub enum BitReaderError { | |
11 | BitstreamEnd, | |
12 | TooManyBitsRequested, | |
13 | InvalidValue, | |
14 | } | |
15 | ||
16 | use self::BitReaderError::*; | |
17 | ||
18 | pub type BitReaderResult<T> = Result<T, BitReaderError>; | |
19 | ||
20 | #[derive(Debug)] | |
21 | pub struct BitReader<'a> { | |
22 | cache: u64, | |
23 | bits: u8, | |
24 | pos: usize, | |
25 | end: usize, | |
26 | src: &'a [u8], | |
27 | mode: BitReaderMode, | |
28 | } | |
29 | ||
30 | impl<'a> BitReader<'a> { | |
31 | ||
32 | pub fn new(src: &'a [u8], size: usize, mode: BitReaderMode) -> Self { | |
33 | if src.len() < size { panic!("size is less than needed"); } | |
34 | BitReader{ cache: 0, pos: 0, bits: 0, end: size, src: src, mode: mode } | |
35 | } | |
36 | ||
37 | pub fn tell(&self) -> usize { | |
38 | self.pos * 8 - (self.bits as usize) | |
39 | } | |
40 | ||
41 | pub fn left(&self) -> isize { | |
42 | ((self.end as isize) - (self.pos as isize)) * 8 + (self.bits as isize) | |
43 | } | |
44 | ||
45 | fn fill32be(&mut self, src: &[u8]) { | |
46 | let nw = (((src[0] as u32) << 24) | | |
47 | ((src[1] as u32) << 16) | | |
48 | ((src[2] as u32) << 8) | | |
49 | ((src[3] as u32) << 0)) as u64; | |
50 | self.cache |= nw << (32 - self.bits); | |
51 | } | |
52 | ||
53 | fn fill32le16(&mut self, src: &[u8], realbits: u8) { | |
54 | let mut nw = (((src[1] as u32) << 24) | | |
55 | ((src[0] as u32) << 16) | | |
56 | ((src[3] as u32) << 8) | | |
57 | ((src[2] as u32) << 0)) as u64; | |
58 | if realbits <= 16 { nw >>= 16; } | |
59 | self.cache |= nw << self.bits; | |
60 | } | |
61 | ||
62 | fn fill32le32(&mut self, src: &[u8]) { | |
63 | let nw = (((src[3] as u32) << 24) | | |
64 | ((src[2] as u32) << 16) | | |
65 | ((src[1] as u32) << 8) | | |
66 | ((src[0] as u32) << 0)) as u64; | |
67 | self.cache |= nw << self.bits; | |
68 | } | |
69 | ||
70 | fn refill(&mut self) -> BitReaderResult<()> { | |
71 | if self.pos >= self.end { return Err(BitstreamEnd) } | |
72 | while self.bits <= 32 { | |
73 | if self.pos + 4 <= self.end { | |
74 | let buf = &self.src[self.pos..]; | |
75 | match self.mode { | |
76 | BitReaderMode::BE => self.fill32be (buf), | |
77 | BitReaderMode::LE16MSB => self.fill32le16(buf, 32), | |
78 | BitReaderMode::LE => self.fill32le32(buf), | |
79 | BitReaderMode::LE32MSB => self.fill32le32(buf), | |
80 | } | |
81 | self.pos += 4; | |
82 | self.bits += 32; | |
83 | } else { | |
84 | let mut buf: [u8; 4] = [0, 0, 0, 0]; | |
85 | let mut newbits: u8 = 0; | |
86 | for i in 0..3 { | |
87 | if self.pos < self.end { | |
88 | buf[i] = self.src[self.pos]; | |
89 | self.pos = self.pos + 1; | |
90 | newbits += 8; | |
91 | } | |
92 | } | |
93 | if newbits == 0 { break; } | |
94 | match self.mode { | |
95 | BitReaderMode::BE => self.fill32be (&buf), | |
96 | BitReaderMode::LE16MSB => self.fill32le16(&buf, newbits), | |
97 | BitReaderMode::LE => self.fill32le32(&buf), | |
98 | BitReaderMode::LE32MSB => self.fill32le32(&buf), | |
99 | } | |
100 | self.bits += newbits; | |
101 | } | |
102 | } | |
103 | Ok(()) | |
104 | } | |
105 | ||
106 | fn read_cache(&mut self, nbits: u8) -> u32 { | |
107 | let res = match self.mode { | |
108 | BitReaderMode::LE => ((1u64 << nbits) - 1) & self.cache, | |
109 | _ => (self.cache as u64) >> (64 - nbits), | |
110 | }; | |
111 | res as u32 | |
112 | } | |
113 | ||
114 | fn read_cache_s(&mut self, nbits: u8) -> i32 { | |
115 | let res = match self.mode { | |
116 | BitReaderMode::LE => ((self.cache as i64) << (64 - nbits)) >> (64 - nbits), | |
117 | _ => (self.cache as i64) >> (64 - nbits), | |
118 | }; | |
119 | res as i32 | |
120 | } | |
121 | ||
122 | fn skip_cache(&mut self, nbits: u8) { | |
123 | match self.mode { | |
124 | BitReaderMode::LE => self.cache >>= nbits, | |
125 | _ => self.cache <<= nbits, | |
126 | }; | |
127 | self.bits -= nbits; | |
128 | } | |
129 | ||
130 | fn reset_cache(&mut self) { | |
131 | self.bits = 0; | |
132 | self.cache = 0; | |
133 | } | |
134 | ||
135 | pub fn read(&mut self, nbits: u8) -> BitReaderResult<u32> { | |
136 | if nbits == 0 { return Ok(0) } | |
137 | if nbits > 32 { return Err(TooManyBitsRequested) } | |
138 | if self.bits < nbits { | |
139 | if let Err(err) = self.refill() { return Err(err) } | |
140 | if self.bits < nbits { return Err(BitstreamEnd) } | |
141 | } | |
142 | let res = self.read_cache(nbits); | |
143 | self.skip_cache(nbits); | |
144 | Ok(res) | |
145 | } | |
146 | ||
147 | pub fn read_s(&mut self, nbits: u8) -> BitReaderResult<i32> { | |
148 | if nbits == 0 || nbits > 32 { return Err(TooManyBitsRequested) } | |
149 | if self.bits < nbits { | |
150 | if let Err(err) = self.refill() { return Err(err) } | |
151 | if self.bits < nbits { return Err(BitstreamEnd) } | |
152 | } | |
153 | let res = self.read_cache_s(nbits); | |
154 | self.skip_cache(nbits); | |
155 | Ok(res) | |
156 | } | |
157 | ||
158 | pub fn peek(&mut self, nbits: u8) -> u32 { | |
159 | if nbits > 32 { return 0 } | |
160 | if self.bits < nbits { let _ = self.refill(); } | |
161 | self.read_cache(nbits) | |
162 | } | |
163 | ||
164 | pub fn skip(&mut self, nbits: u32) -> BitReaderResult<()> { | |
165 | if self.bits as u32 >= nbits { | |
166 | self.skip_cache(nbits as u8); | |
167 | return Ok(()); | |
168 | } | |
169 | let mut skip_bits = nbits - (self.bits as u32); | |
170 | self.reset_cache(); | |
171 | self.pos += ((skip_bits / 32) * 4) as usize; | |
172 | skip_bits = skip_bits & 0x1F; | |
173 | self.refill()?; | |
174 | if skip_bits > 0 { | |
175 | self.skip_cache(skip_bits as u8); | |
176 | } | |
177 | Ok(()) | |
178 | } | |
179 | ||
180 | pub fn seek(&mut self, nbits: u32) -> BitReaderResult<()> { | |
181 | if ((nbits + 7) >> 3) as usize > self.end { return Err(TooManyBitsRequested); } | |
182 | self.reset_cache(); | |
183 | self.pos = ((nbits / 32) * 4) as usize; | |
184 | self.skip(nbits & 0x1F) | |
185 | } | |
186 | } | |
187 | ||
188 | #[cfg(test)] | |
189 | mod test { | |
190 | use super::*; | |
191 | ||
192 | #[test] | |
193 | fn br_works() { | |
194 | const DATA: [u8; 18] = [0b00011011; 18]; | |
195 | let src = &DATA; | |
196 | let mut br = BitReader::new(src, src.len(), BitReaderMode::LE16MSB); | |
197 | ||
198 | for _ in 0..8 { | |
199 | assert_eq!(br.read(16).unwrap(), 0x1B1B); | |
200 | } | |
201 | const DATA2: [u8; 1] = [ 0b00011011 ]; | |
202 | let src = &DATA2; | |
203 | let mut br = BitReader::new(src, src.len(), BitReaderMode::LE); | |
204 | assert_eq!(br.read_s(5).unwrap(), -5); | |
205 | } | |
206 | } |