| 1 | //! Bitstream reader functionality. |
| 2 | //! |
| 3 | //! Bitstream reader operates on `&[u8]` and allows to read bits from the slice in different modes. |
| 4 | //! |
| 5 | //! # Examples |
| 6 | //! |
| 7 | //! Reading 17 bits from a bitstream: |
| 8 | //! ``` |
| 9 | //! use nihav_core::io::bitreader::{BitReader,BitReaderMode}; |
| 10 | //! |
| 11 | //! # use nihav_core::io::bitreader::BitReaderResult; |
| 12 | //! # fn foo() -> BitReaderResult<u32> { |
| 13 | //! let bits: [u8; 4] = [ 42, 43, 44, 45 ]; |
| 14 | //! let mut br = BitReader::new(&bits, BitReaderMode::BE); |
| 15 | //! let value = br.read(17)?; |
| 16 | //! # Ok(value) |
| 17 | //! # } |
| 18 | //! ``` |
| 19 | //! |
| 20 | //! Reading some amount of bits and checking how many bits are left: |
| 21 | //! ``` |
| 22 | //! use nihav_core::io::bitreader::{BitReader,BitReaderMode}; |
| 23 | //! |
| 24 | //! # use nihav_core::io::bitreader::BitReaderResult; |
| 25 | //! # fn foo() -> BitReaderResult<()> { |
| 26 | //! let bits: [u8; 4] = [ 42, 43, 44, 45 ]; |
| 27 | //! let mut br = BitReader::new(&bits, BitReaderMode::BE); |
| 28 | //! let num_skip_bits = br.read(3)?; |
| 29 | //! br.skip(num_skip_bits)?; |
| 30 | //! println!("Now there are {} bits left to read.", br.left()); |
| 31 | //! # Ok(()) |
| 32 | //! # } |
| 33 | //! ``` |
| 34 | |
| 35 | |
| 36 | |
| 37 | /// Bitstream reading modes. |
| 38 | #[derive(Debug,Clone,Copy)] |
| 39 | pub enum BitReaderMode { |
| 40 | /// The stream is big endian MSB first. |
| 41 | BE, |
| 42 | /// The stream is little endian LSB first. |
| 43 | LE, |
| 44 | /// The stream is packed into 16-bit little-endian words MSB first. |
| 45 | LE16MSB, |
| 46 | /// The stream is packed into 32-bit little-endian words MSB first. |
| 47 | LE32MSB, |
| 48 | } |
| 49 | |
| 50 | /// A list specifying general bitstream reading errors. |
| 51 | #[derive(Debug,Clone,Copy)] |
| 52 | pub enum BitReaderError { |
| 53 | /// The reader is at the end of bitstream. |
| 54 | BitstreamEnd, |
| 55 | /// The caller tried to read too many bits at once (e.g. 128). |
| 56 | TooManyBitsRequested, |
| 57 | /// Some argument is invalid. |
| 58 | InvalidValue, |
| 59 | } |
| 60 | |
| 61 | use self::BitReaderError::*; |
| 62 | |
| 63 | /// A specialised `Result` type for bitstream operations. |
| 64 | pub type BitReaderResult<T> = Result<T, BitReaderError>; |
| 65 | |
| 66 | /// Bitstream reader. |
| 67 | #[derive(Debug,Clone)] |
| 68 | pub struct BitReader<'a> { |
| 69 | cache: u64, |
| 70 | bits: u8, |
| 71 | pos: usize, |
| 72 | src: &'a [u8], |
| 73 | mode: BitReaderMode, |
| 74 | } |
| 75 | |
| 76 | #[allow(clippy::identity_op)] |
| 77 | impl<'a> BitReader<'a> { |
| 78 | |
| 79 | /// Constructs a new instance of bitstream reader. |
| 80 | /// |
| 81 | /// # Examples |
| 82 | /// |
| 83 | /// ``` |
| 84 | /// use nihav_core::io::bitreader::{BitReader,BitReaderMode}; |
| 85 | /// |
| 86 | /// let bits: [u8; 4] = [ 42, 43, 44, 45 ]; |
| 87 | /// let mut br = BitReader::new(&bits, BitReaderMode::BE); |
| 88 | /// ``` |
| 89 | pub fn new(src: &'a [u8], mode: BitReaderMode) -> Self { |
| 90 | BitReader{ cache: 0, pos: 0, bits: 0, src, mode } |
| 91 | } |
| 92 | |
| 93 | /// Returns the data bitstream reader uses. |
| 94 | pub fn get_data(&self) -> &'a [u8] { self.src } |
| 95 | |
| 96 | /// Reports the current bit position in the bitstream (usually simply the number of bits read so far). |
| 97 | pub fn tell(&self) -> usize { |
| 98 | self.pos * 8 - (self.bits as usize) |
| 99 | } |
| 100 | |
| 101 | /// Reports the amount of bits left until the end of the bitstream. |
| 102 | pub fn left(&self) -> isize { |
| 103 | ((self.src.len() as isize) - (self.pos as isize)) * 8 + (self.bits as isize) |
| 104 | } |
| 105 | |
| 106 | fn fill32be(&mut self, src: &[u8]) { |
| 107 | let nw = (u32::from(src[0]) << 24) | |
| 108 | (u32::from(src[1]) << 16) | |
| 109 | (u32::from(src[2]) << 8) | |
| 110 | (u32::from(src[3]) << 0); |
| 111 | self.cache |= u64::from(nw) << (32 - self.bits); |
| 112 | } |
| 113 | |
| 114 | fn fill32le16(&mut self, src: &[u8]) { |
| 115 | let nw = (u32::from(src[1]) << 24) | |
| 116 | (u32::from(src[0]) << 16) | |
| 117 | (u32::from(src[3]) << 8) | |
| 118 | (u32::from(src[2]) << 0); |
| 119 | self.cache |= u64::from(nw) << (32 - self.bits); |
| 120 | } |
| 121 | |
| 122 | fn fill32le32(&mut self, src: &[u8], lsb: bool) { |
| 123 | let nw = (u32::from(src[3]) << 24) | |
| 124 | (u32::from(src[2]) << 16) | |
| 125 | (u32::from(src[1]) << 8) | |
| 126 | (u32::from(src[0]) << 0); |
| 127 | if lsb { |
| 128 | self.cache |= u64::from(nw) << self.bits; |
| 129 | } else { |
| 130 | self.cache |= u64::from(nw) << (32 - self.bits); |
| 131 | } |
| 132 | } |
| 133 | |
| 134 | #[inline(always)] |
| 135 | fn refill(&mut self) -> BitReaderResult<()> { |
| 136 | if self.pos >= self.src.len() { return Err(BitstreamEnd) } |
| 137 | while self.bits <= 32 { |
| 138 | if self.pos + 4 <= self.src.len() { |
| 139 | let buf = &self.src[self.pos..]; |
| 140 | match self.mode { |
| 141 | BitReaderMode::BE => self.fill32be (buf), |
| 142 | BitReaderMode::LE16MSB => self.fill32le16(buf), |
| 143 | BitReaderMode::LE => self.fill32le32(buf, true), |
| 144 | BitReaderMode::LE32MSB => self.fill32le32(buf, false), |
| 145 | } |
| 146 | self.pos += 4; |
| 147 | self.bits += 32; |
| 148 | } else { |
| 149 | let mut buf: [u8; 4] = [0, 0, 0, 0]; |
| 150 | let mut newbits: u8 = 0; |
| 151 | for out in buf.iter_mut().take(3) { |
| 152 | if self.pos < self.src.len() { |
| 153 | *out = self.src[self.pos]; |
| 154 | self.pos += 1; |
| 155 | newbits += 8; |
| 156 | } |
| 157 | } |
| 158 | if newbits == 0 { break; } |
| 159 | match self.mode { |
| 160 | BitReaderMode::BE => self.fill32be (&buf), |
| 161 | BitReaderMode::LE16MSB => self.fill32le16(&buf), |
| 162 | BitReaderMode::LE => self.fill32le32(&buf, true), |
| 163 | BitReaderMode::LE32MSB => self.fill32le32(&buf, false), |
| 164 | } |
| 165 | self.bits += newbits; |
| 166 | } |
| 167 | } |
| 168 | Ok(()) |
| 169 | } |
| 170 | |
| 171 | #[inline(always)] |
| 172 | fn read_cache(&mut self, nbits: u8) -> u32 { |
| 173 | let res = match self.mode { |
| 174 | BitReaderMode::LE => ((1u64 << nbits) - 1) & self.cache, |
| 175 | _ => self.cache >> (64 - nbits), |
| 176 | }; |
| 177 | res as u32 |
| 178 | } |
| 179 | |
| 180 | fn read_cache_s(&mut self, nbits: u8) -> i32 { |
| 181 | let res = match self.mode { |
| 182 | BitReaderMode::LE => ((self.cache as i64) << (64 - nbits)) >> (64 - nbits), |
| 183 | _ => (self.cache as i64) >> (64 - nbits), |
| 184 | }; |
| 185 | res as i32 |
| 186 | } |
| 187 | |
| 188 | #[inline(always)] |
| 189 | fn skip_cache(&mut self, nbits: u8) { |
| 190 | match self.mode { |
| 191 | BitReaderMode::LE => self.cache >>= nbits, |
| 192 | _ => self.cache <<= nbits, |
| 193 | }; |
| 194 | self.bits -= nbits; |
| 195 | } |
| 196 | |
| 197 | #[inline(always)] |
| 198 | fn reset_cache(&mut self) { |
| 199 | self.bits = 0; |
| 200 | self.cache = 0; |
| 201 | } |
| 202 | |
| 203 | /// Reads the specified amount of bits as an unsigned value. |
| 204 | /// |
| 205 | /// The amount should fit into 32 bits, if you need more then |
| 206 | /// you should read it as several parts. If the amount of bits |
| 207 | /// requested to read is larger than the amount of bits left the |
| 208 | /// call will return [`BitstreamEnd`]. |
| 209 | /// |
| 210 | /// # Examples |
| 211 | /// |
| 212 | /// ``` |
| 213 | /// use nihav_core::io::bitreader::{BitReader,BitReaderMode}; |
| 214 | /// |
| 215 | /// # use nihav_core::io::bitreader::BitReaderResult; |
| 216 | /// # fn foo() -> BitReaderResult<u32> { |
| 217 | /// let bits: [u8; 4] = [ 42, 43, 44, 45 ]; |
| 218 | /// let mut br = BitReader::new(&bits, BitReaderMode::BE); |
| 219 | /// let value = br.read(17)?; |
| 220 | /// # Ok(value) |
| 221 | /// # } |
| 222 | /// ``` |
| 223 | /// |
| 224 | /// [`BitstreamEnd`]: ./enum.BitReaderError.html#variant.BitstreamEnd |
| 225 | #[inline(always)] |
| 226 | pub fn read(&mut self, nbits: u8) -> BitReaderResult<u32> { |
| 227 | if nbits == 0 { return Ok(0) } |
| 228 | if nbits > 32 { return Err(TooManyBitsRequested) } |
| 229 | if self.bits < nbits { |
| 230 | self.refill()?; |
| 231 | if self.bits < nbits { return Err(BitstreamEnd) } |
| 232 | } |
| 233 | let res = self.read_cache(nbits); |
| 234 | self.skip_cache(nbits); |
| 235 | Ok(res) |
| 236 | } |
| 237 | |
| 238 | /// Reads the specified amount of bits as a signed value. |
| 239 | /// |
| 240 | /// Beside signedness it behaves the same as [`read`]. |
| 241 | /// |
| 242 | /// [`read`]: #method.read |
| 243 | pub fn read_s(&mut self, nbits: u8) -> BitReaderResult<i32> { |
| 244 | if nbits == 0 || nbits > 32 { return Err(TooManyBitsRequested) } |
| 245 | if self.bits < nbits { |
| 246 | self.refill()?; |
| 247 | if self.bits < nbits { return Err(BitstreamEnd) } |
| 248 | } |
| 249 | let res = self.read_cache_s(nbits); |
| 250 | self.skip_cache(nbits); |
| 251 | Ok(res) |
| 252 | } |
| 253 | |
| 254 | /// Reads single bit from the stream and interprets it as a boolean value. |
| 255 | #[inline(always)] |
| 256 | pub fn read_bool(&mut self) -> BitReaderResult<bool> { |
| 257 | if self.bits < 1 { |
| 258 | self.refill()?; |
| 259 | if self.bits < 1 { return Err(BitstreamEnd) } |
| 260 | } |
| 261 | let res = self.read_cache(1); |
| 262 | self.skip_cache(1); |
| 263 | Ok(res == 1) |
| 264 | } |
| 265 | |
| 266 | /// Retrieves the next bits from the stream without advancing. |
| 267 | /// |
| 268 | /// If the bitstream is shorter than the amount of bits requested the result is padded with zeroes. |
| 269 | /// |
| 270 | /// # Examples |
| 271 | /// |
| 272 | /// ``` |
| 273 | /// use nihav_core::io::bitreader::{BitReader,BitReaderMode}; |
| 274 | /// |
| 275 | /// # use nihav_core::io::bitreader::BitReaderResult; |
| 276 | /// # fn foo() -> BitReaderResult<u32> { |
| 277 | /// let bits: [u8; 4] = [ 42, 43, 44, 45 ]; |
| 278 | /// let mut br = BitReader::new(&bits, BitReaderMode::BE); |
| 279 | /// let peek_value = br.peek(8); // this should return 42 |
| 280 | /// let value = br.read(8)?; // also 42 |
| 281 | /// # Ok(value) |
| 282 | /// # } |
| 283 | /// ``` |
| 284 | #[inline(always)] |
| 285 | pub fn peek(&mut self, nbits: u8) -> u32 { |
| 286 | if nbits > 32 { return 0 } |
| 287 | if self.bits < nbits { let _ = self.refill(); } |
| 288 | self.read_cache(nbits) |
| 289 | } |
| 290 | |
| 291 | /// Skips the requested amount of bits. |
| 292 | /// |
| 293 | /// The amount of bits to skip can be arbitrary large. |
| 294 | /// If it skips more bits than there are actually in the stream the call will return [`BitstreamEnd`] |
| 295 | /// |
| 296 | /// [`read`]: #method.read |
| 297 | /// [`BitstreamEnd`]: ./enum.BitReaderError.html#variant.BitstreamEnd |
| 298 | #[inline(always)] |
| 299 | pub fn skip(&mut self, nbits: u32) -> BitReaderResult<()> { |
| 300 | if u32::from(self.bits) >= nbits { |
| 301 | self.skip_cache(nbits as u8); |
| 302 | return Ok(()); |
| 303 | } |
| 304 | let mut skip_bits = nbits - u32::from(self.bits); |
| 305 | self.reset_cache(); |
| 306 | self.pos += ((skip_bits / 32) * 4) as usize; |
| 307 | skip_bits &= 0x1F; |
| 308 | if skip_bits > 0 { |
| 309 | self.refill()?; |
| 310 | if u32::from(self.bits) < skip_bits { |
| 311 | return Err(BitstreamEnd); |
| 312 | } |
| 313 | self.skip_cache(skip_bits as u8); |
| 314 | } |
| 315 | Ok(()) |
| 316 | } |
| 317 | |
| 318 | /// Seeks to the absolute bit position in the stream. |
| 319 | /// If the requested position lies after the bitstream end the function returns [`TooManyBitsRequested`]. |
| 320 | /// |
| 321 | /// # Examples |
| 322 | /// |
| 323 | /// ``` |
| 324 | /// use nihav_core::io::bitreader::{BitReader,BitReaderMode}; |
| 325 | /// |
| 326 | /// # use nihav_core::io::bitreader::BitReaderResult; |
| 327 | /// # fn foo() -> BitReaderResult<u32> { |
| 328 | /// let bits: [u8; 4] = [ 42, 43, 44, 45 ]; |
| 329 | /// let mut br = BitReader::new(&bits, BitReaderMode::BE); |
| 330 | /// br.seek(16)?; |
| 331 | /// let value = br.read(8)?; // this should return 44 |
| 332 | /// # Ok(value) |
| 333 | /// # } |
| 334 | /// ``` |
| 335 | /// |
| 336 | /// [`TooManyBitsRequested`]: ./enum.BitReaderError.html#variant.TooManyBitsRequested |
| 337 | pub fn seek(&mut self, nbits: u32) -> BitReaderResult<()> { |
| 338 | if ((nbits + 7) >> 3) as usize > self.src.len() { return Err(TooManyBitsRequested); } |
| 339 | self.reset_cache(); |
| 340 | self.pos = ((nbits / 32) * 4) as usize; |
| 341 | self.skip(nbits & 0x1F) |
| 342 | } |
| 343 | |
| 344 | /// Aligns the bit position to the next byte boundary. If already at byte boundary the function does nothing. |
| 345 | /// |
| 346 | /// # Examples |
| 347 | /// |
| 348 | /// ``` |
| 349 | /// use nihav_core::io::bitreader::{BitReader,BitReaderMode}; |
| 350 | /// |
| 351 | /// # use nihav_core::io::bitreader::BitReaderResult; |
| 352 | /// # fn foo() -> BitReaderResult<()> { |
| 353 | /// let bits: [u8; 4] = [ 42, 43, 44, 45 ]; |
| 354 | /// let mut br = BitReader::new(&bits, BitReaderMode::BE); |
| 355 | /// br.skip(17)?; // now reader is at bit position 17 |
| 356 | /// br.align(); // now reader is at bit position 24 |
| 357 | /// br.align(); // now reader is still at bit position 24 |
| 358 | /// # Ok(()) |
| 359 | /// # } |
| 360 | /// ``` |
| 361 | pub fn align(&mut self) { |
| 362 | let pos = self.bits & 7; |
| 363 | if pos != 0 { |
| 364 | self.skip_cache(pos); |
| 365 | } |
| 366 | } |
| 367 | } |
| 368 | |
| 369 | /// Returns a variable with `len` amount of low bits in reverse order. |
| 370 | /// |
| 371 | /// # Examples |
| 372 | /// ``` |
| 373 | /// use nihav_core::io::bitreader::reverse_bits; |
| 374 | /// reverse_bits(0b010101, 6); // the result should be 0b101010 |
| 375 | /// ``` |
| 376 | pub fn reverse_bits(inval: u32, len: u8) -> u32 { |
| 377 | if len == 0 { return 0; } |
| 378 | const REV_TAB: [u8; 16] = [ |
| 379 | 0b0000, 0b1000, 0b0100, 0b1100, 0b0010, 0b1010, 0b0110, 0b1110, |
| 380 | 0b0001, 0b1001, 0b0101, 0b1101, 0b0011, 0b1011, 0b0111, 0b1111, |
| 381 | ]; |
| 382 | |
| 383 | let mut ret = 0; |
| 384 | let mut val = inval; |
| 385 | for _ in 0..8 { |
| 386 | ret = (ret << 4) | u32::from(REV_TAB[(val & 0xF) as usize]); |
| 387 | val >>= 4; |
| 388 | } |
| 389 | ret >> (32 - len) |
| 390 | } |
| 391 | |
| 392 | #[cfg(test)] |
| 393 | mod test { |
| 394 | use super::*; |
| 395 | |
| 396 | #[test] |
| 397 | fn br_works() { |
| 398 | const DATA: [u8; 18] = [0b00011011; 18]; |
| 399 | let src = &DATA; |
| 400 | let mut br = BitReader::new(src, BitReaderMode::LE16MSB); |
| 401 | |
| 402 | for _ in 0..8 { |
| 403 | assert_eq!(br.read(16).unwrap(), 0x1B1B); |
| 404 | } |
| 405 | const DATA2: [u8; 1] = [ 0b00011011 ]; |
| 406 | let src = &DATA2; |
| 407 | let mut br = BitReader::new(src, BitReaderMode::LE); |
| 408 | assert_eq!(br.read_s(5).unwrap(), -5); |
| 409 | } |
| 410 | } |