nihav-core/src/io/bitwriter.rs

   1 //! Bitstream writer functionality.
   2 //!
   3 //! Bitstream writer works on `Vec<u8>` and allows to write bits in different modes.
   4 //!
   5 //! # Examples
   6 //!
   7 //! Writing 17-bit value:
   8 //! ```
   9 //! use nihav_core::io::bitwriter::{BitWriter,BitWriterMode};
  10 //!
  11 //! # fn foo() -> Vec<u8> {
  12 //! let mut bw = BitWriter::new(Vec::new(), BitWriterMode::BE);
  13 //! bw.write(42, 17);
  14 //! # bw.end()
  15 //! # }
  16 //! ```
  17
  18 /// Bitstream writing modes.
  19 #[derive(Debug,Clone,Copy,PartialEq)]
  20 pub enum BitWriterMode {
  21     /// The stream is big endian MSB first.
  22     BE,
  23     /// The stream is little endian LSB first.
  24     LE,
  25     /// The stream is packed into 16-bit little-endian words MSB first.
  26     LE16MSB,
  27     /// The stream is packed into 32-bit little-endian words MSB first.
  28     LE32MSB,
  29 }
  30
  31 impl BitWriterMode {
  32     fn is_be(self) -> bool { self != BitWriterMode::LE }
  33 }
  34
  35 /// Bitstream writer.
  36 pub struct BitWriter {
  37     dst:    Vec<u8>,
  38     bitbuf: u32,
  39     bits:   u8,
  40     start:  usize,
  41     mode:   BitWriterMode,
  42 }
  43
  44 impl BitWriter {
  45     /// Creates a new instance of `BitWriter` that will append data to the input vector.
  46     ///
  47     /// # Examples
  48     ///
  49     /// ```
  50     /// use nihav_core::io::bitwriter::{BitWriter,BitWriterMode};
  51     ///
  52     /// let mut bw = BitWriter::new(Vec::with_capacity(100), BitWriterMode::BE);
  53     /// ```
  54     pub fn new(dst: Vec<u8>, mode: BitWriterMode) -> Self {
  55         let start = dst.len();
  56         Self {
  57             dst,
  58             mode,
  59             start,
  60             bitbuf: 0,
  61             bits:   0,
  62         }
  63     }
  64     /// Writes single zero bit to the output.
  65     pub fn write0(&mut self) { self.write_bit(false); }
  66     /// Writes single set bit to the output.
  67     pub fn write1(&mut self) { self.write_bit(true); }
  68     /// Writes single bit.
  69     pub fn write_bit(&mut self, bit: bool) {
  70         if self.mode.is_be() {
  71             self.bitbuf |= (bit as u32) << (31 - self.bits);
  72         } else {
  73             self.bitbuf |= (bit as u32) << self.bits;
  74         }
  75         self.bits += 1;
  76         self.flush();
  77     }
  78     /// Writes `bits` bits of `val` value to the output.
  79     #[allow(clippy::collapsible_if)]
  80     pub fn write(&mut self, val: u32, bits: u8) {
  81         if bits == 0 {
  82             return;
  83         }
  84         if self.mode.is_be() {
  85             if self.bits + bits <= 32 {
  86                 self.bitbuf |= val << (32 - self.bits - bits);
  87                 self.bits += bits;
  88                 self.flush();
  89             } else {
  90                 let cbits = 32 - self.bits;
  91                 let bits2 = bits - cbits;
  92                 self.write(val >> bits2, cbits);
  93                 self.write(val & ((1 << bits2) - 1), bits2);
  94             }
  95         } else {
  96             if self.bits + bits <= 32 {
  97                 self.bitbuf |= val << self.bits;
  98                 self.bits += bits;
  99                 self.flush();
 100             } else {
 101                 let cbits = 32 - self.bits;
 102                 let bits2 = bits - cbits;
 103                 self.write(val & ((1 << cbits) - 1), cbits);
 104                 self.write(val >> cbits, bits2);
 105             }
 106         }
 107     }
 108     /// Writes `bits` bits of signed `val` value to the output.
 109     pub fn write_s(&mut self, val: i32, bits: u8) {
 110         self.write((val as u32) & ((1 << bits) - 1), bits);
 111     }
 112     /// Tells the amount of bits written so far.
 113     pub fn tell(&self) -> usize {
 114         (self.dst.len() - self.start) * 8 + (self.bits as usize)
 115     }
 116     fn flush(&mut self) {
 117         match self.mode {
 118             BitWriterMode::BE => {
 119                 while self.bits >= 8 {
 120                     self.dst.push((self.bitbuf >> 24) as u8);
 121                     self.bitbuf <<= 8;
 122                     self.bits    -= 8;
 123                 }
 124             },
 125             BitWriterMode::LE => {
 126                 while self.bits >= 8 {
 127                     self.dst.push(self.bitbuf as u8);
 128                     self.bitbuf >>= 8;
 129                     self.bits    -= 8;
 130                 }
 131             },
 132             BitWriterMode::LE16MSB => {
 133                 while self.bits >= 16 {
 134                     self.dst.push((self.bitbuf >> 16) as u8);
 135                     self.dst.push((self.bitbuf >> 24) as u8);
 136                     self.bitbuf <<= 16;
 137                     self.bits    -= 16;
 138                 }
 139             },
 140             BitWriterMode::LE32MSB => {
 141                 if self.bits == 32 {
 142                     self.dst.push( self.bitbuf        as u8);
 143                     self.dst.push((self.bitbuf >>  8) as u8);
 144                     self.dst.push((self.bitbuf >> 16) as u8);
 145                     self.dst.push((self.bitbuf >> 24) as u8);
 146                     self.bitbuf = 0;
 147                     self.bits   = 0;
 148                 }
 149             },
 150         };
 151     }
 152     /// Finalises operations and returns the vector containing output data.
 153     pub fn end(mut self) -> Vec<u8> {
 154         self.flush();
 155         if self.bits > 0 {
 156             match self.mode {
 157                 BitWriterMode::BE => {
 158                     self.dst.push((self.bitbuf >> 24) as u8);
 159                 },
 160                 BitWriterMode::LE => {
 161                     self.dst.push(self.bitbuf as u8);
 162                 },
 163                 BitWriterMode::LE16MSB => {
 164                     self.dst.push((self.bitbuf >> 16) as u8);
 165                     self.dst.push((self.bitbuf >> 24) as u8);
 166                 },
 167                 BitWriterMode::LE32MSB => {
 168                     self.dst.push( self.bitbuf        as u8);
 169                     self.dst.push((self.bitbuf >>  8) as u8);
 170                     self.dst.push((self.bitbuf >> 16) as u8);
 171                     self.dst.push((self.bitbuf >> 24) as u8);
 172                 },
 173             };
 174         }
 175         self.dst
 176     }
 177 }
 178
 179 #[cfg(test)]
 180 mod test {
 181     use super::*;
 182
 183     #[test]
 184     fn bw_works() {
 185         let mut bw = BitWriter::new(Vec::new(), BitWriterMode::BE);
 186         bw.write(43, 9);
 187         let data = bw.end();
 188         assert_eq!(&data, &[21, 128]);
 189
 190         let mut bw = BitWriter::new(Vec::new(), BitWriterMode::LE);
 191         bw.write1();
 192         bw.write(43, 9);
 193         let data = bw.end();
 194         assert_eq!(&data, &[87, 0]);
 195
 196         let mut bw = BitWriter::new(Vec::new(), BitWriterMode::LE32MSB);
 197         bw.write(42, 9);
 198         bw.write(42, 9);
 199         let data = bw.end();
 200         assert_eq!(&data, &[0, 128, 10, 21]);
 201     }
 202 }