Module read

Traits and implementations for reading bits from a stream.

Example

Reading the initial STREAMINFO block from a FLAC file, as documented in its specification.

use std::io::{Cursor, Read};
use bitstream_io::*;

#[derive(Debug, PartialEq, Eq)]
struct BlockHeader {
    last_block: bool,  // 1 bit
    block_type: u8,    // 7 bits
    block_size: u32,   // 24 bits
}

impl FromBitStream for BlockHeader {
    type Error = std::io::Error;

    fn from_reader<R: BitRead + ?Sized>(r: &mut R) -> std::io::Result<Self> {
        Ok(Self {
            last_block: r.read_bit()?,
            block_type: r.read_in::<7, _>()?,
            block_size: r.read_in::<24, _>()?,
        })
    }
}

#[derive(Debug, PartialEq, Eq)]
struct Streaminfo {
    minimum_block_size: u16,  // 16 bits
    maximum_block_size: u16,  // 16 bits
    minimum_frame_size: u32,  // 24 bits
    maximum_frame_size: u32,  // 24 bits
    sample_rate: u32,         // 20 bits
    channels: u8,             // 3 bits
    bits_per_sample: u8,      // 5 bits
    total_samples: u64,       // 36 bits
    md5: [u8; 16],            // 16 bytes
}

impl FromBitStream for Streaminfo {
    type Error = std::io::Error;

    fn from_reader<R: BitRead + ?Sized>(r: &mut R) -> std::io::Result<Self> {
        Ok(Self {
            minimum_block_size: r.read_to()?,
            maximum_block_size: r.read_to()?,
            minimum_frame_size: r.read_in::<24, _>()?,
            maximum_frame_size: r.read_in::<24, _>()?,
            sample_rate:        r.read_in::<20, _>()?,
            channels:           r.read_in::<3, u8>()? + 1,
            bits_per_sample:    r.read_in::<5, u8>()? + 1,
            total_samples:      r.read_in::<36, _>()?,
            md5:                r.read_to()?,
        })
    }
}

#[derive(Debug, PartialEq, Eq)]
struct VorbisComment {
    vendor: String,
    comment: Vec<String>,
}

impl FromBitStream for VorbisComment {
   type Error = Box<dyn std::error::Error>;

   fn from_reader<R: BitRead + ?Sized>(r: &mut R) -> Result<Self, Self::Error> {
       use bitstream_io::LE;

       fn read_entry<R: BitRead + ?Sized>(
           r: &mut R,
       ) -> Result<String, Box<dyn std::error::Error>> {
           use std::convert::TryInto;
           let size = r.read_as_to::<LE, u32>()?.try_into()?;
           Ok(String::from_utf8(r.read_to_vec(size)?)?)
       }

       Ok(Self {
           vendor: read_entry(r)?,
           comment: (0..r.read_as_to::<LE, u32>()?)
               .map(|_| read_entry(r))
               .collect::<Result<Vec<_>, _>>()?,
       })
   }
}

// test FLAC file data
let flac: Vec<u8> = vec![0x66,0x4c,0x61,0x43,0x00,0x00,0x00,0x22,
                         0x10,0x00,0x10,0x00,0x00,0x06,0x06,0x00,
                         0x21,0x62,0x0a,0xc4,0x42,0xf0,0x00,0x04,
                         0xa6,0xcc,0xfa,0xf2,0x69,0x2f,0xfd,0xec,
                         0x2d,0x5b,0x30,0x01,0x76,0xb4,0x62,0x88,
                         0x7d,0x92,0x04,0x00,0x00,0x7a,0x20,0x00,
                         0x00,0x00,0x72,0x65,0x66,0x65,0x72,0x65,
                         0x6e,0x63,0x65,0x20,0x6c,0x69,0x62,0x46,
                         0x4c,0x41,0x43,0x20,0x31,0x2e,0x31,0x2e,
                         0x34,0x20,0x32,0x30,0x30,0x37,0x30,0x32,
                         0x31,0x33,0x04,0x00,0x00,0x00,0x16,0x00,
                         0x00,0x00,0x74,0x69,0x74,0x6c,0x65,0x3d,
                         0x32,0x63,0x68,0x20,0x34,0x34,0x31,0x30,
                         0x30,0x20,0x20,0x31,0x36,0x62,0x69,0x74,
                         0x10,0x00,0x00,0x00,0x61,0x6c,0x62,0x75,
                         0x6d,0x3d,0x54,0x65,0x73,0x74,0x20,0x41,
                         0x6c,0x62,0x75,0x6d,0x0f,0x00,0x00,0x00,
                         0x61,0x72,0x74,0x69,0x73,0x74,0x3d,0x41,
                         0x73,0x73,0x6f,0x72,0x74,0x65,0x64,0x0d,
                         0x00,0x00,0x00,0x74,0x72,0x61,0x63,0x6b,
                         0x6e,0x75,0x6d,0x62,0x65,0x72,0x3d,0x31];

let mut cursor = Cursor::new(&flac);

let mut reader = BitReader::endian(&mut cursor, BigEndian);

// stream marker
assert_eq!(&reader.read_to::<[u8; 4]>().unwrap(), b"fLaC");

// metadata block header
assert_eq!(
    reader.parse::<BlockHeader>().unwrap(),
    BlockHeader { last_block: false, block_type: 0, block_size: 34 }
);

// STREAMINFO block
assert_eq!(
    reader.parse::<Streaminfo>().unwrap(),
    Streaminfo {
        minimum_block_size: 4096,
        maximum_block_size: 4096,
        minimum_frame_size: 1542,
        maximum_frame_size: 8546,
        sample_rate: 44100,
        channels: 2,
        bits_per_sample: 16,
        total_samples: 304844,
        md5: *b"\xFA\xF2\x69\x2F\xFD\xEC\x2D\x5B\x30\x01\x76\xB4\x62\x88\x7D\x92",
    }
);

// metadata block header
assert_eq!(
    reader.parse::<BlockHeader>().unwrap(),
    BlockHeader { last_block: false, block_type: 4, block_size: 122 }
);

// VORBIS_COMMENT block
assert_eq!(
   reader.parse::<VorbisComment>().unwrap(),
   VorbisComment {
       vendor: "reference libFLAC 1.1.4 20070213".to_string(),
       comment: vec![
           "title=2ch 44100  16bit".to_string(),
           "album=Test Album".to_string(),
           "artist=Assorted".to_string(),
           "tracknumber=1".to_string(),
       ],
   }
);