bitio

Package bitio provides an optimized bit-level Reader and Writer for Go.

You can use Reader.ReadBits() to read arbitrary number of bits from an io.Reader and return it as an uint64, and Writer.WriteBits() to write arbitrary number of bits of an uint64 value to an io.Writer.

Both Reader and Writer also provide optimized methods for reading / writing 1 bit of information in the form of a bool value: Reader.ReadBool() and Writer.WriteBool(). These make this package ideal for compression algorithms that use Huffman coding for example, where decision whether to step left or right in the Huffman tree is the most frequent operation.

Reader and Writer give a bit-level view of the underlying io.Reader and io.Writer, but they also provide a byte-level view (io.Reader and io.Writer) at the same time. This means you can also use the Reader.Read() and Writer.Write() methods to read and write slices of bytes. These will give you best performance if the underlying io.Reader and io.Writer are aligned to a byte boundary (else all the individual bytes are assembled from / spread to multiple bytes). You can ensure byte boundary alignment by calling the Align() method of Reader and Writer. As an extra, io.ByteReader and io.ByteWriter are also implemented.

Bit order

The more general highest-bits-first order is used. So for example if the input provides the bytes 0x8f and 0x55:

HEXA    8    f     5    5
BINARY  1100 1111  0101 0101
        aaaa bbbc  ccdd dddd

Then ReadBits will return the following values:

r := NewReader(bytes.NewBuffer([]byte{0x8f, 0x55}))
a, err := r.ReadBits(4) //   1100 = 0x08
b, err := r.ReadBits(3) //    111 = 0x07
c, err := r.ReadBits(3) //    101 = 0x05
d, err := r.ReadBits(6) // 010101 = 0x15

Writing the above values would result in the same sequence of bytes:

b := &bytes.Buffer{}
w := NewWriter(b)
err := w.WriteBits(0x08, 4)
err = w.WriteBits(0x07, 3)
err = w.WriteBits(0x05, 3)
err = w.WriteBits(0x15, 6)
err = w.Close()
// b will hold the bytes: 0x8f and 0x55

Error handling

All ReadXXX() and WriteXXX() methods return an error which you are expected to handle. For convenience, there are also matching TryReadXXX() and TryWriteXXX() methods which do not return an error. Instead they store the (first) error in the Reader.TryError / Writer.TryError field which you can inspect later. These TryXXX() methods are a no-op if a TryError has been encountered before, so it's safe to call multiple TryXXX() methods and defer the error checking.

For example:

r := NewReader(bytes.NewBuffer([]byte{0x8f, 0x55}))
a := r.TryReadBits(4) //   1100 = 0x08
b := r.TryReadBits(3) //    111 = 0x07
c := r.TryReadBits(3) //    101 = 0x05
d := r.TryReadBits(6) // 010101 = 0x15
if r.TryError != nil {
    // Handle error
}

This allows you to easily convert the result of individual ReadBits(), like this:

r := NewReader(bytes.NewBuffer([]byte{0x8f, 0x55}))
a := byte(r.TryReadBits(4))   //   1100 = 0x08
b := int32(r.TryReadBits(3))  //    111 = 0x07
c := int64(r.TryReadBits(3))  //    101 = 0x05
d := uint16(r.TryReadBits(6)) // 010101 = 0x15
if r.TryError != nil {
    // Handle error
}

And similarly:

b := &bytes.Buffer{}
w := NewWriter(b)
w.TryWriteBits(0x08, 4)
w.TryWriteBits(0x07, 3)
w.TryWriteBits(0x05, 3)
w.TryWriteBits(0x15, 6)
if w.TryError != nil {
    // Handle error
}
err = w.Close()
// b will hold the bytes: 0x8f and 0x55