bitreader.go

package d2s

import "io"

// bitReader wraps an io.Reader and provides the ability to read values,
// bit-by-bit, from it. Its Read* methods don't return the usual error
// because the error handling was verbose. Instead, any error is kept and can
// be checked afterwards.
type bitReader struct {
	r    io.ByteReader
	n    uint64
	bits uint
	err  error
}

// ReadBits64 reads the given number of bits and returns them in the
// least-significant part of a uint64. In the event of an error, it returns 0
// and the error can be obtained by calling Err().
func (br *bitReader) ReadBits64(bits uint, reverse bool) (n uint64) {
	for bits > br.bits {
		b, err := br.r.ReadByte()

		if reverse {
			b = reverseByte(b)
		}

		if err == io.EOF {
			err = io.ErrUnexpectedEOF
		}
		if err != nil {
			br.err = err
			return 0
		}
		br.n <<= 8
		br.n |= uint64(b)
		br.bits += 8
	}

	// br.n looks like this (assuming that br.bits = 14 and bits = 6):
	// Bit: 111111
	//      5432109876543210
	//
	//         (6 bits, the desired output)
	//        |-----|
	//        V     V
	//      0101101101001110
	//        ^            ^
	//        |------------|
	//           br.bits (num valid bits)
	//
	// This the next line right shifts the desired bits into the
	// least-significant places and masks off anything above.
	n = (br.n >> (br.bits - bits)) & ((1 << bits) - 1)
	br.bits -= bits

	return
}

func reverseByte(b byte) byte {
	var d byte
	for i := 0; i < 8; i++ {
		d <<= 1
		d |= b & 1
		b >>= 1
	}
	return d
}

func reverseBits(b uint64, n uint) uint64 {
	var d uint64
	for i := 0; i < int(n); i++ {
		d <<= 1
		d |= b & 1
		b >>= 1
	}
	return d
}

func (br *bitReader) Err() error {
	return br.err
}