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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package bzip2
import (
"bufio"
"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 byteReader
n uint64
bits uint
err error
}
// bitReader needs to read bytes from an io.Reader. We attempt to convert the
// given io.Reader to this interface and, if it doesn't already fit, we wrap in
// a bufio.Reader.
type byteReader interface {
ReadByte() (byte, error)
}
func newBitReader(r io.Reader) bitReader {
byter, ok := r.(byteReader)
if !ok {
byter = bufio.NewReader(r)
}
return bitReader{r: byter}
}
// 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) (n uint64) {
for bits > br.bits {
b, err := br.r.ReadByte()
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 (br *bitReader) ReadBits(bits uint) (n int) {
n64 := br.ReadBits64(bits)
return int(n64)
}
func (br *bitReader) ReadBit() bool {
n := br.ReadBits(1)
return n != 0
}
func (br *bitReader) Err() error {
return br.err
}
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