| // 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" |
| "os" |
| ) |
| |
| // 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 os.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 os.Error |
| } |
| |
| // bitReader needs to read bytes from an io.Reader. We attempt to cast 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, os.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 Error(). |
| func (br *bitReader) ReadBits64(bits uint) (n uint64) { |
| for bits > br.bits { |
| b, err := br.r.ReadByte() |
| if err == os.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) Error() os.Error { |
| return br.err |
| } |