1 // Copyright 2011 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 package bzip2 6 7 import ( 8 "bufio" 9 "io" 10 ) 11 12 // bitReader wraps an io.Reader and provides the ability to read values, 13 // bit-by-bit, from it. Its Read* methods don't return the usual error 14 // because the error handling was verbose. Instead, any error is kept and can 15 // be checked afterwards. 16 type bitReader struct { 17 r io.ByteReader 18 n uint64 19 bits uint 20 err error 21 } 22 23 // newBitReader returns a new bitReader reading from r. If r is not 24 // already an io.ByteReader, it will be converted via a bufio.Reader. 25 func newBitReader(r io.Reader) bitReader { 26 byter, ok := r.(io.ByteReader) 27 if !ok { 28 byter = bufio.NewReader(r) 29 } 30 return bitReader{r: byter} 31 } 32 33 // ReadBits64 reads the given number of bits and returns them in the 34 // least-significant part of a uint64. In the event of an error, it returns 0 35 // and the error can be obtained by calling Err(). 36 func (br *bitReader) ReadBits64(bits uint) (n uint64) { 37 for bits > br.bits { 38 b, err := br.r.ReadByte() 39 if err == io.EOF { 40 err = io.ErrUnexpectedEOF 41 } 42 if err != nil { 43 br.err = err 44 return 0 45 } 46 br.n <<= 8 47 br.n |= uint64(b) 48 br.bits += 8 49 } 50 51 // br.n looks like this (assuming that br.bits = 14 and bits = 6): 52 // Bit: 111111 53 // 5432109876543210 54 // 55 // (6 bits, the desired output) 56 // |-----| 57 // V V 58 // 0101101101001110 59 // ^ ^ 60 // |------------| 61 // br.bits (num valid bits) 62 // 63 // This the next line right shifts the desired bits into the 64 // least-significant places and masks off anything above. 65 n = (br.n >> (br.bits - bits)) & ((1 << bits) - 1) 66 br.bits -= bits 67 return 68 } 69 70 func (br *bitReader) ReadBits(bits uint) (n int) { 71 n64 := br.ReadBits64(bits) 72 return int(n64) 73 } 74 75 func (br *bitReader) ReadBit() bool { 76 n := br.ReadBits(1) 77 return n != 0 78 } 79 80 func (br *bitReader) Err() error { 81 return br.err 82 } 83