88 lines
2.1 KiB
Go
88 lines
2.1 KiB
Go
// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package bzip2
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import (
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"bufio"
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"io"
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)
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// bitReader wraps an io.Reader and provides the ability to read values,
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// bit-by-bit, from it. Its Read* methods don't return the usual error
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// because the error handling was verbose. Instead, any error is kept and can
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// be checked afterwards.
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type bitReader struct {
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r byteReader
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n uint64
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bits uint
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err error
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}
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// bitReader needs to read bytes from an io.Reader. We attempt to convert the
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// given io.Reader to this interface and, if it doesn't already fit, we wrap in
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// a bufio.Reader.
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type byteReader interface {
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ReadByte() (byte, error)
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}
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func newBitReader(r io.Reader) bitReader {
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byter, ok := r.(byteReader)
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if !ok {
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byter = bufio.NewReader(r)
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}
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return bitReader{r: byter}
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}
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// ReadBits64 reads the given number of bits and returns them in the
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// least-significant part of a uint64. In the event of an error, it returns 0
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// and the error can be obtained by calling Err().
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func (br *bitReader) ReadBits64(bits uint) (n uint64) {
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for bits > br.bits {
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b, err := br.r.ReadByte()
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if err == io.EOF {
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err = io.ErrUnexpectedEOF
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}
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if err != nil {
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br.err = err
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return 0
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}
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br.n <<= 8
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br.n |= uint64(b)
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br.bits += 8
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}
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// br.n looks like this (assuming that br.bits = 14 and bits = 6):
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// Bit: 111111
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// 5432109876543210
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//
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// (6 bits, the desired output)
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// |-----|
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// V V
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// 0101101101001110
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// ^ ^
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// |------------|
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// br.bits (num valid bits)
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//
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// This the next line right shifts the desired bits into the
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// least-significant places and masks off anything above.
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n = (br.n >> (br.bits - bits)) & ((1 << bits) - 1)
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br.bits -= bits
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return
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}
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func (br *bitReader) ReadBits(bits uint) (n int) {
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n64 := br.ReadBits64(bits)
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return int(n64)
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}
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func (br *bitReader) ReadBit() bool {
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n := br.ReadBits(1)
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return n != 0
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}
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func (br *bitReader) Err() error {
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return br.err
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}
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