Buffer
Go标准库Buffer是一个可变大小的字节缓冲区,可以用Wirte和Read方法操作它,在Go标准库中,定义了如下关于Buffer的数据结构。
type Buffer struct { buf []byte // contents are the bytes buf[off : len(buf)] off int // read at &buf[off], write at &buf[len(buf)] runeBytes [utf8.UTFMax]byte // avoid allocation of slice on each WriteByte or Rune bootstrap [64]byte // memory to hold first slice; helps small buffers (Printf) avoid allocation. lastRead readOp // last read operation, so that Unread* can work correctly. } // The readOp constants describe the last action performed on // the buffer, so that UnreadRune and UnreadByte can // check for invalid usage. type readOp int const ( opInvalid readOp = iota // Non-read operation. opReadRune // Read rune. opRead // Any other read operation. )
如上定义,Buffer存储的数据是在off到len(buf)区域之间,其他区域是没有数据,而且只能从&buf[off]开始读取数据和从&buf[len(buf)]写数据,同时为了避免对内存的多次操作,对于小的缓冲区,Buffer定义了bootstrap来避免多次内存的操作,runeBytes的定义也是如此目的,还有一个表示对Buffer的操作标识符lastRead。
Buffer的常见操作
初始化Buffer
func NewBuffer(buf []byte) *Buffer { return &Buffer{buf: buf} } func NewBufferString(s string) *Buffer { return &Buffer{buf: []byte(s)} }
方法NewBuffer使用buf作为参数初始化Buffer,Buffer既可以被读也可以被写,如果是读Buffer,buf需填充一定的数据,如果是写,buf需有一定的容量(capacity),当然也可以通过new(Buffer)来初始化Buffer。另外一个方法NewBufferString用一个string来初始化可读Buffer,并用string的内容填充Buffer.
读写操作
func (b *Buffer) Read(p []byte) (n int, err error) func (b *Buffer) Next(n int) []byte func (b *Buffer) ReadByte() (c byte, err error) func (b *Buffer) ReadRune() (r rune, size int, err error) func (b *Buffer) ReadBytes(delim byte) (line []byte, err error) func (b *Buffer) readSlice(delim byte) (line []byte, err error) func (b *Buffer) ReadString(delim byte) (line string, err error) func (b *Buffer) Write(p []byte) (n int, err error) func (b *Buffer) WriteString(s string) (n int, err error) func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error) func (b *Buffer) WriteTo(w io.Writer) (n int64, err error) func (b *Buffer) WriteByte(c byte) error func (b *Buffer) WriteRune(r rune) (n int, err error)
下面对Read,ReadRune,ReadBytes方法进行分析,对于方法Read, 其主要做三个步骤:第一,判断Buffer是否为空,如果是,则重置Buffer;第二,复制Buffer的buf的数据到p,并调整off的位置标识Buffer的可读位置;第三,设置读标识符为opRead。
func (b *Buffer) Read(p []byte) (n int, err error) { b.lastRead = opInvalid if b.off >= len(b.buf) { // Buffer is empty, reset to recover space. b.Truncate(0) if len(p) == 0 { return } return 0, io.EOF } n = copy(p, b.buf[b.off:]) b.off += n if n > 0 { b.lastRead = opRead } return }
方法ReadRune()定义了如何读取Buffer中UTF8编码的rune数据,同样也需三个步骤,第一,判断Buffer是否为空,若是,重置Buffer;第二,设置读操作符为opReadRune;第三,判断可读位置off处的byte是否小于utf8.Runeself,若是,调整off位置并返回。否则,将Buffer的数据解码成rune,调整off位置,返回解码后的rune及大小。
// ReadRune reads and returns the next UTF-8-encoded // Unicode code point from the buffer. // If no bytes are available, the error returned is io.EOF. // If the bytes are an erroneous UTF-8 encoding, it // consumes one byte and returns U+FFFD, 1. func (b *Buffer) ReadRune() (r rune, size int, err error) { b.lastRead = opInvalid if b.off >= len(b.buf) { // Buffer is empty, reset to recover space. b.Truncate(0) return 0, 0, io.EOF } b.lastRead = opReadRune c := b.buf[b.off] if c < utf8.RuneSelf { b.off++ return rune(c), 1, nil } r, n := utf8.DecodeRune(b.buf[b.off:]) b.off += n return r, n, nil }
方法ReadBytes(delim byte)读取Buffer中从off到第一次delim之间的数据,并且包括delim,ReadBytes调用私有方法readSlice来实现,readSlice方法首先查找delim的位置,如果不存在,则返回从off到len(buf)之间的数据,如果存在,则返回off到off+location(delim)+1之间数据,其中加1是为了包括delim,最后设置操作标识符为opRead。
// ReadBytes reads until the first occurrence of delim in the input, // returning a slice containing the data up to and including the delimiter. // If ReadBytes encounters an error before finding a delimiter, // it returns the data read before the error and the error itself (often io.EOF). // ReadBytes returns err != nil if and only if the returned data does not end in // delim. func (b *Buffer) ReadBytes(delim byte) (line []byte, err error) { slice, err := b.readSlice(delim) // return a copy of slice. The buffer‘s backing array may // be overwritten by later calls. line = append(line, slice...) return } // readSlice is like ReadBytes but returns a reference to internal buffer data. func (b *Buffer) readSlice(delim byte) (line []byte, err error) { i := IndexByte(b.buf[b.off:], delim) end := b.off + i + 1 if i < 0 { end = len(b.buf) err = io.EOF } line = b.buf[b.off:end] b.off = end b.lastRead = opRead return line, err }
同样对相应的Write,WriteRune,ReadFrom, WriteTo写方法进行分析, 对于方法Write,相对Read方法来说,要简单些,主要是扩展Buffer空间,然后将p中的数据复制到Buffer。
// Write appends the contents of p to the buffer, growing the buffer as // needed. The return value n is the length of p; err is always nil. If the // buffer becomes too large, Write will panic with ErrTooLarge. func (b *Buffer) Write(p []byte) (n int, err error) { b.lastRead = opInvalid m := b.grow(len(p)) return copy(b.buf[m:], p), nil }
对于方法WriteRune,首先判断要写的数据rune是否小于utf8.RuneSelf,若是,调用WriteByte将其写入Buffer,若不是,则将要写的数据rune编码成utf8,并调用Write将其写入Buffer。
// WriteRune appends the UTF-8 encoding of Unicode code point r to the // buffer, returning its length and an error, which is always nil but is // included to match bufio.Writer‘s WriteRune. The buffer is grown as needed; // if it becomes too large, WriteRune will panic with ErrTooLarge. func (b *Buffer) WriteRune(r rune) (n int, err error) { if r < utf8.RuneSelf { b.WriteByte(byte(r)) return 1, nil } n = utf8.EncodeRune(b.runeBytes[0:], r) b.Write(b.runeBytes[0:n]) return n, nil }
ReadFrom方法从io.Reader或者实现io.Reader接口的实例中读取所有数据到Buffer,默认情况下最少读取512字节,如果Buffer空间不足512,需增加Buffer空间,该方法返回读取的字节数以及错误信息。从下面可知ReadFrom首先判断Buffer是否为空,若空,则重置Buffer;其次是判断Buffer的free空间是否足够,若小于512且off+free小于512,表示Buffer从0到off之间的空间不足以存放当前Buffer中未读数据的大小,此时设置一临时缓冲区并使其空间Buffer的2倍加上MinRead(512)的空间,将原来Buffer的数据复制到临时缓冲区,然后再把临时缓冲区的数据复制到源Buffer,最后使用io.Reader的Read方法从io.Reader中读取数据,直到遇到io.EOF。
// ReadFrom reads data from r until EOF and appends it to the buffer, growing // the buffer as needed. The return value n is the number of bytes read. Any // error except io.EOF encountered during the read is also returned. If the // buffer becomes too large, ReadFrom will panic with ErrTooLarge. func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error) { b.lastRead = opInvalid // If buffer is empty, reset to recover space. if b.off >= len(b.buf) { b.Truncate(0) } for { if free := cap(b.buf) - len(b.buf); free < MinRead { // not enough space at end newBuf := b.buf if b.off+free < MinRead { // not enough space using beginning of buffer; // double buffer capacity newBuf = makeSlice(2*cap(b.buf) + MinRead) } copy(newBuf, b.buf[b.off:]) b.buf = newBuf[:len(b.buf)-b.off] b.off = 0 } m, e := r.Read(b.buf[len(b.buf):cap(b.buf)]) b.buf = b.buf[0 : len(b.buf)+m] n += int64(m) if e == io.EOF { break } if e != nil { return n, e } } return n, nil // err is EOF, so return nil explicitly }相对ReadFrom方法,WriteTo方法比较简单,WriteTo将Buffer中的数据写到io.Writer,直到Buffer中没有数据,当Buffer为空时,重置Buffer并返回。
// WriteTo writes data to w until the buffer is drained or an error occurs. // The return value n is the number of bytes written; it always fits into an // int, but it is int64 to match the io.WriterTo interface. Any error // encountered during the write is also returned. func (b *Buffer) WriteTo(w io.Writer) (n int64, err error) { b.lastRead = opInvalid if b.off < len(b.buf) { nBytes := b.Len() m, e := w.Write(b.buf[b.off:]) if m > nBytes { panic("bytes.Buffer.WriteTo: invalid Write count") } b.off += m n = int64(m) if e != nil { return n, e } // all bytes should have been written, by definition of // Write method in io.Writer if m != nBytes { return n, io.ErrShortWrite } } // Buffer is now empty; reset. b.Truncate(0) return }
3. 扩展空间和重置
Buffer的重置方法Reset()通过调用Truncate(n int)方法来实现清除Buffer的数据,Truncate丢弃除了从off开始的n个未读数据之外的所有数据, 如果n为0,那就重置Buffer。
// Truncate discards all but the first n unread bytes from the buffer. // It panics if n is negative or greater than the length of the buffer. func (b *Buffer) Truncate(n int) { b.lastRead = opInvalid switch { case n < 0 || n > b.Len(): panic("bytes.Buffer: truncation out of range") case n == 0: // Reuse buffer space. b.off = 0 } b.buf = b.buf[0 : b.off+n] } // Reset resets the buffer so it has no content. // b.Reset() is the same as b.Truncate(0). func (b *Buffer) Reset() { b.Truncate(0) }在对Buffer进行写数据时,通常需要扩展其空间来使所有的数据都能写入Buffer,Buffer用Grow(n int)方法来实现扩展Buffer空间的功能,该方法调用私有方法grow(n int)。
// grow grows the buffer to guarantee space for n more bytes. // It returns the index where bytes should be written. // If the buffer can‘t grow it will panic with ErrTooLarge. func (b *Buffer) grow(n int) int { m := b.Len() // 如果Buffer为空,重置Buffer if m == 0 && b.off != 0 { b.Truncate(0) } //空间增加n后超过Buffer的容量 if len(b.buf)+n > cap(b.buf) { //声明一个临时buf var buf []byte //Buffer的buf只是被声明,还没有初始化,如果n小于bootstrap的空间, //直接将boostrap赋值给buf避免内存的操作而增加负载。 if b.buf == nil && n <= len(b.bootstrap) { buf = b.bootstrap[0:] //如果满足此条件,滑动b.buf的数据而不是分配一个新的slice空间,然后将b.buf的数据复制给你buf。 } else if m+n <= cap(b.buf)/2 { copy(b.buf[:], b.buf[b.off:]) buf = b.buf[:m] } else { //空间不足,重新分配空间 buf = makeSlice(2*cap(b.buf) + n) copy(buf, b.buf[b.off:]) } b.buf = buf b.off = 0 } //扩展n的空间,并返回可以写数据的位置 b.buf = b.buf[0 : b.off+m+n] return b.off + m } // Grow grows the buffer‘s capacity, if necessary, to guarantee space for // another n bytes. After Grow(n), at least n bytes can be written to the // buffer without another allocation. // If n is negative, Grow will panic. // If the buffer can‘t grow it will panic with ErrTooLarge. func (b *Buffer) Grow(n int) { if n < 0 { panic("bytes.Buffer.Grow: negative count") } m := b.grow(n) b.buf = b.buf[0:m] }
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Golang标准库之Buffer,布布扣,bubuko.com
原文:http://aresy.blog.51cto.com/5100031/1405184