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

1. 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.

2. 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
}

// 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) }

// 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]
}

本文出自 “大脑原来不靠谱” 博客，请务必保留此出处http://aresy.blog.51cto.com/5100031/1405184