Buffer
Go標準庫Buffer是一個可變大小的字節緩衝區，可以用Wirte和Read方法操作它，在Go標準庫中,定義瞭如下關於Buffer的數據結構。
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.
)如上定義，Buffer存儲的數據是在off到len(buf)區域之間，其他區域是沒有數據，而且只能從&buf[off]開始讀取數據和從&buf[len(buf)]寫數據，同時爲了避免對內存的多次操作，對於小的緩衝區，Buffer定義了bootstrap來避免多次內存的操作，runeBytes的定義也是如此目的，還有一個表示對Buffer的操作標識符lastRead。
Buffer的常見操作
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作爲參數初始化Buffer，Buffer既可以被讀也可以被寫，如果是讀Buffer，buf需填充一定的數據，如果是寫，buf需有一定的容量(capacity)，當然也可以通過new(Buffer)來初始化Buffer。另外一個方法NewBufferString用一個string來初始化可讀Buffer，並用string的內容填充Buffer.
讀寫操作
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)
下面對Read，ReadRune，ReadBytes方法進行分析，對於方法Read, 其主要做三個步驟：第一，判斷Buffer是否爲空，如果是，則重置Buffer；第二，複製Buffer的buf的數據到p,並調整off的位置標識Buffer的可讀位置；第三，設置讀標識符爲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()定義瞭如何讀取Buffer中UTF8編碼的rune數據，同樣也需三個步驟，第一，判斷Buffer是否爲空，若是，重置Buffer；第二，設置讀操作符爲opReadRune；第三，判斷可讀位置off處的byte是否小於utf8.Runeself，若是，調整off位置並返回。否則，將Buffer的數據解碼成rune，調整off位置，返回解碼後的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)讀取Buffer中從off到第一次delim之間的數據，並且包括delim，ReadBytes調用私有方法readSlice來實現，readSlice方法首先查找delim的位置，如果不存在，則返回從off到len(buf)之間的數據，如果存在，則返回off到off+location(delim)+1之間數據，其中加1是爲了包括delim，最後設置操作標識符爲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
}同樣對相應的Write，WriteRune，ReadFrom， WriteTo寫方法進行分析, 對於方法Write，相對Read方法來說，要簡單些，主要是擴展Buffer空間，然後將p中的數據複製到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
}對於方法WriteRune,首先判斷要寫的數據rune是否小於utf8.RuneSelf，若是，調用WriteByte將其寫入Buffer，若不是，則將要寫的數據rune編碼成utf8,並調用Write將其寫入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方法從io.Reader或者實現io.Reader接口的實例中讀取所有數據到Buffer，默認情況下最少讀取512字節，如果Buffer空間不足512，需增加Buffer空間，該方法返回讀取的字節數以及錯誤信息。從下面可知ReadFrom首先判斷Buffer是否爲空，若空，則重置Buffer；其次是判斷Buffer的free空間是否足夠，若小於512且off+free小於512，表示Buffer從0到off之間的空間不足以存放當前Buffer中未讀數據的大小，此時設置一臨時緩衝區並使其空間Buffer的2倍加上MinRead(512)的空間，將原來Buffer的數據複製到臨時緩衝區，然後再把臨時緩衝區的數據複製到源Buffer，最後使用io.Reader的Read方法從io.Reader中讀取數據，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
} 相對ReadFrom方法，WriteTo方法比較簡單，WriteTo將Buffer中的數據寫到io.Writer，直到Buffer中沒有數據，當Buffer爲空時，重置Buffer並返回。// 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. 擴展空間和重置
Buffer的重置方法Reset()通過調用Truncate(n int)方法來實現清除Buffer的數據，Truncate丟棄除了從off開始的n個未讀數據之外的所有數據, 如果n爲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) } 在對Buffer進行寫數據時，通常需要擴展其空間來使所有的數據都能寫入Buffer，Buffer用Grow(n int)方法來實現擴展Buffer空間的功能，該方法調用私有方法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爲空，重置Buffer
if m == 0 && b.off != 0 {
b.Truncate(0)
}
//空間增加n後超過Buffer的容量
if len(b.buf)+n > cap(b.buf) {
//聲明一個臨時buf
var buf []byte
//Buffer的buf只是被聲明，還沒有初始化，如果n小於bootstrap的空間，
//直接將boostrap賦值給buf避免內存的操作而增加負載。
if b.buf == nil && n <= len(b.bootstrap) {
buf = b.bootstrap[0:]
//如果滿足此條件，滑動b.buf的數據而不是分配一個新的slice空間，然後將b.buf的數據複製給你buf。
} else if m+n <= cap(b.buf)/2 {
copy(b.buf[:], b.buf[b.off:])
buf = b.buf[:m]
} else {
//空間不足，重新分配空間
buf = makeSlice(2*cap(b.buf) + n)
copy(buf, b.buf[b.off:])
}
b.buf = buf
b.off = 0
}
//擴展n的空間，並返回可以寫數據的位置
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]
}