源码包阅读 buffer

发布于 2019-03-20 作者 yhyddr 127次 浏览 版块 分享

这里是我的语雀 https://www.yuque.com/abs

buffer.go

Overview

这是 bytes 包里的 buffer 实现

一图胜千言

看不懂图的再看下面吧

buffer.jpg

核心函数

Buffer 结构

这是 buffer 的内部结构
buf 字节切片,用来存储 buffer 的内容
off 是代表从哪里开始读
bootstrap 用来作为字节切片过小的时候防止多次申请空间减小开销
lastRead 用来记录上一次的操作

// A Buffer is a variable-sized buffer of bytes with Read and Write methods.
// The zero value for Buffer is an empty buffer ready to use.
// 注意 buffer 的零值是空的 buf
type Buffer struct {
	buf       []byte   // contents are the bytes buf[off : len(buf)]
	off       int      // read at &buf[off], write at &buf[len(buf)]
	bootstrap [64]byte // memory to hold first slice; helps small buffers avoid allocation.
	lastRead  readOp   // last read operation, so that Unread* can work correctly.

	// FIXME: it would be advisable to align Buffer to cachelines to avoid false
	// sharing.
}

Grow(n int)

申请扩展缓冲区

// 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.
// 增加容量 n byte
func (b *Buffer) Grow(n int) {
	if n < 0 {
		panic("bytes.Buffer.Grow: negative count")
	}
	m := b.grow(n)
	b.buf = b.buf[:m]
}

WriteString(s string) (n int, err error)

向 buffer 中写字符串

// WriteString appends the contents of s to the buffer, growing the buffer as
// needed. The return value n is the length of s; err is always nil. If the
// buffer becomes too large, WriteString will panic with ErrTooLarge.
// 直接写 string 也行,同时自动扩展
func (b *Buffer) WriteString(s string) (n int, err error) {
	b.lastRead = opInvalid
	//先尝试不用扩展容量的写法
	m, ok := b.tryGrowByReslice(len(s))
	if !ok {
		m = b.grow(len(s))
	}
	// copy 可以直接把 string 类型作为 字节切片拷贝过去
	return copy(b.buf[m:], s), nil
}

也有写字节切片的形式  Write(p []byte) (n int, err error) 

ReadFrom(r io.Reader) (n int64, err error)

从 io.Reader 读取数据到 buffer 中

// 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.
// 从实现了 io.Reader 接口的 r 中读取到 EOF 为止,如果超出了 maxInt 那么大就会返回太
// 大不能通过一个 [maxInt]byte 字节切片来存储了
func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error) {
	b.lastRead = opInvalid
	for {
		i := b.grow(MinRead)
		// grow 申请了 n 个空间之后,会将 buffer 中的字节切片延长长度到 n 个字节之后
		// 所以需要重新赋值一下长度,避免一些误解,保证长度都是有效数据提供的
		b.buf = b.buf[:i]
		// 将 r 中的数据读到 buffer 中去
		m, e := r.Read(b.buf[i:cap(b.buf)])
		if m < 0 {
			panic(errNegativeRead)
		}

		// 手动更改长度
		b.buf = b.buf[:i+m]
		n += int64(m)
		if e == io.EOF {
			return n, nil // e is EOF, so return nil explicitly
		}
		if e != nil {
			return n, e
		}
	}
}

WriteTo(w io.Writer) (n int64, err error)

向 io.Writer 中写数据

// 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 nBytes := b.Len(); nBytes > 0 {
		//从 off 开始读的地方算起,全部写到 io.Writer 中去
		m, e := w.Write(b.buf[b.off:])
		//写的多了就报错
		if m > nBytes {
			panic("bytes.Buffer.WriteTo: invalid Write count")
		}
		//记录写过了多少,位移 offset 指针
		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
		// 因为刚才判断过写多了的情况,所以这里是写少了
		if m != nBytes {
			return n, io.ErrShortWrite
		}
	}
	// Buffer is now empty; reset.
	// 写完之后重置
	b.Reset()
	return n, nil
}

ReadBytes(delim byte) (line []byte, err error)

用来读到终止符就结束,返回的是一个 line 字节切片包含终止符前的数据

// 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.
// 读取到终止符为止,就结束
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 line, err
}

NewBuffer(buf []byte) *Buffer

用来新建一个新的 Buffer ,其实也可以使用 new 和 var 来声明

// NewBuffer creates and initializes a new Buffer using buf as its
// initial contents. The new Buffer takes ownership of buf, and the
// caller should not use buf after this call. NewBuffer is intended to
// prepare a Buffer to read existing data. It can also be used to size
// the internal buffer for writing. To do that, buf should have the
// desired capacity but a length of zero.
//
// In most cases, new(Buffer) (or just declaring a Buffer variable) is
// sufficient to initialize a Buffer.
// 通过字节切片创建一个 buffer ,字节切片会保留初始值
// 在渴望容量但是长度为 0?的情况下
// 也可以当作内核的 buffer 来写入
func NewBuffer(buf []byte) *Buffer { return &Buffer{buf: buf} }

同时也有通过 string 类型的实现
**func NewBufferString(s string) *Buffer {return &Buffer{buf: []byte(s)}}** 

总结

缓冲区,实现了大小控制,字节切片和 string 类型的读写,同时还对情况进行了优化,比如存在 bootstrap,比如 grow 函数中的多次检定。适合多读精读来学习

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