這裡是我的語雀 https://www.yuque.com/abs

buffer.go

Overview

這是 bytes 包裡的 buffer 實現

一圖勝千言

看不懂圖的再看下面吧

核心函式

Buffer 結構

這是 buffer 的內部結構
buf 位元組切片，用來儲存 buffer 的內容
off 是代表從哪裡開始讀
bootstrap 用來作為位元組切片過小的時候防止多次申請空間減小開銷
lastRead 用來記錄上一次的操作

// 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)

申請擴充套件緩衝區

// 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 中寫字串

// 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.
// 直接寫 string 也行，同時自動擴充套件
func (b *Buffer) WriteString(s string) (n int, err error) {
    b.lastRead = opInvalid
    //先嚐試不用擴充套件容量的寫法
    m, ok := b.tryGrowByReslice(len(s))
    if !ok {
        m = b.grow(len(s))
    }
    // copy 可以直接把 string 型別作為 位元組切片拷貝過去
    return copy(b.buf[m:], s), nil
}

也有寫位元組切片的形式  Write(p []byte) (n int, err error) 

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

從 io.Reader 讀取資料到 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.
// 從實現了 io.Reader 介面的 r 中讀取到 EOF 為止，如果超出了 maxInt 那麼大就會返回太
// 大不能通過一個 [maxInt]byte 位元組切片來儲存了
func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error) {
    b.lastRead = opInvalid
    for {
        i := b.grow(MinRead)
        // grow 申請了 n 個空間之後，會將 buffer 中的位元組切片延長長度到 n 個位元組之後
        // 所以需要重新賦值一下長度，避免一些誤解，保證長度都是有效資料提供的
        b.buf = b.buf[:i]
        // 將 r 中的資料讀到 buffer 中去
        m, e := r.Read(b.buf[i:cap(b.buf)])
        if m < 0 {
            panic(errNegativeRead)
        }

        // 手動更改長度
        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 中寫資料

// 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 {
        //從 off 開始讀的地方算起，全部寫到 io.Writer 中去
        m, e := w.Write(b.buf[b.off:])
        //寫的多了就報錯
        if m > nBytes {
            panic("bytes.Buffer.WriteTo: invalid Write count")
        }
        //記錄寫過了多少，位移 offset 指標
        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
        // 因為剛才判斷過寫多了的情況，所以這裡是寫少了
        if m != nBytes {
            return n, io.ErrShortWrite
        }
    }
    // Buffer is now empty; reset.
    // 寫完之後重置
    b.Reset()
    return n, nil
}

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

用來讀到終止符就結束，返回的是一個 line 位元組切片包含終止符前的資料

// 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.
// 讀取到終止符為止，就結束
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

用來新建一個新的 Buffer ，其實也可以使用 new 和 var 來宣告

// 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.
// 通過位元組切片建立一個 buffer ，位元組切片會保留初始值
// 在渴望容量但是長度為 0？的情況下
// 也可以當作核心的 buffer 來寫入
func NewBuffer(buf []byte) *Buffer { return &Buffer{buf: buf} }

同時也有通過 string 型別的實現
**func NewBufferString(s string) *Buffer {return &Buffer{buf: []byte(s)}}** 

總結

緩衝區，實現了大小控制，位元組切片和 string 型別的讀寫，同時還對情況進行了優化，比如存在 bootstrap，比如 grow 函式中的多次檢定。適合多讀精讀來學習