Golang bytes包Buffer源码分析

浅浅的花香味﹌ 2022-11-25 11:47 113阅读 0赞

bytes/buffer.go

Buffer提供了一个可扩展的字节缓冲区，底层是对\[\]byte进行封装，提供读写的功能。

**结构体**

type Buffer struct {
       buf       []byte   // contents are the bytes buf[off : len(buf)]  缓冲区
       off       int      // read at &buf[off], write at &buf[len(buf)]  读写的索引值，指针偏移量
       bootstrap [64]byte // memory to hold first slice; helps small buffers avoid allocation.  存储第一切片，避免小缓冲区分配
       lastRead  readOp   // last read operation, so that Unread* can work correctly.上次读的操作，用于UnReadRune等撤回到上次读操作之前的状态，所以记录最新一次的操作，自动扩容使用
    
       // FIXME: it would be advisable to align Buffer to cachelines to avoid false
       // sharing.
    }

Buffer是一个可变大小的字节缓冲区，具有Read和Write方法。 Buffer的零值是一个可以使用的空缓冲区。

**初始化**

func NewBuffer(buf []byte) *Buffer { return &Buffer{buf: buf} }

根据buf初始化buffer缓冲区

func NewBufferString(s string) *Buffer {
       return &Buffer{buf: []byte(s)}
    }

根据字符串初始化并返回buffer

readOp常量描述了对缓冲区执行的最后一个操作

type readOp int8
    
    // Don't use iota for these, as the values need to correspond with the
    // names and comments, which is easier to see when being explicit.
    const (
       opRead      readOp = -1 // Any other read operation.  任何其他操作
       opInvalid   readOp = 0  // Non-read operation.    没有读操作
       opReadRune1 readOp = 1  // Read rune of size 1.  读取大小为1的字符 （由于UTF-8字符可能包含1-4个字节）
       opReadRune2 readOp = 2  // Read rune of size 2.  读取大小为2的字符
       opReadRune3 readOp = 3  // Read rune of size 3.  读取大小为3的字符
       opReadRune4 readOp = 4  // Read rune of size 4.  读取大小为4的字符
    )

func (b *Buffer) Bytes() []byte { return b.buf[b.off:] }

作用：用于获取未读部分的buffer数据  
Bytes返回一个长度为b.Len()的片段，其中包含缓冲区的未读部分。  
切片仅在下一次缓冲区修改之前有效（即，直到下一次调用Read，Write，Reset或Truncate之类的方法）。

func (b *Buffer) String() string {
       if b == nil {
          // Special case, useful in debugging.
          return "<nil>"
       }
       return string(b.buf[b.off:])
    }

作用：返回缓冲区中未读部分的字符串形式

// empty returns whether the unread portion of the buffer is empty.
    func (b *Buffer) empty() bool { return len(b.buf) <= b.off }

作用：返回缓冲区的未读部分是否为空

//Len returns the number of bytes of the unread portion of the buffer;
    func (b *Buffer) Len() int { return len(b.buf) - b.off }

作用：返回缓冲区未读部分的字节数;

// Cap returns the capacity of the buffer's underlying byte slice, that is, the
    // total space allocated for the buffer's data.
    func (b *Buffer) Cap() int { return cap(b.buf) }

作用：返回缓冲区切片的容量，即为缓冲区数据分配的总空间。

// Truncate discards all but the first n unread bytes from the buffer
    // but continues to use the same allocated storage.
    // It panics if n is negative or greater than the length of the buffer.
    func (b *Buffer) Truncate(n int) {
       if n == 0 {
          b.Reset()
          return
       }
       b.lastRead = opInvalid
       if n < 0 || n > b.Len() {
          panic("bytes.Buffer: truncation out of range")
       }
       b.buf = b.buf[:b.off+n]
    }

作用：从缓冲区中丢弃除前n个未读字节以外的所有字节，但继续使用相同的已分配存储。(已读的数据不会删除)  
如果n为负或大于缓冲区的长度，则会发生panic。

// Reset resets the buffer to be empty,
    // but it retains the underlying storage for use by future writes.
    // Reset is the same as Truncate(0).
    func (b *Buffer) Reset() {
       b.buf = b.buf[:0]
       b.off = 0
       b.lastRead = opInvalid
    }

重置操作

将缓冲区重置为空，但它会保留底层存储以供将来的写入使用。（清空数据，cap不变）  
offset 偏移量置为0  
lastRead置为未读取

// tryGrowByReslice is a inlineable version of grow for the fast-case where the
    // internal buffer only needs to be resliced.
    // It returns the index where bytes should be written and whether it succeeded.
    //适用于内部缓冲区只需要复制的快速情况。
    // 它返回开始写入字节的索引以及是否成功。
    //判断现有的容量是否需要扩容  （ len(b.off)+n < c ）
    func (b *Buffer) tryGrowByReslice(n int) (int, bool) {
    	if l := len(b.buf); n <= cap(b.buf)-l {
    		b.buf = b.buf[:l+n]
    		return l, true
    	}
       return 0,false
    }

// 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()
       // If buffer is empty, reset to recover space.
       if m == 0 && b.off != 0 {
          b.Reset()
       }
       // Try to grow by means of a reslice.
       if i, ok := b.tryGrowByReslice(n); ok {
          return i
       }
       // Check if we can make use of bootstrap array.
       if b.buf == nil && n <= len(b.bootstrap) {
          b.buf = b.bootstrap[:n]
          return 0
       }
       c := cap(b.buf)
       if n <= c/2-m {
          // We can slide things down instead of allocating a new
          // slice. We only need m+n <= c to slide, but
          // we instead let capacity get twice as large so we
          // don't spend all our time copying.
          copy(b.buf, b.buf[b.off:])
       } else if c > maxInt-c-n {
          //panic(ErrTooLarge)
       } else {
          // Not enough space anywhere, we need to allocate.
          buf := makeSlice(2*c + n)
          copy(buf, b.buf[b.off:])
          b.buf = buf
       }
       // Restore b.off and len(b.buf).
       b.off = 0
       b.buf = b.buf[:m+n]
       return m
    }

作用：用于缓冲区扩展n个字节的长度。 它返回应写入字节的索引。  
扩容规则如下：

1.  len(n.buf) + n <= cap(b.buf)   直接返回 b.buf\[:len(b.buf) + n\]  
2.  b.buf == nil && n <= len(b.bootstrap) 直接返回 b.bootstrap\[:n\]  
3.   n + m <= c/2  (m为未读的长度，c是容量),则进行滑动 copy(b.buf, b.buf\[b.off:\]) ，将已读的部分移除  
4. buf := makeSlice(2\*c + n)，容量编程2\*c + n，即时原来容量的2倍+n

// makeSlice allocates a slice of size n. If the allocation fails, it panics
    // with ErrTooLarge.
    func makeSlice(n int) []byte {
       // If the make fails, give a known error.
       defer func() {
          if recover() != nil {
             panic(ErrTooLarge)
          }
       }()
       return make([]byte, n)
    }

分配一个切片大小为n的切片。

// 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[:m]
    }

如果需要，Grow会增加缓冲区的容量，以保证另外n个字节的空间。  
调用grow()进行扩容

// 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
       //ok是否需要扩容，m是开始写的位置索引
       m, ok := b.tryGrowByReslice(len(p))
       if !ok {
          //扩容，m是开始写的位置
          m = b.grow(len(p))
       }
       return copy(b.buf[m:], p), nil
    }

作用：将切片p的内容写到buffer缓冲区

// 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.
    func (b *Buffer) WriteString(s string) (n int, err error) {
       b.lastRead = opInvalid
       m, ok := b.tryGrowByReslice(len(s))
       if !ok {
          m = b.grow(len(s))
       }
       return copy(b.buf[m:], s), nil
    }

作用：将字符串s写入到缓冲区buffer

// MinRead is the minimum slice size passed to a Read call by
    // Buffer.ReadFrom. As long as the Buffer has at least MinRead bytes beyond
    // what is required to hold the contents of r, ReadFrom will not grow the
    // underlying buffer.
    //MinRead是Buffer.ReadFrom传递给Read调用的最小切片大小。
    // 只要Buffer至少具有保存r内容所需的MinRead字节，ReadFrom就不会增长底层缓冲区。
    const MinRead = 512
    
    
    // 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
       for {
          i := b.grow(MinRead)
          m, e := r.Read(b.buf[i:cap(b.buf)])
          if m < 0 {
             panic(errNegativeRead)
          }
    
          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
          }
       }
    }

作用：ReadFrom从r读取数据直到EOF并将其附加到缓冲区，根据需要自动扩容缓冲区。返回值n是读取的字节数。 除了在读取期间遇到的io.EOF之外的任何错误也会被返回。如果缓冲区变得太大，ReadFrom可能会触发ErrTooLarge panic

// 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 {
          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.Reset()
       return n, nil
    }

WriteTo方法将缓冲区的数据写入w，直到缓冲区耗尽或发生错误。返回值n是写入的字节数; 它总是适合int，但它是int64以匹配io.WriterTo接口。写入期间遇到的任何错误也会返回。

// WriteByte appends the byte c to the buffer, growing the buffer as needed.
    // The returned error is always nil, but is included to match bufio.Writer's
    // WriteByte. If the buffer becomes too large, WriteByte will panic with
    // ErrTooLarge.
    func (b *Buffer) WriteByte(c byte) error {
       b.lastRead = opInvalid
       m, ok := b.tryGrowByReslice(1)
       if !ok {
          m = b.grow(1)
       }
       b.buf[m] = c
       return nil
    }

WriteByte将字节c附加到缓冲区，根据需要自动扩容缓冲区。返回的错误总是为nil，如果缓冲区变得太大超过了ErrTooLarge会发生panic。

// 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) {
       //r为一个字节，则调用WriteByte
       if r < utf8.RuneSelf {
          b.WriteByte(byte(r))
          return 1, nil
       }
       //判断是否需要扩容，如果需要则进行扩容，r的最长字节数可能是utf8.UTFMax(即4个字节)
       b.lastRead = opInvalid
       m, ok := b.tryGrowByReslice(utf8.UTFMax)
       if !ok {
          m = b.grow(utf8.UTFMax)
       }
       //将r写入到缓冲区中
       n = utf8.EncodeRune(b.buf[m:m+utf8.UTFMax], r)
       b.buf = b.buf[:m+n]
       return n, nil
    }

WriteRune将Unicode字符r的UTF-8编码附加到缓冲区，返回其长度和错误，  
如果缓冲区变得太大超过了ErrTooLarge会发生panic。

// Read reads the next len(p) bytes from the buffer or until the buffer
    // is drained. The return value n is the number of bytes read. If the
    // buffer has no data to return, err is io.EOF (unless len(p) is zero);
    // otherwise it is nil.
    func (b *Buffer) Read(p []byte) (n int, err error) {
       b.lastRead = opInvalid
       //判断缓冲区内的数据是否为空
       if b.empty() {
          // Buffer is empty, reset to recover space.
          b.Reset()
          if len(p) == 0 {
             return 0, nil
          }
          return 0, io.EOF
       }
       //将缓冲区的数据cope到p
       n = copy(p, b.buf[b.off:])
       b.off += n
       if n > 0 {
          b.lastRead = opRead
       }
       return n, nil
    }

作用：从缓冲区读取len(p) 字节到p切片中，如果缓冲区中剩余的元素个数小于len(p)则将缓冲区中剩余的元素读取到p切片中。 返回值n是读取的字节数。如果缓冲区中没有数据，则err为io.EOF（除非len（p）为零）; 否则它是nil。

// Next returns a slice containing the next n bytes from the buffer,
    // advancing the buffer as if the bytes had been returned by Read.
    // If there are fewer than n bytes in the buffer, Next returns the entire buffer.
    // The slice is only valid until the next call to a read or write method.
    func (b *Buffer) Next(n int) []byte {
       b.lastRead = opInvalid
       //获取缓冲区buffer中未读的长度m
       m := b.Len()
       if n > m {
          n = m
       }
    
       data := b.buf[b.off : b.off+n]
       b.off += n
       if n > 0 {
          b.lastRead = opRead
       }
       return data
    }

Next用于获取缓冲区中接下来n个未读字节，并移动缓冲区的指针偏移量off，就好像Read读取数据一样。如果缓冲区中的字节少于n个，则Next返回整个缓冲区。 切片仅在下次调用read或write方法之前有效，Next也属于读操作，因为指针偏移量会增加

// ReadByte reads and returns the next byte from the buffer.
    // If no byte is available, it returns error io.EOF.
    func (b *Buffer) ReadByte() (byte, error) {
       //如果为空，则复位，返回io.EOF
       if b.empty() {
          // Buffer is empty, reset to recover space.
          b.Reset()
          return 0, io.EOF
       }
       //获取接下来一个未读字节，偏移量索引+1，标记上次读操作是opRead操作
       c := b.buf[b.off]
       b.off++
       b.lastRead = opRead
       return c, nil
    }

读取缓冲区下一个未读字节，如果该缓冲区没有可读的字节，则复位缓冲区，返回io.EOF

// 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) {
       //判断是否为空
       if b.empty() {
          // Buffer is empty, reset to recover space.
          b.Reset()
          return 0, 0, io.EOF
       }
       //如果未读的接下来一个字符是ASCII，
       c := b.buf[b.off]
       if c < utf8.RuneSelf {
          b.off++
          b.lastRead = opReadRune1
          return rune(c), 1, nil
       }
       //读取接下来一个utf-8字符，r是该字符，n是字符长度
       r, n := utf8.DecodeRune(b.buf[b.off:])
       b.off += n
       b.lastRead = readOp(n)
       return r, n, nil
    }

读取一个utf-8字符，返回f该字符和字符的长度size

// UnreadRune unreads the last rune returned by ReadRune.
    // If the most recent read or write operation on the buffer was
    // not a successful ReadRune, UnreadRune returns an error.  (In this regard
    // it is stricter than UnreadByte, which will unread the last byte
    // from any read operation.)
    func (b *Buffer) UnreadRune() error {
       //如果上次操作不是ReadRune()，则返回错误
       if b.lastRead <= opInvalid {
          return errors.New("bytes.Buffer: UnreadRune: previous operation was not a successful ReadRune")
       }
    
       //偏移量off回到ReadRune之前的状态
       if b.off >= int(b.lastRead) {
          b.off -= int(b.lastRead)
       }
       b.lastRead = opInvalid
       return nil
    }

UnreadRune方法用于撤回上次ReadRune操作的数据，如果上次操作不是ReadRune(),则返回错误

// UnreadByte unreads the last byte returned by the most recent successful
    // read operation that read at least one byte. If a write has happened since
    // the last read, if the last read returned an error, or if the read read zero
    // bytes, UnreadByte returns an error.
    func (b *Buffer) UnreadByte() error {
       if b.lastRead == opInvalid {
          return errors.New("bytes.Buffer: UnreadByte: previous operation was not a successful read")
       }
       b.lastRead = opInvalid
       if b.off > 0 {
          b.off--
       }
       return nil
    }

UnreadByte方法用于撤销上次的ReadByte数据，如果上次不是ReadByte()则返回错误

// 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 line, err
    }

ReadBytes方法读取直到第一次出现delim字节，如果delim不在剩余的buffer中，则读完

// 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 {
          //len(b.buf)是缓冲区中数据的长度
          end = len(b.buf)
          err = io.EOF
       }
       line = b.buf[b.off:end]
       b.off = end
       b.lastRead = opRead
       return line, err
    }

读取缓冲区剩余的数据，直到delim字节为止  
如果delim字节找不到，将剩余所有的数据读完  
和ReadBytes不同的是readSlice直接返回buffer数据的引用

// ReadString reads until the first occurrence of delim in the input,
    // returning a string containing the data up to and including the delimiter.
    // If ReadString encounters an error before finding a delimiter,
    // it returns the data read before the error and the error itself (often io.EOF).
    // ReadString returns err != nil if and only if the returned data does not end
    // in delim.
    func (b *Buffer) ReadString(delim byte) (line string, err error) {
       slice, err := b.readSlice(delim)
       return string(slice), err
    }

ReadString方法和ReadBytes功能相似，只是返回字符串