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buffer.go
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buffer.go
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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package bufio
// Simple byte buffer for marshaling data.
import (
"bytes"
"errors"
"io"
"unicode/utf8"
)
// 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.
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.
)
// ErrTooLarge is passed to panic if memory cannot be allocated to store data in a buffer.
var ErrTooLarge = errors.New("bytes.Buffer: too large")
// Bytes returns a slice of the contents of the unread portion of the buffer;
// len(b.Bytes()) == b.Len(). If the caller changes the contents of the
// returned slice, the contents of the buffer will change provided there
// are no intervening method calls on the Buffer.
func (b *Buffer) Bytes() []byte { return b.buf[b.off:] }
// String returns the contents of the unread portion of the buffer
// as a string. If the Buffer is a nil pointer, it returns "<nil>".
func (b *Buffer) String() string {
if b == nil {
// Special case, useful in debugging.
return "<nil>"
}
return string(b.buf[b.off:])
}
// Len returns the number of bytes of the unread portion of the buffer;
// b.Len() == len(b.Bytes()).
func (b *Buffer) Len() int { return len(b.buf) - b.off }
// 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()
// If buffer is empty, reset to recover space.
if m == 0 && b.off != 0 {
b.Truncate(0)
}
if len(b.buf)+n > cap(b.buf) {
var buf []byte
if b.buf == nil && n <= len(b.bootstrap) {
buf = b.bootstrap[0:]
} else if m+n <= cap(b.buf)/2 {
// We can slide things down instead of allocating a new
// slice. We only need m+n <= cap(b.buf) 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:])
buf = b.buf[:m]
} else {
// not enough space anywhere
buf = makeSlice(2*cap(b.buf) + n)
copy(buf, b.buf[b.off:])
}
b.buf = buf
b.off = 0
}
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]
}
// 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
}
// 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 := b.grow(len(s))
return copy(b.buf[m:], s), nil
}
// 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.
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
// 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
}
// 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)
}
// 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
}
// 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 := b.grow(1)
b.buf[m] = c
return nil
}
// 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
}
// 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
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
}
// 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
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
}
// Peek returns the next n bytes without advancing the reader. The bytes stop
// being valid at the next read call. If Peek returns fewer than n bytes, it
// also returns an error explaining why the read is short.
func (b *Buffer) Peek(n int) ([]byte, error) {
b.lastRead = opInvalid
m := b.Len()
var err error
if n > m {
err = io.EOF
n = m
}
data := b.buf[b.off : b.off+n]
if n > 0 {
b.lastRead = opRead
}
return data, err
}
// ReadN tries to read exactly n bytes.
// The bytes stop being valid at the next read call.
// If ReadN encounters an error before reading n bytes,
// it returns all the data in the buffer and the error itself (often io.EOF).
// Because the data returned from ReadN will be overwritten
// by the next I/O operation, most clients should use
// ReadBytes or ReadString instead.
func (b *Buffer) ReadN(n int) ([]byte, error) {
b.lastRead = opInvalid
m := b.Len()
var err error
if n > m {
err = io.EOF
n = m
}
data := b.buf[b.off : b.off+n]
b.off += n
if n > 0 {
b.lastRead = opRead
}
return data, err
}
// ReadByte reads and returns the next byte from the buffer.
// If no byte is available, it returns error io.EOF.
func (b *Buffer) ReadByte() (c byte, err error) {
b.lastRead = opInvalid
if b.off >= len(b.buf) {
// Buffer is empty, reset to recover space.
b.Truncate(0)
return 0, io.EOF
}
c = b.buf[b.off]
b.off++
b.lastRead = opRead
return c, nil
}
// 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
}
// UnreadRune unreads the last rune returned by ReadRune.
// If the most recent read or write operation on the buffer was
// not a 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 {
if b.lastRead != opReadRune {
return errors.New("bytes.Buffer: UnreadRune: previous operation was not ReadRune")
}
b.lastRead = opInvalid
if b.off > 0 {
_, n := utf8.DecodeLastRune(b.buf[0:b.off])
b.off -= n
}
return nil
}
// UnreadByte unreads the last byte returned by the most recent
// read operation. If write has happened since the last read, UnreadByte
// returns an error.
func (b *Buffer) UnreadByte() error {
if b.lastRead != opReadRune && b.lastRead != opRead {
return errors.New("bytes.Buffer: UnreadByte: previous operation was not a read")
}
b.lastRead = opInvalid
if b.off > 0 {
b.off--
}
return nil
}
// 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 := bytes.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
}
// 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
}
// NewBuffer creates and initializes a new Buffer using buf as its initial
// contents. It 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.
func NewBuffer(buf []byte) *Buffer { return &Buffer{buf: buf} }
// NewBufferString creates and initializes a new Buffer using string s as its
// initial contents. It is intended to prepare a buffer to read an existing
// string.
//
// In most cases, new(Buffer) (or just declaring a Buffer variable) is
// sufficient to initialize a Buffer.
func NewBufferString(s string) *Buffer {
return &Buffer{buf: []byte(s)}
}