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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 bytes
// Simple byte buffer for marshaling data.
import (
"errors"
"io"
"unicode/utf8"
)
// smallBufferSize is an initial allocation minimal capacity.
const smallBufferSize = 64
// 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)]
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. opReadRuneX constants are chosen such that
// converted to int they correspond to the rune size that was read.
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.
opInvalid readOp = 0 // Non-read operation.
opReadRune1 readOp = 1 // Read rune of size 1.
opReadRune2 readOp = 2 // Read rune of size 2.
opReadRune3 readOp = 3 // Read rune of size 3.
opReadRune4 readOp = 4 // Read rune of size 4.
)
// ErrTooLarge is passed to panic if memory cannot be allocated to store data in a buffer.
var ErrTooLarge = errors.New("bytes.Buffer: too large")
var errNegativeRead = errors.New("bytes.Buffer: reader returned negative count from Read")
const maxInt = int(^uint(0) >> 1)
// Bytes returns a slice of length b.Len() holding the unread portion of the buffer.
// The slice is valid for use only until the next buffer modification (that is,
// only until the next call to a method like Read, Write, Reset, or Truncate).
// The slice aliases the buffer content at least until the next buffer modification,
// so immediate changes to the slice will affect the result of future reads.
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>".
//
// To build strings more efficiently, see the strings.Builder type.
func (b *Buffer) String() string {
if b == nil {
// Special case, useful in debugging.
return "<nil>"
}
return string(b.buf[b.off:])
}
// empty reports whether the unread portion of the buffer is empty.
func (b *Buffer) empty() bool { return len(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 }
// 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) }
// 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]
}
// 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
}
// 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.
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
}
if b.buf == nil && n <= smallBufferSize {
b.buf = make([]byte, n, smallBufferSize)
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 {
// Add b.off to account for b.buf[:b.off] being sliced off the front.
b.buf = growSlice(b.buf[b.off:], b.off+n)
}
// Restore b.off and len(b.buf).
b.off = 0
b.buf = b.buf[:m+n]
return 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[: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, ok := b.tryGrowByReslice(len(p))
if !ok {
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, ok := b.tryGrowByReslice(len(s))
if !ok {
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
for {
i := b.grow(MinRead)
b.buf = b.buf[:i]
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
}
}
}
// growSlice grows b by n, preserving the original content of b.
// If the allocation fails, it panics with ErrTooLarge.
func growSlice(b []byte, n int) []byte {
defer func() {
if recover() != nil {
panic(ErrTooLarge)
}
}()
// TODO(http://golang.org/issue/51462): We should rely on the append-make
// pattern so that the compiler can call runtime.growslice. For example:
// return append(b, make([]byte, n)...)
// This avoids unnecessary zero-ing of the first len(b) bytes of the
// allocated slice, but this pattern causes b to escape onto the heap.
//
// Instead use the append-make pattern with a nil slice to ensure that
// we allocate buffers rounded up to the closest size class.
c := len(b) + n // ensure enough space for n elements
if c < 2*cap(b) {
// The growth rate has historically always been 2x. In the future,
// we could rely purely on append to determine the growth rate.
c = 2 * cap(b)
}
b2 := append([]byte(nil), make([]byte, c)...)
copy(b2, b)
return b2[:len(b)]
}
// 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
}
// 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
}
// 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) {
// Compare as uint32 to correctly handle negative runes.
if uint32(r) < utf8.RuneSelf {
b.WriteByte(byte(r))
return 1, nil
}
b.lastRead = opInvalid
m, ok := b.tryGrowByReslice(utf8.UTFMax)
if !ok {
m = b.grow(utf8.UTFMax)
}
n = utf8.EncodeRune(b.buf[m:m+utf8.UTFMax], r)
b.buf = b.buf[:m+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.empty() {
// Buffer is empty, reset to recover space.
b.Reset()
if len(p) == 0 {
return 0, nil
}
return 0, io.EOF
}
n = copy(p, b.buf[b.off:])
b.off += n
if n > 0 {
b.lastRead = opRead
}
return n, 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
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
}
// 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) {
if b.empty() {
// Buffer is empty, reset to recover space.
b.Reset()
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) {
if b.empty() {
// Buffer is empty, reset to recover space.
b.Reset()
return 0, 0, io.EOF
}
c := b.buf[b.off]
if c < utf8.RuneSelf {
b.off++
b.lastRead = opReadRune1
return rune(c), 1, nil
}
r, n := utf8.DecodeRune(b.buf[b.off:])
b.off += n
b.lastRead = readOp(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 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 {
if b.lastRead <= opInvalid {
return errors.New("bytes.Buffer: UnreadRune: previous operation was not a successful ReadRune")
}
if b.off >= int(b.lastRead) {
b.off -= int(b.lastRead)
}
b.lastRead = opInvalid
return nil
}
var errUnreadByte = errors.New("bytes.Buffer: UnreadByte: previous operation was not a successful read")
// 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 errUnreadByte
}
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 line, err
}
// 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
}
// 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. 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 set
// the initial size of 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)}
}