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// Copyright 2018 The gVisor Authors.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
package buffer
// Prependable is a buffer that grows backwards, that is, more data can be
// prepended to it. It is useful when building networking packets, where each
// protocol adds its own headers to the front of the higher-level protocol
// header and payload; for example, TCP would prepend its header to the payload,
// then IP would prepend its own, then ethernet.
type Prependable struct {
// Buf is the buffer backing the prependable buffer.
buf View
// usedIdx is the index where the used part of the buffer begins.
usedIdx int
// NewPrependable allocates a new prependable buffer with the given size.
func NewPrependable(size int) Prependable {
return Prependable{buf: NewView(size), usedIdx: size}
// NewPrependableFromView creates an entirely-used Prependable from a View.
// NewPrependableFromView takes ownership of v. Note that since the entire
// prependable is used, further attempts to call Prepend will note that size >
// p.usedIdx and return nil.
func NewPrependableFromView(v View) Prependable {
return Prependable{buf: v, usedIdx: 0}
// NewEmptyPrependableFromView creates a new prependable buffer from a View.
func NewEmptyPrependableFromView(v View) Prependable {
return Prependable{buf: v, usedIdx: len(v)}
// View returns a View of the backing buffer that contains all prepended
// data so far.
func (p Prependable) View() View {
return p.buf[p.usedIdx:]
// UsedLength returns the number of bytes used so far.
func (p Prependable) UsedLength() int {
return len(p.buf) - p.usedIdx
// AvailableLength returns the number of bytes used so far.
func (p Prependable) AvailableLength() int {
return p.usedIdx
// TrimBack removes size bytes from the end.
func (p *Prependable) TrimBack(size int) {
p.buf = p.buf[:len(p.buf)-size]
// Prepend reserves the requested space in front of the buffer, returning a
// slice that represents the reserved space.
func (p *Prependable) Prepend(size int) []byte {
if size > p.usedIdx {
return nil
p.usedIdx -= size
return p.View()[:size:size]
// DeepCopy copies p and the bytes backing it.
func (p Prependable) DeepCopy() Prependable {
p.buf = append(View(nil), p.buf...)
return p