| // Copyright 2016 The Netstack 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 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} |
| } |
| |
| // 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.buf[p.usedIdx:][:size:size] |
| } |
| |
| // View returns a View of the backing buffer that contains all prepended |
| // data so far. |
| func (p *Prependable) View() View { |
| v := p.buf |
| v.TrimFront(p.usedIdx) |
| return v |
| } |
| |
| // UsedBytes returns a slice of the backing buffer that contains all prepended |
| // data so far. |
| func (p *Prependable) UsedBytes() []byte { |
| 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 |
| } |