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// 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
}
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