1 files changed, 53 insertions, 0 deletions
diff --git a/pkg/tcpip/buffer/prependable.go b/pkg/tcpip/buffer/prependable.go
new file mode 100644
index 000000000..fd84585f9
--- /dev/null
+++ b/pkg/tcpip/buffer/prependable.go
@@ -0,0 +1,53 @@
+// 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
+}