github.com/lightlus/netstack@v1.2.0/tcpip/buffer/prependable.go (about)

     1  // Copyright 2018 The gVisor Authors.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  package buffer
    16  
    17  // Prependable is a buffer that grows backwards, that is, more data can be
    18  // prepended to it. It is useful when building networking packets, where each
    19  // protocol adds its own headers to the front of the higher-level protocol
    20  // header and payload; for example, TCP would prepend its header to the payload,
    21  // then IP would prepend its own, then ethernet.
    22  type Prependable struct {
    23  	// Buf is the buffer backing the prependable buffer.
    24  	buf View
    25  
    26  	// usedIdx is the index where the used part of the buffer begins.
    27  	usedIdx int
    28  }
    29  
    30  // NewPrependable allocates a new prependable buffer with the given size.
    31  func NewPrependable(size int) Prependable {
    32  	return Prependable{buf: NewView(size), usedIdx: size}
    33  }
    34  
    35  // NewPrependableFromView creates an entirely-used Prependable from a View.
    36  //
    37  // NewPrependableFromView takes ownership of v. Note that since the entire
    38  // prependable is used, further attempts to call Prepend will note that size >
    39  // p.usedIdx and return nil.
    40  func NewPrependableFromView(v View) Prependable {
    41  	return Prependable{buf: v, usedIdx: 0}
    42  }
    43  
    44  // NewEmptyPrependableFromView creates a new prependable buffer from a View.
    45  func NewEmptyPrependableFromView(v View) Prependable {
    46  	return Prependable{buf: v, usedIdx: len(v)}
    47  }
    48  
    49  // View returns a View of the backing buffer that contains all prepended
    50  // data so far.
    51  func (p Prependable) View() View {
    52  	return p.buf[p.usedIdx:]
    53  }
    54  
    55  // UsedLength returns the number of bytes used so far.
    56  func (p Prependable) UsedLength() int {
    57  	return len(p.buf) - p.usedIdx
    58  }
    59  
    60  // AvailableLength returns the number of bytes used so far.
    61  func (p Prependable) AvailableLength() int {
    62  	return p.usedIdx
    63  }
    64  
    65  // TrimBack removes size bytes from the end.
    66  func (p *Prependable) TrimBack(size int) {
    67  	p.buf = p.buf[:len(p.buf)-size]
    68  }
    69  
    70  // Prepend reserves the requested space in front of the buffer, returning a
    71  // slice that represents the reserved space.
    72  func (p *Prependable) Prepend(size int) []byte {
    73  	if size > p.usedIdx {
    74  		return nil
    75  	}
    76  
    77  	p.usedIdx -= size
    78  	return p.View()[:size:size]
    79  }
    80  
    81  // DeepCopy copies p and the bytes backing it.
    82  func (p Prependable) DeepCopy() Prependable {
    83  	p.buf = append(View(nil), p.buf...)
    84  	return p
    85  }