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encode.go
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encode.go
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// Copyright 2014 The Go 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 hpack
import (
"io"
)
const (
uint32Max = ^uint32(0)
initialHeaderTableSize = 4096
)
type Encoder struct {
dynTab dynamicTable
// minSize is the minimum table size set by
// SetMaxDynamicTableSize after the previous Header Table Size
// Update.
minSize uint32
// maxSizeLimit is the maximum table size this encoder
// supports. This will protect the encoder from too large
// size.
maxSizeLimit uint32
// tableSizeUpdate indicates whether "Header Table Size
// Update" is required.
tableSizeUpdate bool
w io.Writer
buf []byte
}
// NewEncoder returns a new Encoder which performs HPACK encoding. An
// encoded data is written to w.
func NewEncoder(w io.Writer) *Encoder {
e := &Encoder{
minSize: uint32Max,
maxSizeLimit: initialHeaderTableSize,
tableSizeUpdate: false,
w: w,
}
e.dynTab.setMaxSize(initialHeaderTableSize)
return e
}
// WriteField encodes f into a single Write to e's underlying Writer.
// This function may also produce bytes for "Header Table Size Update"
// if necessary. If produced, it is done before encoding f.
func (e *Encoder) WriteField(f HeaderField) error {
e.buf = e.buf[:0]
if e.tableSizeUpdate {
e.tableSizeUpdate = false
if e.minSize < e.dynTab.maxSize {
e.buf = appendTableSize(e.buf, e.minSize)
}
e.minSize = uint32Max
e.buf = appendTableSize(e.buf, e.dynTab.maxSize)
}
idx, nameValueMatch := e.searchTable(f)
if nameValueMatch {
e.buf = appendIndexed(e.buf, idx)
} else {
indexing := e.shouldIndex(f)
if indexing {
e.dynTab.add(f)
}
if idx == 0 {
e.buf = appendNewName(e.buf, f, indexing)
} else {
e.buf = appendIndexedName(e.buf, f, idx, indexing)
}
}
n, err := e.w.Write(e.buf)
if err == nil && n != len(e.buf) {
err = io.ErrShortWrite
}
return err
}
// searchTable searches f in both stable and dynamic header tables.
// The static header table is searched first. Only when there is no
// exact match for both name and value, the dynamic header table is
// then searched. If there is no match, i is 0. If both name and value
// match, i is the matched index and nameValueMatch becomes true. If
// only name matches, i points to that index and nameValueMatch
// becomes false.
func (e *Encoder) searchTable(f HeaderField) (i uint64, nameValueMatch bool) {
for idx, hf := range staticTable {
if !constantTimeStringCompare(hf.Name, f.Name) {
continue
}
if i == 0 {
i = uint64(idx + 1)
}
if f.Sensitive {
continue
}
if !constantTimeStringCompare(hf.Value, f.Value) {
continue
}
i = uint64(idx + 1)
nameValueMatch = true
return
}
j, nameValueMatch := e.dynTab.search(f)
if nameValueMatch || (i == 0 && j != 0) {
i = j + uint64(len(staticTable))
}
return
}
// SetMaxDynamicTableSize changes the dynamic header table size to v.
// The actual size is bounded by the value passed to
// SetMaxDynamicTableSizeLimit.
func (e *Encoder) SetMaxDynamicTableSize(v uint32) {
if v > e.maxSizeLimit {
v = e.maxSizeLimit
}
if v < e.minSize {
e.minSize = v
}
e.tableSizeUpdate = true
e.dynTab.setMaxSize(v)
}
// SetMaxDynamicTableSizeLimit changes the maximum value that can be
// specified in SetMaxDynamicTableSize to v. By default, it is set to
// 4096, which is the same size of the default dynamic header table
// size described in HPACK specification. If the current maximum
// dynamic header table size is strictly greater than v, "Header Table
// Size Update" will be done in the next WriteField call and the
// maximum dynamic header table size is truncated to v.
func (e *Encoder) SetMaxDynamicTableSizeLimit(v uint32) {
e.maxSizeLimit = v
if e.dynTab.maxSize > v {
e.tableSizeUpdate = true
e.dynTab.setMaxSize(v)
}
}
// shouldIndex reports whether f should be indexed.
func (e *Encoder) shouldIndex(f HeaderField) bool {
return !f.Sensitive && f.Size() <= e.dynTab.maxSize
}
// appendIndexed appends index i, as encoded in "Indexed Header Field"
// representation, to dst and returns the extended buffer.
func appendIndexed(dst []byte, i uint64) []byte {
first := len(dst)
dst = appendVarInt(dst, 7, i)
dst[first] |= 0x80
return dst
}
// appendNewName appends f, as encoded in one of "Literal Header field
// - New Name" representation variants, to dst and returns the
// extended buffer.
//
// If f.Sensitive is true, "Never Indexed" representation is used. If
// f.Sensitive is false and indexing is true, "Inremental Indexing"
// representation is used.
func appendNewName(dst []byte, f HeaderField, indexing bool) []byte {
dst = append(dst, encodeTypeByte(indexing, f.Sensitive))
dst = appendHpackString(dst, f.Name)
return appendHpackString(dst, f.Value)
}
// appendIndexedName appends f and index i referring indexed name
// entry, as encoded in one of "Literal Header field - Indexed Name"
// representation variants, to dst and returns the extended buffer.
//
// If f.Sensitive is true, "Never Indexed" representation is used. If
// f.Sensitive is false and indexing is true, "Incremental Indexing"
// representation is used.
func appendIndexedName(dst []byte, f HeaderField, i uint64, indexing bool) []byte {
first := len(dst)
var n byte
if indexing {
n = 6
} else {
n = 4
}
dst = appendVarInt(dst, n, i)
dst[first] |= encodeTypeByte(indexing, f.Sensitive)
return appendHpackString(dst, f.Value)
}
// appendTableSize appends v, as encoded in "Header Table Size Update"
// representation, to dst and returns the extended buffer.
func appendTableSize(dst []byte, v uint32) []byte {
first := len(dst)
dst = appendVarInt(dst, 5, uint64(v))
dst[first] |= 0x20
return dst
}
// appendVarInt appends i, as encoded in variable integer form using n
// bit prefix, to dst and returns the extended buffer.
//
// See
// http://http2.github.io/http2-spec/compression.html#integer.representation
func appendVarInt(dst []byte, n byte, i uint64) []byte {
k := uint64((1 << n) - 1)
if i < k {
return append(dst, byte(i))
}
dst = append(dst, byte(k))
i -= k
for ; i >= 128; i >>= 7 {
dst = append(dst, byte(0x80|(i&0x7f)))
}
return append(dst, byte(i))
}
// appendHpackString appends s, as encoded in "String Literal"
// representation, to dst and returns the the extended buffer.
//
// s will be encoded in Huffman codes only when it produces strictly
// shorter byte string.
func appendHpackString(dst []byte, s string) []byte {
huffmanLength := HuffmanEncodeLength(s)
if huffmanLength < uint64(len(s)) {
first := len(dst)
dst = appendVarInt(dst, 7, huffmanLength)
dst = AppendHuffmanString(dst, s)
dst[first] |= 0x80
} else {
dst = appendVarInt(dst, 7, uint64(len(s)))
dst = append(dst, s...)
}
return dst
}
// encodeTypeByte returns type byte. If sensitive is true, type byte
// for "Never Indexed" representation is returned. If sensitive is
// false and indexing is true, type byte for "Incremental Indexing"
// representation is returned. Otherwise, type byte for "Without
// Indexing" is returned.
func encodeTypeByte(indexing, sensitive bool) byte {
if sensitive {
return 0x10
}
if indexing {
return 0x40
}
return 0
}