route/vendor/github.com/xtaci/kcp-go/fec.go

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package kcp
import (
"encoding/binary"
"sync/atomic"
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"github.com/klauspost/reedsolomon"
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)
const (
fecHeaderSize = 6
fecHeaderSizePlus2 = fecHeaderSize + 2 // plus 2B data size
typeData = 0xf1
typeFEC = 0xf2
)
type (
// fecPacket is a decoded FEC packet
fecPacket struct {
seqid uint32
flag uint16
data []byte
}
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// fecDecoder for decoding incoming packets
fecDecoder struct {
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rxlimit int // queue size limit
dataShards int
parityShards int
shardSize int
rx []fecPacket // ordered receive queue
// caches
decodeCache [][]byte
flagCache []bool
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// zeros
zeros []byte
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// RS decoder
codec reedsolomon.Encoder
}
)
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func newFECDecoder(rxlimit, dataShards, parityShards int) *fecDecoder {
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if dataShards <= 0 || parityShards <= 0 {
return nil
}
if rxlimit < dataShards+parityShards {
return nil
}
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dec := new(fecDecoder)
dec.rxlimit = rxlimit
dec.dataShards = dataShards
dec.parityShards = parityShards
dec.shardSize = dataShards + parityShards
codec, err := reedsolomon.New(dataShards, parityShards)
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if err != nil {
return nil
}
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dec.codec = codec
dec.decodeCache = make([][]byte, dec.shardSize)
dec.flagCache = make([]bool, dec.shardSize)
dec.zeros = make([]byte, mtuLimit)
return dec
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}
// decodeBytes a fec packet
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func (dec *fecDecoder) decodeBytes(data []byte) fecPacket {
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var pkt fecPacket
pkt.seqid = binary.LittleEndian.Uint32(data)
pkt.flag = binary.LittleEndian.Uint16(data[4:])
// allocate memory & copy
buf := xmitBuf.Get().([]byte)[:len(data)-6]
copy(buf, data[6:])
pkt.data = buf
return pkt
}
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// decode a fec packet
func (dec *fecDecoder) decode(pkt fecPacket) (recovered [][]byte) {
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// insertion
n := len(dec.rx) - 1
insertIdx := 0
for i := n; i >= 0; i-- {
if pkt.seqid == dec.rx[i].seqid { // de-duplicate
xmitBuf.Put(pkt.data)
return nil
} else if _itimediff(pkt.seqid, dec.rx[i].seqid) > 0 { // insertion
insertIdx = i + 1
break
}
}
// insert into ordered rx queue
if insertIdx == n+1 {
dec.rx = append(dec.rx, pkt)
} else {
dec.rx = append(dec.rx, fecPacket{})
copy(dec.rx[insertIdx+1:], dec.rx[insertIdx:]) // shift right
dec.rx[insertIdx] = pkt
}
// shard range for current packet
shardBegin := pkt.seqid - pkt.seqid%uint32(dec.shardSize)
shardEnd := shardBegin + uint32(dec.shardSize) - 1
// max search range in ordered queue for current shard
searchBegin := insertIdx - int(pkt.seqid%uint32(dec.shardSize))
if searchBegin < 0 {
searchBegin = 0
}
searchEnd := searchBegin + dec.shardSize - 1
if searchEnd >= len(dec.rx) {
searchEnd = len(dec.rx) - 1
}
// re-construct datashards
if searchEnd-searchBegin+1 >= dec.dataShards {
var numshard, numDataShard, first, maxlen int
// zero cache
shards := dec.decodeCache
shardsflag := dec.flagCache
for k := range dec.decodeCache {
shards[k] = nil
shardsflag[k] = false
}
// shard assembly
for i := searchBegin; i <= searchEnd; i++ {
seqid := dec.rx[i].seqid
if _itimediff(seqid, shardEnd) > 0 {
break
} else if _itimediff(seqid, shardBegin) >= 0 {
shards[seqid%uint32(dec.shardSize)] = dec.rx[i].data
shardsflag[seqid%uint32(dec.shardSize)] = true
numshard++
if dec.rx[i].flag == typeData {
numDataShard++
}
if numshard == 1 {
first = i
}
if len(dec.rx[i].data) > maxlen {
maxlen = len(dec.rx[i].data)
}
}
}
if numDataShard == dec.dataShards {
// case 1: no lost data shards
dec.rx = dec.freeRange(first, numshard, dec.rx)
} else if numshard >= dec.dataShards {
// case 2: data shard lost, but recoverable from parity shard
for k := range shards {
if shards[k] != nil {
dlen := len(shards[k])
shards[k] = shards[k][:maxlen]
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copy(shards[k][dlen:], dec.zeros)
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}
}
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if err := dec.codec.ReconstructData(shards); err == nil {
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for k := range shards[:dec.dataShards] {
if !shardsflag[k] {
recovered = append(recovered, shards[k])
}
}
}
dec.rx = dec.freeRange(first, numshard, dec.rx)
}
}
// keep rxlimit
if len(dec.rx) > dec.rxlimit {
if dec.rx[0].flag == typeData { // record unrecoverable data
atomic.AddUint64(&DefaultSnmp.FECShortShards, 1)
}
dec.rx = dec.freeRange(0, 1, dec.rx)
}
return
}
// free a range of fecPacket, and zero for GC recycling
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func (dec *fecDecoder) freeRange(first, n int, q []fecPacket) []fecPacket {
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for i := first; i < first+n; i++ { // free
xmitBuf.Put(q[i].data)
}
copy(q[first:], q[first+n:])
for i := 0; i < n; i++ { // dereference data
q[len(q)-1-i].data = nil
}
return q[:len(q)-n]
}
type (
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// fecEncoder for encoding outgoing packets
fecEncoder struct {
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dataShards int
parityShards int
shardSize int
paws uint32 // Protect Against Wrapped Sequence numbers
next uint32 // next seqid
shardCount int // count the number of datashards collected
maxSize int // record maximum data length in datashard
headerOffset int // FEC header offset
payloadOffset int // FEC payload offset
// caches
shardCache [][]byte
encodeCache [][]byte
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// zeros
zeros []byte
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// RS encoder
codec reedsolomon.Encoder
}
)
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func newFECEncoder(dataShards, parityShards, offset int) *fecEncoder {
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if dataShards <= 0 || parityShards <= 0 {
return nil
}
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enc := new(fecEncoder)
enc.dataShards = dataShards
enc.parityShards = parityShards
enc.shardSize = dataShards + parityShards
enc.paws = (0xffffffff/uint32(enc.shardSize) - 1) * uint32(enc.shardSize)
enc.headerOffset = offset
enc.payloadOffset = enc.headerOffset + fecHeaderSize
codec, err := reedsolomon.New(dataShards, parityShards)
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if err != nil {
return nil
}
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enc.codec = codec
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// caches
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enc.encodeCache = make([][]byte, enc.shardSize)
enc.shardCache = make([][]byte, enc.shardSize)
for k := range enc.shardCache {
enc.shardCache[k] = make([]byte, mtuLimit)
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}
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enc.zeros = make([]byte, mtuLimit)
return enc
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}
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// encode the packet, output parity shards if we have enough datashards
// the content of returned parityshards will change in next encode
func (enc *fecEncoder) encode(b []byte) (ps [][]byte) {
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enc.markData(b[enc.headerOffset:])
binary.LittleEndian.PutUint16(b[enc.payloadOffset:], uint16(len(b[enc.payloadOffset:])))
// copy data to fec datashards
sz := len(b)
enc.shardCache[enc.shardCount] = enc.shardCache[enc.shardCount][:sz]
copy(enc.shardCache[enc.shardCount], b)
enc.shardCount++
// record max datashard length
if sz > enc.maxSize {
enc.maxSize = sz
}
// calculate Reed-Solomon Erasure Code
if enc.shardCount == enc.dataShards {
// bzero each datashard's tail
for i := 0; i < enc.dataShards; i++ {
shard := enc.shardCache[i]
slen := len(shard)
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copy(shard[slen:enc.maxSize], enc.zeros)
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}
// construct equal-sized slice with stripped header
cache := enc.encodeCache
for k := range cache {
cache[k] = enc.shardCache[k][enc.payloadOffset:enc.maxSize]
}
// rs encode
if err := enc.codec.Encode(cache); err == nil {
ps = enc.shardCache[enc.dataShards:]
for k := range ps {
enc.markFEC(ps[k][enc.headerOffset:])
ps[k] = ps[k][:enc.maxSize]
}
}
// reset counters to zero
enc.shardCount = 0
enc.maxSize = 0
}
return
}
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func (enc *fecEncoder) markData(data []byte) {
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binary.LittleEndian.PutUint32(data, enc.next)
binary.LittleEndian.PutUint16(data[4:], typeData)
enc.next++
}
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func (enc *fecEncoder) markFEC(data []byte) {
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binary.LittleEndian.PutUint32(data, enc.next)
binary.LittleEndian.PutUint16(data[4:], typeFEC)
enc.next = (enc.next + 1) % enc.paws
}