prometheus/chunks/xor.go

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package chunks
import (
"encoding/binary"
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"math"
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bits "github.com/dgryski/go-bits"
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)
// XORChunk holds XOR encoded sample data.
type XORChunk struct {
b *bstream
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num uint16
sz int
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}
// NewXORChunk returns a new chunk with XOR encoding of the given size.
func NewXORChunk(size int) *XORChunk {
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b := make([]byte, 3, 128)
b[0] = byte(EncXOR)
return &XORChunk{
b: &bstream{stream: b, count: 0},
sz: size,
num: 0,
}
}
// Bytes returns the underlying byte slice of the chunk.
func (c *XORChunk) Bytes() []byte {
b := c.b.bytes()
// Lazily populate length bytes probably not necessary to have the
// cache value in struct.
binary.LittleEndian.PutUint16(b[1:3], c.num)
return b
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}
// Appender implements the Chunk interface.
func (c *XORChunk) Appender() (Appender, error) {
it := c.iterator()
// To get an appender we must know the state it would have if we had
// appended all existing data from scratch.
// We iterate through the end and populate via the iterator's state.
for it.Next() {
}
if err := it.Err(); err != nil {
return nil, err
}
a := &xorAppender{
c: c,
b: c.b,
t: it.t,
v: it.val,
tDelta: it.tDelta,
leading: it.leading,
trailing: it.trailing,
}
if c.num == 0 {
a.leading = 0xff
}
return a, nil
}
func (c *XORChunk) iterator() *xorIterator {
// Should iterators guarantee to act on a copy of the data so it doesn't lock append?
// When using striped locks to guard access to chunks, probably yes.
// Could only copy data if the chunk is not completed yet.
return &xorIterator{
br: newBReader(c.b.bytes()[3:]),
numTotal: c.num,
}
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}
// Iterator implements the Chunk interface.
func (c *XORChunk) Iterator() Iterator {
return fancyIterator{c.iterator()}
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}
type xorAppender struct {
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c *XORChunk
b *bstream
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t int64
v float64
tDelta uint64
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leading uint8
trailing uint8
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}
func (a *xorAppender) Append(t int64, v float64) error {
var tDelta uint64
l := len(a.b.bytes())
if a.c.num == 0 {
buf := make([]byte, binary.MaxVarintLen64)
for _, b := range buf[:binary.PutVarint(buf, t)] {
a.b.writeByte(b)
}
a.b.writeBits(math.Float64bits(v), 64)
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} else if a.c.num == 1 {
tDelta = uint64(t - a.t)
buf := make([]byte, binary.MaxVarintLen64)
for _, b := range buf[:binary.PutUvarint(buf, tDelta)] {
a.b.writeByte(b)
}
a.writeVDelta(v)
} else {
tDelta = uint64(t - a.t)
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dod := int64(tDelta - a.tDelta)
// Gorilla has a max resolution of seconds, Prometheus milliseconds.
// Thus we use higher value range steps with larger bit size.
switch {
case dod == 0:
a.b.writeBit(zero)
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case bitRange(dod, 14):
a.b.writeBits(0x02, 2) // '10'
a.b.writeBits(uint64(dod), 14)
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case bitRange(dod, 17):
a.b.writeBits(0x06, 3) // '110'
a.b.writeBits(uint64(dod), 17)
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case bitRange(dod, 20):
a.b.writeBits(0x0e, 4) // '1110'
a.b.writeBits(uint64(dod), 20)
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default:
a.b.writeBits(0x0f, 4) // '1111'
a.b.writeBits(uint64(dod), 64)
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}
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a.writeVDelta(v)
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}
if len(a.b.bytes()) > a.c.sz {
// If the appended data exceeded the size limit, we truncate
// the underlying data slice back to the length we started with.
a.b.stream = a.b.stream[:l]
return ErrChunkFull
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}
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a.t = t
a.v = v
a.c.num++
a.tDelta = tDelta
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return nil
}
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func bitRange(x int64, nbits uint8) bool {
return -((1<<(nbits-1))-1) <= x && x <= 1<<(nbits-1)
}
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func (a *xorAppender) writeVDelta(v float64) {
vDelta := math.Float64bits(v) ^ math.Float64bits(a.v)
if vDelta == 0 {
a.b.writeBit(zero)
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return
}
a.b.writeBit(one)
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leading := uint8(bits.Clz(vDelta))
trailing := uint8(bits.Ctz(vDelta))
// Clamp number of leading zeros to avoid overflow when encoding.
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if leading >= 32 {
leading = 31
}
if a.leading != 0xff && leading >= a.leading && trailing >= a.trailing {
a.b.writeBit(zero)
a.b.writeBits(vDelta>>a.trailing, 64-int(a.leading)-int(a.trailing))
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} else {
a.leading, a.trailing = leading, trailing
a.b.writeBit(one)
a.b.writeBits(uint64(leading), 5)
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// Note that if leading == trailing == 0, then sigbits == 64. But that value doesn't actually fit into the 6 bits we have.
// Luckily, we never need to encode 0 significant bits, since that would put us in the other case (vdelta == 0).
// So instead we write out a 0 and adjust it back to 64 on unpacking.
sigbits := 64 - leading - trailing
a.b.writeBits(uint64(sigbits), 6)
a.b.writeBits(vDelta>>trailing, int(sigbits))
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}
}
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type xorIterator struct {
br *bstream
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numTotal uint16
numRead uint16
t int64
val float64
leading uint8
trailing uint8
tDelta uint64
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err error
}
func (it *xorIterator) Values() (int64, float64) {
return it.t, it.val
}
func (it *xorIterator) Err() error {
return it.err
}
func (it *xorIterator) Next() bool {
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if it.err != nil || it.numRead == it.numTotal {
return false
}
if it.numRead == 0 {
t, err := binary.ReadVarint(it.br)
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if err != nil {
it.err = err
return false
}
v, err := it.br.readBits(64)
if err != nil {
it.err = err
return false
}
it.t = int64(t)
it.val = math.Float64frombits(v)
it.numRead++
return true
}
if it.numRead == 1 {
tDelta, err := binary.ReadUvarint(it.br)
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if err != nil {
it.err = err
return false
}
it.tDelta = tDelta
it.t = it.t + int64(it.tDelta)
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return it.readValue()
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}
var d byte
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// read delta-of-delta
for i := 0; i < 4; i++ {
d <<= 1
bit, err := it.br.readBit()
if err != nil {
it.err = err
return false
}
if bit == zero {
break
}
d |= 1
}
var sz uint8
var dod int64
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switch d {
case 0x00:
// dod == 0
case 0x02:
sz = 14
case 0x06:
sz = 17
case 0x0e:
sz = 20
case 0x0f:
bits, err := it.br.readBits(64)
if err != nil {
it.err = err
return false
}
dod = int64(bits)
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}
if sz != 0 {
bits, err := it.br.readBits(int(sz))
if err != nil {
it.err = err
return false
}
if bits > (1 << (sz - 1)) {
// or something
bits = bits - (1 << sz)
}
dod = int64(bits)
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}
it.tDelta = uint64(int64(it.tDelta) + dod)
it.t = it.t + int64(it.tDelta)
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return it.readValue()
}
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func (it *xorIterator) readValue() bool {
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bit, err := it.br.readBit()
if err != nil {
it.err = err
return false
}
if bit == zero {
// it.val = it.val
} else {
bit, err := it.br.readBit()
if err != nil {
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it.err = err
return false
}
if bit == zero {
// reuse leading/trailing zero bits
// it.leading, it.trailing = it.leading, it.trailing
} else {
bits, err := it.br.readBits(5)
if err != nil {
it.err = err
return false
}
it.leading = uint8(bits)
bits, err = it.br.readBits(6)
if err != nil {
it.err = err
return false
}
mbits := uint8(bits)
// 0 significant bits here means we overflowed and we actually need 64; see comment in encoder
if mbits == 0 {
mbits = 64
}
it.trailing = 64 - it.leading - mbits
}
mbits := int(64 - it.leading - it.trailing)
bits, err := it.br.readBits(mbits)
if err != nil {
it.err = err
return false
}
vbits := math.Float64bits(it.val)
vbits ^= (bits << it.trailing)
it.val = math.Float64frombits(vbits)
}
it.numRead++
return true
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}