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