1st

atree/cursor.go

@@ -0,0 +1,187 @@
+package atree
+
+import (
+	"errors"
+	"fmt"
+
+	octopus "gordenko.dev/dima/qb"
+	"gordenko.dev/dima/qb/bin"
+	"gordenko.dev/dima/qb/enc"
+)
+
+type BackwardCursor struct {
+	metricType            octopus.MetricType
+	fracDigits            byte
+	atree                 *Atree
+	pageNo                uint32
+	pageData              []byte
+	timestampDecompressor octopus.TimestampDecompressor
+	valueDecompressor     octopus.ValueDecompressor
+}
+
+type BackwardCursorOptions struct {
+	MetricType octopus.MetricType
+	FracDigits byte
+	PageNo     uint32
+	PageData   []byte
+	Atree      *Atree
+}
+
+func NewBackwardCursor(opt BackwardCursorOptions) (*BackwardCursor, error) {
+	switch opt.MetricType {
+	case octopus.Instant, octopus.Cumulative:
+		// ok
+	default:
+		return nil, fmt.Errorf("MetricType option has wrong value: %d", opt.MetricType)
+	}
+	if opt.FracDigits > octopus.MaxFracDigits {
+		return nil, errors.New("FracDigits option is required")
+	}
+	if opt.Atree == nil {
+		return nil, errors.New("Atree option is required")
+	}
+	if opt.PageNo == 0 {
+		return nil, errors.New("PageNo option is required")
+	}
+	if len(opt.PageData) == 0 {
+		return nil, errors.New("PageData option is required")
+	}
+
+	s := &BackwardCursor{
+		metricType: opt.MetricType,
+		fracDigits: opt.FracDigits,
+		atree:      opt.Atree,
+		pageNo:     opt.PageNo,
+		pageData:   opt.PageData,
+	}
+	err := s.makeDecompressors()
+	if err != nil {
+		return nil, err
+	}
+	return s, nil
+}
+
+// timestamp, value, done, error
+func (s *BackwardCursor) Prev() (uint32, float64, bool, error) {
+	var (
+		timestamp uint32
+		value     float64
+		done      bool
+		err       error
+	)
+
+	timestamp, done = s.timestampDecompressor.NextValue()
+	if !done {
+		value, done = s.valueDecompressor.NextValue()
+		if done {
+			return 0, 0, false,
+				fmt.Errorf("corrupted data page %d: has timestamp, no value",
+					s.pageNo)
+		}
+		return timestamp, value, false, nil
+	}
+
+	prevPageNo := bin.GetUint32(s.pageData[prevPageIdx:])
+	if prevPageNo == 0 {
+		return 0, 0, true, nil
+	}
+	s.atree.releaseDataPage(s.pageNo)
+
+	s.pageNo = prevPageNo
+	s.pageData, err = s.atree.fetchDataPage(s.pageNo)
+	if err != nil {
+		return 0, 0, false, fmt.Errorf("atree.fetchDataPage(%d): %s", s.pageNo, err)
+	}
+
+	err = s.makeDecompressors()
+	if err != nil {
+		return 0, 0, false, err
+	}
+
+	timestamp, done = s.timestampDecompressor.NextValue()
+	if done {
+		return 0, 0, false,
+			fmt.Errorf("corrupted data page %d: no timestamps",
+				s.pageNo)
+	}
+	value, done = s.valueDecompressor.NextValue()
+	if done {
+		return 0, 0, false,
+			fmt.Errorf("corrupted data page %d: no values",
+				s.pageNo)
+	}
+	return timestamp, value, false, nil
+}
+
+func (s *BackwardCursor) Close() {
+	s.atree.releaseDataPage(s.pageNo)
+}
+
+// HELPER
+
+func (s *BackwardCursor) makeDecompressors() error {
+	timestampsPayloadSize := bin.GetUint16(s.pageData[timestampsSizeIdx:])
+	valuesPayloadSize := bin.GetUint16(s.pageData[valuesSizeIdx:])
+
+	payloadSize := timestampsPayloadSize + valuesPayloadSize
+
+	if payloadSize > dataFooterIdx {
+		return fmt.Errorf("corrupted data page %d: timestamps + values size %d gt payload size",
+			s.pageNo, payloadSize)
+	}
+
+	s.timestampDecompressor = enc.NewReverseTimeDeltaOfDeltaDecompressor(
+		s.pageData[:timestampsPayloadSize],
+	)
+
+	vbuf := s.pageData[timestampsPayloadSize : timestampsPayloadSize+valuesPayloadSize]
+
+	switch s.metricType {
+	case octopus.Instant:
+		s.valueDecompressor = enc.NewReverseInstantDeltaDecompressor(
+			vbuf, s.fracDigits)
+
+	case octopus.Cumulative:
+		s.valueDecompressor = enc.NewReverseCumulativeDeltaDecompressor(
+			vbuf, s.fracDigits)
+
+	default:
+		return fmt.Errorf("bug: wrong metricType %d", s.metricType)
+	}
+	return nil
+}
+
+func makeDecompressors(pageData []byte, metricType octopus.MetricType, fracDigits byte) (
+	octopus.TimestampDecompressor, octopus.ValueDecompressor, error,
+) {
+	timestampsPayloadSize := bin.GetUint16(pageData[timestampsSizeIdx:])
+	valuesPayloadSize := bin.GetUint16(pageData[valuesSizeIdx:])
+
+	payloadSize := timestampsPayloadSize + valuesPayloadSize
+
+	if payloadSize > dataFooterIdx {
+		return nil, nil, fmt.Errorf("corrupted: timestamps + values size %d > payload size",
+			payloadSize)
+	}
+
+	timestampDecompressor := enc.NewReverseTimeDeltaOfDeltaDecompressor(
+		pageData[:timestampsPayloadSize],
+	)
+
+	vbuf := pageData[timestampsPayloadSize : timestampsPayloadSize+valuesPayloadSize]
+
+	var valueDecompressor octopus.ValueDecompressor
+	switch metricType {
+	case octopus.Instant:
+		valueDecompressor = enc.NewReverseInstantDeltaDecompressor(
+			vbuf, fracDigits)
+
+	case octopus.Cumulative:
+		valueDecompressor = enc.NewReverseCumulativeDeltaDecompressor(
+			vbuf, fracDigits)
+
+	default:
+		return nil, nil, fmt.Errorf("bug: wrong metricType %d", metricType)
+	}
+	return timestampDecompressor, valueDecompressor, nil
+}