wp

storage/cursor.go

@@ -0,0 +1,136 @@
+package storage
+
+import (
+	"errors"
+	"fmt"
+
+	bin "gordenko.dev/dima/bin/little"
+	"gordenko.dev/dima/qb"
+)
+
+type BackwardCursor struct {
+	metricType            qb.MetricType
+	fracDigits            byte
+	fetchDataPage         func(uint32) ([]byte, error)
+	releasePage           func(uint32)
+	pageNo                uint32
+	pageData              []byte
+	timestampDecompressor qb.TimestampDecompressor
+	valueDecompressor     qb.ValueDecompressor
+}
+
+type BackwardCursorOptions struct {
+	MetricType    qb.MetricType
+	FracDigits    byte
+	PageNo        uint32
+	PageData      []byte
+	FetchDataPage func(uint32) ([]byte, error)
+	ReleasePage   func(uint32)
+}
+
+func NewBackwardCursor(opt BackwardCursorOptions) (*BackwardCursor, error) {
+	switch opt.MetricType {
+	case qb.Instant, qb.Cumulative:
+		// ok
+	default:
+		return nil, fmt.Errorf("MetricType option has wrong value: %d", opt.MetricType)
+	}
+	if opt.FracDigits > qb.MaxFracDigits {
+		return nil, errors.New("FracDigits option is required")
+	}
+	if opt.FetchDataPage == nil {
+		return nil, errors.New("FetchDataPage option is required")
+	}
+	if opt.ReleasePage == nil {
+		return nil, errors.New("ReleasePage 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,
+		fetchDataPage: opt.FetchDataPage,
+		releasePage:   opt.ReleasePage,
+		pageNo:        opt.PageNo,
+		pageData:      opt.PageData,
+	}
+	var err error
+	s.timestampDecompressor, err = CreateTimestampDecompressor(s.pageData)
+	if err != nil {
+		return nil, fmt.Errorf("CreateTimestampDecompressor: %s", err)
+	}
+	s.valueDecompressor, err = CreateValueDecompressor(s.pageData, s.metricType, s.fracDigits)
+	if err != nil {
+		return nil, fmt.Errorf("CreateValueDecompressor: %s", 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.releasePage(s.pageNo)
+
+	s.pageNo = prevPageNo
+	s.pageData, err = s.fetchDataPage(s.pageNo)
+	if err != nil {
+		return 0, 0, false,
+			fmt.Errorf("fetchDataPage(%d): %s", s.pageNo, err)
+	}
+	s.timestampDecompressor, err = CreateTimestampDecompressor(s.pageData)
+	if err != nil {
+		return 0, 0, false,
+			fmt.Errorf("CreateTimestampDecompressor: %s", err)
+	}
+	s.valueDecompressor, err = CreateValueDecompressor(s.pageData, s.metricType, s.fracDigits)
+	if err != nil {
+		return 0, 0, false,
+			fmt.Errorf("CreateValueDecompressor: %s", 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.releasePage(s.pageNo)
+}
+
+// HELPER
+
+//func (s *BackwardCursor) makeDecompressors() error {
+
+//}