go-whisperをコードリーディングしてみた

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/*
  Open an existing Whisper database and read it's header
*/
func Open(path string) (whisper *Whisper, err error) {
    file, err := os.OpenFile(path, os.O_RDWR, 0666)
    if err != nil {
        return
    }

    defer func() {
        if err != nil {
            whisper = nil
            file.Close()
        }
    }()

    whisper = new(Whisper)
    whisper.file = file

    offset := 0

    // read the metadata
    b := make([]byte, MetadataSize)
    readed, err := file.Read(b)

    if err != nil {
        err = fmt.Errorf("Unable to read header: %s", err.Error())
        return
    }
    if readed != MetadataSize {
        err = fmt.Errorf("Unable to read header: EOF")
        return
    }

    a := unpackInt(b[offset : offset+IntSize])
    if a > 1024 { // support very old format. File starts with lastUpdate and has only average aggregation method
        whisper.aggregationMethod = Average
    } else {
        whisper.aggregationMethod = AggregationMethod(a)
    }
    offset += IntSize
    whisper.maxRetention = unpackInt(b[offset : offset+IntSize])
    offset += IntSize
    whisper.xFilesFactor = unpackFloat32(b[offset : offset+FloatSize])
    offset += FloatSize
    archiveCount := unpackInt(b[offset : offset+IntSize])
    offset += IntSize

    // read the archive info
    b = make([]byte, ArchiveInfoSize)

    whisper.archives = make([]*archiveInfo, 0)
    for i := 0; i < archiveCount; i++ {
        readed, err = file.Read(b)
        if err != nil || readed != ArchiveInfoSize {
            err = fmt.Errorf("Unable to read archive %d metadata", i)
            return
        }
        whisper.archives = append(whisper.archives, unpackArchiveInfo(b))
    }

    return whisper, nil
}

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/*
    Represents a Whisper database file.
*/
type Whisper struct {
    file *os.File

    // Metadata
    aggregationMethod AggregationMethod
    maxRetention      int
    xFilesFactor      float32
    archives          []*archiveInfo
}

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type AggregationMethod int

const (
    Average AggregationMethod = iota + 1
    Sum
    Last
    Max
    Min
)

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/*
  Describes a time series in a file.
  The only addition this type has over a Retention is the offset at which it exists within the
  whisper file.
*/
type archiveInfo struct {
    Retention
    offset int
}

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/*
  A retention level.
  Retention levels describe a given archive in the database. How detailed it is and how far back
  it records.
*/
type Retention struct {
    secondsPerPoint int
    numberOfPoints  int
}

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func (whisper *Whisper) UpdateMany(points []*TimeSeriesPoint) (err error) {
    // recover panics and return as error
    defer func() {
        if e := recover(); e != nil {
            err = errors.New(e.(string))
        }
    }()

    // sort the points, newest first
    reversePoints(points)
    sort.Stable(timeSeriesPointsNewestFirst{points})

    now := int(time.Now().Unix()) // TODO: danger of 2030 something overflow

    var currentPoints []*TimeSeriesPoint
    for _, archive := range whisper.archives {
        currentPoints, points = extractPoints(points, now, archive.MaxRetention())
        if len(currentPoints) == 0 {
            continue
        }
        // reverse currentPoints
        reversePoints(currentPoints)
        err = whisper.archiveUpdateMany(archive, currentPoints)
        if err != nil {
            return
        }

        if len(points) == 0 { // nothing left to do
            break
        }
    }
    return
}

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type TimeSeriesPoint struct {
    Time  int
    Value float64
}

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func reversePoints(points []*TimeSeriesPoint) {
    size := len(points)
    end := size / 2

    for i := 0; i < end; i++ {
        points[i], points[size-i-1] = points[size-i-1], points[i]
    }
}

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type timeSeriesPoints []*TimeSeriesPoint

func (p timeSeriesPoints) Len() int {
    return len(p)
}

func (p timeSeriesPoints) Swap(i, j int) {
    p[i], p[j] = p[j], p[i]
}

type timeSeriesPointsNewestFirst struct {
    timeSeriesPoints
}

func (p timeSeriesPointsNewestFirst) Less(i, j int) bool {
    return p.timeSeriesPoints[i].Time > p.timeSeriesPoints[j].Time
}

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func extractPoints(points []*TimeSeriesPoint, now int, maxRetention int) (currentPoints []*TimeSeriesPoint, remainingPoints []*TimeSeriesPoint) {
    maxAge := now - maxRetention
    for i, point := range points {
        if point.Time < maxAge {
            if i > 0 {
                return points[:i-1], points[i-1:]
            } else {
                return []*TimeSeriesPoint{}, points
            }
        }
    }
    return points, remainingPoints
}

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func (whisper *Whisper) archiveUpdateMany(archive *archiveInfo, points []*TimeSeriesPoint) error {
    alignedPoints := alignPoints(archive, points)
    intervals, packedBlocks := packSequences(archive, alignedPoints)

    baseInterval := whisper.getBaseInterval(archive)
    if baseInterval == 0 {
        baseInterval = intervals[0]
    }

    for i := range intervals {
        myOffset := archive.PointOffset(baseInterval, intervals[i])
        bytesBeyond := int(myOffset-archive.End()) + len(packedBlocks[i])
        if bytesBeyond > 0 {
            pos := len(packedBlocks[i]) - bytesBeyond
            err := whisper.fileWriteAt(packedBlocks[i][:pos], myOffset)
            if err != nil {
                return err
            }
            err = whisper.fileWriteAt(packedBlocks[i][pos:], archive.Offset())
            if err != nil {
                return err
            }
        } else {
            err := whisper.fileWriteAt(packedBlocks[i], myOffset)
            if err != nil {
                return err
            }
        }
    }

    higher := archive
    lowerArchives := whisper.lowerArchives(archive)

    for _, lower := range lowerArchives {
        seen := make(map[int]bool)
        propagateFurther := false
        for _, point := range alignedPoints {
            interval := point.interval - mod(point.interval, lower.secondsPerPoint)
            if !seen[interval] {
                if propagated, err := whisper.propagate(interval, higher, lower); err != nil {
                    panic("Failed to propagate")
                } else if propagated {
                    propagateFurther = true
                }
            }
        }
        if !propagateFurther {
            break
        }
        higher = lower
    }
    return nil
}

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func alignPoints(archive *archiveInfo, points []*TimeSeriesPoint) []dataPoint {
    alignedPoints := make([]dataPoint, 0, len(points))
    positions := make(map[int]int)
    for _, point := range points {
        dPoint := dataPoint{point.Time - mod(point.Time, archive.secondsPerPoint), point.Value}
        if p, ok := positions[dPoint.interval]; ok {
            alignedPoints[p] = dPoint
        } else {
            alignedPoints = append(alignedPoints, dPoint)
            positions[dPoint.interval] = len(alignedPoints) - 1
        }
    }
    return alignedPoints
}

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type dataPoint struct {
    interval int
    value    float64
}

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/*
    Implementation of modulo that works like Python
    Thanks @timmow for this
*/
func mod(a, b int) int {
    return a - (b * int(math.Floor(float64(a)/float64(b))))
}

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func packSequences(archive *archiveInfo, points []dataPoint) (intervals []int, packedBlocks [][]byte) {
    intervals = make([]int, 0)
    packedBlocks = make([][]byte, 0)
    for i, point := range points {
        if i == 0 || point.interval != intervals[len(intervals)-1]+archive.secondsPerPoint {
            intervals = append(intervals, point.interval)
            packedBlocks = append(packedBlocks, point.Bytes())
        } else {
            packedBlocks[len(packedBlocks)-1] = append(packedBlocks[len(packedBlocks)-1], point.Bytes()...)
        }
    }
    return
}

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func (point *dataPoint) Bytes() []byte {
    b := make([]byte, PointSize)
    packInt(b, point.interval, 0)
    packFloat64(b, point.value, IntSize)
    return b
}

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// Wrappers for whisper.file operations
func (whisper *Whisper) fileWriteAt(b []byte, off int64) error {
    _, err := whisper.file.WriteAt(b, off)
    return err
}

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func (whisper *Whisper) lowerArchives(archive *archiveInfo) (lowerArchives []*archiveInfo) {
    for i, lower := range whisper.archives {
        if lower.secondsPerPoint > archive.secondsPerPoint {
            return whisper.archives[i:]
        }
    }
    return
}

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func (whisper *Whisper) readSeries(start, end int64, archive *archiveInfo) ([]dataPoint, error) {
    var b []byte
    if start < end {
        b = make([]byte, end-start)
        err := whisper.fileReadAt(b, start)
        if err != nil {
            return nil, err
        }
    } else {
        b = make([]byte, archive.End()-start)
        err := whisper.fileReadAt(b, start)
        if err != nil {
            return nil, err
        }
        b2 := make([]byte, end-archive.Offset())
        err = whisper.fileReadAt(b2, archive.Offset())
        if err != nil {
            return nil, err
        }
        b = append(b, b2...)
    }
    return unpackDataPoints(b), nil
}