b3cea3d1b6
Treat the piece size as a weight, and perform weighted reservoir sampling as given in Algorithm A-Chao (https://en.wikipedia.org/wiki/Reservoir_sampling#Algorithm_A-Chao) Change-Id: I299d0026d9e02d03b3d2130b0f32192928e6e326
82 lines
2.3 KiB
Go
82 lines
2.3 KiB
Go
// Copyright (C) 2019 Storj Labs, Inc.
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// See LICENSE for copying information.
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package audit
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import (
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"math/rand"
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"time"
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"storj.io/common/uuid"
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"storj.io/storj/satellite/metabase"
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"storj.io/storj/satellite/metabase/segmentloop"
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)
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const maxReservoirSize = 3
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// Reservoir holds a certain number of segments to reflect a random sample.
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type Reservoir struct {
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Segments [maxReservoirSize]Segment
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size int8
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index int64
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wSum int64
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}
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// NewReservoir instantiates a Reservoir.
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func NewReservoir(size int) *Reservoir {
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if size < 1 {
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size = 1
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} else if size > maxReservoirSize {
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size = maxReservoirSize
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}
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return &Reservoir{
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size: int8(size),
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index: 0,
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}
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}
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// Sample makes sure that for every segment in metainfo from index i=size..n-1,
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// compute the relative weight based on segment size, and pick a random floating
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// point number r = rand(0..1), and if r < the relative weight of the segment,
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// select uniformly a random segment reservoir.Segments[rand(0..i)] to replace with
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// segment. See https://en.wikipedia.org/wiki/Reservoir_sampling#Algorithm_A-Chao
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// for the algorithm used.
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func (reservoir *Reservoir) Sample(r *rand.Rand, segment Segment) {
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if reservoir.index < int64(reservoir.size) {
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reservoir.Segments[reservoir.index] = segment
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reservoir.wSum += int64(segment.EncryptedSize)
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} else {
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reservoir.wSum += int64(segment.EncryptedSize)
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p := float64(segment.EncryptedSize) / float64(reservoir.wSum)
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random := r.Float64()
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if random < p {
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index := r.Int31n(int32(reservoir.size))
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reservoir.Segments[index] = segment
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}
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}
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reservoir.index++
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}
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// Segment is a segment to audit.
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type Segment struct {
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StreamID uuid.UUID
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Position metabase.SegmentPosition
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ExpiresAt *time.Time
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EncryptedSize int32 // size of the whole segment (not a piece)
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}
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// NewSegment creates a new segment to audit from a metainfo loop segment.
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func NewSegment(loopSegment *segmentloop.Segment) Segment {
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return Segment{
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StreamID: loopSegment.StreamID,
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Position: loopSegment.Position,
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ExpiresAt: loopSegment.ExpiresAt,
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EncryptedSize: loopSegment.EncryptedSize,
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}
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}
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// Expired checks if segment is expired relative to now.
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func (segment *Segment) Expired(now time.Time) bool {
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return segment.ExpiresAt != nil && segment.ExpiresAt.Before(now)
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}
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