335 lines
9.9 KiB
Go
335 lines
9.9 KiB
Go
// Copyright 2021 The Prometheus Authors
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package chunkenc
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import (
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"math"
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"github.com/prometheus/prometheus/model/histogram"
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)
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func writeHistogramChunkLayout(b *bstream, schema int32, zeroThreshold float64, positiveSpans, negativeSpans []histogram.Span) {
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putZeroThreshold(b, zeroThreshold)
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putVarbitInt(b, int64(schema))
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putHistogramChunkLayoutSpans(b, positiveSpans)
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putHistogramChunkLayoutSpans(b, negativeSpans)
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}
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func readHistogramChunkLayout(b *bstreamReader) (
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schema int32, zeroThreshold float64,
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positiveSpans, negativeSpans []histogram.Span,
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err error,
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) {
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zeroThreshold, err = readZeroThreshold(b)
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if err != nil {
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return
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}
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v, err := readVarbitInt(b)
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if err != nil {
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return
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}
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schema = int32(v)
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positiveSpans, err = readHistogramChunkLayoutSpans(b)
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if err != nil {
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return
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}
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negativeSpans, err = readHistogramChunkLayoutSpans(b)
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if err != nil {
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return
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}
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return
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}
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func putHistogramChunkLayoutSpans(b *bstream, spans []histogram.Span) {
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putVarbitUint(b, uint64(len(spans)))
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for _, s := range spans {
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putVarbitUint(b, uint64(s.Length))
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putVarbitInt(b, int64(s.Offset))
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}
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}
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func readHistogramChunkLayoutSpans(b *bstreamReader) ([]histogram.Span, error) {
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var spans []histogram.Span
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num, err := readVarbitUint(b)
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if err != nil {
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return nil, err
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}
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for i := 0; i < int(num); i++ {
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length, err := readVarbitUint(b)
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if err != nil {
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return nil, err
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}
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offset, err := readVarbitInt(b)
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if err != nil {
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return nil, err
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}
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spans = append(spans, histogram.Span{
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Length: uint32(length),
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Offset: int32(offset),
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})
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}
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return spans, nil
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}
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// putZeroThreshold writes the zero threshold to the bstream. It stores typical
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// values in just one byte, but needs 9 bytes for other values. In detail:
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//
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// * If the threshold is 0, store a single zero byte.
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//
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// - If the threshold is a power of 2 between (and including) 2^-243 and 2^10,
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// take the exponent from the IEEE 754 representation of the threshold, which
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// covers a range between (and including) -242 and 11. (2^-243 is 0.5*2^-242
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// in IEEE 754 representation, and 2^10 is 0.5*2^11.) Add 243 to the exponent
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// and store the result (which will be between 1 and 254) as a single
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// byte. Note that small powers of two are preferred values for the zero
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// threshold. The default value for the zero threshold is 2^-128 (or
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// 0.5*2^-127 in IEEE 754 representation) and will therefore be encoded as a
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// single byte (with value 116).
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//
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// - In all other cases, store 255 as a single byte, followed by the 8 bytes of
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// the threshold as a float64, i.e. taking 9 bytes in total.
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func putZeroThreshold(b *bstream, threshold float64) {
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if threshold == 0 {
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b.writeByte(0)
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return
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}
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frac, exp := math.Frexp(threshold)
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if frac != 0.5 || exp < -242 || exp > 11 {
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b.writeByte(255)
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b.writeBits(math.Float64bits(threshold), 64)
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return
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}
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b.writeByte(byte(exp + 243))
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}
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// readZeroThreshold reads the zero threshold written with putZeroThreshold.
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func readZeroThreshold(br *bstreamReader) (float64, error) {
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b, err := br.ReadByte()
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if err != nil {
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return 0, err
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}
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switch b {
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case 0:
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return 0, nil
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case 255:
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v, err := br.readBits(64)
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if err != nil {
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return 0, err
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}
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return math.Float64frombits(v), nil
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default:
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return math.Ldexp(0.5, int(b)-243), nil
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}
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}
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type bucketIterator struct {
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spans []histogram.Span
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span int // Span position of last yielded bucket.
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bucket int // Bucket position within span of last yielded bucket.
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idx int // Bucket index (globally across all spans) of last yielded bucket.
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}
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func newBucketIterator(spans []histogram.Span) *bucketIterator {
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b := bucketIterator{
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spans: spans,
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span: 0,
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bucket: -1,
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idx: -1,
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}
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if len(spans) > 0 {
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b.idx += int(spans[0].Offset)
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}
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return &b
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}
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func (b *bucketIterator) Next() (int, bool) {
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// We're already out of bounds.
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if b.span >= len(b.spans) {
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return 0, false
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}
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try:
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if b.bucket < int(b.spans[b.span].Length-1) { // Try to move within same span.
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b.bucket++
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b.idx++
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return b.idx, true
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} else if b.span < len(b.spans)-1 { // Try to move from one span to the next.
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b.span++
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b.idx += int(b.spans[b.span].Offset + 1)
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b.bucket = 0
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if b.spans[b.span].Length == 0 {
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// Pathological case that should never happen. We can't use this span, let's try again.
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goto try
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}
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return b.idx, true
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}
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// We're out of options.
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return 0, false
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}
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// An Interjection describes how many new buckets have to be introduced before
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// processing the pos'th delta from the original slice.
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type Interjection struct {
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pos int
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num int
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}
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// compareSpans returns the interjections to convert a slice of deltas to a new
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// slice representing an expanded set of buckets, or false if incompatible
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// (e.g. if buckets were removed).
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//
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// Example:
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//
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// Let's say the old buckets look like this:
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//
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// span syntax: [offset, length]
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// spans : [ 0 , 2 ] [2,1] [ 3 , 2 ] [3,1] [1,1]
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// bucket idx : [0] [1] 2 3 [4] 5 6 7 [8] [9] 10 11 12 [13] 14 [15]
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// raw values 6 3 3 2 4 5 1
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// deltas 6 -3 0 -1 2 1 -4
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//
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// But now we introduce a new bucket layout. (Carefully chosen example where we
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// have a span appended, one unchanged[*], one prepended, and two merge - in
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// that order.)
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//
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// [*] unchanged in terms of which bucket indices they represent. but to achieve
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// that, their offset needs to change if "disrupted" by spans changing ahead of
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// them
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//
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// \/ this one is "unchanged"
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// spans : [ 0 , 3 ] [1,1] [ 1 , 4 ] [ 3 , 3 ]
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// bucket idx : [0] [1] [2] 3 [4] 5 [6] [7] [8] [9] 10 11 12 [13] [14] [15]
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// raw values 6 3 0 3 0 0 2 4 5 0 1
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// deltas 6 -3 -3 3 -3 0 2 2 1 -5 1
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// delta mods: / \ / \ / \
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//
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// Note that whenever any new buckets are introduced, the subsequent "old"
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// bucket needs to readjust its delta to the new base of 0. Thus, for the caller
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// who wants to transform the set of original deltas to a new set of deltas to
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// match a new span layout that adds buckets, we simply need to generate a list
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// of interjections.
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//
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// Note: Within compareSpans we don't have to worry about the changes to the
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// spans themselves, thanks to the iterators we get to work with the more useful
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// bucket indices (which of course directly correspond to the buckets we have to
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// adjust).
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func compareSpans(a, b []histogram.Span) ([]Interjection, bool) {
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ai := newBucketIterator(a)
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bi := newBucketIterator(b)
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var interjections []Interjection
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// When inter.num becomes > 0, this becomes a valid interjection that
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// should be yielded when we finish a streak of new buckets.
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var inter Interjection
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av, aOK := ai.Next()
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bv, bOK := bi.Next()
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loop:
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for {
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switch {
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case aOK && bOK:
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switch {
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case av == bv: // Both have an identical value. move on!
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// Finish WIP interjection and reset.
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if inter.num > 0 {
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interjections = append(interjections, inter)
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}
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inter.num = 0
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av, aOK = ai.Next()
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bv, bOK = bi.Next()
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inter.pos++
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case av < bv: // b misses a value that is in a.
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return interjections, false
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case av > bv: // a misses a value that is in b. Forward b and recompare.
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inter.num++
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bv, bOK = bi.Next()
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}
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case aOK && !bOK: // b misses a value that is in a.
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return interjections, false
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case !aOK && bOK: // a misses a value that is in b. Forward b and recompare.
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inter.num++
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bv, bOK = bi.Next()
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default: // Both iterators ran out. We're done.
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if inter.num > 0 {
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interjections = append(interjections, inter)
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}
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break loop
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}
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}
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return interjections, true
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}
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// interject merges 'in' with the provided interjections and writes them into
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// 'out', which must already have the appropriate length.
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func interject(in, out []int64, interjections []Interjection) []int64 {
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var (
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j int // Position in out.
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v int64 // The last value seen.
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interj int // The next interjection to process.
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)
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for i, d := range in {
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if interj < len(interjections) && i == interjections[interj].pos {
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// We have an interjection!
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// Add interjection.num new delta values such that their
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// bucket values equate 0.
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out[j] = int64(-v)
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j++
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for x := 1; x < interjections[interj].num; x++ {
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out[j] = 0
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j++
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}
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interj++
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// Now save the value from the input. The delta value we
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// should save is the original delta value + the last
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// value of the point before the interjection (to undo
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// the delta that was introduced by the interjection).
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out[j] = d + v
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j++
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v = d + v
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continue
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}
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// If there was no interjection, the original delta is still
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// valid.
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out[j] = d
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j++
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v += d
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}
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switch interj {
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case len(interjections):
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// All interjections processed. Nothing more to do.
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case len(interjections) - 1:
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// One more interjection to process at the end.
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out[j] = int64(-v)
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j++
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for x := 1; x < interjections[interj].num; x++ {
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out[j] = 0
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j++
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}
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default:
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panic("unprocessed interjections left")
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}
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return out
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}
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