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241 lines
8.0 KiB
241 lines
8.0 KiB
// 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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// The code in this file was largely written by Damian Gryski as part of
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// https://github.com/dgryski/go-tsz and published under the license below.
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// It was modified to accommodate reading from byte slices without modifying
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// the underlying bytes, which would panic when reading from mmap'd
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// read-only byte slices.
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package chunkenc
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import "github.com/prometheus/prometheus/pkg/histogram"
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func writeHistoChunkMeta(b *bstream, schema int32, posSpans, negSpans []histogram.Span) {
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putInt64VBBucket(b, int64(schema))
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putHistoChunkMetaSpans(b, posSpans)
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putHistoChunkMetaSpans(b, negSpans)
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}
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func putHistoChunkMetaSpans(b *bstream, spans []histogram.Span) {
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putInt64VBBucket(b, int64(len(spans)))
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for _, s := range spans {
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putInt64VBBucket(b, int64(s.Length))
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putInt64VBBucket(b, int64(s.Offset))
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}
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}
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func readHistoChunkMeta(b *bstreamReader) (int32, []histogram.Span, []histogram.Span, error) {
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v, err := readInt64VBBucket(b)
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if err != nil {
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return 0, nil, nil, err
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}
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schema := int32(v)
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posSpans, err := readHistoChunkMetaSpans(b)
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if err != nil {
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return 0, nil, nil, err
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}
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negSpans, err := readHistoChunkMetaSpans(b)
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if err != nil {
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return 0, nil, nil, err
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}
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return schema, posSpans, negSpans, nil
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}
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func readHistoChunkMetaSpans(b *bstreamReader) ([]histogram.Span, error) {
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var spans []histogram.Span
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num, err := readInt64VBBucket(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 := readInt64VBBucket(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 := readInt64VBBucket(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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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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// interjection describes that num new buckets are introduced before 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 slice representing an expanded set of buckets, or false if incompatible (e.g. if buckets were removed)
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// For example:
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// Let's say the old buckets look like this:
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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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// But now we introduce a new bucket layout. (carefully chosen example where we have a span appended, one unchanged[*], one prepended, and two merge - in that order)
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// [*] unchanged in terms of which bucket indices they represent. but to achieve that, their offset needs to change if "disrupted" by spans changing ahead of them
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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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// note that whenever any new buckets are introduced, the subsequent "old" bucket needs to readjust its delta to the new base of 0
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// thus, for the caller, who wants to transform the set of original deltas to a new set of deltas to match a new span layout that adds buckets, we simply
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// need to generate a list of interjections
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// note: within compareSpans we don't have to worry about the changes to the spans themselves,
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// thanks to the iterators, we get to work with the more useful bucket indices (which of course directly correspond to the buckets we have to 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 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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for {
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if aok && bok {
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if 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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if aok {
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inter.pos++
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}
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continue
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}
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if av < bv { // b misses a value that is in a.
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return interjections, false
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}
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if 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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continue
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}
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} else if aok && !bok { // b misses a value that is in a.
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return interjections, false
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} else if !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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continue
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} else { // 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
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}
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}
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return interjections, true
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}
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// caller is responsible for making sure len(in) and len(out) are appropriate for the provided interjections!
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func interject(in, out []int64, interjections []interjection) []int64 {
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var j int // position in out
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var v int64 // the last value seen
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var interj int // the next interjection to process
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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 as their 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 should save is
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// the original delta value + the last value of the point before the interjection (to undo 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 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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return out
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}
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