// Copyright 2022 The Prometheus Authors // Licensed under the Apache License, Version 2.0 (the "License"); // you may not use this file except in compliance with the License. // You may obtain a copy of the License at // // http://www.apache.org/licenses/LICENSE-2.0 // // Unless required by applicable law or agreed to in writing, software // distributed under the License is distributed on an "AS IS" BASIS, // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. // See the License for the specific language governing permissions and // limitations under the License. package histogram import ( "fmt" "math" "strings" ) // BucketCount is a type constraint for the count in a bucket, which can be // float64 (for type FloatHistogram) or uint64 (for type Histogram). type BucketCount interface { float64 | uint64 } // internalBucketCount is used internally by Histogram and FloatHistogram. The // difference to the BucketCount above is that Histogram internally uses deltas // between buckets rather than absolute counts (while FloatHistogram uses // absolute counts directly). Go type parameters don't allow type // specialization. Therefore, where special treatment of deltas between buckets // vs. absolute counts is important, this information has to be provided as a // separate boolean parameter "deltaBuckets" type internalBucketCount interface { float64 | int64 } // Bucket represents a bucket with lower and upper limit and the absolute count // of samples in the bucket. It also specifies if each limit is inclusive or // not. (Mathematically, inclusive limits create a closed interval, and // non-inclusive limits an open interval.) // // To represent cumulative buckets, Lower is set to -Inf, and the Count is then // cumulative (including the counts of all buckets for smaller values). type Bucket[BC BucketCount] struct { Lower, Upper float64 LowerInclusive, UpperInclusive bool Count BC // Index within schema. To easily compare buckets that share the same // schema and sign (positive or negative). Irrelevant for the zero bucket. Index int32 } // String returns a string representation of a Bucket, using the usual // mathematical notation of '['/']' for inclusive bounds and '('/')' for // non-inclusive bounds. func (b Bucket[BC]) String() string { var sb strings.Builder if b.LowerInclusive { sb.WriteRune('[') } else { sb.WriteRune('(') } fmt.Fprintf(&sb, "%g,%g", b.Lower, b.Upper) if b.UpperInclusive { sb.WriteRune(']') } else { sb.WriteRune(')') } fmt.Fprintf(&sb, ":%v", b.Count) return sb.String() } // BucketIterator iterates over the buckets of a Histogram, returning decoded // buckets. type BucketIterator[BC BucketCount] interface { // Next advances the iterator by one. Next() bool // At returns the current bucket. At() Bucket[BC] } // baseBucketIterator provides a struct that is shared by most BucketIterator // implementations, together with an implementation of the At method. This // iterator can be embedded in full implementations of BucketIterator to save on // code replication. type baseBucketIterator[BC BucketCount, IBC internalBucketCount] struct { schema int32 spans []Span buckets []IBC positive bool // Whether this is for positive buckets. spansIdx int // Current span within spans slice. idxInSpan uint32 // Index in the current span. 0 <= idxInSpan < span.Length. bucketsIdx int // Current bucket within buckets slice. currCount IBC // Count in the current bucket. currIdx int32 // The actual bucket index. } func (b baseBucketIterator[BC, IBC]) At() Bucket[BC] { bucket := Bucket[BC]{ Count: BC(b.currCount), Index: b.currIdx, } if b.positive { bucket.Upper = getBound(b.currIdx, b.schema) bucket.Lower = getBound(b.currIdx-1, b.schema) } else { bucket.Lower = -getBound(b.currIdx, b.schema) bucket.Upper = -getBound(b.currIdx-1, b.schema) } bucket.LowerInclusive = bucket.Lower < 0 bucket.UpperInclusive = bucket.Upper > 0 return bucket } // compactBuckets is a generic function used by both Histogram.Compact and // FloatHistogram.Compact. Set deltaBuckets to true if the provided buckets are // deltas. Set it to false if the buckets contain absolute counts. func compactBuckets[IBC internalBucketCount](buckets []IBC, spans []Span, maxEmptyBuckets int, deltaBuckets bool) ([]IBC, []Span) { // Fast path: If there are no empty buckets AND no offset in any span is // <= maxEmptyBuckets AND no span has length 0, there is nothing to do and we can return // immediately. We check that first because it's cheap and presumably // common. nothingToDo := true var currentBucketAbsolute IBC for _, bucket := range buckets { if deltaBuckets { currentBucketAbsolute += bucket } else { currentBucketAbsolute = bucket } if currentBucketAbsolute == 0 { nothingToDo = false break } } if nothingToDo { for _, span := range spans { if int(span.Offset) <= maxEmptyBuckets || span.Length == 0 { nothingToDo = false break } } if nothingToDo { return buckets, spans } } var iBucket, iSpan int var posInSpan uint32 currentBucketAbsolute = 0 // Helper function. emptyBucketsHere := func() int { i := 0 abs := currentBucketAbsolute for uint32(i)+posInSpan < spans[iSpan].Length && abs == 0 { i++ if i+iBucket >= len(buckets) { break } abs = buckets[i+iBucket] } return i } // Merge spans with zero-offset to avoid special cases later. if len(spans) > 1 { for i, span := range spans[1:] { if span.Offset == 0 { spans[iSpan].Length += span.Length continue } iSpan++ if i+1 != iSpan { spans[iSpan] = span } } spans = spans[:iSpan+1] iSpan = 0 } // Merge spans with zero-length to avoid special cases later. for i, span := range spans { if span.Length == 0 { if i+1 < len(spans) { spans[i+1].Offset += span.Offset } continue } if i != iSpan { spans[iSpan] = span } iSpan++ } spans = spans[:iSpan] iSpan = 0 // Cut out empty buckets from start and end of spans, no matter // what. Also cut out empty buckets from the middle of a span but only // if there are more than maxEmptyBuckets consecutive empty buckets. for iBucket < len(buckets) { if deltaBuckets { currentBucketAbsolute += buckets[iBucket] } else { currentBucketAbsolute = buckets[iBucket] } if nEmpty := emptyBucketsHere(); nEmpty > 0 { if posInSpan > 0 && nEmpty < int(spans[iSpan].Length-posInSpan) && nEmpty <= maxEmptyBuckets { // The empty buckets are in the middle of a // span, and there are few enough to not bother. // Just fast-forward. iBucket += nEmpty if deltaBuckets { currentBucketAbsolute = 0 } posInSpan += uint32(nEmpty) continue } // In all other cases, we cut out the empty buckets. if deltaBuckets && iBucket+nEmpty < len(buckets) { currentBucketAbsolute = -buckets[iBucket] buckets[iBucket+nEmpty] += buckets[iBucket] } buckets = append(buckets[:iBucket], buckets[iBucket+nEmpty:]...) if posInSpan == 0 { // Start of span. if nEmpty == int(spans[iSpan].Length) { // The whole span is empty. offset := spans[iSpan].Offset spans = append(spans[:iSpan], spans[iSpan+1:]...) if len(spans) > iSpan { spans[iSpan].Offset += offset + int32(nEmpty) } continue } spans[iSpan].Length -= uint32(nEmpty) spans[iSpan].Offset += int32(nEmpty) continue } // It's in the middle or in the end of the span. // Split the current span. newSpan := Span{ Offset: int32(nEmpty), Length: spans[iSpan].Length - posInSpan - uint32(nEmpty), } spans[iSpan].Length = posInSpan // In any case, we have to split to the next span. iSpan++ posInSpan = 0 if newSpan.Length == 0 { // The span is empty, so we were already at the end of a span. // We don't have to insert the new span, just adjust the next // span's offset, if there is one. if iSpan < len(spans) { spans[iSpan].Offset += int32(nEmpty) } continue } // Insert the new span. spans = append(spans, Span{}) if iSpan+1 < len(spans) { copy(spans[iSpan+1:], spans[iSpan:]) } spans[iSpan] = newSpan continue } iBucket++ posInSpan++ if posInSpan >= spans[iSpan].Length { posInSpan = 0 iSpan++ } } if maxEmptyBuckets == 0 || len(buckets) == 0 { return buckets, spans } // Finally, check if any offsets between spans are small enough to merge // the spans. iBucket = int(spans[0].Length) if deltaBuckets { currentBucketAbsolute = 0 for _, bucket := range buckets[:iBucket] { currentBucketAbsolute += bucket } } iSpan = 1 for iSpan < len(spans) { if int(spans[iSpan].Offset) > maxEmptyBuckets { l := int(spans[iSpan].Length) if deltaBuckets { for _, bucket := range buckets[iBucket : iBucket+l] { currentBucketAbsolute += bucket } } iBucket += l iSpan++ continue } // Merge span with previous one and insert empty buckets. offset := int(spans[iSpan].Offset) spans[iSpan-1].Length += uint32(offset) + spans[iSpan].Length spans = append(spans[:iSpan], spans[iSpan+1:]...) newBuckets := make([]IBC, len(buckets)+offset) copy(newBuckets, buckets[:iBucket]) copy(newBuckets[iBucket+offset:], buckets[iBucket:]) if deltaBuckets { newBuckets[iBucket] = -currentBucketAbsolute newBuckets[iBucket+offset] += currentBucketAbsolute } iBucket += offset buckets = newBuckets currentBucketAbsolute = buckets[iBucket] // Note that with many merges, it would be more efficient to // first record all the chunks of empty buckets to insert and // then do it in one go through all the buckets. } return buckets, spans } func getBound(idx, schema int32) float64 { // Here a bit of context about the behavior for the last bucket counting // regular numbers (called simply "last bucket" below) and the bucket // counting observations of ±Inf (called "inf bucket" below, with an idx // one higher than that of the "last bucket"): // // If we apply the usual formula to the last bucket, its upper bound // would be calculated as +Inf. The reason is that the max possible // regular float64 number (math.MaxFloat64) doesn't coincide with one of // the calculated bucket boundaries. So the calculated boundary has to // be larger than math.MaxFloat64, and the only float64 larger than // math.MaxFloat64 is +Inf. However, we want to count actual // observations of ±Inf in the inf bucket. Therefore, we have to treat // the upper bound of the last bucket specially and set it to // math.MaxFloat64. (The upper bound of the inf bucket, with its idx // being one higher than that of the last bucket, naturally comes out as // +Inf by the usual formula. So that's fine.) // // math.MaxFloat64 has a frac of 0.9999999999999999 and an exp of // 1024. If there were a float64 number following math.MaxFloat64, it // would have a frac of 1.0 and an exp of 1024, or equivalently a frac // of 0.5 and an exp of 1025. However, since frac must be smaller than // 1, and exp must be smaller than 1025, either representation overflows // a float64. (Which, in turn, is the reason that math.MaxFloat64 is the // largest possible float64. Q.E.D.) However, the formula for // calculating the upper bound from the idx and schema of the last // bucket results in precisely that. It is either frac=1.0 & exp=1024 // (for schema < 0) or frac=0.5 & exp=1025 (for schema >=0). (This is, // by the way, a power of two where the exponent itself is a power of // two, 2¹⁰ in fact, which coinicides with a bucket boundary in all // schemas.) So these are the special cases we have to catch below. if schema < 0 { exp := int(idx) << -schema if exp == 1024 { // This is the last bucket before the overflow bucket // (for ±Inf observations). Return math.MaxFloat64 as // explained above. return math.MaxFloat64 } return math.Ldexp(1, exp) } fracIdx := idx & ((1 << schema) - 1) frac := exponentialBounds[schema][fracIdx] exp := (int(idx) >> schema) + 1 if frac == 0.5 && exp == 1025 { // This is the last bucket before the overflow bucket (for ±Inf // observations). Return math.MaxFloat64 as explained above. return math.MaxFloat64 } return math.Ldexp(frac, exp) } // exponentialBounds is a precalculated table of bucket bounds in the interval // [0.5,1) in schema 0 to 8. var exponentialBounds = [][]float64{ // Schema "0": {0.5}, // Schema 1: {0.5, 0.7071067811865475}, // Schema 2: {0.5, 0.5946035575013605, 0.7071067811865475, 0.8408964152537144}, // Schema 3: { 0.5, 0.5452538663326288, 0.5946035575013605, 0.6484197773255048, 0.7071067811865475, 0.7711054127039704, 0.8408964152537144, 0.9170040432046711, }, // Schema 4: { 0.5, 0.5221368912137069, 0.5452538663326288, 0.5693943173783458, 0.5946035575013605, 0.620928906036742, 0.6484197773255048, 0.6771277734684463, 0.7071067811865475, 0.7384130729697496, 0.7711054127039704, 0.805245165974627, 0.8408964152537144, 0.8781260801866495, 0.9170040432046711, 0.9576032806985735, }, // Schema 5: { 0.5, 0.5109485743270583, 0.5221368912137069, 0.5335702003384117, 0.5452538663326288, 0.5571933712979462, 0.5693943173783458, 0.5818624293887887, 0.5946035575013605, 0.6076236799902344, 0.620928906036742, 0.6345254785958666, 0.6484197773255048, 0.6626183215798706, 0.6771277734684463, 0.6919549409819159, 0.7071067811865475, 0.7225904034885232, 0.7384130729697496, 0.7545822137967112, 0.7711054127039704, 0.7879904225539431, 0.805245165974627, 0.8228777390769823, 0.8408964152537144, 0.8593096490612387, 0.8781260801866495, 0.8973545375015533, 0.9170040432046711, 0.9370838170551498, 0.9576032806985735, 0.9785720620876999, }, // Schema 6: { 0.5, 0.5054446430258502, 0.5109485743270583, 0.5165124395106142, 0.5221368912137069, 0.5278225891802786, 0.5335702003384117, 0.5393803988785598, 0.5452538663326288, 0.5511912916539204, 0.5571933712979462, 0.5632608093041209, 0.5693943173783458, 0.5755946149764913, 0.5818624293887887, 0.5881984958251406, 0.5946035575013605, 0.6010783657263515, 0.6076236799902344, 0.6142402680534349, 0.620928906036742, 0.6276903785123455, 0.6345254785958666, 0.6414350080393891, 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