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Source file src/cloud.google.com/go/pubsub/internal/distribution/distribution.go

Documentation: cloud.google.com/go/pubsub/internal/distribution

     1  // Copyright 2017 Google LLC
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //      http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  package distribution
    16  
    17  import (
    18  	"log"
    19  	"math"
    20  	"sort"
    21  	"sync"
    22  	"sync/atomic"
    23  )
    24  
    25  // D is a distribution. Methods of D can be called concurrently by multiple
    26  // goroutines.
    27  type D struct {
    28  	buckets []uint64
    29  	// sumsReuse is the scratch space that is reused
    30  	// to store sums during invocations of Percentile.
    31  	// After an invocation of New(n):
    32  	//    len(buckets) == len(sumsReuse) == n
    33  	sumsReuse []uint64
    34  	mu        sync.Mutex
    35  }
    36  
    37  // New creates a new distribution capable of holding values from 0 to n-1.
    38  func New(n int) *D {
    39  	return &D{
    40  		buckets:   make([]uint64, n),
    41  		sumsReuse: make([]uint64, n),
    42  	}
    43  }
    44  
    45  // Record records value v to the distribution.
    46  // To help with distributions with long tails, if v is larger than the maximum value,
    47  // Record records the maximum value instead.
    48  // If v is negative, Record panics.
    49  func (d *D) Record(v int) {
    50  	if v < 0 {
    51  		log.Panicf("Record: value out of range: %d", v)
    52  	} else if v >= len(d.buckets) {
    53  		v = len(d.buckets) - 1
    54  	}
    55  	atomic.AddUint64(&d.buckets[v], 1)
    56  }
    57  
    58  // Percentile computes the p-th percentile of the distribution where
    59  // p is between 0 and 1. This method may be called by multiple goroutines.
    60  func (d *D) Percentile(p float64) int {
    61  	// NOTE: This implementation uses the nearest-rank method.
    62  	// https://en.wikipedia.org/wiki/Percentile#The_nearest-rank_method
    63  
    64  	if p < 0 || p > 1 {
    65  		log.Panicf("Percentile: percentile out of range: %f", p)
    66  	}
    67  
    68  	d.mu.Lock()
    69  	defer d.mu.Unlock()
    70  
    71  	var sum uint64
    72  	for i := range d.sumsReuse {
    73  		sum += atomic.LoadUint64(&d.buckets[i])
    74  		d.sumsReuse[i] = sum
    75  	}
    76  
    77  	target := uint64(math.Ceil(float64(sum) * p))
    78  	return sort.Search(len(d.sumsReuse), func(i int) bool { return d.sumsReuse[i] >= target })
    79  }
    80  

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