mirror of https://github.com/tidwall/tile38.git
80 lines
2.2 KiB
Go
80 lines
2.2 KiB
Go
// Copyright 2017 Google LLC
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//
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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 distribution
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import (
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"log"
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"math"
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"sort"
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"sync"
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"sync/atomic"
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)
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// D is a distribution. Methods of D can be called concurrently by multiple
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// goroutines.
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type D struct {
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buckets []uint64
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// sumsReuse is the scratch space that is reused
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// to store sums during invocations of Percentile.
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// After an invocation of New(n):
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// len(buckets) == len(sumsReuse) == n
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sumsReuse []uint64
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mu sync.Mutex
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}
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// New creates a new distribution capable of holding values from 0 to n-1.
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func New(n int) *D {
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return &D{
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buckets: make([]uint64, n),
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sumsReuse: make([]uint64, n),
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}
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}
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// Record records value v to the distribution.
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// To help with distributions with long tails, if v is larger than the maximum value,
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// Record records the maximum value instead.
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// If v is negative, Record panics.
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func (d *D) Record(v int) {
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if v < 0 {
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log.Panicf("Record: value out of range: %d", v)
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} else if v >= len(d.buckets) {
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v = len(d.buckets) - 1
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}
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atomic.AddUint64(&d.buckets[v], 1)
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}
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// Percentile computes the p-th percentile of the distribution where
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// p is between 0 and 1. This method may be called by multiple goroutines.
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func (d *D) Percentile(p float64) int {
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// NOTE: This implementation uses the nearest-rank method.
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// https://en.wikipedia.org/wiki/Percentile#The_nearest-rank_method
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if p < 0 || p > 1 {
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log.Panicf("Percentile: percentile out of range: %f", p)
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}
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d.mu.Lock()
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defer d.mu.Unlock()
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var sum uint64
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for i := range d.sumsReuse {
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sum += atomic.LoadUint64(&d.buckets[i])
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d.sumsReuse[i] = sum
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}
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target := uint64(math.Ceil(float64(sum) * p))
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return sort.Search(len(d.sumsReuse), func(i int) bool { return d.sumsReuse[i] >= target })
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}
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