Line data    Source code

       1             : // Copyright 2018 The Cockroach Authors.
       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
      12             : // implied. See the License for the specific language governing
      13             : // permissions and limitations under the License. See the AUTHORS file
      14             : // for names of contributors.
      15             : 
      16             : package randvar
      17             : 
      18             : import (
      19             :         "sync"
      20             : 
      21             :         "golang.org/x/exp/rand"
      22             : )
      23             : 
      24             : // SkewedLatest is a random number generator that generates numbers in
      25             : // the range [min, max], but skews it towards max using a zipfian
      26             : // distribution.
      27             : type SkewedLatest struct {
      28             :         mu struct {
      29             :                 sync.RWMutex
      30             :                 max  uint64
      31             :                 zipf *Zipf
      32             :         }
      33             : }
      34             : 
      35             : // NewDefaultSkewedLatest constructs a new SkewedLatest generator with the
      36             : // default parameters.
      37           0 : func NewDefaultSkewedLatest() (*SkewedLatest, error) {
      38           0 :         return NewSkewedLatest(1, defaultMax, defaultTheta)
      39           0 : }
      40             : 
      41             : // NewSkewedLatest constructs a new SkewedLatest generator with the given
      42             : // parameters. It returns an error if the parameters are outside the accepted
      43             : // range.
      44           2 : func NewSkewedLatest(min, max uint64, theta float64) (*SkewedLatest, error) {
      45           2 :         z := &SkewedLatest{}
      46           2 :         z.mu.max = max
      47           2 :         zipf, err := NewZipf(0, max-min, theta)
      48           2 :         if err != nil {
      49           0 :                 return nil, err
      50           0 :         }
      51           2 :         z.mu.zipf = zipf
      52           2 :         return z, nil
      53             : }
      54             : 
      55             : // IncMax increments max.
      56           1 : func (z *SkewedLatest) IncMax(delta int) {
      57           1 :         z.mu.Lock()
      58           1 :         z.mu.zipf.IncMax(delta)
      59           1 :         z.mu.max += uint64(delta)
      60           1 :         z.mu.Unlock()
      61           1 : }
      62             : 
      63             : // Max returns max.
      64           1 : func (z *SkewedLatest) Max() uint64 {
      65           1 :         z.mu.Lock()
      66           1 :         defer z.mu.Unlock()
      67           1 :         return z.mu.zipf.Max()
      68           1 : }
      69             : 
      70             : // Uint64 returns a random Uint64 between min and max, where keys near max are
      71             : // most likely to be drawn.
      72           1 : func (z *SkewedLatest) Uint64(rng *rand.Rand) uint64 {
      73           1 :         z.mu.RLock()
      74           1 :         result := z.mu.max - z.mu.zipf.Uint64(rng)
      75           1 :         z.mu.RUnlock()
      76           1 :         return result
      77           1 : }