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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// W.Hormann, G.Derflinger:
// "Rejection-Inversion to Generate Variates
// from Monotone Discrete Distributions"
// http://eeyore.wu-wien.ac.at/papers/96-04-04.wh-der.ps.gz
package rand
import "math"
// A Zipf generates Zipf distributed variates.
type Zipf struct {
r *Rand
imax float64
v float64
q float64
s float64
oneminusQ float64
oneminusQinv float64
hxm float64
hx0minusHxm float64
}
func (z *Zipf) h(x float64) float64 {
return math.Exp(z.oneminusQ*math.Log(z.v+x)) * z.oneminusQinv
}
func (z *Zipf) hinv(x float64) float64 {
return math.Exp(z.oneminusQinv*math.Log(z.oneminusQ*x)) - z.v
}
// NewZipf returns a Zipf generating variates p(k) on [0, imax]
// proportional to (v+k)**(-s) where s>1 and k>=0, and v>=1.
func NewZipf(r *Rand, s float64, v float64, imax uint64) *Zipf {
z := new(Zipf)
if s <= 1.0 || v < 1 {
return nil
}
z.r = r
z.imax = float64(imax)
z.v = v
z.q = s
z.oneminusQ = 1.0 - z.q
z.oneminusQinv = 1.0 / z.oneminusQ
z.hxm = z.h(z.imax + 0.5)
z.hx0minusHxm = z.h(0.5) - math.Exp(math.Log(z.v)*(-z.q)) - z.hxm
z.s = 1 - z.hinv(z.h(1.5)-math.Exp(-z.q*math.Log(z.v+1.0)))
return z
}
// Uint64 returns a value drawn from the Zipf distribution described
// by the Zipf object.
func (z *Zipf) Uint64() uint64 {
if z == nil {
panic("rand: nil Zipf")
}
k := 0.0
for {
r := z.r.Float64() // r on [0,1]
ur := z.hxm + r*z.hx0minusHxm
x := z.hinv(ur)
k = math.Floor(x + 0.5)
if k-x <= z.s {
break
}
if ur >= z.h(k+0.5)-math.Exp(-math.Log(k+z.v)*z.q) {
break
}
}
return uint64(k)
}
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