github.com/aclements/go-misc@v0.0.0-20240129233631-2f6ede80790c/findflakes/geodist.go (about)

     1  // Copyright 2015 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package main
     6  
     7  import (
     8  	"math"
     9  	"math/rand"
    10  )
    11  
    12  // GeometricDist is a geometric distribution with success probability
    13  // P.
    14  type GeometricDist struct {
    15  	P float64
    16  
    17  	// Start is the start of the distribution's support. There are
    18  	// two conventional definitions of the geometric distribution:
    19  	//
    20  	// For Start=0, the distribution gives the number of failures
    21  	// before the first success in a Bernoulli process with
    22  	// success probability P.
    23  	//
    24  	// For Start=1, the distribution gives the number of trials
    25  	// needed to get one success. This is often called the
    26  	// "shifted geometric distribution."
    27  	//
    28  	// Other values of Start are allowed, but have no conventional
    29  	// meaning.
    30  	Start int
    31  }
    32  
    33  func (d *GeometricDist) PMF(k int) float64 {
    34  	if k < d.Start {
    35  		return 0
    36  	}
    37  	return math.Pow(1-d.P, float64(k-d.Start)) * d.P
    38  }
    39  
    40  func (d *GeometricDist) CDF(k int) float64 {
    41  	if k < d.Start {
    42  		return 0
    43  	}
    44  	return 1 - math.Pow(1-d.P, float64(k-d.Start+1))
    45  }
    46  
    47  func (d *GeometricDist) SF(k int) float64 {
    48  	if k < d.Start {
    49  		return 1
    50  	}
    51  	return math.Pow(1-d.P, float64(k-d.Start+1))
    52  }
    53  
    54  func (d *GeometricDist) InvCDF(y float64) int {
    55  	return int(math.Ceil(math.Log(1-y)/math.Log(1-d.P)-1)) + d.Start
    56  }
    57  
    58  func (d *GeometricDist) Rand() int {
    59  	u := 1 - rand.Float64()
    60  	return int(math.Log(u)/math.Log(1-d.P)) + d.Start
    61  }