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// Copyright ©2017 The Gonum Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package sampleuv
import (
"sort"
"golang.org/x/exp/rand"
)
// WithoutReplacement samples len(idxs) integers from [0, n) without replacement.
// That is, upon return the elements of idxs will be unique integers. If source
// is non-nil it will be used to generate random numbers, otherwise the default
// source from the math/rand package will be used.
//
// WithoutReplacement will panic if len(idxs) > n.
func WithoutReplacement(idxs []int, n int, src rand.Source) {
if len(idxs) == 0 {
panic("withoutreplacement: zero length input")
}
if len(idxs) > n {
panic("withoutreplacement: impossible size inputs")
}
// There are two algorithms. One is to generate a random permutation
// and take the first len(idxs) elements. The second is to generate
// individual random numbers for each element and check uniqueness. The first
// method scales as O(n), and the second scales as O(len(idxs)^2). Choose
// the algorithm accordingly.
if n < len(idxs)*len(idxs) {
var perm []int
if src != nil {
perm = rand.New(src).Perm(n)
} else {
perm = rand.Perm(n)
}
copy(idxs, perm)
return
}
// Instead, generate the random numbers directly.
sorted := make([]int, 0, len(idxs))
for i := range idxs {
var r int
if src != nil {
r = rand.New(src).Intn(n - i)
} else {
r = rand.Intn(n - i)
}
for _, v := range sorted {
if r >= v {
r++
}
}
idxs[i] = r
sorted = append(sorted, r)
sort.Ints(sorted)
}
}
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