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uniform_replacer.go
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uniform_replacer.go
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// (c) 2019-2020, Ava Labs, Inc. All rights reserved.
// See the file LICENSE for licensing terms.
package sampler
import (
"math"
"math/rand"
"time"
)
func init() { rand.Seed(time.Now().UnixNano()) }
type defaultMap map[uint64]uint64
func (m defaultMap) get(key uint64, defaultVal uint64) uint64 {
if val, ok := m[key]; ok {
return val
}
return defaultVal
}
// uniformReplacer allows for sampling over a uniform distribution without
// replacement.
//
// Sampling is performed by lazily performing an array shuffle of the array
// [0, 1, ..., length - 1]. By performing the first count swaps of this shuffle,
// we can create an array of length count with elements sampled with uniform
// probability.
//
// Initialization takes O(1) time.
//
// Sampling is performed in O(count) time and O(count) space.
type uniformReplacer struct {
length uint64
drawn defaultMap
}
func (s *uniformReplacer) Initialize(length uint64) error {
if length > math.MaxInt64 {
return errOutOfRange
}
s.length = length
return nil
}
func (s *uniformReplacer) Sample(count int) ([]uint64, error) {
if count < 0 || s.length < uint64(count) {
return nil, errOutOfRange
}
if s.drawn == nil {
s.drawn = make(defaultMap, count)
} else {
for k := range s.drawn {
delete(s.drawn, k)
}
}
results := make([]uint64, count)
for i := 0; i < count; i++ {
// We don't use a cryptographically secure source of randomness here, as
// there's no need to ensure a truly random sampling.
draw := uint64(rand.Int63n(int64(s.length-uint64(i)))) + uint64(i) // #nosec G404
ret := s.drawn.get(draw, draw)
s.drawn[draw] = s.drawn.get(uint64(i), uint64(i))
results[i] = ret
}
return results, nil
}