staticrand.go

package staticrand

import (
	"math/rand"
	"sync/atomic"
)

const (
	_MAX  = 1 << 63
	_MASK = _MAX - 1
)

const randLen = 5000

type Source struct {
	i int32
	r [randLen]uint64
}

func (src *Source) Seed(seed int64) {
	r := rand.New(rand.NewSource(seed))
	for i := 0; i < randLen; i++ {
		src.r[i] = r.Uint64()
	}
}

func (src *Source) Int63() int64 {
	return int64(src.Uint64() & _MASK)
}

func (src *Source) Uint64() uint64 {
	i := atomic.AddInt32(&src.i, -1)
	for i < 0 {
		old := i
		i = randLen - 1
		if !atomic.CompareAndSwapInt32(&src.i, old, i) {
			// if swap failed, we got a conflict, try again
			i = atomic.AddInt32(&src.i, -1)
			continue
		}
		break
	}
	return src.r[i]
}