MaxHalford/eaopt

package eaopt

import (
    "fmt"
    "math/rand"
    "time"
)

// newRand returns a new random number generator with a random seed.
func newRand() *rand.Rand {
    return rand.New(rand.NewSource(time.Now().UnixNano()))
}

// Sample k unique integers in range [min, max) using reservoir sampling,
// specifically Vitter's Algorithm R.
func randomInts(k uint, min, max int, rng *rand.Rand) []int {
    var ints = make([]int, k)
    for i := 0; i < int(k); i++ {
        ints[i] = i + min
    }
    for i := int(k); i < max-min; i++ {
        var j = rng.Intn(i + 1)
        if j < int(k) {
            ints[j] = i + min
        }
    }
    return ints
}

// Sample k unique integers from a slice of n integers without replacement.
func sampleInts(ints []int, k uint, rng *rand.Rand) ([]int, []int, error) {
    if int(k) > len(ints) {
        return nil, nil, fmt.Errorf("cannot sample %d elements from array of length %d", k, len(ints))
    }
    var (
        sample = make([]int, k)
        idxs   = make([]int, k)
    )
    for i, idx := range randomInts(k, 0, len(ints), rng) {
        sample[i] = ints[idx]
        idxs[i] = idx
    }
    return sample, idxs, nil
}

// Generate random weights that sum up to 1.
func randomWeights(size int) []float64 {
    var (
        weights = make([]float64, size)
        total   float64
    )
    for i := range weights {
        weights[i] = rand.Float64()
        total += weights[i]
    }
    var normalized = divide(weights, total)
    return normalized
}

const (
    letterBytes   = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
    letterIdxBits = 6                    // 6 bits to represent a letter index
    letterIdxMask = 1<<letterIdxBits - 1 // All 1-bits, as many as letterIdxBits
    letterIdxMax  = 63 / letterIdxBits   // # of letter indices fitting in 63 bits
)

func randString(n int, rng *rand.Rand) string {
    b := make([]byte, n)
    for i, cache, remain := n-1, rng.Int63(), letterIdxMax; i >= 0; {
        if remain == 0 {
            cache, remain = rng.Int63(), letterIdxMax
        }
        if idx := int(cache & letterIdxMask); idx < len(letterBytes) {
            b[i] = letterBytes[idx]
            i--
        }
        cache >>= letterIdxBits
        remain--
    }
    return string(b)
}