randomizers.go

package main

import (
	"math/rand"
	"time"
)

const letterBytes = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"

const (
	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
)

var src = rand.NewSource(time.Now().UnixNano())

// RandStringBytesMaskImprSrc is a function highly optimized for generation of
// random strings in golang
func RandStringBytesMaskImprSrc(n int) string {
	b := make([]byte, n)
	// A src.Int63() generates 63 random bits, enough for letterIdxMax characters!
	for i, cache, remain := n-1, src.Int63(), letterIdxMax; i >= 0; {
		if remain == 0 {
			cache, remain = src.Int63(), letterIdxMax
		}
		if idx := int(cache & letterIdxMask); idx < len(letterBytes) {
			b[i] = letterBytes[idx]
			i--
		}
		cache >>= letterIdxBits
		remain--
	}

	return string(b)
}

// RandStringBytesMaskImprSrcBytes is a function highly optimized for generation of
// random slice of bytes using letters as source in golang
func RandStringBytesMaskImprSrcBytes(n int) []byte {
	b := make([]byte, n)
	// A src.Int63() generates 63 random bits, enough for letterIdxMax characters!
	for i, cache, remain := n-1, src.Int63(), letterIdxMax; i >= 0; {
		if remain == 0 {
			cache, remain = src.Int63(), letterIdxMax
		}
		if idx := int(cache & letterIdxMask); idx < len(letterBytes) {
			b[i] = letterBytes[idx]
			i--
		}
		cache >>= letterIdxBits
		remain--
	}

	return b
}

// RandRangeString returns a random string with using the min and max values as limit.
func RandRangeString(min, max int) string {
	randsize := rand.Intn(max - min)
	return RandStringBytesMaskImprSrc(min + randsize)
}

// RandRangeBytes returns a random []byte with the specified random size
func RandRangeBytes(min, max int) []byte {
	randsize := rand.Intn(max - min)
	return RandStringBytesMaskImprSrcBytes(min + randsize)
}