[go] fast random byte string generator

go/rand_bytes.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"fmt"
+	"math/rand"
+	"time"
+)
+
+// reference: https://medium.com/@kpbird/golang-generate-fixed-size-random-string-dd6dbd5e63c0
+
+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 randBytes(n int) []byte {
+	// rand.NewSource is not thread-safe
+	// see https://github.com/golang/go/issues/21099#issuecomment-317973851
+	src := rand.NewSource(time.Now().UnixNano())
+
+	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
+}
+
+func main() {
+	fmt.Println(string(randBytes(10)))
+}