diff --git a/main.go b/main.go
index ed8bcf1..7e375a0 100644
--- a/main.go
+++ b/main.go
@@ -30,6 +30,11 @@ type redisPoolConf struct {
 }
 
 const letterBytes = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
+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
+)
 
 const defaultPort int = 8002
 const defaultExpire = 90
@@ -160,9 +165,20 @@ func longToShort(longUrl string, ttl int) string {
 // 产生一个63位随机整数，除以字符数取余获取对应字符
 func generate(bits int) string {
 	b := make([]byte, bits)
-	for i := range b {
-		b[i] = letterBytes[rand.Int63()%int64(len(letterBytes))]
+
+	// A rand.Int63() generates 63 random bits, enough for letterIdxMax letters!
+	for i, cache, remain := bits-1, rand.Int63(), letterIdxMax; i >= 0; {
+		if remain == 0 {
+			cache, remain = rand.Int63(), letterIdxMax
+		}
+		if idx := int(cache & letterIdxMask); idx < len(letterBytes) {
+			b[i] = letterBytes[idx]
+			i--
+		}
+		cache >>= letterIdxBits
+		remain--
 	}
+
 	return string(b)
 }