Optimize random string generator to avoid multiple locks & use int bi…

…t-masking

staging/src/k8s.io/apimachinery/pkg/util/rand/rand.go

@@ -70,17 +70,42 @@ func Perm(n int) []int {
 	return rng.rand.Perm(n)
 }
 
-// We omit vowels from the set of available characters to reduce the chances
-// of "bad words" being formed.
-var alphanums = []rune("bcdfghjklmnpqrstvwxz2456789")
+const (
+	// We omit vowels from the set of available characters to reduce the chances
+	// of "bad words" being formed.
+	alphanums = "bcdfghjklmnpqrstvwxz2456789"
+	// No. of bits required to index into alphanums string.
+	alphanumsIdxBits = 5
+	// Mask used to extract last alphanumsIdxBits of an int.
+	alphanumsIdxMask = 1<<alphanumsIdxBits - 1
+	// No. of random letters we can extract from a single int63.
+	maxAlphanumsPerInt = 63 / alphanumsIdxBits
+)
 
 // String generates a random alphanumeric string, without vowels, which is n
 // characters long.  This will panic if n is less than zero.
-func String(length int) string {
-	b := make([]rune, length)
-	for i := range b {
-		b[i] = alphanums[Intn(len(alphanums))]
-	}
+// How the random string is created:
+// - we generate random int63's
+// - from each int63, we are extracting multiple random letters by bit-shifting and masking
+// - if some index is out of range of alphanums we neglect it (unlikely to happen multiple times in a row)
+func String(n int) string {
+	b := make([]byte, n)
+	rng.Lock()
+	defer rng.Unlock()
+
+	randomInt63 := rng.rand.Int63()
+	remaining := maxAlphanumsPerInt
+    for i := 0; i < n; {
+        if remaining == 0 {
+            randomInt63, remaining = rng.rand.Int63(), maxAlphanumsPerInt
+        }
+        if idx := int(randomInt63 & alphanumsIdxMask); idx < len(alphanums) {
+            b[i] = alphanums[idx]
+            i++
+        }
+        randomInt63 >>= alphanumsIdxBits
+        remaining--
+    }
 	return string(b)
 }