Each time you set the same seed, you get the same sequence. So of course if you’re setting the seed to the time in a fast loop, you’ll probably call it with the same seed many times.
In your case, as you’re calling your randInt function until you have a different value, you’re waiting for the time (as returned by Nano) to change.
As for all pseudo-random libraries, you have to set the seed only once, for example when initializing your program unless you specifically need to reproduce a given sequence (which is usually only done for debugging and unit testing).
After that you simply call Intn to get the next random integer.
Move the rand.Seed(time.Now().UTC().UnixNano()) line from the randInt function to the start of the main and everything will be faster. And lose the .UTC() call since:
UnixNano returns t as a Unix time, the number of nanoseconds elapsed since January 1, 1970 UTC.
Note also that I think you can simplify your string building:
package main
import (
"fmt"
"math/rand"
"time"
)
func main() {
rand.Seed(time.Now().UnixNano())
fmt.Println(randomString(10))
}
func randomString(l int) string {
bytes := make([]byte, l)
for i := 0; i < l; i++ {
bytes[i] = byte(randInt(65, 90))
}
return string(bytes)
}
func randInt(min int, max int) int {
return min + rand.Intn(max-min)
}