par_rand_append_loop.go

package main

import (
    "fmt"
    "math/rand"
    "strings"
    "sync"
)

func main() {
    str := "REALLY LONG STRING"

    strLength := len(str)

    var result []string

    var mutex = &sync.Mutex{}
    var wg sync.WaitGroup

    wg.Add(strLength)

    for i := 0; i < strLength; i++ {
        go func(i int) {
            defer wg.Done()
            char := str[i]

            random := rand.Intn(1000)

            mutex.Lock()

            obf := string(fmt.Sprintf("%v%v", string(char), random))

            result = append(result, obf)

            mutex.Unlock()
        }(i)
    }

    wg.Wait()

    allSaidAndDone := strings.Join(result, "")

    fmt.Println(allSaidAndDone)
}