Rewriting the main() function for better testing

There exists a clever way that you can rewrite each main() function in order to make testing (and benchmarking) a lot easier. The main() function has a restriction, which is that you cannot call it from test code—this technique presents a solution to that problem using the code found in main.go. The import block is omitted to save space.

func main() {
    err := run(os.Args, os.Stdout)
    if err != nil {
        fmt.Printf("%s\n", err)
        return
    }
}

As we cannot have an executable program without a main() function, we have to create a minimalistic one. What main() does is call run(), which is our own customized version of main(), send the desired os.Args to it, and collect the return value of run():

func run(args []string, stdout io.Writer) error {
    if len(args) == 1 {
        return errors.New("No input!")
    }
    // Continue with the implementation of run()
    // as you would...