fix issues/3: Starting from the second round, the first object will be fetched twice

[superbear]

Description

Starting from the second round, the first object will be fetched twice

Code

// go version: 1.18
package main

import (
	"fmt"
	"net/url"

	roundrobin "github.com/thegeekyasian/round-robin-go"
)

func main() {
	rr, _ := roundrobin.New(
		&url.URL{Host: "192.168.0.1"},
		&url.URL{Host: "192.168.0.2"},
		&url.URL{Host: "192.168.0.3"},
		&url.URL{Host: "192.168.0.4"},
	)

	for i := 0; i < 20; i++ {
		fmt.Println(rr.Next().Host)
	}
}

// output
192.168.0.1
192.168.0.2
192.168.0.3
192.168.0.4
192.168.0.1
192.168.0.1
192.168.0.2
192.168.0.3
192.168.0.4
192.168.0.1
192.168.0.1

Why？

when the next field is greater than the length of the object, the first object will be fetched and next will be reset to 0. The next time the Next method is called, next field will be 1, 1 - 1 = 0, so the first object will be fetched.

// Next returns the next object.
func (r *RoundRobin[O]) Next() *O {
	n := atomic.AddUint32(&r.next, 1)

	if int(n) > len(r.objects) {
		atomic.StoreUint32(&r.next, 0) // reset next field
		n = 1
	}
	return r.objects[(int(n)-1)%len(r.objects)]
}

[thegeekyasian]

@superbear Good catch, thank you for raising the PR with the fix 🚀