feat: check unused package (#232)

* feat: check unused package

update edganiukov/fcm to appleboy/go-fcm

* update readme

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* update comment

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

vendor/github.com/appleboy/go-fcm/retry.go

@@ -0,0 +1,34 @@
+package fcm
+
+import (
+	"net"
+	"time"
+)
+
+const (
+	minBackoff = 100 * time.Millisecond
+	maxBackoff = 1 * time.Minute
+	factor     = 2.7
+)
+
+func retry(fn func() error, attempts int) error {
+	var attempt int
+	for {
+		err := fn()
+		if err == nil {
+			return nil
+		}
+
+		if tErr, ok := err.(net.Error); !ok || !tErr.Temporary() {
+			return err
+		}
+
+		attempt++
+		backoff := minBackoff * time.Duration(attempt*attempt)
+		if attempt > attempts || backoff > maxBackoff {
+			return err
+		}
+
+		time.Sleep(backoff)
+	}
+}