Add delay and backoff strategies

backoff.go

@@ -3,6 +3,16 @@ package retry
 import "time"
 
 // NoBackoff defines no backoff strategy
-func NoBackoff() time.Duration {
+func NoBackoff(_ uint, _ time.Duration) time.Duration {
 	return time.Duration(0)
 }
+
+// ConstantBackoff defines constant backoff strategy
+func ConstantBackoff(_ uint, delay time.Duration) time.Duration {
+	return delay
+}
+
+// ExponentialBackoff defines exponential backoff strategy
+func ExponentialBackoff(n uint, delay time.Duration) time.Duration {
+	return delay * (1 << n)
+}

retryer.go

@@ -9,7 +9,10 @@ type Retryer struct {
 
 	// Backoff defines a backoff function that returns `time.Duration`
 	// This is applied on subsequent attempts.
-	Backoff func() time.Duration
+	Backoff func(n uint, delay time.Duration) time.Duration
+
+	// Delay defines duration to delay
+	Delay time.Duration
 }
 
 // Do executes `fn` and returns success if `fn` executed succesfully and any errors that occurred
@@ -30,7 +33,7 @@ func (retryer Retryer) Do(fn func() error) (bool, Errors) {
 	for n < retryer.Attempts {
 		// Backoff after first attempt only
 		if n > 0 {
-			time.Sleep(retryer.Backoff())
+			time.Sleep(retryer.Backoff(n, retryer.Delay))
 		}
 
 		err := fn()

retryer_test.go

@@ -9,7 +9,9 @@ import (
 )
 
 func TestRetry_Do_Success(t *testing.T) {
-	retryer := retry.Retryer{}
+	retryer := retry.Retryer{
+		Backoff: retry.ConstantBackoff,
+	}
 	ok, err := retryer.Do(func() error {
 		return nil
 	})