feat: Add ConstantBackoff: always return the same duration

policies/constant.go

@@ -0,0 +1,13 @@
+package policies
+
+import "time"
+
+func GetConstantPolicy(duration time.Duration) Policy {
+	return func(count int) time.Duration {
+		if count == 0 {
+			return 0
+		}
+
+		return duration
+	}
+}

policies/constant_test.go

@@ -0,0 +1,27 @@
+package policies_test
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/honestbank/backoff-policy/policies"
+)
+
+func TestGetConstantPolicy(t *testing.T) {
+	t.Run("returns correct duration based on count", func(t *testing.T) {
+		policy := policies.GetConstantPolicy(time.Millisecond * 100)
+		assert.Equal(t, time.Millisecond*0, policy(0))
+		assert.Equal(t, time.Millisecond*100, policy(1))
+		assert.Equal(t, time.Millisecond*100, policy(2))
+		assert.Equal(t, time.Millisecond*100, policy(3))
+	})
+	t.Run("value is capped at max count", func(t *testing.T) {
+		policy := policies.GetConstantPolicy(time.Millisecond * 100)
+		assert.Equal(t, policy(10), policy(10))
+		assert.Equal(t, policy(10), policy(11))
+		assert.Equal(t, policy(10), policy(12))
+		assert.Equal(t, policy(10), policy(13))
+	})
+}