add funcs CoalesceStrategy and FirstStrategy

Author: pmallela

Diff for: backoff/strategy.go

@@ -98,3 +98,24 @@ func WithTransforms(s Strategy, transforms ...linger.DurationTransform) Strategy
 		return d
 	}
 }
+
+// CoalesceStrategy returns a strategy that iterates over the given strategies
+// and runs them, returning the first positive duration.
+func CoalesceStrategy(strategies ...Strategy) Strategy {
+	return FirstStrategy(linger.Positive, strategies...)
+}
+
+// FirstStrategy returns a strategy that iterates over the given strategies
+// and runs them, returning the first duration which satisfies the predicate.
+// Return zero if no duration satisfies the predicate.
+func FirstStrategy(p linger.DurationPredicate, strategies ...Strategy) Strategy {
+	return func(e error, n uint) time.Duration {
+		for _, s := range strategies {
+			d := s(e, n)
+			if p(d) {
+				return d
+			}
+		}
+		return 0
+	}
+}

Diff for: backoff/strategy_test.go

@@ -108,3 +108,41 @@ var _ = Describe("func WithTransform()", func() {
 		Expect(s(nil, 2)).To(Equal(25 * time.Second))
 	})
 })
+
+var _ = Describe("func CoalesceStrategy()", func() {
+	It("returns a strategy that yields the first positive duration", func() {
+
+		stgyOne := func(_ error, n uint) time.Duration {
+			if n == 1 {
+				return 6 * time.Second
+			}
+			return 0
+		}
+
+		stgyTwo := func(_ error, n uint) time.Duration {
+			if n == 2 {
+				return 9 * time.Second
+			}
+			return 0
+		}
+
+		s := CoalesceStrategy(
+			stgyOne,
+			stgyTwo,
+		)
+
+		sC := CoalesceStrategy(
+			s,
+			Constant(3*time.Second),
+		)
+
+		Expect(s(nil, 0)).To(Equal(0 * time.Second))
+		Expect(s(nil, 1)).To(Equal(6 * time.Second))
+		Expect(s(nil, 2)).To(Equal(9 * time.Second))
+
+		Expect(sC(nil, 0)).To(Equal(3 * time.Second))
+		Expect(sC(nil, 1)).To(Equal(6 * time.Second))
+		Expect(sC(nil, 2)).To(Equal(9 * time.Second))
+
+	})
+})