add Substract method for EntropyAccumulator

ackhandler/entropy_accumulator.go

@@ -12,6 +12,11 @@ func (e *EntropyAccumulator) Add(packetNumber protocol.PacketNumber, entropyFlag
 	}
 }
 
+// Add the contribution of the entropy flag of a given packet number
+func (e *EntropyAccumulator) Substract(packetNumber protocol.PacketNumber, entropyFlag bool) {
+	e.Add(packetNumber, entropyFlag)
+}
+
 // Get the byte of entropy
 func (e *EntropyAccumulator) Get() byte {
 	return byte(*e)

ackhandler/entropy_accumulator_test.go

@@ -11,15 +11,31 @@ var _ = Describe("EntropyAccumulator", func() {
 		Expect(e.Get()).To(BeZero())
 	})
 
-	It("adds entropy", func() {
-		var e EntropyAccumulator
-		e.Add(9, true)
-		Expect(e.Get()).To(Equal(byte(0x02)))
+	Context("Add", func() {
+		It("adds entropy", func() {
+			var e EntropyAccumulator
+			e.Add(9, true)
+			Expect(e.Get()).To(Equal(byte(0x02)))
+		})
+
+		It("doesn't add entropy for zero entropy flags", func() {
+			var e EntropyAccumulator
+			e.Add(9, false)
+			Expect(e.Get()).To(BeZero())
+		})
 	})
 
-	It("doesn't add entropy for zero entropy flags", func() {
-		var e EntropyAccumulator
-		e.Add(9, false)
-		Expect(e.Get()).To(BeZero())
+	Context("Substract", func() {
+		It("calculates the correct entropy", func() {
+			var e1 EntropyAccumulator
+			e1.Add(3, true)
+
+			var e2 EntropyAccumulator
+			e2.Add(1, true)
+			e2.Add(3, true)
+			e2.Substract(1, true)
+
+			Expect(e1).To(Equal(e2))
+		})
 	})
 })