move EntropyAccumulator to AckHandler package

ackhandler/entropy_accumulator.go

@@ -0,0 +1,18 @@
+package ackhandler
+
+import "github.com/lucas-clemente/quic-go/protocol"
+
+// EntropyAccumulator accumulates the entropy according to the QUIC docs
+type EntropyAccumulator byte
+
+// Add the contribution of the entropy flag of a given packet number
+func (e *EntropyAccumulator) Add(packetNumber protocol.PacketNumber, entropyFlag bool) {
+	if entropyFlag {
+		(*e) ^= 0x01 << (packetNumber % 8)
+	}
+}
+
+// Get the byte of entropy
+func (e *EntropyAccumulator) Get() byte {
+	return byte(*e)
+}

ackhandler/entropy_accumulator_test.go

@@ -0,0 +1,25 @@
+package ackhandler
+
+import (
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("EntropyAccumulator", func() {
+	It("initializes as zero", func() {
+		var e EntropyAccumulator
+		Expect(e.Get()).To(BeZero())
+	})
+
+	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())
+	})
+})