move the packet number generator to the ackhandler package

internal/ackhandler/packet_number_generator.go

@@ -0,0 +1,62 @@
+package ackhandler
+
+import (
+	"crypto/rand"
+	"math"
+
+	"github.com/lucas-clemente/quic-go/internal/protocol"
+)
+
+// The packetNumberGenerator generates the packet number for the next packet
+// it randomly skips a packet number every averagePeriod packets (on average)
+// it is guarantued to never skip two consecutive packet numbers
+type packetNumberGenerator struct {
+	averagePeriod protocol.PacketNumber
+
+	next       protocol.PacketNumber
+	nextToSkip protocol.PacketNumber
+}
+
+func newPacketNumberGenerator(initial, averagePeriod protocol.PacketNumber) *packetNumberGenerator {
+	g := &packetNumberGenerator{
+		next:          initial,
+		averagePeriod: averagePeriod,
+	}
+	g.generateNewSkip()
+	return g
+}
+
+func (p *packetNumberGenerator) Peek() protocol.PacketNumber {
+	return p.next
+}
+
+func (p *packetNumberGenerator) Pop() protocol.PacketNumber {
+	next := p.next
+
+	// generate a new packet number for the next packet
+	p.next++
+
+	if p.next == p.nextToSkip {
+		p.next++
+		p.generateNewSkip()
+	}
+
+	return next
+}
+
+func (p *packetNumberGenerator) generateNewSkip() {
+	num := p.getRandomNumber()
+	skip := protocol.PacketNumber(num) * (p.averagePeriod - 1) / (math.MaxUint16 / 2)
+	// make sure that there are never two consecutive packet numbers that are skipped
+	p.nextToSkip = p.next + 2 + skip
+}
+
+// getRandomNumber() generates a cryptographically secure random number between 0 and MaxUint16 (= 65535)
+// The expectation value is 65535/2
+func (p *packetNumberGenerator) getRandomNumber() uint16 {
+	b := make([]byte, 2)
+	rand.Read(b) // ignore the error here
+
+	num := uint16(b[0])<<8 + uint16(b[1])
+	return num
+}