Implement MapContinuation.

This is used to map a continuation packet in VP8, which does
not necessarily carry a valid PID.

packetmap/packetmap.go

@@ -61,6 +61,33 @@ func (m *Map) Map(seqno uint16, pid uint16) (bool, uint16, uint16) {
 	return m.direct(seqno)
 }
 
+// MapContinuation is like map, but used for padding packets that don't
+// have a pid.
+func (m *Map) MapContinuation(seqno uint16) (bool, uint16, uint16) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if m.delta == 0 && m.entries == nil {
+		if m.next == seqno {
+			m.next = seqno + 1
+		}
+		return true, seqno, 0
+	}
+
+	if m.next == seqno {
+		addMapping(m, seqno, m.delta, m.pidDelta)
+		m.next = seqno + 1
+		return true, seqno + m.delta, m.pidDelta
+	}
+
+	if compare(m.next, seqno) <= 0 {
+		return false, 0, 0
+	}
+
+	return m.direct(seqno)
+}
+
+
 func (m *Map) reset() {
 	m.next = 0
 	m.nextPid = 0

packetmap/packetmap_test.go

@@ -141,6 +141,40 @@ func TestDrop(t *testing.T) {
 	}
 }
 
+func TestPadding(t *testing.T) {
+	m := Map{}
+
+	ok, s, p := m.Map(42, 1001)
+	if !ok || s != 42 || p != 0 {
+		t.Errorf("Expected 42, 0, got %v, %v, %v", ok, s, p)
+	}
+
+	ok, s, p = m.MapContinuation(43)
+	if !ok || s != 43 || p != 0 {
+		t.Errorf("Expected 43, 0, got %v %v %v", ok, s, p)
+	}
+
+	ok, s, p = m.Map(43, 1001)
+	if !ok || s != 43 || p != 0 {
+		t.Errorf("Expected 43, 0, got %v %v %v", ok, s, p)
+	}
+
+	ok, s, p = m.Map(44, 1002)
+	if !ok || s != 44 || p != 0 {
+		t.Errorf("Expected 44, 0, got %v %v %v", ok, s, p)
+	}
+
+	ok = m.Drop(45, 1003)
+	if !ok {
+		t.Errorf("Not ok")
+	}
+
+	ok, s, p = m.MapContinuation(46)
+	if !ok || s != 45 || p != 1 {
+		t.Errorf("Expected 45, 0, got %v %v %v", ok, s, p)
+	}
+}
+
 func TestWraparound(t *testing.T) {
 	m := Map{}