feat: Add utils for received stream stats (#3)

* feat: Add received stream stats

* feat: Make constructor parameter implicit

vpx_rtp/received_rtp_stream_statistics.py

@@ -0,0 +1,69 @@
+from vpx_rtp.rtp import RtpPacket, VIDEO_CLOCK_RATE
+
+# From https://github.com/aiortc/aiortc/blob/22699ea879f93b6d6dd1af4a200d37b9ff560870/src/aiortc/rtcrtpreceiver.py
+
+class ReceivedRtpStreamStatistics:
+    def __init__(self) -> None:
+        self.base_seq: Optional[int] = None
+        self.max_seq: Optional[int] = None
+        self.cycles = 0
+        self.packets_received = 0
+
+        # jitter
+        self._jitter_q4 = 0
+        self._last_arrival: Optional[int] = None
+        self._last_timestamp: Optional[int] = None
+
+        # fraction lost
+        self._expected_prior = 0
+        self._received_prior = 0
+
+    def add(self, packet: RtpPacket) -> None:
+        in_order = self.max_seq is None or uint16_gt(
+            packet.sequence_number, self.max_seq
+        )
+        self.packets_received += 1
+
+        if self.base_seq is None:
+            self.base_seq = packet.sequence_number
+
+        if in_order:
+            arrival = int(time.time() * VIDEO_CLOCK_RATE)
+
+            if self.max_seq is not None and packet.sequence_number < self.max_seq:
+                self.cycles += 1 << 16
+            self.max_seq = packet.sequence_number
+
+            if packet.timestamp != self._last_timestamp and self.packets_received > 1:
+                diff = abs(
+                    (arrival - self._last_arrival)
+                    - (packet.timestamp - self._last_timestamp)
+                )
+                self._jitter_q4 += diff - ((self._jitter_q4 + 8) >> 4)
+
+            self._last_arrival = arrival
+            self._last_timestamp = packet.timestamp
+
+    @property
+    def fraction_lost(self) -> int:
+        expected_interval = self.packets_expected - self._expected_prior
+        self._expected_prior = self.packets_expected
+        received_interval = self.packets_received - self._received_prior
+        self._received_prior = self.packets_received
+        lost_interval = expected_interval - received_interval
+        if expected_interval == 0 or lost_interval <= 0:
+            return 0
+        else:
+            return (lost_interval << 8) // expected_interval
+
+    @property
+    def jitter(self) -> int:
+        return self._jitter_q4 >> 4
+
+    @property
+    def packets_expected(self) -> int:
+        return self.cycles + self.max_seq - self.base_seq + 1
+
+    @property
+    def packets_lost(self) -> int:
+        return clamp_packets_lost(self.packets_expected - self.packets_received)