refactor to extract ntp update logic

adafruit_ntp.py

@@ -1,4 +1,4 @@
 # SPDX-FileCopyrightText: 2022 Scott Shawcroft for Adafruit Industries
 #
 # SPDX-License-Identifier: MIT
 
@@ -32,13 +32,13 @@
 PACKET_SIZE = const(48)
 
 
-class NTP:
+class NTP:  # pylint:disable=too-many-instance-attributes
     """Network Time Protocol (NTP) helper module for CircuitPython.
     This module does not handle daylight savings or local time. It simply requests
     UTC from a NTP server.
     """
 
-    def __init__(
+    def __init__(  # pylint:disable=too-many-arguments
         self,
         socketpool,
         *,
@@ -70,45 +70,69 @@
 
         # This is our estimated start time for the monotonic clock. We adjust it based on the ntp
         # responses.
-        self._monotonic_start = 0
+        self._monotonic_start_ns = 0
 
         self.next_sync = 0
 
+    def _update_time_sync(self) -> None:
+        """Update the time sync value. Raises OSError exception if no response
+        is received within socket_timeout seconds, ArithmeticError for substantially incorrect
+        NTP results."""
+        if self._socket_address is None:
+            self._socket_address = self._pool.getaddrinfo(
+                self._server, self._port)[0][4]
+
+        self._packet[0] = 0b00100011  # Not leap second, NTP version 4, Client mode
+        for i in range(1, PACKET_SIZE):
+            self._packet[i] = 0
+        with self._pool.socket(self._pool.AF_INET, self._pool.SOCK_DGRAM) as sock:
+            sock.settimeout(self._socket_timeout)
+            local_send_ns = time.monotonic_ns()  # expanded
+            sock.sendto(self._packet, self._socket_address)
+            sock.recv_into(self._packet)
+            # Get the time in the context to minimize the difference between it and receiving
+            # the packet.
+            local_recv_ns = time.monotonic_ns()  # was destination
+
+        poll = struct.unpack_from("!B", self._packet, offset=2)[0]
+
+        cache_offset_s = max(2**poll, self._cache_seconds)
+        self.next_sync = local_recv_ns + cache_offset_s * 1_000_000_000
+
+        srv_recv_s, srv_recv_f = struct.unpack_from("!II", self._packet, offset=32)
+        srv_send_s, srv_send_f = struct.unpack_from("!II", self._packet, offset=40)
+
+        # Convert the server times from NTP to UTC for local use
+        srv_recv_ns = (srv_recv_s - NTP_TO_UNIX_EPOCH) * 1_000_000_000 + \
+                      (srv_recv_f * 1_000_000_000 // 2**32)
+        srv_send_ns = (srv_send_s - NTP_TO_UNIX_EPOCH) * 1_000_000_000 + \
+                      (srv_send_f * 1_000_000_000 // 2**32)
+
+        # _round_trip_delay = (local_recv_ns - local_send_ns) - (srv_send_ns - srv_recv_ns)
+        # Calculate (best estimate) offset between server UTC and board monotonic_ns time
+        clock_offset = ((srv_recv_ns - local_send_ns) + (srv_send_ns - local_recv_ns)) // 2
+
+        self._monotonic_start_ns = clock_offset + self._tz_offset * 1_000_000_000
+
     @property
     def datetime(self) -> time.struct_time:
         """Current time from NTP server. Accessing this property causes the NTP time request,
-        unless there has already been a recent request. Raises OSError exception if no response
-        is received within socket_timeout seconds, ArithmeticError for substantially incorrect
-        NTP results."""
+        unless there has already been a recent request."""
         if time.monotonic_ns() > self.next_sync:
-            if self._socket_address is None:
-                self._socket_address = self._pool.getaddrinfo(self._server, self._port)[
-                    0
-                ][4]
-
-            self._packet[0] = 0b00100011  # Not leap second, NTP version 4, Client mode
-            for i in range(1, PACKET_SIZE):
-                self._packet[i] = 0
-            with self._pool.socket(self._pool.AF_INET, self._pool.SOCK_DGRAM) as sock:
-                sock.settimeout(self._socket_timeout)
-                sock.sendto(self._packet, self._socket_address)
-                sock.recv_into(self._packet)
-                # Get the time in the context to minimize the difference between it and receiving
-                # the packet.
-                destination = time.monotonic_ns()
-            poll = struct.unpack_from("!B", self._packet, offset=2)[0]
-
-            cache_offset = max(2**poll, self._cache_seconds)
-            self.next_sync = destination + cache_offset * 1_000_000_000
-            seconds = struct.unpack_from("!I", self._packet, offset=PACKET_SIZE - 8)[0]
-
-            self._monotonic_start = (
-                seconds
-                + self._tz_offset
-                - NTP_TO_UNIX_EPOCH
-                - (destination // 1_000_000_000)
-            )
-
-        return time.localtime(
-            time.monotonic_ns() // 1_000_000_000 + self._monotonic_start
-        )
+            self._update_time_sync()
+
+        # Calculate the current time based on the current and start monotonic times
+        current_time_s = (time.monotonic_ns() + self._monotonic_start_ns) // 1_000_000_000
+
+        return time.localtime(current_time_s)
+
+    @property
+    def utc_ns(self) -> int:
+        """UTC (unix epoch) time in nanoseconds. Accessing this property causes the NTP time
+        request, unless there has already been a recent request. Raises OSError exception if
+        no response is received within socket_timeout seconds, ArithmeticError for substantially
+        incorrect NTP results."""
+        if time.monotonic_ns() > self.next_sync:
+            self._update_time_sync()
+
+        return time.monotonic_ns() + self._monotonic_start_ns