move stream parser/deframer to the protocol module

[loctrkd.git] / gps303 / gps303proto.py

diff --git a/gps303/gps303proto.py b/gps303/gps303proto.py
index 0a222d05634930679995e6e4807d939d35104346..e6a80fe5118f4df0c8370539290a13c44e6de821 100755 (executable)
--- a/gps303/gps303proto.py
+++ b/gps303/gps303proto.py
@@ -18,6 +18,7 @@ from datetime import datetime, timezone
 from enum import Enum
 from inspect import isclass
 from struct import error, pack, unpack
+from time import time
 from typing import (
     Any,
     Callable,
@@ -31,6 +32,8 @@ from typing import (
 )
 
 __all__ = (
+    "GPS303Conn",
+    "StreamError",
     "class_by_prefix",
     "inline_response",
     "parse_message",
@@ -77,6 +80,76 @@ __all__ = (
     "UNKNOWN_B3",
 )
 
+### Deframer ###
+
+MAXBUFFER: int = 4096
+
+
+class StreamError(Exception):
+    pass
+
+
+class GPS303Conn:
+    def __init__(self) -> None:
+        self.buffer = b""
+
+    @staticmethod
+    def enframe(buffer: bytes) -> bytes:
+        return b"xx" + buffer + b"\r\n"
+
+    def recv(self, segment: bytes) -> List[bytes]:
+        when = time()
+        self.buffer += segment
+        if len(self.buffer) > MAXBUFFER:
+            # We are receiving junk. Let's drop it or we run out of memory.
+            self.buffer = b""
+            raise StreamError(
+                f"More than {MAXBUFFER} unparseable data, dropping"
+            )
+        msgs = []
+        while True:
+            framestart = self.buffer.find(b"xx")
+            if framestart == -1:  # No frames, return whatever we have
+                break
+            if framestart > 0:  # Should not happen, report
+                self.buffer = self.buffer[framestart:]
+                raise StreamError(
+                    f'Undecodable data ({framestart}) "{self.buffer[:framestart][:64].hex()}"'
+                )
+            # At this point, buffer starts with a packet
+            if len(self.buffer) < 6:  # no len and proto - cannot proceed
+                break
+            exp_end = self.buffer[2] + 3  # Expect '\r\n' here
+            frameend = 0
+            # Length field can legitimeely be much less than the
+            # length of the packet (e.g. WiFi positioning), but
+            # it _should not_ be greater. Still sometimes it is.
+            # Luckily, not by too much: by maybe two or three bytes?
+            # Do this embarrassing hack to avoid accidental match
+            # of some binary data in the packet against '\r\n'.
+            while True:
+                frameend = self.buffer.find(b"\r\n", frameend + 1)
+                if frameend == -1 or frameend >= (
+                    exp_end - 3
+                ):  # Found realistic match or none
+                    break
+            if frameend == -1:  # Incomplete frame, return what we have
+                break
+            packet = self.buffer[2:frameend]
+            self.buffer = self.buffer[frameend + 2 :]
+            if len(packet) < 2:  # frameend comes too early
+                raise StreamError(f"Packet too short: {packet.hex()}")
+            msgs.append(packet)
+        return msgs
+
+    def close(self) -> bytes:
+        ret = self.buffer
+        self.buffer = b""
+        return ret
+
+
+### Parser/Constructor ###
+
 
 class DecodeError(Exception):
     def __init__(self, e: Exception, **kwargs: Any) -> None: