port: net: replace useless checksum with slightly more useful crc16

port/src/net/net.c

@@ -368,11 +368,14 @@ static void netServerQueryResponse(ENetAddress *address)
 
 	// calculate and rewrite checksum
 	buf.wp = ebuf.dataLength - sizeof(u16);
-	u32 chk = 1;
+	u16 crc = 0xFFFF;
+	u16 x;
 	for (u32 i = 0; i < buf.wp; ++i) {
-		chk = (chk + buf.data[i]) % 65521;
+		x = crc >> 8 ^ buf.data[i];
+		x ^= x >> 4;
+		crc += (crc << 8) ^ (x << 12) ^ (x << 5) ^ x;
 	}
-	netbufWriteU16(&buf, chk);
+	netbufWriteU16(&buf, crc);
 
 	enet_socket_send(g_NetHost->socket, address, &ebuf, 1);
 }

tools/query.py

@@ -9,10 +9,13 @@ QUERY_MAGIC = b"PDQM\x01"
 MAX_WAIT = 5.0
 
 def checksum(data):
-  chk = 1
+  crc = 0xFFFF
   for b in data[:-2]:
-    chk = (chk + b) % 65521
-  return chk
+    x = crc >> 8 ^ b
+    x ^= x >> 4
+    crc += (crc << 8) ^ (x << 12) ^ (x << 5) ^ x
+    crc &= 0xFFFF
+  return crc
 
 def eat_string(data):
   strlen = struct.unpack_from("<H", data)[0]