add incremental crc calculation

re-using the enet code

source/enet/include/enet/enet.h

@@ -519,6 +519,7 @@ ENET_API ENetPacket * enet_packet_create (const void *, size_t, enet_uint32);
 ENET_API void         enet_packet_destroy (ENetPacket *);
 ENET_API int          enet_packet_resize  (ENetPacket *, size_t);
 ENET_API enet_uint32  enet_crc32 (const ENetBuffer *, size_t);
+ENET_API void         enet_crc32_inc (enet_uint32 *crc, enet_uint8 byte);
 
 ENET_API ENetHost * enet_host_create (const ENetAddress *, size_t, size_t, enet_uint32, enet_uint32);
 ENET_API void       enet_host_destroy (ENetHost *);

source/enet/packet.c

@@ -161,4 +161,12 @@ enet_crc32 (const ENetBuffer * buffers, size_t bufferCount)
     return ENET_HOST_TO_NET_32 (~ crc);
 }
 
+void
+enet_crc32_inc (enet_uint32 *crc, enet_uint8 byte)
+{
+    if (! initializedCRC32) initialize_crc32 ();
+
+    *crc = (*crc >> 8) ^ crcTable [(*crc & 0xFF) ^ byte];
+}
+
 /** @} */