decodePacket.c

00001 /**
00002  * Rapid-prototyping protection schemes with IEC 61850
00003  *
00004  * Copyright (c) 2012 Steven Blair
00005  *
00006  * This program is free software; you can redistribute it and/or
00007  * modify it under the terms of the GNU General Public License
00008  * as published by the Free Software Foundation; either version 2
00009  * of the License, or (at your option) any later version.
00010 
00011  * This program is distributed in the hope that it will be useful,
00012  * but WITHOUT ANY WARRANTY; without even the implied warranty of
00013  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
00014  * GNU General Public License for more details.
00015 
00016  * You should have received a copy of the GNU General Public License
00017  * along with this program; if not, write to the Free Software
00018  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
00019  */
00020 
00021 #include "decodePacket.h"
00022 //#include "svEncodeBasic.h"
00023 
00024 
00025 // returns size of length field, from first byte
00026 int getLengthFieldSize(unsigned char byte) {
00027     if (byte <= 126) {
00028         return 1;
00029     }
00030     else {
00031         return 1 + (byte & 0x7F);
00032     }
00033 }
00034 
00035 //TODO    check this works for all inputs, e.g., buf[1] = 0xFF
00036 int decodeLength(unsigned char *buf) {
00037     if (buf[0] <= 126) {
00038         return buf[0];
00039     }
00040     else {
00041         int bytes = (buf[0] & 0x7F);
00042 
00043         if (bytes == 1) {
00044             return buf[1];
00045         }
00046         else if (bytes == 2) {    // assumes max length of 2^15
00047             return ((buf[1] & 0xFF) << 8) | (buf[2] & 0xFF);
00048         }
00049     }
00050 
00051     return 0;
00052 }