Rewrite CRC to use bytewise CRC instead of bitwise. ~8x faster.

include/gr_air_modes/modes_crc.h

@@ -23,7 +23,7 @@
 #ifndef INCLUDED_MODES_CRC_H
 #define INCLUDED_MODES_CRC_H
 extern const unsigned int modes_crc_table[112];
-int modes_check_crc(unsigned char data[], int length);
+unsigned int modes_check_crc(unsigned char data[], int length);
 bruteResultTypeDef modes_ec_brute(modes_packet &err_packet);
 unsigned next_set_of_n_elements(unsigned x);
 

lib/modes_crc.cc

@@ -1,25 +1,24 @@
 /*
- * Copyright 2007 Free Software Foundation, Inc.
+ * Copyright 2013 Nick Foster
  *
- * This file is part of GNU Radio
+ * This file is part of gr-air-modes
  *
- * GNU Radio is free software; you can redistribute it and/or modify
+ * gr-air-modes is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
  * the Free Software Foundation; either version 2, or (at your option)
  * any later version.
  *
- * GNU Radio is distributed in the hope that it will be useful,
+ * gr-air-modes is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
- * along with GNU Radio; see the file COPYING.  If not, write to
+ * along with gr-air-modes; see the file COPYING.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street,
  * Boston, MA 02110-1301, USA.
  */
 
-//this is copied almost verbatim from Eric Cottrell's gr-air platform.
 
 #ifdef HAVE_CONFIG_H
 #include "config.h"
@@ -31,135 +30,34 @@
 #include <math.h>
 #include <stdlib.h>
 
-/*  Mode S Parity Table
- *   Index is bit position with bit 0 being the first bit after preamble
- *   On short frames an offset of 56 is used.
-*/
-const unsigned int modes_crc_table[112] =
-{
-    0x3935ea,  // Start of Long Frame CRC
-    0x1c9af5,
-    0xf1b77e,
-    0x78dbbf,
-    0xc397db,
-    0x9e31e9,
-    0xb0e2f0,
-    0x587178,
-    0x2c38bc,
-    0x161c5e,
-    0x0b0e2f,
-    0xfa7d13,
-    0x82c48d,
-    0xbe9842,
-    0x5f4c21,
-    0xd05c14,
-    0x682e0a,
-    0x341705,
-    0xe5f186,
-    0x72f8c3,
-    0xc68665,
-    0x9cb936,
-    0x4e5c9b,
-    0xd8d449,
-    0x939020,
-    0x49c810,
-    0x24e408,
-    0x127204,
-    0x093902,
-    0x049c81,
-    0xfdb444,
-    0x7eda22,
-    0x3f6d11, // Extended 56 bit field
-    0xe04c8c,
-    0x702646,
-    0x381323,
-    0xe3f395,
-    0x8e03ce,
-    0x4701e7,
-    0xdc7af7,
-    0x91c77f,
-    0xb719bb,
-    0xa476d9,
-    0xadc168,
-    0x56e0b4,
-    0x2b705a,
-    0x15b82d,
-    0xf52612,
-    0x7a9309,
-    0xc2b380,
-    0x6159c0,
-    0x30ace0,
-    0x185670,
-    0x0c2b38,
-    0x06159c,
-    0x030ace,
-    0x018567,
-    0xff38b7,  // Start of Short Frame CRC
-    0x80665f,
-    0xbfc92b,
-    0xa01e91,
-    0xaff54c,
-    0x57faa6,
-    0x2bfd53,
-    0xea04ad,
-    0x8af852,
-    0x457c29,
-    0xdd4410,
-    0x6ea208,
-    0x375104,
-    0x1ba882,
-    0x0dd441,
-    0xf91024,
-    0x7c8812,
-    0x3e4409,
-    0xe0d800,
-    0x706c00,
-    0x383600,
-    0x1c1b00,
-    0x0e0d80,
-    0x0706c0,
-    0x038360,
-    0x01c1b0,
-    0x00e0d8,
-    0x00706c,
-    0x003836,
-    0x001c1b,
-    0xfff409,
-    0x800000,   // 24 PI or PA bits
-    0x400000,
-    0x200000,
-    0x100000,
-    0x080000,
-    0x040000,
-    0x020000,
-    0x010000,
-    0x008000,
-    0x004000,
-    0x002000,
-    0x001000,
-    0x000800,
-    0x000400,
-    0x000200,
-    0x000100,
-    0x000080,
-    0x000040,
-    0x000020,
-    0x000010,
-    0x000008,
-    0x000004,
-    0x000002,
-    0x000001,
-};
+unsigned int crc_table[256];
+const unsigned int POLY=0xFFF409;
 
-int modes_check_crc(unsigned char data[], int length)
+//generate a bytewise lookup CRC table
+
+void generate_crc_table(void)
 {
-	int crc=0, i;
-	for(i = 0; i < length; i++)
-	{
-		if(data[i/8] & (1 << (7-(i%8))))
-		{
-			crc ^= modes_crc_table[i+(112-length)];
-		}
-	}
-    return crc;
+    unsigned int crc = 0;
+    for(int n=0; n<256; n++) {
+        crc = n<<16;
+        for(int k=0; k<8; k++) {
+            if(crc & 0x800000) {
+                crc = ((crc<<1) ^ POLY) & 0xFFFFFF;
+            } else {
+                crc = (crc<<1) & 0xFFFFFF;
+            }
+        }
+        crc_table[n] = crc & 0xFFFFFF;
+    }
+}
+
+//Perform a bytewise CRC check
+unsigned int modes_check_crc(unsigned char data[], int length)
+{
+    if(crc_table[1] != POLY) generate_crc_table();
+    unsigned int crc=0;
+    for(int i=0; i<length; i++) {
+        crc = crc_table[((crc>>16) ^ data[i]) & 0xff] ^ (crc << 8);
+    }
+    return crc & 0xFFFFFF;
 }

lib/slicer_impl.cc

@@ -172,7 +172,11 @@ int air_modes::slicer_impl::work(int noutput_items,
         if(rx_packet.type == Short_Packet && rx_packet.message_type != 11 && rx_packet.numlowconf > 0) {continue;}
         if(rx_packet.message_type == 11 && rx_packet.numlowconf >= 10) {continue;}
 
-        rx_packet.crc = modes_check_crc(rx_packet.data, packet_length);
+        rx_packet.crc = modes_check_crc(rx_packet.data, (packet_length/8)-3);
+        unsigned int ap = rx_packet.data[packet_length/8-3] << 16
+                        | rx_packet.data[packet_length/8-2] << 8
+                        | rx_packet.data[packet_length/8-1] << 0;
+        rx_packet.crc ^= ap;
 
         //crc for packets that aren't type 11 or type 17 is encoded with the transponder ID, which we don't know
         //therefore we toss 'em if there's syndrome