Error correction for network packets, proof of concept (untested)

Signed-off-by: Stephen L. <lrq3000@gmail.com>

code/rscode/rs.c

@@ -0,0 +1,201 @@
+/* 
+ * Reed Solomon Encoder/Decoder 
+ *
+ * Copyright Henry Minsky (hqm@alum.mit.edu) 1991-2009
+ *
+ * This software library is licensed under terms of the GNU GENERAL
+ * PUBLIC LICENSE
+ *
+ * RSCODE 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 3 of the License, or
+ * (at your option) any later version.
+ *
+ * RSCODE 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 Rscode.  If not, see <http://www.gnu.org/licenses/>.
+
+ * Commercial licensing is available under a separate license, please
+ * contact author for details.
+ *
+ * Source code is available at http://rscode.sourceforge.net
+ */
+
+#include <stdio.h>
+#include <ctype.h>
+#include "ecc.h"
+
+/* Encoder parity bytes */
+int pBytes[MAXDEG];
+
+/* Decoder syndrome bytes */
+int synBytes[MAXDEG];
+
+/* generator polynomial */
+int genPoly[MAXDEG*2];
+
+int DEBUG = FALSE;
+
+static void
+compute_genpoly (int nbytes, int genpoly[]);
+
+/* Initialize lookup tables, polynomials, etc. */
+void
+initialize_ecc ()
+{
+  /* Initialize the galois field arithmetic tables */
+    init_galois_tables();
+
+    /* Compute the encoder generator polynomial */
+    compute_genpoly(NPAR, genPoly);
+}
+
+void
+zero_fill_from (unsigned char buf[], int from, int to)
+{
+  int i;
+  for (i = from; i < to; i++) buf[i] = 0;
+}
+
+/* debugging routines */
+void
+print_parity (void)
+{ 
+  int i;
+  printf("Parity Bytes: ");
+  for (i = 0; i < NPAR; i++) 
+    printf("[%d]:%x, ",i,pBytes[i]);
+  printf("\n");
+}
+
+
+void
+print_syndrome (void)
+{ 
+  int i;
+  printf("Syndrome Bytes: ");
+  for (i = 0; i < NPAR; i++) 
+    printf("[%d]:%x, ",i,synBytes[i]);
+  printf("\n");
+}
+
+/* Append the parity bytes onto the end of the message */
+void
+build_codeword (unsigned char msg[], int nbytes, unsigned char dst[])
+{
+  int i;
+	
+  for (i = 0; i < nbytes; i++) dst[i] = msg[i];
+	
+  for (i = 0; i < NPAR; i++) {
+    dst[i+nbytes] = pBytes[NPAR-1-i];
+  }
+}
+	
+/**********************************************************
+ * Reed Solomon Decoder 
+ *
+ * Computes the syndrome of a codeword. Puts the results
+ * into the synBytes[] array.
+ */
+ 
+void
+decode_data(unsigned char data[], int nbytes)
+{
+  int i, j, sum;
+  for (j = 0; j < NPAR;  j++) {
+    sum	= 0;
+    for (i = 0; i < nbytes; i++) {
+      sum = data[i] ^ gmult(gexp[j+1], sum);
+    }
+    synBytes[j]  = sum;
+  }
+}
+
+
+/* Check if the syndrome is zero */
+int
+check_syndrome (void)
+{
+ int i, nz = 0;
+ for (i =0 ; i < NPAR; i++) {
+  if (synBytes[i] != 0) {
+      nz = 1;
+      break;
+  }
+ }
+ return nz;
+}
+
+
+void
+debug_check_syndrome (void)
+{	
+  int i;
+	
+  for (i = 0; i < 3; i++) {
+    printf(" inv log S[%d]/S[%d] = %d\n", i, i+1, 
+	   glog[gmult(synBytes[i], ginv(synBytes[i+1]))]);
+  }
+}
+
+
+/* Create a generator polynomial for an n byte RS code. 
+ * The coefficients are returned in the genPoly arg.
+ * Make sure that the genPoly array which is passed in is 
+ * at least n+1 bytes long.
+ */
+
+static void
+compute_genpoly (int nbytes, int genpoly[])
+{
+  int i, tp[256], tp1[256];
+	
+  /* multiply (x + a^n) for n = 1 to nbytes */
+
+  zero_poly(tp1);
+  tp1[0] = 1;
+
+  for (i = 1; i <= nbytes; i++) {
+    zero_poly(tp);
+    tp[0] = gexp[i];		/* set up x+a^n */
+    tp[1] = 1;
+	  
+    mult_polys(genpoly, tp, tp1);
+    copy_poly(tp1, genpoly);
+  }
+}
+
+/* Simulate a LFSR with generator polynomial for n byte RS code. 
+ * Pass in a pointer to the data array, and amount of data. 
+ *
+ * The parity bytes are deposited into pBytes[], and the whole message
+ * and parity are copied to dest to make a codeword.
+ * 
+ */
+
+void
+encode_data (unsigned char msg[], int nbytes, unsigned char dst[])
+{
+  int i, LFSR[NPAR+1],dbyte, j;
+	
+  for(i=0; i < NPAR+1; i++) LFSR[i]=0;
+
+  for (i = 0; i < nbytes; i++) {
+    dbyte = msg[i] ^ LFSR[NPAR-1];
+    for (j = NPAR-1; j > 0; j--) {
+      LFSR[j] = LFSR[j-1] ^ gmult(genPoly[j], dbyte);
+    }
+    LFSR[0] = gmult(genPoly[0], dbyte);
+  }
+
+  for (i = 0; i < NPAR; i++) 
+    pBytes[i] = LFSR[i];
+	
+  build_codeword(msg, nbytes, dst);
+}
+