Initial Commit

lib/ftrsd/init_rs.c

@@ -0,0 +1,119 @@
+/* Initialize a RS codec
+ *
+ * Copyright 2002 Phil Karn, KA9Q
+ * May be used under the terms of the GNU General Public License (GPL)
+ */
+#include <stdlib.h>
+
+#ifdef CCSDS
+#include "ccsds.h"
+#elif defined(BIGSYM)
+#include "int.h"
+#else
+#include "char.h"
+#endif
+
+void FREE_RS(void *p){
+  struct rs *rs = (struct rs *)p;
+
+  free(rs->alpha_to);
+  free(rs->index_of);
+  free(rs->genpoly);
+  free(rs);
+}
+
+/* Initialize a Reed-Solomon codec
+ * symsize = symbol size, bits (1-8)
+ * gfpoly = Field generator polynomial coefficients
+ * fcr = first root of RS code generator polynomial, index form
+ * prim = primitive element to generate polynomial roots
+ * nroots = RS code generator polynomial degree (number of roots)
+ */
+void *INIT_RS(unsigned int symsize,unsigned int gfpoly,unsigned fcr,unsigned prim,
+		unsigned int nroots){
+  struct rs *rs;
+  int i, j, sr,root,iprim;
+
+  if(symsize > 8*sizeof(DTYPE))
+    return NULL; /* Need version with ints rather than chars */
+
+  if(fcr >= (1<<symsize))
+    return NULL;
+  if(prim == 0 || prim >= (1<<symsize))
+    return NULL;
+  if(nroots >= (1<<symsize))
+    return NULL; /* Can't have more roots than symbol values! */
+
+  rs = (struct rs *)calloc(1,sizeof(struct rs));
+  rs->mm = symsize;
+  rs->nn = (1<<symsize)-1;
+
+  rs->alpha_to = (DTYPE *)malloc(sizeof(DTYPE)*(rs->nn+1));
+  if(rs->alpha_to == NULL){
+    free(rs);
+    return NULL;
+  }
+  rs->index_of = (DTYPE *)malloc(sizeof(DTYPE)*(rs->nn+1));
+  if(rs->index_of == NULL){
+    free(rs->alpha_to);
+    free(rs);
+    return NULL;
+  }
+
+  /* Generate Galois field lookup tables */
+  rs->index_of[0] = A0; /* log(zero) = -inf */
+  rs->alpha_to[A0] = 0; /* alpha**-inf = 0 */
+  sr = 1;
+  for(i=0;i<rs->nn;i++){
+    rs->index_of[sr] = i;
+    rs->alpha_to[i] = sr;
+    sr <<= 1;
+    if(sr & (1<<symsize))
+      sr ^= gfpoly;
+    sr &= rs->nn;
+  }
+  if(sr != 1){
+    /* field generator polynomial is not primitive! */
+    free(rs->alpha_to);
+    free(rs->index_of);
+    free(rs);
+    return NULL;
+  }
+
+  /* Form RS code generator polynomial from its roots */
+  rs->genpoly = (DTYPE *)malloc(sizeof(DTYPE)*(nroots+1));
+  if(rs->genpoly == NULL){
+    free(rs->alpha_to);
+    free(rs->index_of);
+    free(rs);
+    return NULL;
+  }
+  rs->fcr = fcr;
+  rs->prim = prim;
+  rs->nroots = nroots;
+
+  /* Find prim-th root of 1, used in decoding */
+  for(iprim=1;(iprim % prim) != 0;iprim += rs->nn)
+    ;
+  rs->iprim = iprim / prim;
+
+  rs->genpoly[0] = 1;
+  for (i = 0,root=fcr*prim; i < nroots; i++,root += prim) {
+    rs->genpoly[i+1] = 1;
+
+    /* Multiply rs->genpoly[] by  @**(root + x) */
+    for (j = i; j > 0; j--){
+      if (rs->genpoly[j] != 0)
+	rs->genpoly[j] = rs->genpoly[j-1] ^ rs->alpha_to[modnn(rs,rs->index_of[rs->genpoly[j]] + root)];
+      else
+	rs->genpoly[j] = rs->genpoly[j-1];
+    }
+    /* rs->genpoly[0] can never be zero */
+    rs->genpoly[0] = rs->alpha_to[modnn(rs,rs->index_of[rs->genpoly[0]] + root)];
+  }
+  /* convert rs->genpoly[] to index form for quicker encoding */
+  for (i = 0; i <= nroots; i++)
+    rs->genpoly[i] = rs->index_of[rs->genpoly[i]];
+
+  return rs;
+}