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2026-06-22 11:39:50 +03:00
commit ba6f1bde22
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+/* The guts of the Reed-Solomon encoder, meant to be #included
+ * into a function body with the following typedefs, macros and variables supplied
+ * according to the code parameters:
+
+ * data_t - a typedef for the data symbol
+ * data_t data[] - array of NN-NROOTS-PAD and type data_t to be encoded
+ * data_t parity[] - an array of NROOTS and type data_t to be written with parity symbols
+ * NROOTS - the number of roots in the RS code generator polynomial,
+ *          which is the same as the number of parity symbols in a block.
+            Integer variable or literal.
+	    * 
+ * NN - the total number of symbols in a RS block. Integer variable or literal.
+ * PAD - the number of pad symbols in a block. Integer variable or literal.
+ * ALPHA_TO - The address of an array of NN elements to convert Galois field
+ *            elements in index (log) form to polynomial form. Read only.
+ * INDEX_OF - The address of an array of NN elements to convert Galois field
+ *            elements in polynomial form to index (log) form. Read only.
+ * MODNN - a function to reduce its argument modulo NN. May be inline or a macro.
+ * GENPOLY - an array of NROOTS+1 elements containing the generator polynomial in index form
+
+ * The memset() and memmove() functions are used. The appropriate header
+ * file declaring these functions (usually <string.h>) must be included by the calling
+ * program.
+
+ * Copyright 2004, Phil Karn, KA9Q
+ * May be used under the terms of the GNU Lesser General Public License (LGPL)
+ */
+
+
+#undef A0
+#define A0 (NN) /* Special reserved value encoding zero in index form */
+
+{
+  int i, j;
+  data_t feedback;
+
+  memset(parity,0,NROOTS*sizeof(data_t));
+
+  for(i=0;i<NN-NROOTS-PAD;i++){
+    feedback = INDEX_OF[data[i] ^ parity[0]];
+    if(feedback != A0){      /* feedback term is non-zero */
+#ifdef UNNORMALIZED
+      /* This line is unnecessary when GENPOLY[NROOTS] is unity, as it must
+       * always be for the polynomials constructed by init_rs()
+       */
+      feedback = MODNN(NN - GENPOLY[NROOTS] + feedback);
+#endif
+      for(j=1;j<NROOTS;j++)
+	parity[j] ^= ALPHA_TO[MODNN(feedback + GENPOLY[NROOTS-j])];
+    }
+    /* Shift */
+    memmove(&parity[0],&parity[1],sizeof(data_t)*(NROOTS-1));
+    if(feedback != A0)
+      parity[NROOTS-1] = ALPHA_TO[MODNN(feedback + GENPOLY[0])];
+    else
+      parity[NROOTS-1] = 0;
+  }
+}