Small improvement of FX.25 RS encoder speed

The old design walked the entire feedback array XORing all values, then
shifted them all down by one index. This design does the shift-by-one
while doing the XOR work to save a step.

Signed-off-by: Justin Brzozoski <justin.brzozoski@gmail.com>

Author: jbrzozoski

Diff for: src/fx25_encode.c

@@ -69,15 +69,15 @@ void ENCODE_RS(struct rs * restrict rs, DTYPE * restrict data, DTYPE * restrict
   for(i=0;i<NN-NROOTS;i++){
     feedback = INDEX_OF[data[i] ^ bb[0]];
     if(feedback != A0){      /* feedback term is non-zero */
+      /* Shift and XOR */
       for(j=1;j<NROOTS;j++)
-	    bb[j] ^= ALPHA_TO[MODNN(feedback + GENPOLY[NROOTS-j])];
-    }
-    /* Shift */
-    memmove(&bb[0],&bb[1],sizeof(DTYPE)*(NROOTS-1));
-    if(feedback != A0)
+	    bb[j-1] = bb[j] ^ ALPHA_TO[MODNN(feedback + GENPOLY[NROOTS-j])];
       bb[NROOTS-1] = ALPHA_TO[MODNN(feedback + GENPOLY[0])];
-    else
+    } else{
+      /* Shift only */
+      memmove(&bb[0],&bb[1],sizeof(DTYPE)*(NROOTS-1));
       bb[NROOTS-1] = 0;
+    }
   }
 }