Optimize short code decoder performance.

Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

src/shortcode.cc

@@ -24,7 +24,7 @@
 using std::vector;
 
 #if 0
-/* codetables.de */
+/* codetables.de / magma online calculator BKLC (GF(2), N, K) */
 static vector<vector<int>> block_48_16_16 = {
   { 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 1, 1, 1, 0, 0, 1, 0, 1, 1, 0, 0, 0, 1, 1, 0, 1, 0, 0, 0, 1, 1, 0, 1, 0, 1, 0, 1, 0, 0, 0, 0, 1 },
   { 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 1, 1, 1, 0, 1, 1, 0, 1, 0, 1, 0, 0, 1, 1, 0, 0 },
@@ -148,7 +148,7 @@ short_decode_blk (const vector<int>& coded_bits)
   vector<int> out_bits;
   for (size_t c = 0; c < (1 << gen_in_count); c++)
     {
-      vector<int> w;
+      bool match = true;
       for (size_t j = 0; j < gen_out_count; j++)
         {
           int x = 0;
@@ -159,9 +159,13 @@ short_decode_blk (const vector<int>& coded_bits)
                   x ^= gen_matrix[bit][j];
                 }
             }
-          w.push_back (x);
+          if (coded_bits[j] != x)
+            {
+              match = false;
+              break;
+            }
         }
-      if (w == coded_bits)
+      if (match)
         {
           for (size_t bit = 0; bit < gen_in_count; bit++)
             {

src/testshortcode.cc

@@ -114,14 +114,14 @@ main (int argc, char **argv)
     }
   if (argc == 2 && string (argv[1]) == "perf")
     {
-      vector<int> in_bits;
-      while (in_bits.size() != K)
-        in_bits.push_back (rand() & 1);
-
       const double start_t = gettime();
-      const size_t runs = 20;
+      const size_t runs = 100;
       for (size_t i = 0; i < runs; i++)
         {
+          vector<int> in_bits;
+          while (in_bits.size() != K)
+            in_bits.push_back (rand() & 1);
+
           vector<int> out_bits = short_decode_blk (short_encode_blk (in_bits));
           assert (out_bits == in_bits);
         }
@@ -155,6 +155,11 @@ main (int argc, char **argv)
           if (weight[i])
             printf ("W %3zd %6d %20s\n", i, weight[i], number_format (factorial (N) / (factorial (i) * factorial (N - i)) / weight[i]).c_str());
         }
+      /* decoding test */
+      for (auto it : table)
+        {
+          assert (short_decode_blk (it.first) == it.second);
+        }
 
       const size_t runs = 50LL * 1000 * 1000 * 1000;
       size_t match = 0;