Improve performance for speed detection bits compare.

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

src/wmspeed.cc

@@ -282,11 +282,18 @@ SpeedSync::compare_bits (vector<CmpState>& cmp_states, double relative_speed)
 
   for (size_t mi = 0; mi < sync_bits.size(); mi++)
     {
-      for (auto& cs : cmp_states)
+      auto start = std::lower_bound (cmp_states.begin(), cmp_states.end(), 0,
+        [&] (auto& cs, auto idx)
           {
             int index = (cs.offset + OFFSET + sync_bits[mi].frame * steps_per_frame) * relative_speed_inv + 0.5;
-          if (index >= 0)
+            return index < idx;
+          });
+
+      for (auto it = start; it != cmp_states.end(); it++)
         {
+          CmpState& cs = *it;
+          int index = (cs.offset + OFFSET + sync_bits[mi].frame * steps_per_frame) * relative_speed_inv + 0.5;
+
           if (index >= sync_matrix.rows())
             break;
 
@@ -305,7 +312,6 @@ SpeedSync::compare_bits (vector<CmpState>& cmp_states, double relative_speed)
           bv.count++;
         }
     }
-    }
 }
 
 void