fix error when compiling without NDEBUG

parallel_hashmap/phmap.h

@@ -2151,6 +2151,30 @@ private:
     }
 };
 
+// ----------------------------------------------------------------------------
+// ----------------------------------------------------------------------------
+constexpr size_t Group::kWidth;
+
+// Returns "random" seed.
+inline size_t RandomSeed() 
+{
+#if PHMAP_HAVE_THREAD_LOCAL
+    static thread_local size_t counter = 0;
+    size_t value = ++counter;
+#else   // PHMAP_HAVE_THREAD_LOCAL
+    static std::atomic<size_t> counter(0);
+    size_t value = counter.fetch_add(1, std::memory_order_relaxed);
+#endif  // PHMAP_HAVE_THREAD_LOCAL
+    return value ^ static_cast<size_t>(reinterpret_cast<uintptr_t>(&counter));
+}
+
+bool ShouldInsertBackwards(size_t hash, ctrl_t* ctrl) 
+{
+    // To avoid problems with weak hashes and single bit tests, we use % 13.
+    // TODO(kfm,sbenza): revisit after we do unconditional mixing
+    return (H1(hash, ctrl) ^ RandomSeed()) % 13 > 6;
+}
+
 // ----------------------------------------------------------------------------
 // ----------------------------------------------------------------------------
 template <size_t N,