Improve unpack reserve handling [0.18]

src/serialization/container.h

@@ -57,8 +57,23 @@ namespace serialization
       return true;
     }
 
-    template <typename C>
-    void do_reserve(C &c, size_t N) {}
+    //! @brief Reserve space for N elements if applicable for container.
+    template<typename... C>
+    void do_reserve(const C&...) {}
+    template<typename C>
+    auto do_reserve(C &c, std::size_t N, std::size_t B) -> decltype(c.reserve(N))
+    {
+      using T = typename C::value_type;
+
+      // max compression ratio for upfront memory usage
+      B /= sizeof(T);
+      if (std::numeric_limits<std::size_t>::max() / 4 <= B)
+        B = std::numeric_limits<std::size_t>::max();
+      else
+        B *= 4;
+
+      return c.reserve(std::min(N, B));
+    }
   }
 }
 
@@ -77,7 +92,7 @@ bool do_serialize_container(Archive<false> &ar, C &v)
     return false;
   }
 
-  ::serialization::detail::do_reserve(v, cnt);
+  ::serialization::detail::do_reserve(v, cnt, ar.remaining_bytes());
 
   for (size_t i = 0; i < cnt; i++) {
     if (i > 0)

src/serialization/containers.h

@@ -87,7 +87,6 @@ namespace serialization
 {
   namespace detail
   {
-    template <typename T> void do_reserve(std::vector<T> &c, size_t N) { c.reserve(N); }
     template <typename T> void do_add(std::vector<T> &c, T &&e) { c.emplace_back(std::forward<T>(e)); }
 
     template <typename T> void do_add(std::deque<T> &c, T &&e) { c.emplace_back(std::forward<T>(e)); }

tests/unit_tests/serialization.cpp

@@ -304,6 +304,20 @@ TEST(Serialization, serializes_vector_int64_as_fixed_int)
   ASSERT_EQ(57, blob.size());
 }
 
+TEST(Serialization, deserializes_vector_reserve)
+{
+  std::vector<int64_t> v;
+  string blob;
+
+  tools::write_varint(std::back_inserter(blob), unsigned(100));
+  blob.append(std::string(100, 0));
+
+  ASSERT_LT(v.capacity(), 20);
+  ASSERT_FALSE(serialization::parse_binary(blob, v));
+  ASSERT_EQ(48, v.capacity());
+}
+
+
 namespace
 {
   template<typename T>