base::JoinString: Pre-allocate the string to its final size.

base/strings/string_util.cc

Index: base/strings/string_util.cc
diff --git a/base/strings/string_util.cc b/base/strings/string_util.cc
index 49ae9435e934febcd1383437616287af2d22ede5..71ae894dd6e7559e1efa0a2623e6328353ce61d4 100644
--- a/base/strings/string_util.cc
+++ b/base/strings/string_util.cc
@@ -875,13 +875,20 @@ char16* WriteInto(string16* str, size_t length_with_null) {
 template <typename list_type, typename string_type>
 static string_type JoinStringT(const list_type& parts,
                                BasicStringPiece<string_type> sep) {
-  auto iter = parts.begin();
-  // Early-exit to avoid bad access to the first element.
-  if (iter == parts.end())
+  if (parts.size() == 0)
     return string_type();
 
-  // Begin constructing the result from the first element.
-  string_type result(iter->data(), iter->size());
+  // Pre-allocate the eventual size of the string. Start with the size of all of
+  // the separators (note that this *assumes* parts.size() > 0).
+  size_t total_size = (parts.size() - 1) * sep.size();
+  for (const auto& part : parts)
+    total_size += part.size();
+  string_type result;
+  result.reserve(total_size);
+
+  auto iter = parts.begin();
+  DCHECK(iter != parts.end());
+  AppendToString(&result, *iter);
   ++iter;
 
   for (; iter != parts.end(); ++iter) {
@@ -892,6 +899,9 @@ static string_type JoinStringT(const list_type& parts,
     AppendToString(&result, *iter);
   }
 
+  // Sanity-check that we pre-allocated correctly.
+  DCHECK_EQ(total_size, result.size());
+
   return result;
 }