Improvements to uses of base::SmallMap

storage/browser/blob/blob_memory_controller.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "storage/browser/blob/blob_memory_controller.h"
 
 #include <algorithm>
 #include <numeric>
 
 #include "base/bind.h"
@@ skipping 227 matching lines @@
       bytes_written < 0 || !success ? File::FILE_ERROR_FAILED : File::FILE_OK;
   creation_info.last_modified = info.last_modified;
   return std::make_pair(std::move(creation_info), disk_availability);
 }
 
 uint64_t GetTotalSizeAndFileSizes(
     const std::vector<scoped_refptr<ShareableBlobDataItem>>&
         unreserved_file_items,
     std::vector<uint64_t>* file_sizes_output) {
   uint64_t total_size_output = 0;
-  base::SmallMap<std::map<uint64_t, uint64_t>> file_id_to_sizes;
+  base::small_map<std::map<uint64_t, uint64_t>> file_id_to_sizes;

[danakj, 2017/04/18 21:35:50]

Aside.. but this is interesting. What it really wants is something like

vector<pair<int, int>> sizes;
for (each item i)
  sizes.push_back(file_id, offset+length);
sort(sizes, by_pair_first);  // group up same ids together
accumulate(sizes);  // reduce consecutive things with same first by adding their
second.

That could give much better performance I think than a map. I wonder if such
helpful STL tools exist.

[brettw, 2017/04/19 16:29:48]

An STL nerd would say yes, but my guess is it would be 76 lines of unreadable
std::for_each code :(

   for (const auto& item : unreserved_file_items) {
     const DataElement& element = item->item()->data_element();
     uint64_t file_id = BlobDataBuilder::GetFutureFileID(element);
     auto it = file_id_to_sizes.find(file_id);
     if (it != file_id_to_sizes.end())
       it->second = std::max(it->second, element.offset() + element.length());
     else
       file_id_to_sizes[file_id] = element.offset() + element.length();
     total_size_output += element.length();
   }
@@ skipping 716 matching lines @@
   MaybeGrantPendingMemoryRequests();
 }
 
 void BlobMemoryController::OnBlobFileDelete(uint64_t size,
                                             const FilePath& path) {
   DCHECK_LE(size, disk_used_);
   disk_used_ -= size;
 }
 
 }  // namespace storage