VersionBuilder: optmize SaveTo() to linear time. (#4366)

Summary:
Because `base_files` and `added_files` both are sorted, using a merge
operation to these two sorted arrays is more effective. The complexity
is reduced to linear time.

    - optmize the merge complexity.
    - move the `NDEBUG` of sorted `added_files` out of merge process.
Signed-off-by: NJiYou <jiyou09@gmail.com>
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4366

Differential Revision: D9833592

Pulled By: ajkr

fbshipit-source-id: dd32b67ebdca4c20e5e9546ab8082cecefe99fd0

db/version_builder.cc

@@ -324,8 +324,6 @@ class VersionBuilder::Rep {
       // Merge the set of added files with the set of pre-existing files.
       // Drop any deleted files.  Store the result in *v.
       const auto& base_files = base_vstorage_->LevelFiles(level);
-      auto base_iter = base_files.begin();
-      auto base_end = base_files.end();
       const auto& unordered_added_files = levels_[level].added_files;
       vstorage->Reserve(level,
                         base_files.size() + unordered_added_files.size());
@@ -339,30 +337,27 @@ class VersionBuilder::Rep {
       std::sort(added_files.begin(), added_files.end(), cmp);
 
 #ifndef NDEBUG
-      FileMetaData* prev_file = nullptr;
-#endif
-
+      FileMetaData* prev_added_file = nullptr;
       for (const auto& added : added_files) {
-#ifndef NDEBUG
-        if (level > 0 && prev_file != nullptr) {
+        if (level > 0 && prev_added_file != nullptr) {
           assert(base_vstorage_->InternalComparator()->Compare(
-                     prev_file->smallest, added->smallest) <= 0);
+                     prev_added_file->smallest, added->smallest) <= 0);
         }
-        prev_file = added;
+        prev_added_file = added;
+      }
 #endif
 
-        // Add all smaller files listed in base_
-        for (auto bpos = std::upper_bound(base_iter, base_end, added, cmp);
-             base_iter != bpos; ++base_iter) {
-          MaybeAddFile(vstorage, level, *base_iter);
+      auto base_iter = base_files.begin();
+      auto base_end = base_files.end();
+      auto added_iter = added_files.begin();
+      auto added_end = added_files.end();
+      while (added_iter != added_end || base_iter != base_end) {
+        if (base_iter == base_end ||
+                (added_iter != added_end && cmp(*added_iter, *base_iter))) {
+          MaybeAddFile(vstorage, level, *added_iter++);
+        } else {
+          MaybeAddFile(vstorage, level, *base_iter++);
         }
-
-        MaybeAddFile(vstorage, level, added);
-      }
-
-      // Add remaining base files
-      for (; base_iter != base_end; ++base_iter) {
-        MaybeAddFile(vstorage, level, *base_iter);
       }
     }