Avoid vector::insert in base::OffsetAdjuster::MergeSequentialAdjustments

This CL trades space for time in |MergeSequentialAdjustments|. By
allocating a new |Adjustments| vector, we can avoid calling
|std::vector::insert| in a loop, which costs roughly O(n^2) time. With
the additional vector, we only need to call |std::vector::push_back|,
costing roughly O(n) time in total.

Bug: 1017193
Change-Id: I7beb2c3c0b6a9cb7616a22f4a4355285fd281894
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2070605Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Dan McArdle <dmcardle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746171}

base/strings/utf_offset_string_conversions.cc

@@ -90,16 +90,22 @@ void OffsetAdjuster::MergeSequentialAdjustments(
   auto adjusted_iter = adjustments_on_adjusted_string->begin();
   auto first_iter = first_adjustments.begin();
   // Simultaneously iterate over all |adjustments_on_adjusted_string| and
-  // |first_adjustments|, adding adjustments to or correcting the adjustments
-  // in |adjustments_on_adjusted_string| as we go.  |shift| keeps track of the
-  // current number of characters collapsed by |first_adjustments| up to this
-  // point.  |currently_collapsing| keeps track of the number of characters
-  // collapsed by |first_adjustments| into the current |adjusted_iter|'s
-  // length.  These are characters that will change |shift| as soon as we're
-  // done processing the current |adjusted_iter|; they are not yet reflected in
-  // |shift|.
+  // |first_adjustments|, pushing adjustments at the end of
+  // |adjustments_builder| as we go.  |shift| keeps track of the current number
+  // of characters collapsed by |first_adjustments| up to this point.
+  // |currently_collapsing| keeps track of the number of characters collapsed by
+  // |first_adjustments| into the current |adjusted_iter|'s length.  These are
+  // characters that will change |shift| as soon as we're done processing the
+  // current |adjusted_iter|; they are not yet reflected in |shift|.
   size_t shift = 0;
   size_t currently_collapsing = 0;
+  // While we *could* update |adjustments_on_adjusted_string| in place by
+  // inserting new adjustments into the middle, we would be repeatedly calling
+  // |std::vector::insert|. That would cost O(n) time per insert, relative to
+  // distance from end of the string.  By instead allocating
+  // |adjustments_builder| and calling |std::vector::push_back|, we only pay
+  // amortized constant time per push. We are trading space for time.
+  Adjustments adjustments_builder;
   while (adjusted_iter != adjustments_on_adjusted_string->end()) {
     if ((first_iter == first_adjustments.end()) ||
         ((adjusted_iter->original_offset + shift +
@@ -112,6 +118,7 @@ void OffsetAdjuster::MergeSequentialAdjustments(
       adjusted_iter->original_offset += shift;
       shift += currently_collapsing;
       currently_collapsing = 0;
+      adjustments_builder.push_back(*adjusted_iter);
       ++adjusted_iter;
     } else if ((adjusted_iter->original_offset + shift) >
                first_iter->original_offset) {
@@ -127,15 +134,9 @@ void OffsetAdjuster::MergeSequentialAdjustments(
       DCHECK_LE(first_iter->original_offset + first_iter->output_length,
                 adjusted_iter->original_offset + shift);
 
-      // Add the |first_adjustment_iter| to the full set of adjustments while
-      // making sure |adjusted_iter| continues pointing to the same element.
-      // We do this by inserting the |first_adjustment_iter| right before
-      // |adjusted_iter|, then incrementing |adjusted_iter| so it points to
-      // the following element.
+      // Add the |first_iter| to the full set of adjustments.
       shift += first_iter->original_length - first_iter->output_length;
-      adjusted_iter = adjustments_on_adjusted_string->insert(
-          adjusted_iter, *first_iter);
-      ++adjusted_iter;
+      adjustments_builder.push_back(*first_iter);
       ++first_iter;
     } else {
       // The first adjustment adjusted something that then got further adjusted
@@ -168,10 +169,10 @@ void OffsetAdjuster::MergeSequentialAdjustments(
     // (Their offsets are already correct with respect to the original string.)
     // Append them all.
     DCHECK(adjusted_iter == adjustments_on_adjusted_string->end());
-    adjustments_on_adjusted_string->insert(
-        adjustments_on_adjusted_string->end(), first_iter,
+    adjustments_builder.insert(adjustments_builder.end(), first_iter,
                                first_adjustments.end());
   }
+  *adjustments_on_adjusted_string = std::move(adjustments_builder);
 }
 
 // Converts the given source Unicode character type to the given destination