[Sync] Creating sync_sessions component, moving revisit logic.

components/sync_driver/revisit/offset_tab_matcher.cc

-// Copyright 2015 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 "components/sync_driver/revisit/offset_tab_matcher.h"
-
-#include "base/metrics/histogram_macros.h"
-#include "base/metrics/sparse_histogram.h"
-#include "components/sessions/core/serialized_navigation_entry.h"
-
-namespace sync_driver {
-
-namespace {
-
-// This is an upper bound of the max size of positive offset we will emit
-// correct metrics for. Anything larger than this will be clamped to this value.
-// This value doesn't exactly correspond to what we actually expect, this value
-// is currently larger than expected. This value is more for the safety of our
-// sparse histogram usage. It is assumed that the max negative offset is
-// symmetrical and can be found by taking the negative of this value.
-const int kMaxOffset = 10;
-
-}  // namespace
-
-OffsetTabMatcher::OffsetTabMatcher(const PageEquality& page_equality)
-    : page_equality_(page_equality) {}
-
-void OffsetTabMatcher::Check(const sessions::SessionTab* tab) {
-  const int current_index = tab->normalized_navigation_index();
-  for (std::size_t i = 0; i < tab->navigations.size(); ++i) {
-    // Ignore the entry if it is the current entry. There's actually some
-    // ambiguity here, the index of a tab is located in two places. Hopefully
-    // they are equal, but it is possible for the index() accessor of an entry
-    // to be different from the index in the tab's vector. Theoretically this
-    // should not happen outside of tab construction logic, but to be safe all
-    // matcher logic treats the index in the vector as the authoritative index.
-    // We chose this because the other matcher wants efficient random access.
-    if (current_index >= 0 && (std::size_t)current_index == i) {
-      continue;
-    }
-    const int offset = i - current_index;
-    if (page_equality_.IsSamePage(tab->navigations[i].virtual_url()) &&
-        (best_tab_ == nullptr || best_tab_->timestamp < tab->timestamp ||
-         (best_tab_->timestamp == tab->timestamp && best_offset_ < offset))) {
-      best_tab_ = tab;
-      best_offset_ = offset;
-    }
-  }
-}
-
-void OffsetTabMatcher::Emit(
-    const PageVisitObserver::TransitionType transition) {
-  if (best_tab_ == nullptr) {
-    UMA_HISTOGRAM_ENUMERATION("Sync.PageRevisitNavigationMissTransition",
-                              transition,
-                              PageVisitObserver::kTransitionTypeLast);
-  } else {
-    // The sparse macro allows us to handle negative offsets. However, we need
-    // to be careful when doing this because of the unrestricted nature of
-    // sparse we could end up with a very large output space across many
-    // clients. So we clamp on a resonable bound that's larger than we expect to
-    // be sure no unexpected data causes problems.
-    UMA_HISTOGRAM_SPARSE_SLOWLY("Sync.PageRevisitNavigationMatchOffset",
-                                Clamp(best_offset_, -kMaxOffset, kMaxOffset));
-    UMA_HISTOGRAM_CUSTOM_TIMES("Sync.PageRevisitNavigationMatchAge",
-                               (base::Time::Now() - best_tab_->timestamp),
-                               base::TimeDelta::FromSeconds(1),
-                               base::TimeDelta::FromDays(14), 100);
-    UMA_HISTOGRAM_ENUMERATION("Sync.PageRevisitNavigationMatchTransition",
-                              transition,
-                              PageVisitObserver::kTransitionTypeLast);
-  }
-}
-
-int OffsetTabMatcher::Clamp(const int input, const int lower, const int upper) {
-  return std::max(lower, std::min(upper, input));
-}
-
-}  // namespace sync_driver