Add a SiteEngagementEvictionPolicy.

chrome/browser/engagement/site_engagement_eviction_policy_unittest.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 "chrome/browser/engagement/site_engagement_eviction_policy.h"
+#include "chrome/browser/engagement/site_engagement_service.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace {
+
+const int64 kGlobalQuota = 25 * 1024;
+
+}  // namespace
+
+class TestSiteEngagementService : public SiteEngagementService {
+ public:
+  TestSiteEngagementService() : SiteEngagementService(nullptr) {}
+
+  ~TestSiteEngagementService() override {}
+
+  int GetScore(const GURL& url) override {
+    return engagement_score_map_[url];
+  }
+
+  int GetTotalEngagementPoints() override {
+    int total = 0;
+    for (const auto& site : engagement_score_map_)
+      total += site.second;
+    return total;
+  }
+
+  void SetScore(const GURL& origin, int score) {
+    engagement_score_map_[origin] = score;
+  }
+
+ private:
+  std::map<GURL, int> engagement_score_map_;
+
+  DISALLOW_COPY_AND_ASSIGN(TestSiteEngagementService);
+};
+
+class SiteEngagementEvictionPolicyTest : public testing::Test {
+ public:
+  SiteEngagementEvictionPolicyTest()
+      : service_(new TestSiteEngagementService()) {}
+
+  ~SiteEngagementEvictionPolicyTest() override {
+  }
+
+
+  GURL CalculateEvictionOrigin(const std::map<GURL, int64>& usage) {
+    return SiteEngagementEvictionPolicy::GetEvictionOriginTask::
+                      CalculateEvictionOrigin(service_, usage, kGlobalQuota);

[raymes, 2015/07/21 03:16:00]

Ohh I see, this one is static for testing. That seems ok then. If we do remove
GetEvictionOriginTask perhaps we can make this a static function on
SiteEngagementEvictionPolicy or else a little class
SiteEngagementEvictionCalculator and make it clear that we're just testing that
unit of functionality

[calamity, 2015/07/21 04:57:09]

Done.

+  }
+
+  TestSiteEngagementService* service() {
+    return service_;
+  }
+
+ private:
+  TestSiteEngagementService* service_;
+
+  DISALLOW_COPY_AND_ASSIGN(SiteEngagementEvictionPolicyTest);
+};
+
+TEST_F(SiteEngagementEvictionPolicyTest, GetEvictionOrigin) {
+  GURL url1("http://www.google.com");
+  GURL url2("http://www.example.com");
+  GURL url3("http://www.spam.me");
+
+  std::map<GURL, int64> usage;
+  usage[url1] = 10 * 1024;
+  usage[url2] = 10 * 1024;
+  usage[url3] = 10 * 1024;
+
+  service()->SetScore(url1, 50);
+  service()->SetScore(url2, 25);
+
+  // When 3 sites have equal usage, evict the site with the least engagement.
+  EXPECT_EQ(url3, CalculateEvictionOrigin(usage));
+
+  usage[url2] = usage[url3] + 10;
+
+  // When a site has more engagement, it may get preference over a site with
+  // less usage.

[raymes, 2015/07/21 03:16:00]

This comment is a bit hard to read in the context of the code. Perhaps:
// Now |url2| has the most usage but |url3| has the least engagement score so
one of them should be evicted. In this case the heuristic chooses |url3|.
...

[calamity, 2015/07/21 04:57:09]

Done.

+  EXPECT_EQ(url3, CalculateEvictionOrigin(usage));
+
+  usage[url2] = 15 * 1024;
+  // But exceeding allocated usage too much will still result in being evicted.

[raymes, 2015/07/21 03:16:00]

result in being evicted even though the engagement with |url2| is higher.

[calamity, 2015/07/21 04:57:09]

Done.

+  EXPECT_EQ(url2, CalculateEvictionOrigin(usage));
+}

[raymes, 2015/07/21 03:16:00]

I can think of at least one more test to add: All sites have the same score but
different usage. Make sure that the highest usage is thrown away.

[calamity, 2015/07/21 04:57:09]

Done.