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diff --git a/spec/lib/gitlab/usage_data_counters/hll_redis_counter_spec.rb b/spec/lib/gitlab/usage_data_counters/hll_redis_counter_spec.rb
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+++ b/spec/lib/gitlab/usage_data_counters/hll_redis_counter_spec.rb
@@ -0,0 +1,113 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::UsageDataCounters::HLLRedisCounter, :clean_gitlab_redis_shared_state do
+  let(:entity1) { 'dfb9d2d2-f56c-4c77-8aeb-6cddc4a1f857' }
+  let(:entity2) { '1dd9afb2-a3ee-4de1-8ae3-a405579c8584' }
+  let(:entity3) { '34rfjuuy-ce56-sa35-ds34-dfer567dfrf2' }
+  let(:entity4) { '8b9a2671-2abf-4bec-a682-22f6a8f7bf31' }
+
+  let(:weekly_event) { 'g_analytics_contribution' }
+  let(:daily_event) { 'g_search' }
+  let(:different_aggregation) { 'different_aggregation' }
+
+  let(:known_events) do
+    [
+      { name: "g_analytics_contribution", redis_slot: "analytics", category: "analytics", expiry: 84, aggregation: "weekly" },
+      { name: "g_analytics_valuestream", redis_slot: "analytics", category: "analytics", expiry: 84, aggregation: "daily" },
+      { name: "g_analytics_productivity", redis_slot: "analytics", category: "productivity", expiry: 84, aggregation: "weekly" },
+      { name: "g_compliance_dashboard", redis_slot: "compliance", category: "compliance", aggregation: "weekly" },
+      { name: "g_search", category: "global", aggregation: "daily" },
+      { name: "different_aggregation", category: "global", aggregation: "monthly" }
+    ].map(&:with_indifferent_access)
+  end
+
+  before do
+    allow(described_class).to receive(:known_events).and_return(known_events)
+  end
+
+  around do |example|
+    # We need to freeze to a reference time
+    # because visits are grouped by the week number in the year
+    # Without freezing the time, the test may behave inconsistently
+    # depending on which day of the week test is run.
+    # Monday 6th of June
+    reference_time = Time.utc(2020, 6, 1)
+    Timecop.freeze(reference_time) { example.run }
+  end
+
+  describe '.track_event' do
+    it "raise error if metrics don't have same aggregation" do
+      expect { described_class.track_event(entity1, different_aggregation, Date.current) } .to raise_error(Gitlab::UsageDataCounters::HLLRedisCounter::UnknownAggregation)
+    end
+
+    it 'raise error if metrics of unknown aggregation' do
+      expect { described_class.track_event(entity1, 'unknown', Date.current) } .to raise_error(Gitlab::UsageDataCounters::HLLRedisCounter::UnknownEvent)
+    end
+  end
+
+  describe '.unique_events' do
+    before do
+      # events in current week, should not be counted as week is not complete
+      described_class.track_event(entity1, weekly_event, Date.current)
+      described_class.track_event(entity2, weekly_event, Date.current)
+
+      # Events last week
+      described_class.track_event(entity1, weekly_event, 2.days.ago)
+      described_class.track_event(entity1, weekly_event, 2.days.ago)
+
+      # Events 2 weeks ago
+      described_class.track_event(entity1, weekly_event, 2.weeks.ago)
+
+      # Events 4 weeks ago
+      described_class.track_event(entity3, weekly_event, 4.weeks.ago)
+      described_class.track_event(entity4, weekly_event, 29.days.ago)
+
+      # events in current day should be counted in daily aggregation
+      described_class.track_event(entity1, daily_event, Date.current)
+      described_class.track_event(entity2, daily_event, Date.current)
+
+      # Events last week
+      described_class.track_event(entity1, daily_event, 2.days.ago)
+      described_class.track_event(entity1, daily_event, 2.days.ago)
+
+      # Events 2 weeks ago
+      described_class.track_event(entity1, daily_event, 14.days.ago)
+
+      # Events 4 weeks ago
+      described_class.track_event(entity3, daily_event, 28.days.ago)
+      described_class.track_event(entity4, daily_event, 29.days.ago)
+    end
+
+    it 'raise error if metrics are not in the same slot' do
+      expect { described_class.unique_events(event_names: %w(g_analytics_contribution g_compliance_dashboard), start_date: 4.weeks.ago, end_date: Date.current) }.to raise_error('Events should be in same slot')
+    end
+
+    it 'raise error if metrics are not in the same category' do
+      expect { described_class.unique_events(event_names: %w(g_analytics_contribution g_analytics_productivity), start_date: 4.weeks.ago, end_date: Date.current) }.to raise_error('Events should be in same category')
+    end
+
+    it "raise error if metrics don't have same aggregation" do
+      expect { described_class.unique_events(event_names: %w(g_analytics_contribution g_analytics_valuestream), start_date: 4.weeks.ago, end_date: Date.current) }.to raise_error('Events should have same aggregation level')
+    end
+
+    context 'when data for the last complete week' do
+      it { expect(described_class.unique_events(event_names: weekly_event, start_date: 1.week.ago, end_date: Date.current)).to eq(1) }
+    end
+
+    context 'when data for the last 4 complete weeks' do
+      it { expect(described_class.unique_events(event_names: weekly_event, start_date: 4.weeks.ago, end_date: Date.current)).to eq(2) }
+    end
+
+    context 'when data for the week 4 weeks ago' do
+      it { expect(described_class.unique_events(event_names: weekly_event, start_date: 4.weeks.ago, end_date: 3.weeks.ago)).to eq(1) }
+    end
+
+    context 'when using daily aggregation' do
+      it { expect(described_class.unique_events(event_names: daily_event, start_date: 7.days.ago, end_date: Date.current)).to eq(2) }
+      it { expect(described_class.unique_events(event_names: daily_event, start_date: 28.days.ago, end_date: Date.current)).to eq(3) }
+      it { expect(described_class.unique_events(event_names: daily_event, start_date: 28.days.ago, end_date: 21.days.ago)).to eq(1) }
+    end
+  end
+end