Introduce hash_code for VSA stages and events

This change adds the hash_code methods for stages and events.

app/models/concerns/analytics/cycle_analytics/stage.rb

@@ -50,6 +50,10 @@ module Analytics
         end
       end
 
+      def events_hash_code
+        Digest::SHA256.hexdigest("#{start_event.hash_code}-#{end_event.hash_code}")
+      end
+
       def start_event_label_based?
         start_event_identifier && start_event.label_based?
       end

ee/lib/gitlab/analytics/cycle_analytics/stage_events/label_based_stage_event.rb

@@ -20,6 +20,10 @@ module Gitlab
             true
           end
 
+          def hash_code
+            Digest::SHA256.hexdigest("#{self.class.identifier}-#{label_id}")
+          end
+
           def column_list
             [Arel.sql("#{join_expression_name}.created_at")]
           end

ee/spec/lib/gitlab/analytics/cycle_analytics/stage_events/issue_label_added_spec.rb

@@ -4,6 +4,8 @@ require 'spec_helper'
 
 RSpec.describe Gitlab::Analytics::CycleAnalytics::StageEvents::IssueLabelAdded do
   it_behaves_like 'value stream analytics event' do
-    let(:params) { { label: GroupLabel.new } }
+    let(:label_id) { 10 }
+    let(:params) { { label: GroupLabel.new(id: label_id) } }
+    let(:expected_hash_code) { Digest::SHA256.hexdigest("#{instance.class.identifier}-#{label_id}") }
   end
 end

ee/spec/lib/gitlab/analytics/cycle_analytics/stage_events/issue_label_removed_spec.rb

@@ -4,6 +4,8 @@ require 'spec_helper'
 
 RSpec.describe Gitlab::Analytics::CycleAnalytics::StageEvents::IssueLabelRemoved do
   it_behaves_like 'value stream analytics event' do
-    let(:params) { { label: GroupLabel.new } }
+    let(:label_id) { 10 }
+    let(:params) { { label: GroupLabel.new(id: label_id) } }
+    let(:expected_hash_code) { Digest::SHA256.hexdigest("#{instance.class.identifier}-#{label_id}") }
   end
 end

ee/spec/lib/gitlab/analytics/cycle_analytics/stage_events/merge_request_label_added_spec.rb

@@ -4,6 +4,8 @@ require 'spec_helper'
 
 RSpec.describe Gitlab::Analytics::CycleAnalytics::StageEvents::MergeRequestLabelAdded do
   it_behaves_like 'value stream analytics event' do
-    let(:params) { { label: GroupLabel.new } }
+    let(:label_id) { 10 }
+    let(:params) { { label: GroupLabel.new(id: label_id) } }
+    let(:expected_hash_code) { Digest::SHA256.hexdigest("#{instance.class.identifier}-#{label_id}") }
   end
 end

ee/spec/lib/gitlab/analytics/cycle_analytics/stage_events/merge_request_label_removed_spec.rb

@@ -4,6 +4,8 @@ require 'spec_helper'
 
 RSpec.describe Gitlab::Analytics::CycleAnalytics::StageEvents::MergeRequestLabelRemoved do
   it_behaves_like 'value stream analytics event' do
-    let(:params) { { label: GroupLabel.new } }
+    let(:label_id) { 10 }
+    let(:params) { { label: GroupLabel.new(id: label_id) } }
+    let(:expected_hash_code) { Digest::SHA256.hexdigest("#{instance.class.identifier}-#{label_id}") }
   end
 end

lib/gitlab/analytics/cycle_analytics/stage_events/stage_event.rb

@@ -31,6 +31,10 @@ module Gitlab
             raise NotImplementedError
           end
 
+          def hash_code
+            Digest::SHA256.hexdigest(self.class.identifier.to_s)
+          end
+
           # Each StageEvent must expose a timestamp or a timestamp like expression in order to build a range query.
           # Example: get me all the Issue records between start event end end event
           def timestamp_projection

spec/support/shared_examples/lib/gitlab/cycle_analytics/event_shared_examples.rb

@@ -3,6 +3,7 @@
 RSpec.shared_examples_for 'value stream analytics event' do
   let(:params) { {} }
   let(:instance) { described_class.new(params) }
+  let(:expected_hash_code) { Digest::SHA256.hexdigest(instance.class.identifier.to_s) }
 
   it { expect(described_class.name).to be_a_kind_of(String) }
   it { expect(described_class.identifier).to be_a_kind_of(Symbol) }
@@ -19,4 +20,16 @@ RSpec.shared_examples_for 'value stream analytics event' do
       expect(output_query).to be_a_kind_of(ActiveRecord::Relation)
     end
   end
+
+  describe '#hash_code' do
+    it 'returns a hash that uniquely identifies an event' do
+      expect(instance.hash_code).to eq(expected_hash_code)
+    end
+
+    it 'does not differ when the same object is built with the same params' do
+      another_instance_with_same_params = described_class.new(params)
+
+      expect(another_instance_with_same_params.hash_code).to eq(instance.hash_code)
+    end
+  end
 end

spec/support/shared_examples/models/cycle_analytics_stage_shared_examples.rb

@@ -122,6 +122,22 @@ RSpec.shared_examples 'value stream analytics stage' do
       expect(stage.parent_id).to eq(parent.id)
     end
   end
+
+  describe '#hash_code' do
+    it 'does not differ when the same object is built with the same params' do
+      stage_1 = build(factory)
+      stage_2 = build(factory)
+
+      expect(stage_1.events_hash_code).to eq(stage_2.events_hash_code)
+    end
+
+    it 'differs when the stage events are different' do
+      stage_1 = build(factory, start_event_identifier: :merge_request_created, end_event_identifier: :merge_request_merged)
+      stage_2 = build(factory, start_event_identifier: :issue_created, end_event_identifier: :issue_first_mentioned_in_commit)
+
+      expect(stage_1.events_hash_code).not_to eq(stage_2.events_hash_code)
+    end
+  end
 end
 
 RSpec.shared_examples 'value stream analytics label based stage' do