Add tests for rebalancing service

app/services/issue_rebalancing_service.rb

 # frozen_string_literal: true
 
-module Issue
-  class RebalancingService < Issues::BaseService
+class IssueRebalancingService < Issues::BaseService
   MAX_ISSUE_COUNT = 100_000
   TooManyIssues = Class.new(StandardError)
 
@@ -57,5 +56,4 @@ module Issue
     end
   end
   # rubocop: enable CodeReuse/ActiveRecord
-  end
 end

spec/services/issue_rebalancing_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe IssueRebalancingService do
+  let_it_be(:project) { create(:project) }
+  let_it_be(:user) { project.creator }
+  let_it_be(:start) { RelativePositioning::START_POSITION }
+  let_it_be(:max_pos) { RelativePositioning::MAX_POSITION }
+  let_it_be(:min_pos) { RelativePositioning::MIN_POSITION }
+  let_it_be(:clump_size) { 300 }
+
+  let_it_be(:unclumped) do
+    (0..clump_size).to_a.map do |i|
+      create(:issue, project: project, author: user, relative_position: start + (1024 * i))
+    end
+  end
+
+  let_it_be(:end_clump) do
+    (0..clump_size).to_a.map do |i|
+      create(:issue, project: project, author: user, relative_position: max_pos - i)
+    end
+  end
+
+  let_it_be(:start_clump) do
+    (0..clump_size).to_a.map do |i|
+      create(:issue, project: project, author: user, relative_position: min_pos + i)
+    end
+  end
+
+  def issues_in_position_order
+    project.reload.issues.reorder(relative_position: :asc).to_a
+  end
+
+  it 'rebalances a set of issues with clumps at the end and start' do
+    all_issues = start_clump + unclumped + end_clump.reverse
+
+    service = described_class.new(project.issues.first)
+
+    expect { service.execute }.not_to change { issues_in_position_order.map(&:id) }
+
+    all_issues.each(&:reset)
+
+    gaps = all_issues.take(all_issues.count - 1).zip(all_issues.drop(1)).map do |a, b|
+      b.relative_position - a.relative_position
+    end
+
+    expect(gaps).to all(be > RelativePositioning::MIN_GAP)
+    expect(all_issues.first.relative_position).to be > RelativePositioning::MIN_POSITION
+    expect(all_issues.last.relative_position).to be < RelativePositioning::MAX_POSITION
+  end
+end