1 files changed, 71 insertions, 0 deletions
diff --git a/app/services/issue_rebalancing_service.rb b/app/services/issue_rebalancing_service.rb
new file mode 100644
index 00000000000..4138c6441c8
--- /dev/null
+++ b/app/services/issue_rebalancing_service.rb
@@ -0,0 +1,71 @@
+# frozen_string_literal: true
+
+class IssueRebalancingService
+  MAX_ISSUE_COUNT = 10_000
+  TooManyIssues = Class.new(StandardError)
+
+  def initialize(issue)
+    @issue = issue
+    @base = Issue.relative_positioning_query_base(issue)
+  end
+
+  def execute
+    gates = [issue.project, issue.project.group].compact
+    return unless gates.any? { |gate| Feature.enabled?(:rebalance_issues, gate) }
+
+    raise TooManyIssues, "#{issue_count} issues" if issue_count > MAX_ISSUE_COUNT
+
+    start = RelativePositioning::START_POSITION - (gaps / 2) * gap_size
+
+    Issue.transaction do
+      indexed_ids.each_slice(100) { |pairs| assign_positions(start, pairs) }
+    end
+  end
+
+  private
+
+  attr_reader :issue, :base
+
+  # rubocop: disable CodeReuse/ActiveRecord
+  def indexed_ids
+    base.reorder(:relative_position, :id).pluck(:id).each_with_index
+  end
+  # rubocop: enable CodeReuse/ActiveRecord
+
+  # rubocop: disable CodeReuse/ActiveRecord
+  def assign_positions(start, positions)
+    values = positions.map do |id, index|
+      "(#{id}, #{start + (index * gap_size)})"
+    end.join(', ')
+
+    Issue.connection.exec_query(<<~SQL, "rebalance issue positions")
+      WITH cte(cte_id, new_pos) AS (
+       SELECT *
+       FROM (VALUES #{values}) as t (id, pos)
+      )
+      UPDATE #{Issue.table_name}
+      SET relative_position = cte.new_pos
+      FROM cte
+      WHERE cte_id = id
+    SQL
+  end
+  # rubocop: enable CodeReuse/ActiveRecord
+
+  def issue_count
+    @issue_count ||= base.count
+  end
+
+  def gaps
+    issue_count - 1
+  end
+
+  def gap_size
+    # We could try to split the available range over the number of gaps we need,
+    # but IDEAL_DISTANCE * MAX_ISSUE_COUNT is only 0.1% of the available range,
+    # so we are guaranteed not to exhaust it by using this static value.
+    #
+    # If we raise MAX_ISSUE_COUNT or IDEAL_DISTANCE significantly, this may
+    # change!
+    RelativePositioning::IDEAL_DISTANCE
+  end
+end