Create burnup chart service for milestones

This computes the aggregated data per day so that we don't have to send
all the events and compute this in the frontend

ee/app/services/milestones/burnup_chart_service.rb

+# frozen_string_literal: true
+
+# This service computes the milestone's daily total number of issues and their weights.
+# For each day, this returns the totals for all issues that are assigned to the milestone at that point in time.
+# This represents the scope for this milestone. This also returns separate totals for closed issues which represent the completed work.
+#
+# This is implemented by iterating over all relevant resource events ordered by time. We need to do this
+# so that we can keep track of the issue's state during that point in time and handle the events based on that.
+
+class Milestones::BurnupChartService
+  def initialize(milestone)
+    raise ArgumentError, 'Milestone must have a start and due date' if milestone.start_date.blank? || milestone.due_date.blank?
+
+    @milestone = milestone
+  end
+
+  def execute
+    @issue_states = {}
+    @chart_data = []
+
+    resource_events.each do |event|
+      case event['event_type']
+      when 'milestone'
+        handle_milestone_event(event)
+      when 'state'
+        handle_state_event(event)
+      when 'weight'
+        handle_weight_event(event)
+      end
+    end
+
+    chart_data
+  end
+
+  private
+
+  attr_reader :milestone, :issue_states, :chart_data
+
+  def handle_milestone_event(event)
+    issue_state = find_issue_state(event['issue_id'])
+
+    return if issue_state[:milestone] == milestone.id && event['action'] == ResourceMilestoneEvent.actions[:add] && event['value'] == milestone.id
+
+    if event['action'] == ResourceMilestoneEvent.actions[:add] && event['value'] == milestone.id
+      handle_add_milestone_event(event)
+    elsif issue_state[:milestone] == milestone.id
+      # If the issue is currently assigned to the milestone, then treat any event here as a removal.
+      # We do not have a separate `:remove` event when replacing milestones with another one.
+      handle_remove_milestone_event(event)
+    end
+
+    issue_state[:milestone] = event['value']
+  end
+
+  def handle_add_milestone_event(event)
+    issue_state = find_issue_state(event['issue_id'])
+
+    increment_scope(event['created_at'], issue_state[:weight])
+
+    if issue_state[:state] == ResourceStateEvent.states[:closed]
+      increment_completed(event['created_at'], issue_state[:weight])
+    end
+  end
+
+  def handle_remove_milestone_event(event)
+    issue_state = find_issue_state(event['issue_id'])
+
+    decrement_scope(event['created_at'], issue_state[:weight])
+
+    if issue_state[:state] == ResourceStateEvent.states[:closed]
+      decrement_completed(event['created_at'], issue_state[:weight])
+    end
+  end
+
+  def handle_state_event(event)
+    issue_state = find_issue_state(event['issue_id'])
+    old_state = issue_state[:state]
+    issue_state[:state] = event['value']
+
+    return if issue_state[:milestone] != milestone.id
+
+    if old_state == ResourceStateEvent.states[:closed] && event['value'] == ResourceStateEvent.states[:reopened]
+      decrement_completed(event['created_at'], issue_state[:weight])
+    elsif ResourceStateEvent.states.values_at(:opened, :reopened).include?(old_state) && event['value'] == ResourceStateEvent.states[:closed]
+      increment_completed(event['created_at'], issue_state[:weight])
+    end
+  end
+
+  def handle_weight_event(event)
+    issue_state = find_issue_state(event['issue_id'])
+    old_weight = issue_state[:weight]
+    issue_state[:weight] = event['value'] || 0
+
+    return if issue_state[:milestone] != milestone.id
+
+    add_chart_data(event['created_at'], :scope_weight, issue_state[:weight] - old_weight)
+
+    if issue_state[:state] == ResourceStateEvent.states[:closed]
+      add_chart_data(event['created_at'], :completed_weight, issue_state[:weight] - old_weight)
+    end
+  end
+
+  def increment_scope(timestamp, weight)
+    add_chart_data(timestamp, :scope_count, 1)
+    add_chart_data(timestamp, :scope_weight, weight)
+  end
+
+  def decrement_scope(timestamp, weight)
+    add_chart_data(timestamp, :scope_count, -1)
+    add_chart_data(timestamp, :scope_weight, -weight)
+  end
+
+  def increment_completed(timestamp, weight)
+    add_chart_data(timestamp, :completed_count, 1)
+    add_chart_data(timestamp, :completed_weight, weight)
+  end
+
+  def decrement_completed(timestamp, weight)
+    add_chart_data(timestamp, :completed_count, -1)
+    add_chart_data(timestamp, :completed_weight, -weight)
+  end
+
+  def add_chart_data(timestamp, field, value)
+    date = timestamp.to_date
+    date = milestone.start_date if date < milestone.start_date
+
+    if chart_data.empty?
+      chart_data.push({
+        date: date,
+        scope_count: 0,
+        scope_weight: 0,
+        completed_count: 0,
+        completed_weight: 0
+      })
+    elsif chart_data.last[:date] != date
+      # To start a new day entry we copy the previous day's data because the numbers are cumulative
+      chart_data.push(
+        chart_data.last.merge(date: date)
+      )
+    end
+
+    chart_data.last[field] += value
+  end
+
+  def find_issue_state(issue_id)
+    issue_states[issue_id] ||= {
+      milestone: nil,
+      weight: 0,
+      state: ResourceStateEvent.states[:opened]
+    }
+  end
+
+  # rubocop: disable CodeReuse/ActiveRecord
+  def resource_events
+    union = Gitlab::SQL::Union.new([milestone_events, state_events, weight_events]) # rubocop: disable Gitlab/Union
+
+    ActiveRecord::Base.connection.execute("(#{union.to_sql}) ORDER BY created_at")
+  end
+  # rubocop: enable CodeReuse/ActiveRecord
+
+  def milestone_events
+    ResourceMilestoneEvent.by_issue_ids_and_created_at_earlier_or_equal_to(issue_ids, end_time)
+      .select('\'milestone\' AS event_type, created_at, milestone_id AS value, action, issue_id')
+  end
+
+  def state_events
+    ResourceStateEvent.by_issue_ids_and_created_at_earlier_or_equal_to(issue_ids, end_time)
+      .select('\'state\' AS event_type, created_at, state AS value, NULL AS action, issue_id')
+  end
+
+  def weight_events
+    ResourceWeightEvent.by_issue_ids_and_created_at_earlier_or_equal_to(issue_ids, end_time)
+      .select('\'weight\' AS event_type, created_at, weight AS value, NULL AS action, issue_id')
+  end
+
+  # rubocop: disable CodeReuse/ActiveRecord
+  def issue_ids
+    # We find all issues that have this milestone added before this milestone's due date.
+    # We cannot just filter by `issues.milestone_id` because there might be issues that have
+    # since been moved to other milestones and we still need to include these in this graph.
+    ResourceMilestoneEvent
+      .select(:issue_id)
+      .where({
+        milestone_id: milestone.id,
+        action: :add
+      })
+      .where('created_at <= ?', end_time)
+  end
+  # rubocop: enable CodeReuse/ActiveRecord
+
+  def end_time
+    @end_time ||= milestone.due_date.end_of_day
+  end
+end