feat: implement method of calculating which instructors to prompt

After a professor has deployed a case in their classroom, we want to
invite them to write up a retrospective of their experience as a post in
the instructors-only community “CaseLog.”

We look for deployments whose members were much more active in the case
14 to 7 days ago than they were in the last 7 days. This is defined as
an 80% drop-off of use from wee to week, where the previous week
measured over 500 events. We exclude any deployments with fewer than
five readers as a sanity check.

These magic numbers were chosen empirically based on recorded data at an
early stage and should be subject to review.

app/services/send_retrospective_prompts.rb

@@ -0,0 +1,65 @@
+# frozen_string_literal: true
+
+# After a professor has deployed a case in their classroom, we want to invite
+# them to write up a retrospective of their experience as a post in the
+# instructors-only community “CaseLog.” This service sends such notifications.
+#
+# Running once a week, on Wednesdays at noon (Eastern), we look for deployments
+# whose members were much more active in the case 14 to 7 days ago than they
+# were in the last 7 days. This is defined as an 80% drop-off of use from week
+# to week, where the previous week measured over 500 events. This was chosen
+# empirically based on recorded data at an early stage and should be subject to
+# review. We exclude any deployments with fewer than five readers as a sanity
+# check. When we send prompts to the instructors, we set the
+# `retrospective_prompt_sent` flag so never to bug people more than once.
+class SendRetrospectivePrompts
+  def self.call(time_basis: Time.zone.now)
+    new(time_basis).call
+  end
+
+  def initialize(now)
+    @now = now
+  end
+
+  def call
+    deployments_needing_prompts.map { |x| [x.group.name, x.case.slug] }
+    # deployments_needing_prompts.each do |deployment|
+    #   deployment.update retrospective_prompt_set: true
+    #   DeploymentMailer.retrospective_prompt(deployment).deliver
+    # end
+  end
+
+  private
+
+  def deployments_needing_prompts
+    candidate_deployments_with_enough_readers
+      .select { |d| much_more_use_two_weeks_ago d }
+  end
+
+  def candidate_deployments_with_enough_readers
+    count_readers_by_group = GroupMembership.group(:group_id).count(:reader_id)
+    Deployment.where(retrospective_prompt_sent_at: nil)
+              .select { |d| count_readers_by_group[d.group_id] > 5 }
+  end
+
+  def much_more_use_two_weeks_ago(deployment)
+    two_weeks_ago = count_interesting_events(
+      deployment,
+      (@now - 2.weeks)..(@now - 1.week)
+    )
+    last_week = count_interesting_events(
+      deployment,
+      (@now - 1.week)..@now
+    )
+
+    two_weeks_ago > 500 && (two_weeks_ago - last_week) / two_weeks_ago > -0.8
+  end
+
+  def count_interesting_events(deployment, range)
+    Ahoy::Event.interesting
+               .where(user_id: deployment.group.readers.pluck(:id))
+               .where_properties(case_slug: deployment.case.slug)
+               .where(time: range)
+               .count
+  end
+end