mentorenwahl/backend/src/backend/worker/jobs/assignment_job.cr

# Mentorenwahl: A fullstack application for assigning mentors to students based on their whishes.
# Copyright (C) 2022  Dominic Grimm
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <https://www.gnu.org/licenses/>.

module Backend
  module Worker
    module Jobs
      # Assigns students to teachers when all students voted
      class AssignmentJob < Mosquito::QueuedJob
        alias TeacherVote = {student: Int32, priority: Int32}
        alias Assignment = {teacher: Int32, priority: Int32}

        def rescheduleable?
          false
        end

        # :ditto:
        def perform : Nil
          if Db::Config.query.where { active }.first!.can_vote
            log "Voting still allowed, skipping assignment"
            fail
          end

          teacher_count = Db::Teacher.query.count
          student_count = Db::Student.query.count
          vote_count = Db::Vote.query.count
          if teacher_count == 0
            log "No teachers found, skipping assignment"
            fail
          elsif student_count == 0
            log "No students found, skipping assignment"
            fail
          elsif student_count != Db::User.query.where(role: Db::UserRole::Student).count
            log "Not all students registered, skipping assignment"
            fail
          elsif vote_count < student_count
            log "Not all students voted, skipping assignment"
            fail
          end

          if Db::Teacher.query.sum(:max_students, Int64) < student_count
            log "Capacity too low, teachers need to adopt more students, skipping assignment"
            fail
          end

          teachers = Db::Teacher.query
            .where do
              raw("EXISTS (SELECT 1 FROM teacher_votes WHERE teacher_id = teachers.id)") &
                (max_students > 0)
            end
            .with_teacher_votes
            .to_a
          teacher_ids = teachers.map(&.id)
          teacher_max_students = Hash.zip(teacher_ids, teachers.map(&.max_students))
          teacher_votes : Hash(Int32, Array(TeacherVote)) = Hash.zip(
            teacher_ids,
            teachers.map do |t|
              t.teacher_votes.map do |tv|
                {
                  student:  tv.vote.student.id,
                  priority: tv.priority,
                }
              end
            end
          )
          log "Got teachers"

          students = Db::Student.query
            .with_vote(&.with_teacher_votes(&.order_by(priority: :desc)))
            .to_a
          student_ids = students.map(&.id)
          votes = Hash.zip(
            student_ids,
            students.map do |s|
              s.vote.not_nil!.teacher_votes
                .to_a
                .select! { |tv| teacher_votes.has_key?(tv.teacher.id) }
                .map do |tv|
                  {
                    teacher:  tv.teacher.id,
                    priority: tv.priority,
                  }
                end
            end
          )
          votes_a = votes.to_a
          log "Got students' votes"

          best : {assignment: Hash(Int32, Assignment), priority_score: Int64, teacher_score: Int64}? = nil
          empty_assignment_count = Hash.zip(teachers.map(&.id), [0] * teachers.size)
          log "Starting assignment"
          valid_count = 0_u64
          invalid_count = 0_u64

          max_span = Time::Span.new(minutes: Backend.config.assignment_run_time)
          start_time = Time.utc

          loop do
            assignment = {} of Int32 => Assignment
            assignment_count = empty_assignment_count.clone
            votes_a.shuffle(Random::Secure).each do |s, tvs|
              tvs.each do |tv|
                if assignment_count[tv[:teacher]] < teacher_max_students[tv[:teacher]]
                  if assignment[s]?.nil?
                    assignment_count[tv[:teacher]] += 1
                    assignment[s] = {teacher: tv[:teacher], priority: tv[:priority]}
                  else
                    assignment_count[assignment[s][:teacher]] -= 1
                    assignment_count[tv[:teacher]] += 1
                    assignment[s] = {teacher: tv[:teacher], priority: tv[:priority]}
                  end
                end
              end
            end

            if assignment.size != student_count
              invalid_count += 1
              log "Invalid combination, next (#{assignment.size} != #{student_count}, valid: #{valid_count} invalid: #{invalid_count})"
            else
              log "Valid combination (valid: #{valid_count} invalid: #{invalid_count})!"

              priority_score = 0_i64
              assignment.each do |_, a|
                priority_score += a[:priority] ** 2
              end

              teacher_score = 0_i64
              assignment_count.each do |t, c|
                teacher_score += (teacher_max_students[t] ** c) * teacher_max_students[t]
              end

              if best.nil?
                best = {
                  assignment:     assignment,
                  priority_score: priority_score,
                  teacher_score:  teacher_score,
                }
              elsif priority_score < best.not_nil![:priority_score] && teacher_score < best.not_nil![:teacher_score]
                best = {
                  assignment:     assignment,
                  priority_score: priority_score,
                  teacher_score:  teacher_score,
                }
              end

              valid_count += 1
            end

            break if (Time.utc - start_time) > max_span && valid_count > 0
          end

          pp! best

          log "Saving best assignment into database"
          Db::Assignment.query.where { active }.to_update.set(active: false).execute
          assignment_id = Db::Assignment.create!({
            active:         true,
            priority_score: best.not_nil![:priority_score],
            teacher_score:  best.not_nil![:teacher_score],
          }).id
          Db::StudentAssignment.import(
            best.not_nil![:assignment].map do |s, a|
              Db::StudentAssignment.new({
                assignment_id: assignment_id,
                student_id:    s,
                teacher_id:    a[:teacher],
              })
            end
          )
        end
      end
    end
  end
end