Define the constraints and calculate the score

A score represents the quality of a specific solution. The higher the better. Timefold looks for the best solution, which is the solution with the highest score found in the available time. It might be the optimal solution.

Because this use case has hard and soft constraints, use the HardSoftScore class to represent the score:

Hard constraints must not be broken. For example: A room can have at most one lesson at the same time.
Soft constraints should not be broken. For example: A teacher prefers to teach in a single room.

Hard constraints are weighted against other hard constraints. Soft constraints are weighted too, against other soft constraints. Hard constraints always outweigh soft constraints, regardless of their respective weights.

To calculate the score, you could implement an EasyScoreCalculator class:

public class TimeTableEasyScoreCalculator implements EasyScoreCalculator<TimeTable, HardSoftScore> {

    @Override
    public HardSoftScore calculateScore(TimeTable timeTable) {
        List<Lesson> lessonList = timeTable.getLessonList();
        int hardScore = 0;
        for (Lesson a : lessonList) {
            for (Lesson b : lessonList) {
                if (a.getTimeslot() != null && a.getTimeslot().equals(b.getTimeslot())
                        && a.getId() < b.getId()) {
                    // A room can accommodate at most one lesson at the same time.
                    if (a.getRoom() != null && a.getRoom().equals(b.getRoom())) {
                        hardScore--;
                    }
                    // A teacher can teach at most one lesson at the same time.
                    if (a.getTeacher().equals(b.getTeacher())) {
                        hardScore--;
                    }
                    // A student can attend at most one lesson at the same time.
                    if (a.getStudentGroup().equals(b.getStudentGroup())) {
                        hardScore--;
                    }
                }
            }
        }
        int softScore = 0;
        // Soft constraints are only implemented in the timefold-quickstarts code
        return HardSoftScore.of(hardScore, softScore);
    }

}

Unfortunately that does not scale well, because it is non-incremental: every time a lesson is assigned to a different time slot or room, all lessons are re-evaluated to calculate the new score.

Instead, create a src/main/java/org/acme/schooltimetabling/solver/TimeTableConstraintProvider.java class to perform incremental score calculation. It uses Timefold’s ConstraintStream API which is inspired by Java Streams and SQL:

package org.acme.schooltimetabling.solver;

import org.acme.schooltimetabling.domain.Lesson;
import ai.timefold.solver.core.api.score.buildin.hardsoft.HardSoftScore;
import ai.timefold.solver.core.api.score.stream.Constraint;
import ai.timefold.solver.core.api.score.stream.ConstraintFactory;
import ai.timefold.solver.core.api.score.stream.ConstraintProvider;
import ai.timefold.solver.core.api.score.stream.Joiners;

public class TimeTableConstraintProvider implements ConstraintProvider {

    @Override
    public Constraint[] defineConstraints(ConstraintFactory constraintFactory) {
        return new Constraint[] {
                // Hard constraints
                roomConflict(constraintFactory),
                teacherConflict(constraintFactory),
                studentGroupConflict(constraintFactory),
                // Soft constraints are only implemented in the timefold-quickstarts code
        };
    }

    private Constraint roomConflict(ConstraintFactory constraintFactory) {
        // A room can accommodate at most one lesson at the same time.

        // Select a lesson ...
        return constraintFactory
                .forEach(Lesson.class)
                // ... and pair it with another lesson ...
                .join(Lesson.class,
                        // ... in the same timeslot ...
                        Joiners.equal(Lesson::getTimeslot),
                        // ... in the same room ...
                        Joiners.equal(Lesson::getRoom),
                        // ... and the pair is unique (different id, no reverse pairs) ...
                        Joiners.lessThan(Lesson::getId))
                // ... then penalize each pair with a hard weight.
                .penalize(HardSoftScore.ONE_HARD)
                .asConstraint("Room conflict");
    }

    private Constraint teacherConflict(ConstraintFactory constraintFactory) {
        // A teacher can teach at most one lesson at the same time.
        return constraintFactory.forEach(Lesson.class)
                .join(Lesson.class,
                        Joiners.equal(Lesson::getTimeslot),
                        Joiners.equal(Lesson::getTeacher),
                        Joiners.lessThan(Lesson::getId))
                .penalize(HardSoftScore.ONE_HARD)
                .asConstraint("Teacher conflict");
    }

    private Constraint studentGroupConflict(ConstraintFactory constraintFactory) {
        // A student can attend at most one lesson at the same time.
        return constraintFactory.forEach(Lesson.class)
                .join(Lesson.class,
                        Joiners.equal(Lesson::getTimeslot),
                        Joiners.equal(Lesson::getStudentGroup),
                        Joiners.lessThan(Lesson::getId))
                .penalize(HardSoftScore.ONE_HARD)
                .asConstraint("Student group conflict");
    }

}

The ConstraintProvider scales an order of magnitude better than the EasyScoreCalculator: O(n) instead of O(n²).