Every initialized Solution has a score. That score is an objective way to compare 2 solutions: the solution with the higher score is better. The Solver aims to find the Solution with the highest Score of all possible solutions. The best solution is the Solution with the highest Score that Solver has encountered during solving, which might be the optimal solution.

Planner cannot automatically know which Solution is best for your business, so you need to tell it how to calculate the score of a given Solution according to your business needs. There are multiple score techniques that you can use and combine.

All score techniques are based on constraints. Such a constraint can be a simple pattern (such as Maximize the apple harvest in the solution) or a more complex pattern. a positive constraint is a constraint you're trying to maximize. A negative constraint is a constraint you're trying to minimize.

Notice in the image above, that the optimal solution always has the highest score, regardless if the constraints are positive or negative.

Most planning problems have only negative constraints and therefore have a negative score. In that case, the score is usually the sum of the weight of the negative constraints being broken, with a perfect score of 0. This explains why the score of a solution of 4 queens is the negative (and not the positive!) of the number of queen couples which can attack each other.

Negative and positive constraints can be combined, even in the same score level.

When a constraint activates (because the negative constraint is broken or the positive constraint is fulfilled) on a certain planning entity set, it is called a constraint occurrence.

Not all score constraints are equally important. If breaking one constraint is equally bad as breaking another constraint x times, then those 2 constraints have a different weight (but they are in the same score level). For example in vehicle routing, you can make 2 "unhappy driver" constraint occurrences count as much as 1 "fuel tank usage" constraint occurrence:

Score weighting is often used in use cases where you can put a price tag on everything. In that case, the positive constraints maximize revenue and the negative constraints minimize expenses: together they maximize profit. Alternatively, score weighting is also often used to create social fairness. For example: nurses that request a free day on New Year's eve pay a higher weight than on a normal day.

The weight of a constraint occurrence can be dynamically based on the planning entities involved. For example in cloud balance: the weight of the soft constraint occurrence for an active Computer is the cost of that Computer.

Sometimes a score constraint outranks another score constraint, no matter how many times the other is broken. In that case, those score constraints are in different levels. For example: a nurse cannot do 2 shifts at the same time (due to the constraints of physical reality), this outranks nurse happiness constraints.

Most use cases have only 2 score levels: hard and soft. When comparing 2 scores, they are compared lexicographically: the first score level gets compared first. If those differ, the others score levels are ignored. For example: a score that breaks 0 hard constraints and 1000000 soft constraints is better than a score that breaks 1 hard constraint and 0 soft constraints.

Score levels often employ score weighting per level. In such case, the hard constraint level usually makes the solution feasible and the soft constraint level maximizes profit by weighting the constraints on price.

3 or more score levels is supported. For example: a company might decide that profit outranks employee satisfaction (or visa versa), while both are outranked by the constraints of physical reality.

Far less common is the use case of pareto optimization, which is also known under the more confusing term multi-objective optimization. In pareto scoring, score constraints are in the same score level, yet they are not weighted against each other. When 2 scores are compared, each of the score constraints are compared individually and the score with the most dominating score constraints wins. Pareto scoring can even be combined with score levels and score constraint weighting.

Consider this example with positive constraints, where we want to get the most apples and oranges. Since it's impossible to compare apples and oranges, we can't weight them against each other. Yet, despite that we can't compare them, we can state that 2 apples are better then 1 apple. Similarly, we can state that 2 apples and 1 orange are better than just 1 orange. So despite our inability to compare some Scores conclusively (at which point we declare them equal), we can find a set of optimal scores. Those are called pareto optimal.

Scores are considered equal far more often. It's left up to a human to choose the better out of a set of best solutions (with equal scores) found by Planner. In the example above, the user must choose between solution A (3 apples and 1 orange) and solution B (1 apples and 6 oranges). It's guaranteed that Planner has not found another solution which has more apples or more oranges or even a better combination of both (such as 2 apples and 3 oranges).

To implement pareto scoring in Planner, implement a custom ScoreDefinition and Score. Future versions will provide out-of-the-box support.

A score is represented by the Score interface, which naturally extends Comparable:

public interface Score<...> extends Comparable<...> {

    ...

}

The Score implementation to use depends on your use case. Your score might not efficiently fit in a single double value. Planner has several build-in Score implementations, but you can implement a custom Score too. Most use cases will just use the build-in HardAndSoftScore.

The Score implementation (for example DefaultHardAndSoftScore) must be the same throughout a Solver runtime. The Score implementation is configured in the solver configuration as a ScoreDefinition:

  <scoreDirectorFactory>

    <scoreDefinitionType>HARD_AND_SOFT</scoreDefinitionType>

    ...

  </scoreDirectorFactory>

Based on your score constraints and score level requirements, you'll choose a certain ScoreDefinition:

The SimpleScoreDefinition defines the Score as a SimpleScore which has a single int value, for example -123. It has a single score level.

  <scoreDirectorFactory>

    <scoreDefinitionType>SIMPLE</scoreDefinitionType>

    ...

  </scoreDirectorFactory>

Variants:

The HardAndSoftScoreDefinition defines the Score as a HardAndSoftScore which has a hard int value and a soft int value, for example -123hard/-456soft. It has 2 score levels (hard and soft).

  <scoreDirectorFactory>

    <scoreDefinitionType>HARD_AND_SOFT</scoreDefinitionType>

    ...

  </scoreDirectorFactory>

Variants:

The ScoreDefinition interface defines the score representation.

To implement a custom Score, you'll also need to implement a custom ScoreDefinition. Extend AbstractScoreDefinition (preferable by copy pasting HardAndSoftScoreDefinition or SimpleScoreDefinition) and start from there.

Then hook you custom ScoreDefinition in your SolverConfig.xml:

  <scoreDirectorFactory>

    <scoreDefinitionClass>org.drools.planner.examples.my.score.definition.MyScoreDefinition</scoreDefinitionClass>

    ...

  </scoreDirectorFactory>

There are several ways to calculate the Score of a Solution:

Every score calculation type can use any Score definition. For example, simple score calculation can output a HardAndSoftScore.

All score calculation types are Object Orientated and can reuse existing Java code.

A simple way to implement your score calculation in Java.

Just implement one method of the interface SimpleScoreCalculator:

public interface SimpleScoreCalculator<Sol extends Solution> {


    Score calculateScore(Sol solution);

   

}

For example in n queens:

public class NQueensSimpleScoreCalculator implements SimpleScoreCalculator<NQueens> {


    public SimpleScore calculateScore(NQueens nQueens) {

        int n = nQueens.getN();

        List<Queen> queenList = nQueens.getQueenList();

        

        int score = 0;

        for (int i = 0; i < n; i++) {

            for (int j = i + 1; j < n; j++) {

                Queen leftQueen = queenList.get(i);

                Queen rightQueen = queenList.get(j);

                if (leftQueen.getRow() != null && rightQueen.getRow() != null) {

                    if (leftQueen.getRowIndex() == rightQueen.getRowIndex()) {

                        score--;

                    }

                    if (leftQueen.getAscendingDiagonalIndex() == rightQueen.getAscendingDiagonalIndex()) {

                        score--;

                    }

                    if (leftQueen.getDescendingDiagonalIndex() == rightQueen.getDescendingDiagonalIndex()) {

                        score--;

                    }

                }

            }

        }

        return DefaultSimpleScore.valueOf(score);

    }


}

Configure it in your solver configuration:

  <scoreDirectorFactory>

    <scoreDefinitionType>...</scoreDefinitionType>

    <simpleScoreCalculatorClass>org.drools.planner.examples.nqueens.solver.score.NQueensSimpleScoreCalculator</simpleScoreCalculatorClass>

  </scoreDirectorFactory>

Alternatively, build a SimpleScoreCalculator instance at runtime and set it with the programmatic API:

    solverFactory.getSolverConfig().getScoreDirectorFactoryConfig.setSimpleScoreCalculator(simpleScoreCalculator);

A way to implement your score calculation incrementally in Java.

Implement all the methods of the interface IncrementalScoreCalculator and extend the class AbstractIncrementalScoreCalculator:

public interface IncrementalScoreCalculator<Sol extends Solution> {


    void resetWorkingSolution(Sol workingSolution);


    void beforeEntityAdded(Object entity);


    void afterEntityAdded(Object entity);


    void beforeAllVariablesChanged(Object entity);


    void afterAllVariablesChanged(Object entity);


    void beforeVariableChanged(Object entity, String variableName);


    void afterVariableChanged(Object entity, String variableName);


    void beforeEntityRemoved(Object entity);


    void afterEntityRemoved(Object entity);


    Score calculateScore();

    

}

For example in n queens:

public class NQueensAdvancedIncrementalScoreCalculator extends AbstractIncrementalScoreCalculator<NQueens> {


    private Map<Integer, List<Queen>> rowIndexMap;

    private Map<Integer, List<Queen>> ascendingDiagonalIndexMap;

    private Map<Integer, List<Queen>> descendingDiagonalIndexMap;


    private int score;


    public void resetWorkingSolution(NQueens nQueens) {

        int n = nQueens.getN();

        rowIndexMap = new HashMap<Integer, List<Queen>>(n);

        ascendingDiagonalIndexMap = new HashMap<Integer, List<Queen>>(n * 2);

        descendingDiagonalIndexMap = new HashMap<Integer, List<Queen>>(n * 2);

        for (int i = 0; i < n; i++) {

            rowIndexMap.put(i, new ArrayList<Queen>(n));

            ascendingDiagonalIndexMap.put(i, new ArrayList<Queen>(n));

            descendingDiagonalIndexMap.put(i, new ArrayList<Queen>(n));

            if (i != 0) {

                ascendingDiagonalIndexMap.put(n - 1 + i, new ArrayList<Queen>(n));

                descendingDiagonalIndexMap.put((-i), new ArrayList<Queen>(n));

            }

        }

        score = 0;

        for (Queen queen : nQueens.getQueenList()) {

            insert(queen);

        }

    }


    public void beforeEntityAdded(Object entity) {

        // Do nothing

    }


    public void afterEntityAdded(Object entity) {

        insert((Queen) entity);

    }


    public void beforeAllVariablesChanged(Object entity) {

        retract((Queen) entity);

    }


    public void afterAllVariablesChanged(Object entity) {

        insert((Queen) entity);

    }


    public void beforeVariableChanged(Object entity, String variableName) {

        retract((Queen) entity);

    }


    public void afterVariableChanged(Object entity, String variableName) {

        insert((Queen) entity);

    }


    public void beforeEntityRemoved(Object entity) {

        retract((Queen) entity);

    }


    public void afterEntityRemoved(Object entity) {

        // Do nothing

    }


    private void insert(Queen queen) {

        Row row = queen.getRow();

        if (row != null) {

            int rowIndex = queen.getRowIndex();

            List<Queen> rowIndexList = rowIndexMap.get(rowIndex);

            score -= rowIndexList.size();

            rowIndexList.add(queen);

            List<Queen> ascendingDiagonalIndexList = ascendingDiagonalIndexMap.get(queen.getAscendingDiagonalIndex());

            score -= ascendingDiagonalIndexList.size();

            ascendingDiagonalIndexList.add(queen);

            List<Queen> descendingDiagonalIndexList = descendingDiagonalIndexMap.get(queen.getDescendingDiagonalIndex());

            score -= descendingDiagonalIndexList.size();

            descendingDiagonalIndexList.add(queen);

        }

    }


    private void retract(Queen queen) {

        Row row = queen.getRow();

        if (row != null) {

            List<Queen> rowIndexList = rowIndexMap.get(queen.getRowIndex());

            rowIndexList.remove(queen);

            score += rowIndexList.size();

            List<Queen> ascendingDiagonalIndexList = ascendingDiagonalIndexMap.get(queen.getAscendingDiagonalIndex());

            ascendingDiagonalIndexList.remove(queen);

            score += ascendingDiagonalIndexList.size();

            List<Queen> descendingDiagonalIndexList = descendingDiagonalIndexMap.get(queen.getDescendingDiagonalIndex());

            descendingDiagonalIndexList.remove(queen);

            score += descendingDiagonalIndexList.size();

        }

    }


    public SimpleScore calculateScore() {

        return DefaultSimpleScore.valueOf(score);

    }


}

Configure it in your solver configuration:

  <scoreDirectorFactory>

    <scoreDefinitionType>...</scoreDefinitionType>

    <incrementalScoreCalculatorClass>org.drools.planner.examples.nqueens.solver.score.NQueensAdvancedIncrementalScoreCalculator</incrementalScoreCalculatorClass>

  </scoreDirectorFactory>

Optionally, to get better output when the IncrementalScoreCalculator is corrupted in environmentMode DEBUG or TRACE, you can overwrite the method buildScoreCorruptionAnalysis from AbstractIncrementalScoreCalculator.

Put the environmentMode in TRACE (or DEBUG) to detect corruption in the incremental score calculation. On the difference between TRACE and DEBUG, see the section about environmentMode. However, that will not detect if your score calculator implements your score constraints as your business actually desires.

A piece of incremental score calculator code can be difficult to write and to review. You can assert its correctness by using a different implementation (for example a SimpleScoreCalculator) to do the assertions trigged by the environmentMode. Just configure it as assertionScoreDirectorFactory:

  <environmentMode>DEBUG</environmentMode>

  ...

  <scoreDirectorFactory>

    <scoreDefinitionType>...</scoreDefinitionType>

    <scoreDrl>/org/drools/planner/examples/nqueens/solver/nQueensScoreRules.drl</scoreDrl>

    <assertionScoreDirectorFactory>

      <scoreDefinitionType>...</scoreDefinitionType>

      <simpleScoreCalculatorClass>org.drools.planner.examples.nqueens.solver.score.NQueensSimpleScoreCalculator</simpleScoreCalculatorClass>

    </assertionScoreDirectorFactory>

  </scoreDirectorFactory>

The Solver will normally spend most of its execution time running the score calculation (which is called in its deepest loops). Faster score calculation will return the same solution in less time with the same algorithm, which normally means a better solution in equal time.

After solving a problem, the Solver will log the average calculation count per second. This is a good measurement of Score calculation performance, despite that it is affected by non score calculation execution time. It depends on the problem scale of the problem dataset. Normally, even for high scale problems, it is higher than 1000, except when you're using SimpleScoreCalculator.

When a Solution changes, incremental score calculation (AKA delta based score calculation), will calculate the delta with the previous state to find the new Score, instead of recalculating the entire score on every solution evaluation.

For example, if a single queen A moves from row 1 to 2, it won't bother to check if queen B and C can attack each other, since neither of them changed.

Figure 5.1. Incremental score calculation for the 4 queens puzzle

Do not call remote services in your score calculation (except if you're bridging SimpleScoreCalculator to a legacy system). The network latency will kill your score calculation performance. Cache the results of those remote services if possible.

If some parts of a constraint can be calculated once, when the Solver starts, and never change during solving, then turn them into cached problem facts.

If you know a certain constraint can never be broken, don't bother writing a score constraint for it. For example in n queens, the score calculation doesn't check if multiple queens occupy the same column, because a Queen's column never changes and every Solution starts with each Queen on a different column.

Instead of implementing a hard constraint, you can sometimes make it build-in too. For example: If Course A should never be assigned to Room X, but it uses ValueRange from Solution, the Solver will often try to assign it to Room X too (only to find out that it breaks a hard constraint). Use filtered selection to define that Course A should only be assigned a Room other than X.

This tends to give a good performance gain, not just because the score calculation is faster, but mainly because most optimization algorithms will spend less time evaluating unfeasible solutions.

Be watchful for score traps. A score trap is a state in which several moves need to be done to resolve or lower the weight of a single constraint occurrence. Some examples of score traps:

Not all score constraints have the same performance cost. Sometimes 1 score constraint can kill the score calculation performance outright. Use the benchmarker to do a 1 minute run and check what happens to the average calculation count per second if you comment out all but 1 of the score constraints.