chore: make constraint metadata more flexible

core/src/main/java/ai/timefold/solver/core/api/domain/solution/ConstraintWeightOverrides.java

@@ -6,6 +6,7 @@
 
 import ai.timefold.solver.core.api.score.Score;
 import ai.timefold.solver.core.api.score.stream.ConstraintProvider;
+import ai.timefold.solver.core.api.score.stream.ConstraintRef;
 import ai.timefold.solver.core.api.score.stream.uni.UniConstraintStream;
 import ai.timefold.solver.core.api.solver.change.ProblemChange;
 import ai.timefold.solver.core.impl.domain.solution.DefaultConstraintWeightOverrides;
@@ -44,16 +45,24 @@ static <Score_ extends Score<Score_>> ConstraintWeightOverrides<Score_> of(Map<S
     /**
      * Return a constraint weight for a particular constraint.
      *
-     * @return null if the constraint name is not known
+     * @return null if the constraint id is not known
      */
     @Nullable
-    Score_ getConstraintWeight(String constraintName);
+    Score_ getConstraintWeight(String constraintId);
+
+    /**
+     * As defined by {@link #getConstraintWeight(String)},
+     * but accepts {@link ConstraintRef} instead of the ID directly.
+     */
+    default @Nullable Score_ getConstraintWeight(ConstraintRef constraintRef) {
+        return getConstraintWeight(constraintRef.id());
+    }
 
     /**
      * Returns all known constraints.
      *
-     * @return All constraint names for which {@link #getConstraintWeight(String)} returns a non-null value.
+     * @return All constraint IDs for which {@link #getConstraintWeight(String)} returns a non-null value.
      */
-    Set<String> getKnownConstraintNames();
+    Set<String> getKnownConstraintIds();
 
 }

core/src/main/java/ai/timefold/solver/core/api/score/analysis/ConstraintAnalysis.java

@@ -52,11 +52,13 @@ public interface ConstraintAnalysis<Score_ extends Score<Score_>> {
     int matchCount();
 
     /**
-     * Return name of the constraint that this analysis is for.
+     * Return id of the constraint that this analysis is for.
      *
-     * @return equal to {@code constraintRef.constraintName()}
+     * @return equal to {@code constraintRef.id()}
      */
-    String constraintName();
+    default String constraintId() {
+        return constraintRef().id();
+    }
 
     /**
      * Returns a diagnostic text that explains part of the score quality through the {@link ConstraintAnalysis} API.

core/src/main/java/ai/timefold/solver/core/api/score/analysis/ScoreAnalysis.java

@@ -101,7 +101,7 @@ public interface ScoreAnalysis<Score_ extends Score<Score_>> {
      * @return null if no constraint matches of such constraint are present
      */
     @Nullable
-    ConstraintAnalysis<Score_> getConstraintAnalysis(String constraintName);
+    ConstraintAnalysis<Score_> getConstraintAnalysis(String constraintId);
 
     /**
      * Compare this {@link ScoreAnalysis} to another {@link ScoreAnalysis}

core/src/main/java/ai/timefold/solver/core/api/score/stream/Constraint.java

@@ -2,7 +2,7 @@
 
 import ai.timefold.solver.core.api.score.Score;
 
-import org.jspecify.annotations.NonNull;
+import org.jspecify.annotations.NullMarked;
 import org.jspecify.annotations.Nullable;
 
 /**
@@ -11,34 +11,27 @@
  * It is defined in {@link ConstraintProvider#defineConstraints(ConstraintFactory)}
  * by calling {@link ConstraintFactory#forEach(Class)}.
  */
+@NullMarked
 public interface Constraint {
 
-    String DEFAULT_CONSTRAINT_GROUP = "default";
-
     ConstraintRef getConstraintRef();
 
     /**
-     * Returns a human-friendly description of the constraint.
-     * The format of the description is left unspecified and will not be parsed in any way.
+     * Returns the metadata for this constraint, as provided to
+     * {@link ConstraintBuilder#asConstraint(ConstraintMetadata)}.
+     * The constraint's identity ({@link ConstraintMetadata#id()}) is fixed at build time;
+     * any later mutation of the returned object does not affect the constraint's identity.
      *
-     * @return may be left empty
+     * @return never null
      */
-    default @NonNull String getDescription() {
-        return "";
-    }
-
-    default @NonNull String getConstraintGroup() {
-        return DEFAULT_CONSTRAINT_GROUP;
-    }
+    ConstraintMetadata getConstraintMetadata();
 
     /**
      * Returns the weight of the constraint as defined in the {@link ConstraintProvider},
      * without any overrides.
      *
      * @return null if the constraint does not have a weight defined
      */
-    default <Score_ extends Score<Score_>> @Nullable Score_ getConstraintWeight() {
-        return null;
-    }
+    <Score_ extends Score<Score_>> @Nullable Score_ getConstraintWeight();
 
 }

core/src/main/java/ai/timefold/solver/core/api/score/stream/ConstraintBuilder.java

@@ -1,6 +1,7 @@
 package ai.timefold.solver.core.api.score.stream;
 
 import ai.timefold.solver.core.api.score.analysis.ScoreAnalysis;
+import ai.timefold.solver.core.impl.score.stream.common.DefaultConstraintMetadata;
 
 import org.jspecify.annotations.NullMarked;
 
@@ -9,40 +10,23 @@ public interface ConstraintBuilder {
 
     /**
      * Builds a {@link Constraint} from the constraint stream.
-     * The constraint will be placed in the {@link Constraint#DEFAULT_CONSTRAINT_GROUP default constraint group}.
+     * Shorthand for {@link #asConstraint(ConstraintMetadata)}.
      *
-     * @param constraintName shows up in {@link ScoreAnalysis}
+     * @param id shows up in {@link ScoreAnalysis}
      */
-    default Constraint asConstraint(String constraintName) {
-        return asConstraintDescribed(constraintName, "");
-    }
-
-    /**
-     * As defined by {@link #asConstraintDescribed(String, String, String)},
-     * placing the constraint in the {@link Constraint#DEFAULT_CONSTRAINT_GROUP default constraint group}.
-     *
-     * @param constraintName shows up in {@link ScoreAnalysis}
-     * @param constraintDescription can contain any character, but it is recommended to keep it short and concise.
-     */
-    default Constraint asConstraintDescribed(String constraintName, String constraintDescription) {
-        return asConstraintDescribed(constraintName, constraintDescription, Constraint.DEFAULT_CONSTRAINT_GROUP);
+    default Constraint asConstraint(String id) {
+        return asConstraint(new DefaultConstraintMetadata(id));
     }
 
     /**
      * Builds a {@link Constraint} from the constraint stream.
-     * Both the constraint name and the constraint group are only allowed
-     * to contain alphanumeric characters, " ", "-" or "_".
-     * The constraint description can contain any character, but it is recommended to keep it short and concise.
-     * <p>
-     * Unlike the constraint name and group,
-     * the constraint description is unlikely to be used externally as an identifier,
-     * and therefore doesn't need to be URL-friendly, or protected against injection attacks.
+     * {@link ConstraintMetadata#id()} is called exactly once at this point;
+     * the returned value is validated and snapshotted as the constraint's permanent identity.
+     * Subsequent changes to the description's {@link ConstraintMetadata#id()} return value are ignored.
      *
-     * @param constraintName shows up in {@link ScoreAnalysis}
-     * @param constraintDescription can contain any character, but it is recommended to keep it short and concise.
-     * @param constraintGroup not used by the solver directly, but may be used by external tools to group constraints together,
-     *        such as by their source or by their purpose
+     * @param metadata identifies and describes the constraint;
+     *        {@link ConstraintMetadata#id()} shows up in {@link ScoreAnalysis}
      */
-    Constraint asConstraintDescribed(String constraintName, String constraintDescription, String constraintGroup);
+    Constraint asConstraint(ConstraintMetadata metadata);
 
 }

core/src/main/java/ai/timefold/solver/core/api/score/stream/ConstraintMetaModel.java

@@ -1,15 +1,15 @@
 package ai.timefold.solver.core.api.score.stream;
 
 import java.util.Collection;
-import java.util.Set;
 
-import org.jspecify.annotations.NonNull;
+import org.jspecify.annotations.NullMarked;
 import org.jspecify.annotations.Nullable;
 
 /**
  * Provides information about the known constraints.
  * Works in combination with {@link ConstraintProvider}.
  */
+@NullMarked
 public interface ConstraintMetaModel {
 
     /**
@@ -18,30 +18,23 @@ public interface ConstraintMetaModel {
      * @return null if such constraint does not exist
      */
     @Nullable
-    Constraint getConstraint(@NonNull ConstraintRef constraintRef);
+    Constraint getConstraint(ConstraintRef constraintRef);
 
     /**
-     * Returns all constraints defined in the {@link ConstraintProvider}.
+     * Returns the constraint with the given id.
+     * Convenience shorthand for {@link #getConstraint(ConstraintRef)}.
      *
-     * @return iteration order is undefined
-     */
-    @NonNull
-    Collection<Constraint> getConstraints();
-
-    /**
-     * Returns all constraints from {@link #getConstraints()} that belong to the given group.
-     *
-     * @return iteration order is undefined
+     * @return null if such constraint does not exist
      */
-    @NonNull
-    Collection<Constraint> getConstraintsPerGroup(@NonNull String constraintGroup);
+    default @Nullable Constraint getConstraint(String id) {
+        return getConstraint(ConstraintRef.of(id));
+    }
 
     /**
-     * Returns constraint groups with at least one constraint in it.
+     * Returns all constraints defined in the {@link ConstraintProvider}.
      *
      * @return iteration order is undefined
      */
-    @NonNull
-    Set<String> getConstraintGroups();
+    Collection<Constraint> getConstraints();
 
 }

core/src/main/java/ai/timefold/solver/core/api/score/stream/ConstraintMetadata.java

@@ -0,0 +1,27 @@
+package ai.timefold.solver.core.api.score.stream;
+
+import org.jspecify.annotations.NullMarked;
+
+/**
+ * Identifies a {@link Constraint} and optionally carries metadata about it.
+ * Users implement this interface, adding any fields they require.
+ * <p>
+ * <strong>Immutability contract:</strong> {@link #id()} must return the same value
+ * for the lifetime of the object.
+ * The first value returned is snapshotted
+ * when the constraint is built via {@link ConstraintBuilder#asConstraint(ConstraintMetadata)};
+ * any later change to the return value of {@link #id()} will be silently ignored
+ * and will NOT affect the constraint's identity.
+ */
+@NullMarked
+public interface ConstraintMetadata {
+
+    /**
+     * Returns the unique identifier of the constraint.
+     * Must be non-null, non-empty, and stable (see class Javadoc).
+     *
+     * @return the constraint's unique identifier
+     */
+    String id();
+
+}

core/src/main/java/ai/timefold/solver/core/api/score/stream/ConstraintRef.java

@@ -9,24 +9,24 @@
  * <p>
  * If you need an instance created, use {@link ConstraintRef#of(String)} and not the record's constructors.
  *
- * @param constraintName The constraint name. It must be unique.
+ * @param id The constraint id. It must be unique.
  */
 @NullMarked
-public record ConstraintRef(String constraintName)
+public record ConstraintRef(String id)
         implements
             Comparable<ConstraintRef> {
 
-    public static ConstraintRef of(String constraintName) {
-        return new ConstraintRef(constraintName);
+    public static ConstraintRef of(String id) {
+        return new ConstraintRef(id);
     }
 
     public ConstraintRef {
-        constraintName = AbstractConstraintBuilder.sanitize("constraintName", constraintName);
+        id = AbstractConstraintBuilder.sanitize("id", id);
     }
 
     @Override
     public int compareTo(ConstraintRef other) {
-        return constraintName.compareTo(other.constraintName);
+        return id.compareTo(other.id);
     }
 
 }

core/src/main/java/ai/timefold/solver/core/impl/bavet/common/BavetAbstractConstraintStream.java

@@ -14,7 +14,7 @@
 import ai.timefold.solver.core.api.score.Score;
 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.ConstraintRef;
+import ai.timefold.solver.core.api.score.stream.ConstraintMetadata;
 import ai.timefold.solver.core.api.score.stream.ConstraintStream;
 import ai.timefold.solver.core.api.score.stream.bi.BiConstraintStream;
 import ai.timefold.solver.core.api.score.stream.quad.QuadConstraintStream;
@@ -104,14 +104,13 @@
         }
     }
 
-    protected <Score_ extends Score<Score_>> Constraint buildConstraint(String constraintName, String description,
-            String constraintGroup, Score_ constraintWeight, ScoreImpactType impactType, Object justificationFunction,
+    protected <Score_ extends Score<Score_>> Constraint buildConstraint(ConstraintMetadata description,
+            Score_ constraintWeight, ScoreImpactType impactType, Object justificationFunction,
             BavetScoringConstraintStream<Solution_> stream) {
         var resolvedJustificationMapping =
                 Objects.requireNonNullElseGet(justificationFunction, this::getDefaultJustificationMapping);
         var isConstraintWeightConfigurable = constraintWeight == null;
-        var constraintRef = ConstraintRef.of(constraintName);
-        var constraint = new BavetConstraint<>(constraintFactory, constraintRef, description, constraintGroup,
+        var constraint = new BavetConstraint<>(constraintFactory, description,
                 isConstraintWeightConfigurable ? null : constraintWeight, impactType, resolvedJustificationMapping,
                 stream);
         stream.setConstraint(constraint);

core/src/main/java/ai/timefold/solver/core/impl/bavet/visual/NodeGraph.java

@@ -145,7 +145,7 @@ private static <Solution_> String getMetadata(GraphSink<Solution_> sink, Solutio
         var constraint = sink.constraint();
         var metadata = getBaseDOTProperties("#3423a6", true);
         metadata.put("label", "<B>%s</B><BR />(Weight: %s)"
-                .formatted(constraint.getConstraintRef().constraintName(), constraint.extractConstraintWeight(solution)));
+                .formatted(constraint.getConstraintRef().id(), constraint.extractConstraintWeight(solution)));
         return mergeMetadata(metadata);
     }
 

core/src/main/java/ai/timefold/solver/core/impl/domain/solution/DefaultConstraintWeightOverrides.java

@@ -21,12 +21,12 @@ public DefaultConstraintWeightOverrides(Map<String, Score_> constraintWeightMap)
     }
 
     @Override
-    public @Nullable Score_ getConstraintWeight(String constraintName) {
-        return constraintWeightMap.get(constraintName);
+    public @Nullable Score_ getConstraintWeight(String constraintId) {
+        return constraintWeightMap.get(constraintId);
     }
 
     @Override
-    public Set<String> getKnownConstraintNames() {
+    public Set<String> getKnownConstraintIds() {
         return Collections.unmodifiableSet(constraintWeightMap.keySet());
     }
 

core/src/main/java/ai/timefold/solver/core/impl/domain/solution/descriptor/ConstraintWeightSupplier.java

@@ -62,19 +62,19 @@ private ConstraintWeightSupplier(SolutionDescriptor<Solution_> solutionDescripto
      * @param userDefinedConstraints never null
      */
     public void validate(@Nullable Solution_ workingSolution, Set<ConstraintRef> userDefinedConstraints) {
-        var userDefinedConstraintNames =
-                userDefinedConstraints.stream().map(ConstraintRef::constraintName).collect(Collectors.toSet());
+        var userDefinedConstraintIds =
+                userDefinedConstraints.stream().map(ConstraintRef::id).collect(Collectors.toSet());
         // Constraint verifier is known to cause null here.
         var overrides = workingSolution == null ? ConstraintWeightOverrides.none()
                 : Objects.requireNonNull(getConstraintWeights(workingSolution));
-        var supportedConstraints = overrides.getKnownConstraintNames();
+        var supportedConstraints = overrides.getKnownConstraintIds();
         var excessiveConstraints = supportedConstraints.stream()
-                .filter(constraintName -> !userDefinedConstraintNames.contains(constraintName)).collect(Collectors.toSet());
+                .filter(constraintId -> !userDefinedConstraintIds.contains(constraintId)).collect(Collectors.toSet());
         if (!excessiveConstraints.isEmpty()) {
             throw new IllegalStateException("""
                     The constraint weight overrides contain the following constraints (%s) \
                     that are not in the user-defined constraints (%s).
-                    Maybe check your %s for missing constraints.""".formatted(excessiveConstraints, userDefinedConstraintNames,
+                    Maybe check your %s for missing constraints.""".formatted(excessiveConstraints, userDefinedConstraintIds,
                     ConstraintProvider.class.getSimpleName()));
         }
         // Constraints are allowed to be missing; the default value provided by the ConstraintProvider will be used.
@@ -107,7 +107,7 @@ public Class<? extends ConstraintWeightOverrides<Score_>> getProblemFactClass()
         if (workingSolution == null) { // ConstraintVerifier is known to cause null here.
             return null;
         }
-        var weight = getConstraintWeights(workingSolution).getConstraintWeight(constraintRef.constraintName());
+        var weight = getConstraintWeights(workingSolution).getConstraintWeight(constraintRef.id());
         if (weight == null) { // This is fine; use default value from ConstraintProvider.
             return null;
         }

core/src/main/java/ai/timefold/solver/core/impl/localsearch/DefaultLocalSearchPhase.java

@@ -188,8 +188,8 @@ private void collectMetrics(LocalSearchStepScope<Solution_> stepScope) {
             if (scoreDirector.getConstraintMatchPolicy().isEnabled()) {
                 for (ConstraintMatchTotal<?> constraintMatchTotal : scoreDirector.getConstraintMatchTotalMap()
                         .values()) {
-                    var tags = solverScope.getMonitoringTags().and("constraint.name",
-                            constraintMatchTotal.getConstraintRef().constraintName());
+                    var tags = solverScope.getMonitoringTags().and("constraint.id",
+                            constraintMatchTotal.getConstraintRef().id());
                     collectConstraintMatchTotalMetrics(SolverMetric.CONSTRAINT_MATCH_TOTAL_BEST_SCORE, tags,
                             constraintMatchTotalTagsToBestCount,
                             constraintMatchTotalBestScoreMap, constraintMatchTotal, scoreDefinition, solverScope);

core/src/main/java/ai/timefold/solver/core/impl/score/constraint/ConstraintMatch.java

@@ -89,7 +89,7 @@ public int compareTo(ConstraintMatch<Score_> other) {
 
     @Override
     public String toString() {
-        return "%s/%s=%s".formatted(getConstraintRef().constraintName(), justification, score);
+        return "%s/%s=%s".formatted(getConstraintRef().id(), justification, score);
     }
 
 }