/*
    * Licensed to the Apache Software Foundation (ASF) under one
    * or more contributor license agreements.  See the NOTICE file
    * distributed with this work for additional information
    * regarding copyright ownership.  The ASF licenses this file
    * to you under the Apache License, Version 2.0 (the
    * "License"); you may not use this file except in compliance
    * with the License.  You may obtain a copy of the License at
    *
   *   http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing,
   * software distributed under the License is distributed on an
   * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
   * KIND, either express or implied.  See the License for the
   * specific language governing permissions and limitations
   * under the License.
   */
  package org.eclipse.aether.util.graph.transformer;
  
  import java.util.Arrays;
  import java.util.Collection;
  
  import org.eclipse.aether.ConfigurationProperties;
  import org.eclipse.aether.RepositoryException;
  import org.eclipse.aether.RepositorySystemSession;
  import org.eclipse.aether.collection.DependencyGraphTransformationContext;
  import org.eclipse.aether.collection.DependencyGraphTransformer;
  import org.eclipse.aether.graph.Dependency;
  import org.eclipse.aether.graph.DependencyNode;
  import org.eclipse.aether.util.ConfigUtils;
  
  import static java.util.Objects.requireNonNull;
  
  /**
   * Abstract base class for dependency graph transformers that resolve version and scope conflicts among dependencies.
   * For a given set of conflicting nodes, one node will be chosen as the winner. How losing nodes are handled depends
   * on the configured verbosity level: they may be removed entirely, have their children removed, or be left in place
   * with conflict information. The exact rules by which a winning node and its effective scope are determined are
   * controlled by user-supplied implementations of {@link VersionSelector}, {@link ScopeSelector},
   * {@link OptionalitySelector} and {@link ScopeDeriver}.
   * <p>
   * <strong>Available Implementations:</strong>
   * <ul>
   * <li><strong>{@link PathConflictResolver}</strong> - Recommended high-performance implementation with O(N) complexity</li>
   * <li><strong>{@link ClassicConflictResolver}</strong> - Legacy implementation for backward compatibility (O(N²) worst-case)</li>
   * </ul>
   * <p>
   * <strong>Implementation Selection Guide:</strong>
   * <ul>
   * <li><strong>New Projects:</strong> Use {@link PathConflictResolver} for optimal performance</li>
   * <li><strong>Large Multi-Module Projects:</strong> Use {@link PathConflictResolver} to avoid performance bottlenecks</li>
   * <li><strong>Maven 4+ Environments:</strong> Use {@link PathConflictResolver} for best build performance</li>
   * <li><strong>Legacy Compatibility:</strong> Use {@link ClassicConflictResolver} only when exact Maven 3.x behavior is required</li>
   * </ul>
   * <p>
   * <strong>Usage Example:</strong>
   * <pre>{@code
   * // Recommended: High-performance path-based resolver
   * DependencyGraphTransformer transformer = new ChainedDependencyGraphTransformer(
   *     new PathConflictResolver(
   *         new NearestVersionSelector(),
   *         new JavaScopeSelector(),
   *         new SimpleOptionalitySelector(),
   *         new JavaScopeDeriver()),
   *     // other transformers...
   * );
   *
   * // Legacy: Classic resolver for backward compatibility
   * DependencyGraphTransformer legacyTransformer = new ChainedDependencyGraphTransformer(
   *     new ClassicConflictResolver(
   *         new NearestVersionSelector(),
   *         new JavaScopeSelector(),
   *         new SimpleOptionalitySelector(),
   *         new JavaScopeDeriver()),
   *     // other transformers...
   * );
   * }</pre>
   * <p>
   * <strong>Verbosity Levels and Conflict Handling:</strong>
   * <ul>
   * <li><strong>NONE (default):</strong> Creates a clean dependency tree without duplicate artifacts.
   *     Losing nodes are completely removed from the graph, so are cycles as well.</li>
   * <li><strong>STANDARD:</strong> Retains losing nodes for analysis but removes their children to prevent
   *     duplicate dependencies. Special handling for version ranges: redundant nodes may still be removed
   *     if multiple versions of the same artifact exist. Losing nodes link back to the winner via
   *     {@link #NODE_DATA_WINNER} and preserve original scope/optionality information. This mode removes cycles only,
   *     while conflict nodes/duplicates are left in place. Graphs in this verbosity level cannot be resolved,
   *     their purpose is for analysis only.</li>
   * <li><strong>FULL:</strong> Preserves the complete original graph structure including all conflicts and cycles.
   *     All nodes remain with their children, but conflict information is recorded for analysis.
   *     Graphs in this verbosity level cannot be resolved, their purpose is for analysis only.</li>
   * </ul>
   * The verbosity level is controlled by the {@link #CONFIG_PROP_VERBOSE} configuration property.
   * <p>
   * <strong>Conflict Metadata:</strong> In STANDARD and FULL modes, the keys {@link #NODE_DATA_ORIGINAL_SCOPE}
   * and {@link #NODE_DATA_ORIGINAL_OPTIONALITY} are used to store the original scope and optionality of each node.
   * Obviously, dependency trees with verbosity STANDARD or FULL are not suitable for artifact resolution unless
   * a filter is employed to exclude the duplicate dependencies.
  * <p>
  * <strong>Conflict ID Processing Pipeline:</strong>
  * <ol>
  * <li><strong>{@link ConflictMarker}:</strong> Assigns conflict IDs based on GACE (groupId:artifactId:classifier:extension)
  *     coordinates, grouping artifacts that differ only in version (partitions the graph, assigning same conflict IDs
  *     to nodes belonging to same conflict group).</li>
  * <li><strong>{@link ConflictIdSorter}:</strong> Creates topological ordering of conflict IDs and detects cycles</li>
  * <li><strong>ConflictResolver implementation:</strong> Uses the sorted conflict IDs to resolve conflicts in dependency order</li>
  * </ol>
  * This transformer will query the keys {@link TransformationContextKeys#CONFLICT_IDS},
  * {@link TransformationContextKeys#SORTED_CONFLICT_IDS}, {@link TransformationContextKeys#CYCLIC_CONFLICT_IDS} for
  * existing information about conflict ids. In absence of this information, it will automatically invoke the
  * {@link ConflictIdSorter} to calculate it.
  *
  * @see PathConflictResolver
  * @see ClassicConflictResolver
  */
 public class ConflictResolver implements DependencyGraphTransformer {
 
     /**
      * The key in the repository session's {@link org.eclipse.aether.RepositorySystemSession#getConfigProperties()
      * configuration properties} used to store a {@link Boolean} flag controlling the transformer's verbose mode.
      * Accepted values are Boolean types, String type (where "true" would be interpreted as {@code true})
      * or Verbosity enum instances.
      *
      * @configurationSource {@link RepositorySystemSession#getConfigProperties()}
      * @configurationType {@link java.lang.Object}
      * @configurationDefaultValue "NONE"
      */
     public static final String CONFIG_PROP_VERBOSE = ConfigurationProperties.PREFIX_AETHER + "conflictResolver.verbose";
 
     /**
      * The name of the conflict resolver implementation to use: "path" (default) or "classic" (same as Maven 3).
      *
      * @since 2.0.11
      * @configurationSource {@link RepositorySystemSession#getConfigProperties()}
      * @configurationType {@link java.lang.String}
      * @configurationDefaultValue {@link #DEFAULT_CONFLICT_RESOLVER_IMPL}
      */
     public static final String CONFIG_PROP_CONFLICT_RESOLVER_IMPL =
             ConfigurationProperties.PREFIX_AETHER + "conflictResolver.impl";
 
     public static final String CLASSIC_CONFLICT_RESOLVER = "classic";
     public static final String PATH_CONFLICT_RESOLVER = "path";
 
     public static final String DEFAULT_CONFLICT_RESOLVER_IMPL = PATH_CONFLICT_RESOLVER;
 
     /**
      * The enum representing verbosity levels of conflict resolver.
      *
      * @since 1.9.8
      */
     public enum Verbosity {
         /**
          * Verbosity level to be used in all "common" resolving use cases (ie. dependencies to build class path). The
          * {@link ConflictResolver} in this mode will trim down the graph to the barest minimum: will not leave
          * any conflicting node in place, hence no conflicts will be present in transformed graph either.
          */
         NONE,
 
         /**
          * Verbosity level to be used in "analyze" resolving use cases (ie. dependency convergence calculations). The
          * {@link ConflictResolver} in this mode will remove any redundant collected nodes, in turn it will leave one
          * with recorded conflicting information. This mode corresponds to "classic verbose" mode when
          * {@link #CONFIG_PROP_VERBOSE} was set to {@code true}. Obviously, the resulting dependency tree is not
          * suitable for artifact resolution unless a filter is employed to exclude the duplicate dependencies.
          */
         STANDARD,
 
         /**
          * Verbosity level to be used in "analyze" resolving use cases (ie. dependency convergence calculations). The
          * {@link ConflictResolver} in this mode will not remove any collected node, in turn it will record on all
          * eliminated nodes the conflicting information. Obviously, the resulting dependency tree is not suitable
          * for artifact resolution unless a filter is employed to exclude the duplicate dependencies.
          */
         FULL
     }
 
     /**
      * Helper method that uses {@link RepositorySystemSession} and {@link #CONFIG_PROP_VERBOSE} key to figure out
      * current {@link Verbosity}: if {@link Boolean} or {@code String} found, returns {@link Verbosity#STANDARD}
      * or {@link Verbosity#NONE}, depending on value (string is parsed with {@link Boolean#parseBoolean(String)}
      * for {@code true} or {@code false} correspondingly. This is to retain "existing" behavior, where the config
      * key accepted only these values.
      * Since 1.9.8 release, this key may contain {@link Verbosity} enum instance as well, in which case that instance
      * is returned.
      * This method never returns {@code null}.
      */
     public static Verbosity getVerbosity(RepositorySystemSession session) {
         final Object verbosityValue = session.getConfigProperties().get(CONFIG_PROP_VERBOSE);
         if (verbosityValue instanceof Boolean) {
             return (Boolean) verbosityValue ? Verbosity.STANDARD : Verbosity.NONE;
         } else if (verbosityValue instanceof String) {
             return Boolean.parseBoolean(verbosityValue.toString()) ? Verbosity.STANDARD : Verbosity.NONE;
         } else if (verbosityValue instanceof Verbosity) {
             return (Verbosity) verbosityValue;
         } else if (verbosityValue != null) {
             throw new IllegalArgumentException("Unsupported Verbosity configuration: " + verbosityValue);
         }
         return Verbosity.NONE;
     }
 
     /**
      * The key in the dependency node's {@link DependencyNode#getData() custom data} under which a reference to the
      * {@link DependencyNode} which has won the conflict is stored.
      */
     public static final String NODE_DATA_WINNER = "conflict.winner";
 
     /**
      * The key in the dependency node's {@link DependencyNode#getData() custom data} under which the scope of the
      * dependency before scope derivation and conflict resolution is stored.
      */
     public static final String NODE_DATA_ORIGINAL_SCOPE = "conflict.originalScope";
 
     /**
      * The key in the dependency node's {@link DependencyNode#getData() custom data} under which the optional flag of
      * the dependency before derivation and conflict resolution is stored.
      */
     public static final String NODE_DATA_ORIGINAL_OPTIONALITY = "conflict.originalOptionality";
 
     private final ConflictResolver.VersionSelector versionSelector;
     private final ConflictResolver.ScopeSelector scopeSelector;
     private final ConflictResolver.ScopeDeriver scopeDeriver;
     private final ConflictResolver.OptionalitySelector optionalitySelector;
 
     /**
      * No arg ctor for subclasses and default cases.
      */
     protected ConflictResolver() {
         this.versionSelector = null;
         this.scopeSelector = null;
         this.scopeDeriver = null;
         this.optionalitySelector = null;
     }
 
     /**
      * Creates a new conflict resolver instance with the specified hooks that delegates to configured conflict resolver
      * dynamically.
      *
      * @param versionSelector The version selector to use, must not be {@code null}.
      * @param scopeSelector The scope selector to use, must not be {@code null}.
      * @param optionalitySelector The optionality selector ot use, must not be {@code null}.
      * @param scopeDeriver The scope deriver to use, must not be {@code null}.
      */
     public ConflictResolver(
             VersionSelector versionSelector,
             ScopeSelector scopeSelector,
             OptionalitySelector optionalitySelector,
             ScopeDeriver scopeDeriver) {
         this.versionSelector = requireNonNull(versionSelector, "version selector cannot be null");
         this.scopeSelector = requireNonNull(scopeSelector, "scope selector cannot be null");
         this.optionalitySelector = requireNonNull(optionalitySelector, "optionality selector cannot be null");
         this.scopeDeriver = requireNonNull(scopeDeriver, "scope deriver cannot be null");
     }
 
     @Override
     public DependencyNode transformGraph(DependencyNode node, DependencyGraphTransformationContext context)
             throws RepositoryException {
         String cf = ConfigUtils.getString(
                 context.getSession(), DEFAULT_CONFLICT_RESOLVER_IMPL, CONFIG_PROP_CONFLICT_RESOLVER_IMPL);
         ConflictResolver delegate;
         if (PATH_CONFLICT_RESOLVER.equals(cf)) {
             delegate = new PathConflictResolver(versionSelector, scopeSelector, optionalitySelector, scopeDeriver);
         } else if (CLASSIC_CONFLICT_RESOLVER.equals(cf)) {
             delegate = new ClassicConflictResolver(versionSelector, scopeSelector, optionalitySelector, scopeDeriver);
         } else {
             throw new IllegalArgumentException("Unknown conflict resolver: " + cf + "; known are "
                     + Arrays.asList(PATH_CONFLICT_RESOLVER, CLASSIC_CONFLICT_RESOLVER));
         }
         return delegate.transformGraph(node, context);
     }
 
     /**
      * A context used to hold information that is relevant for deriving the scope of a child dependency.
      *
      * @see ScopeDeriver
      * @noinstantiate This class is not intended to be instantiated by clients in production code, the constructor may
      *                change without notice and only exists to enable unit testing.
      */
     public abstract static class ScopeContext {
         /**
          * Gets the scope of the parent dependency. This is usually the scope that was derived by earlier invocations of
          * the scope deriver.
          *
          * @return The scope of the parent dependency, never {@code null}.
          */
         public abstract String getParentScope();
 
         /**
          * Gets the original scope of the child dependency. This is the scope that was declared in the artifact
          * descriptor of the parent dependency.
          *
          * @return The original scope of the child dependency, never {@code null}.
          */
         public abstract String getChildScope();
 
         /**
          * Gets the derived scope of the child dependency. This is initially equal to {@link #getChildScope()} until the
          * scope deriver makes changes.
          *
          * @return The derived scope of the child dependency, never {@code null}.
          */
         public abstract String getDerivedScope();
 
         /**
          * Sets the derived scope of the child dependency.
          *
          * @param derivedScope The derived scope of the dependency, may be {@code null}.
          */
         public abstract void setDerivedScope(String derivedScope);
     }
 
     /**
      * A conflicting dependency.
      *
      * @noinstantiate This class is not intended to be instantiated by clients in production code, the constructor may
      *                change without notice and only exists to enable unit testing.
      */
     public abstract static class ConflictItem {
         /**
          * Determines whether the specified conflict item is a sibling of this item.
          *
          * @param item The other conflict item, must not be {@code null}.
          * @return {@code true} if the given item has the same parent as this item, {@code false} otherwise.
          */
         public abstract boolean isSibling(ConflictItem item);
 
         /**
          * Gets the dependency node involved in the conflict.
          *
          * @return The involved dependency node, never {@code null}.
          */
         public abstract DependencyNode getNode();
 
         /**
          * Gets the dependency involved in the conflict, short for {@code getNode.getDependency()}.
          *
          * @return The involved dependency, never {@code null}.
          */
         public abstract Dependency getDependency();
 
         /**
          * Gets the zero-based depth at which the conflicting node occurs in the graph. As such, the depth denotes the
          * number of parent nodes. If actually multiple paths lead to the node, the return value denotes the smallest
          * possible depth.
          *
          * @return The zero-based depth of the node in the graph.
          */
         public abstract int getDepth();
 
         /**
          * Gets the derived scopes of the dependency. In general, the same dependency node could be reached via
          * different paths and each path might result in a different derived scope.
          *
          * @see ScopeDeriver
          * @return The (read-only) set of derived scopes of the dependency, never {@code null}.
          */
         public abstract Collection<String> getScopes();
 
         /**
          * Bit flag indicating whether one or more paths consider the dependency non-optional.
          */
         public static final int OPTIONAL_FALSE = 0x01;
 
         /**
          * Bit flag indicating whether one or more paths consider the dependency optional.
          */
         public static final int OPTIONAL_TRUE = 0x02;
 
         /**
          * Gets the derived optionalities of the dependency. In general, the same dependency node could be reached via
          * different paths and each path might result in a different derived optionality.
          *
          * @return A bit field consisting of {@link ConflictResolver.ConflictItem#OPTIONAL_FALSE} and/or
          *         {@link ConflictResolver.ConflictItem#OPTIONAL_TRUE} indicating the derived optionalities the
          *         dependency was encountered with.
          */
         public abstract int getOptionalities();
     }
 
     /**
      * A context used to hold information that is relevant for resolving version and scope conflicts.
      *
      * @see VersionSelector
      * @see ScopeSelector
      * @noinstantiate This class is not intended to be instantiated by clients in production code, the constructor may
      *                change without notice and only exists to enable unit testing.
      */
     public abstract static class ConflictContext {
         /**
          * Gets the root node of the dependency graph being transformed.
          *
          * @return The root node of the dependency graph, never {@code null}.
          */
         public abstract DependencyNode getRoot();
 
         /**
          * Determines whether the specified dependency node belongs to this conflict context.
          *
          * @param node The dependency node to check, must not be {@code null}.
          * @return {@code true} if the given node belongs to this conflict context, {@code false} otherwise.
          */
         public abstract boolean isIncluded(DependencyNode node);
 
         /**
          * Gets the collection of conflict items in this context.
          *
          * @return The (read-only) collection of conflict items in this context, never {@code null}.
          */
         public abstract Collection<ConflictItem> getItems();
 
         /**
          * Gets the conflict item which has been selected as the winner among the conflicting dependencies.
          *
          * @return The winning conflict item or {@code null} if not set yet.
          */
         public abstract ConflictItem getWinner();
 
         /**
          * Sets the conflict item which has been selected as the winner among the conflicting dependencies.
          *
          * @param winner The winning conflict item, may be {@code null}.
          */
         public abstract void setWinner(ConflictItem winner);
 
         /**
          * Gets the effective scope of the winning dependency.
          *
          * @return The effective scope of the winning dependency or {@code null} if none.
          */
         public abstract String getScope();
 
         /**
          * Sets the effective scope of the winning dependency.
          *
          * @param scope The effective scope, may be {@code null}.
          */
         public abstract void setScope(String scope);
 
         /**
          * Gets the effective optional flag of the winning dependency.
          *
          * @return The effective optional flag or {@code null} if none.
          */
         public abstract Boolean getOptional();
 
         /**
          * Sets the effective optional flag of the winning dependency.
          *
          * @param optional The effective optional flag, may be {@code null}.
          */
         public abstract void setOptional(Boolean optional);
     }
 
     /**
      * An extension point of {@link ConflictResolver} that determines the winner among conflicting dependencies. The
      * winning node (and its children) will be retained in the dependency graph, the other nodes will get removed. The
      * version selector does not need to deal with potential scope conflicts, these will be addressed afterwards by the
      * {@link ScopeSelector}.
      * <p>
      * <strong>Note:</strong> Implementations must be stateless.
      */
     public abstract static class VersionSelector {
 
         /**
          * Retrieves the version selector for use during the specified graph transformation. The conflict resolver calls
          * this method once per
          * {@link ConflictResolver#transformGraph(DependencyNode, DependencyGraphTransformationContext)} invocation to
          * allow implementations to prepare any auxiliary data that is needed for their operation. Given that
          * implementations must be stateless, a new instance needs to be returned to hold such auxiliary data. The
          * default implementation simply returns the current instance which is appropriate for implementations which do
          * not require auxiliary data.
          *
          * @param root The root node of the (possibly cyclic!) graph to transform, must not be {@code null}.
          * @param context The graph transformation context, must not be {@code null}.
          * @return The scope deriver to use for the given graph transformation, never {@code null}.
          * @throws RepositoryException If the instance could not be retrieved.
          */
         public VersionSelector getInstance(DependencyNode root, DependencyGraphTransformationContext context)
                 throws RepositoryException {
             return this;
         }
 
         /**
          * Determines the winning node among conflicting dependencies. Implementations will usually iterate
          * {@link ConflictContext#getItems()}, inspect {@link ConflictItem#getNode()} and eventually call
          * {@link ConflictContext#setWinner(ConflictResolver.ConflictItem)} to deliver the winner. Failure to select a
          * winner will automatically fail the entire conflict resolution.
          *
          * @param context The conflict context, must not be {@code null}.
          * @throws RepositoryException If the version selection failed.
          */
         public abstract void selectVersion(ConflictContext context) throws RepositoryException;
     }
 
     /**
      * An extension point of {@link ConflictResolver} that determines the effective scope of a dependency from a
      * potentially conflicting set of {@link ScopeDeriver derived scopes}. The scope selector gets invoked after the
      * {@link VersionSelector} has picked the winning node.
      * <p>
      * <strong>Note:</strong> Implementations must be stateless.
      */
     public abstract static class ScopeSelector {
 
         /**
          * Retrieves the scope selector for use during the specified graph transformation. The conflict resolver calls
          * this method once per
          * {@link ConflictResolver#transformGraph(DependencyNode, DependencyGraphTransformationContext)} invocation to
          * allow implementations to prepare any auxiliary data that is needed for their operation. Given that
          * implementations must be stateless, a new instance needs to be returned to hold such auxiliary data. The
          * default implementation simply returns the current instance which is appropriate for implementations which do
          * not require auxiliary data.
          *
          * @param root The root node of the (possibly cyclic!) graph to transform, must not be {@code null}.
          * @param context The graph transformation context, must not be {@code null}.
          * @return The scope selector to use for the given graph transformation, never {@code null}.
          * @throws RepositoryException If the instance could not be retrieved.
          */
         public ScopeSelector getInstance(DependencyNode root, DependencyGraphTransformationContext context)
                 throws RepositoryException {
             return this;
         }
 
         /**
          * Determines the effective scope of the dependency given by {@link ConflictContext#getWinner()}.
          * Implementations will usually iterate {@link ConflictContext#getItems()}, inspect
          * {@link ConflictItem#getScopes()} and eventually call {@link ConflictContext#setScope(String)} to deliver the
          * effective scope.
          *
          * @param context The conflict context, must not be {@code null}.
          * @throws RepositoryException If the scope selection failed.
          */
         public abstract void selectScope(ConflictContext context) throws RepositoryException;
     }
 
     /**
      * An extension point of {@link ConflictResolver} that determines the scope of a dependency in relation to the scope
      * of its parent.
      * <p>
      * <strong>Note:</strong> Implementations must be stateless.
      */
     public abstract static class ScopeDeriver {
 
         /**
          * Retrieves the scope deriver for use during the specified graph transformation. The conflict resolver calls
          * this method once per
          * {@link ConflictResolver#transformGraph(DependencyNode, DependencyGraphTransformationContext)} invocation to
          * allow implementations to prepare any auxiliary data that is needed for their operation. Given that
          * implementations must be stateless, a new instance needs to be returned to hold such auxiliary data. The
          * default implementation simply returns the current instance which is appropriate for implementations which do
          * not require auxiliary data.
          *
          * @param root The root node of the (possibly cyclic!) graph to transform, must not be {@code null}.
          * @param context The graph transformation context, must not be {@code null}.
          * @return The scope deriver to use for the given graph transformation, never {@code null}.
          * @throws RepositoryException If the instance could not be retrieved.
          */
         public ScopeDeriver getInstance(DependencyNode root, DependencyGraphTransformationContext context)
                 throws RepositoryException {
             return this;
         }
 
         /**
          * Determines the scope of a dependency in relation to the scope of its parent. Implementors need to call
          * {@link ScopeContext#setDerivedScope(String)} to deliver the result of their calculation. If said method is
          * not invoked, the conflict resolver will assume the scope of the child dependency remains unchanged.
          *
          * @param context The scope context, must not be {@code null}.
          * @throws RepositoryException If the scope deriviation failed.
          */
         public abstract void deriveScope(ScopeContext context) throws RepositoryException;
     }
 
     /**
      * An extension point of {@link ConflictResolver} that determines the effective optional flag of a dependency from a
      * potentially conflicting set of derived optionalities. The optionality selector gets invoked after the
      * {@link VersionSelector} has picked the winning node.
      * <p>
      * <strong>Note:</strong> Implementations must be stateless.
      */
     public abstract static class OptionalitySelector {
 
         /**
          * Retrieves the optionality selector for use during the specified graph transformation. The conflict resolver
          * calls this method once per
          * {@link ConflictResolver#transformGraph(DependencyNode, DependencyGraphTransformationContext)} invocation to
          * allow implementations to prepare any auxiliary data that is needed for their operation. Given that
          * implementations must be stateless, a new instance needs to be returned to hold such auxiliary data. The
          * default implementation simply returns the current instance which is appropriate for implementations which do
          * not require auxiliary data.
          *
          * @param root The root node of the (possibly cyclic!) graph to transform, must not be {@code null}.
          * @param context The graph transformation context, must not be {@code null}.
          * @return The optionality selector to use for the given graph transformation, never {@code null}.
          * @throws RepositoryException If the instance could not be retrieved.
          */
         public OptionalitySelector getInstance(DependencyNode root, DependencyGraphTransformationContext context)
                 throws RepositoryException {
             return this;
         }
 
         /**
          * Determines the effective optional flag of the dependency given by {@link ConflictContext#getWinner()}.
          * Implementations will usually iterate {@link ConflictContext#getItems()}, inspect
          * {@link ConflictItem#getOptionalities()} and eventually call {@link ConflictContext#setOptional(Boolean)} to
          * deliver the effective optional flag.
          *
          * @param context The conflict context, must not be {@code null}.
          * @throws RepositoryException If the optionality selection failed.
          */
         public abstract void selectOptionality(ConflictContext context) throws RepositoryException;
     }
 }