package org.apache.maven.model.validation;

/*
 * 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.
 */

import org.apache.maven.model.Activation;
import org.apache.maven.model.ActivationFile;
import org.apache.maven.model.Build;
import org.apache.maven.model.BuildBase;
import org.apache.maven.model.Dependency;
import org.apache.maven.model.DependencyManagement;
import org.apache.maven.model.DistributionManagement;
import org.apache.maven.model.Exclusion;
import org.apache.maven.model.InputLocation;
import org.apache.maven.model.InputLocationTracker;
import org.apache.maven.model.Model;
import org.apache.maven.model.Parent;
import org.apache.maven.model.Plugin;
import org.apache.maven.model.PluginExecution;
import org.apache.maven.model.PluginManagement;
import org.apache.maven.model.Profile;
import org.apache.maven.model.ReportPlugin;
import org.apache.maven.model.Reporting;
import org.apache.maven.model.Repository;
import org.apache.maven.model.Resource;
import org.apache.maven.model.building.ModelBuildingRequest;
import org.apache.maven.model.building.ModelProblem.Severity;
import org.apache.maven.model.building.ModelProblem.Version;
import org.apache.maven.model.building.ModelProblemCollector;
import org.apache.maven.model.building.ModelProblemCollectorRequest;
import org.apache.maven.model.interpolation.AbstractStringBasedModelInterpolator;
import org.codehaus.plexus.util.StringUtils;

import java.io.File;
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.Set;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import javax.inject.Named;
import javax.inject.Singleton;

/**
 * @author <a href="mailto:trygvis@inamo.no">Trygve Laugst&oslash;l</a>
 */
@Named
@Singleton
public class DefaultModelValidator
    implements ModelValidator
{

    private static final Pattern CI_FRIENDLY_EXPRESSION = Pattern.compile( "\\$\\{(.+?)\\}" );

    private static final List<String> CI_FRIENDLY_POSSIBLE_PROPERTY_NAMES =
        Arrays.asList( AbstractStringBasedModelInterpolator.REVISION_PROPERTY,
                       AbstractStringBasedModelInterpolator.CHANGELIST_PROPERTY,
                       AbstractStringBasedModelInterpolator.SHA1_PROPERTY );

    private static final String ILLEGAL_FS_CHARS = "\\/:\"<>|?*";

    private static final String ILLEGAL_VERSION_CHARS = ILLEGAL_FS_CHARS;

    private static final String ILLEGAL_REPO_ID_CHARS = ILLEGAL_FS_CHARS;

    private static final String EMPTY = "";

    private final Set<String> validIds = new HashSet<>();

    @Override
    public void validateFileModel( Model m, ModelBuildingRequest request, ModelProblemCollector problems )
    {
        Parent parent = m.getParent();
        if ( parent != null )
        {
            validateStringNotEmpty( "parent.groupId", problems, Severity.FATAL, Version.BASE, parent.getGroupId(),
                                    parent );

            validateStringNotEmpty( "parent.artifactId", problems, Severity.FATAL, Version.BASE, parent.getArtifactId(),
                                    parent );

            if ( equals( parent.getGroupId(), m.getGroupId() ) && equals( parent.getArtifactId(), m.getArtifactId() ) )
            {
                addViolation( problems, Severity.FATAL, Version.BASE, "parent.artifactId", null,
                              "must be changed"
                                  + ", the parent element cannot have the same groupId:artifactId as the project.",
                              parent );
            }

            if ( equals( "LATEST", parent.getVersion() ) || equals( "RELEASE", parent.getVersion() ) )
            {
                addViolation( problems, Severity.WARNING, Version.BASE, "parent.version", null,
                              "is either LATEST or RELEASE (both of them are being deprecated)", parent );
            }

        }

        if ( request.getValidationLevel() >= ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_2_0 )
        {
            Set<String> modules = new HashSet<>();
            for ( int i = 0, n = m.getModules().size(); i < n; i++ )
            {
                String module = m.getModules().get( i );
                if ( !modules.add( module ) )
                {
                    addViolation( problems, Severity.ERROR, Version.V20, "modules.module[" + i + "]", null,
                                  "specifies duplicate child module " + module, m.getLocation( "modules" ) );
                }
            }

            Severity errOn30 = getSeverity( request, ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_3_0 );

            // [MNG-6074] Maven should produce an error if no model version has been set in a POM file used to build an
            // effective model.
            //
            // As of 3.4, the model version is mandatory even in raw models. The XML element still is optional in the
            // XML schema and this will not change anytime soon. We do not want to build effective models based on
            // models without a version starting with 3.4.
            validateStringNotEmpty( "modelVersion", problems, Severity.ERROR, Version.V20, m.getModelVersion(), m );

            validateModelVersion( problems, m.getModelVersion(), m, "4.0.0" );

            validateStringNoExpression( "groupId", problems, Severity.WARNING, Version.V20, m.getGroupId(), m );
            if ( parent == null )
            {
                validateStringNotEmpty( "groupId", problems, Severity.FATAL, Version.V20, m.getGroupId(), m );
            }

            validateStringNoExpression( "artifactId", problems, Severity.WARNING, Version.V20, m.getArtifactId(), m );
            validateStringNotEmpty( "artifactId", problems, Severity.FATAL, Version.V20, m.getArtifactId(), m );

            validateVersionNoExpression( "version", problems, Severity.WARNING, Version.V20, m.getVersion(), m );
            if ( parent == null )
            {
                validateStringNotEmpty( "version", problems, Severity.FATAL, Version.V20, m.getVersion(), m );
            }

            validate20RawDependencies( problems, m.getDependencies(), "dependencies.dependency.", EMPTY, request );

            validate20RawDependenciesSelfReferencing( problems, m, m.getDependencies(), "dependencies.dependency",
                                                      request );

            if ( m.getDependencyManagement() != null )
            {
                validate20RawDependencies( problems, m.getDependencyManagement().getDependencies(),
                                           "dependencyManagement.dependencies.dependency.", EMPTY, request );
            }

            validateRawRepositories( problems, m.getRepositories(), "repositories.repository.", EMPTY, request );

            validateRawRepositories( problems, m.getPluginRepositories(), "pluginRepositories.pluginRepository.",
                                     EMPTY, request );

            Build build = m.getBuild();
            if ( build != null )
            {
                validate20RawPlugins( problems, build.getPlugins(), "build.plugins.plugin.", EMPTY, request );

                PluginManagement mgmt = build.getPluginManagement();
                if ( mgmt != null )
                {
                    validate20RawPlugins( problems, mgmt.getPlugins(), "build.pluginManagement.plugins.plugin.",
                                          EMPTY, request );
                }
            }

            Set<String> profileIds = new HashSet<>();

            for ( Profile profile : m.getProfiles() )
            {
                String prefix = "profiles.profile[" + profile.getId() + "].";

                validateId( prefix, "id", problems, Severity.ERROR, Version.V40, profile.getId(), null, m );

                if ( !profileIds.add( profile.getId() ) )
                {
                    addViolation( problems, errOn30, Version.V20, "profiles.profile.id", null,
                                  "must be unique but found duplicate profile with id " + profile.getId(), profile );
                }

                validate30RawProfileActivation( problems, profile.getActivation(), profile.getId(),
                                                prefix, "activation", request );

                validate20RawDependencies( problems, profile.getDependencies(), prefix, "dependencies.dependency.",
                                           request );

                if ( profile.getDependencyManagement() != null )
                {
                    validate20RawDependencies( problems, profile.getDependencyManagement().getDependencies(),
                                               prefix, "dependencyManagement.dependencies.dependency.", request );
                }

                validateRawRepositories( problems, profile.getRepositories(), prefix, "repositories.repository.",
                                         request );

                validateRawRepositories( problems, profile.getPluginRepositories(),
                                         prefix, "pluginRepositories.pluginRepository.", request );

                BuildBase buildBase = profile.getBuild();
                if ( buildBase != null )
                {
                    validate20RawPlugins( problems, buildBase.getPlugins(), prefix, "plugins.plugin.", request );

                    PluginManagement mgmt = buildBase.getPluginManagement();
                    if ( mgmt != null )
                    {
                        validate20RawPlugins( problems, mgmt.getPlugins(), prefix, "pluginManagement.plugins.plugin.",
                                              request );
                    }
                }
            }
        }
    }

    @Override
    public void validateRawModel( Model m, ModelBuildingRequest request, ModelProblemCollector problems )
    {
        Parent parent = m.getParent();

        if ( parent != null )
        {
            validateStringNotEmpty( "parent.version", problems, Severity.FATAL, Version.BASE, parent.getVersion(),
                                    parent );
        }
    }

    private void validate30RawProfileActivation( ModelProblemCollector problems, Activation activation,
                                                 String sourceHint, String prefix, String fieldName,
                                                 ModelBuildingRequest request )
    {
        if ( activation == null )
        {
            return;
        }

        ActivationFile file = activation.getFile();

        if ( file != null )
        {
            String path;
            boolean missing;

            if ( StringUtils.isNotEmpty( file.getExists() ) )
            {
                path = file.getExists();
                missing = false;
            }
            else if ( StringUtils.isNotEmpty( file.getMissing() ) )
            {
                path = file.getMissing();
                missing = true;
            }
            else
            {
                return;
            }

            if ( path.contains( "${project.basedir}" ) )
            {
                addViolation( problems, Severity.WARNING, Version.V30,
                              prefix + fieldName + ( missing ? ".file.missing" : ".file.exists" ), null,
                              "Failed to interpolate file location " + path + " for profile " + sourceHint
                                  + ": ${project.basedir} expression not supported during profile activation, "
                                  + "use ${basedir} instead",
                              file.getLocation( missing ? "missing" : "exists" ) );
            }
            else if ( hasProjectExpression( path ) )
            {
                addViolation( problems, Severity.WARNING, Version.V30,
                              prefix + fieldName + ( missing ? ".file.missing" : ".file.exists" ), null,
                              "Failed to interpolate file location " + path + " for profile " + sourceHint
                                  + ": ${project.*} expressions are not supported during profile activation",
                              file.getLocation( missing ? "missing" : "exists" ) );
            }
        }
    }

    private void validate20RawPlugins( ModelProblemCollector problems, List<Plugin> plugins, String prefix,
                                       String prefix2, ModelBuildingRequest request )
    {
        Severity errOn31 = getSeverity( request, ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_3_1 );

        Map<String, Plugin> index = new HashMap<>();

        for ( Plugin plugin : plugins )
        {
            if ( plugin.getGroupId() == null
                || ( plugin.getGroupId() != null && plugin.getGroupId().trim().isEmpty() ) )
            {
                addViolation( problems, Severity.FATAL, Version.V20, prefix + prefix2 + "(groupId:artifactId)", null,
                              "groupId of a plugin must be defined. ", plugin );
            }

            if ( plugin.getArtifactId() == null
                || ( plugin.getArtifactId() != null && plugin.getArtifactId().trim().isEmpty() ) )
            {
                addViolation( problems, Severity.FATAL, Version.V20, prefix + prefix2 + "(groupId:artifactId)", null,
                              "artifactId of a plugin must be defined. ", plugin );
            }

            // This will catch cases like <version></version> or <version/>
            if ( plugin.getVersion() != null && plugin.getVersion().trim().isEmpty() )
            {
                addViolation( problems, Severity.FATAL, Version.V20, prefix + prefix2 + "(groupId:artifactId)", null,
                              "version of a plugin must be defined. ", plugin );
            }

            String key = plugin.getKey();

            Plugin existing = index.get( key );

            if ( existing != null )
            {
                addViolation( problems, errOn31, Version.V20, prefix + prefix2 + "(groupId:artifactId)", null,
                              "must be unique but found duplicate declaration of plugin " + key, plugin );
            }
            else
            {
                index.put( key, plugin );
            }

            Set<String> executionIds = new HashSet<>();

            for ( PluginExecution exec : plugin.getExecutions() )
            {
                if ( !executionIds.add( exec.getId() ) )
                {
                    addViolation( problems, Severity.ERROR, Version.V20,
                                  prefix + prefix2 + "[" + plugin.getKey() + "].executions.execution.id", null,
                                  "must be unique but found duplicate execution with id " + exec.getId(), exec );
                }
            }
        }
    }

    @Override
    public void validateEffectiveModel( Model m, ModelBuildingRequest request, ModelProblemCollector problems )
    {
        validateStringNotEmpty( "modelVersion", problems, Severity.ERROR, Version.BASE, m.getModelVersion(), m );

        validateId( "groupId", problems, m.getGroupId(), m );

        validateId( "artifactId", problems, m.getArtifactId(), m );

        validateStringNotEmpty( "packaging", problems, Severity.ERROR, Version.BASE, m.getPackaging(), m );

        if ( !m.getModules().isEmpty() )
        {
            if ( !"pom".equals( m.getPackaging() ) )
            {
                addViolation( problems, Severity.ERROR, Version.BASE, "packaging", null, "with value '"
                    + m.getPackaging() + "' is invalid. Aggregator projects " + "require 'pom' as packaging.", m );
            }

            for ( int i = 0, n = m.getModules().size(); i < n; i++ )
            {
                String module = m.getModules().get( i );
                if ( StringUtils.isBlank( module ) )
                {
                    addViolation( problems, Severity.ERROR, Version.BASE, "modules.module[" + i + "]", null,
                                  "has been specified without a path to the project directory.",
                                  m.getLocation( "modules" ) );
                }
            }
        }

        validateStringNotEmpty( "version", problems, Severity.ERROR, Version.BASE, m.getVersion(), m );

        Severity errOn30 = getSeverity( request, ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_3_0 );

        validateEffectiveDependencies( problems, m, m.getDependencies(), false, request );

        DependencyManagement mgmt = m.getDependencyManagement();
        if ( mgmt != null )
        {
            validateEffectiveDependencies( problems, m, mgmt.getDependencies(), true, request );
        }

        if ( request.getValidationLevel() >= ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_2_0 )
        {
            Severity errOn31 = getSeverity( request, ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_3_1 );

            validateBannedCharacters( EMPTY, "version", problems, errOn31, Version.V20, m.getVersion(), null, m,
                                      ILLEGAL_VERSION_CHARS );
            validate20ProperSnapshotVersion( "version", problems, errOn31, Version.V20, m.getVersion(), null, m );

            Build build = m.getBuild();
            if ( build != null )
            {
                for ( Plugin p : build.getPlugins() )
                {
                    validateStringNotEmpty( "build.plugins.plugin.artifactId", problems, Severity.ERROR, Version.V20,
                                            p.getArtifactId(), p );

                    validateStringNotEmpty( "build.plugins.plugin.groupId", problems, Severity.ERROR, Version.V20,
                                            p.getGroupId(), p );

                    validate20PluginVersion( "build.plugins.plugin.version", problems, p.getVersion(), p.getKey(), p,
                                             request );

                    validateBoolean( "build.plugins.plugin.inherited", EMPTY, problems, errOn30, Version.V20,
                                     p.getInherited(), p.getKey(), p );

                    validateBoolean( "build.plugins.plugin.extensions", EMPTY, problems, errOn30, Version.V20,
                                     p.getExtensions(), p.getKey(), p );

                    validate20EffectivePluginDependencies( problems, p, request );
                }

                validate20RawResources( problems, build.getResources(), "build.resources.resource.", request );

                validate20RawResources( problems, build.getTestResources(), "build.testResources.testResource.",
                                        request );
            }

            Reporting reporting = m.getReporting();
            if ( reporting != null )
            {
                for ( ReportPlugin p : reporting.getPlugins() )
                {
                    validateStringNotEmpty( "reporting.plugins.plugin.artifactId", problems, Severity.ERROR,
                                            Version.V20, p.getArtifactId(), p );

                    validateStringNotEmpty( "reporting.plugins.plugin.groupId", problems, Severity.ERROR, Version.V20,
                                            p.getGroupId(), p );
                }
            }

            for ( Repository repository : m.getRepositories() )
            {
                validate20EffectiveRepository( problems, repository, "repositories.repository.", request );
            }

            for ( Repository repository : m.getPluginRepositories() )
            {
                validate20EffectiveRepository( problems, repository, "pluginRepositories.pluginRepository.", request );
            }

            DistributionManagement distMgmt = m.getDistributionManagement();
            if ( distMgmt != null )
            {
                if ( distMgmt.getStatus() != null )
                {
                    addViolation( problems, Severity.ERROR, Version.V20, "distributionManagement.status", null,
                                  "must not be specified.", distMgmt );
                }

                validate20EffectiveRepository( problems, distMgmt.getRepository(), "distributionManagement.repository.",
                                               request );
                validate20EffectiveRepository( problems, distMgmt.getSnapshotRepository(),
                                               "distributionManagement.snapshotRepository.", request );
            }
        }
    }

    private void validate20RawDependencies( ModelProblemCollector problems, List<Dependency> dependencies,
                                            String prefix, String prefix2, ModelBuildingRequest request )
    {
        Severity errOn30 = getSeverity( request, ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_3_0 );
        Severity errOn31 = getSeverity( request, ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_3_1 );

        Map<String, Dependency> index = new HashMap<>();

        for ( Dependency dependency : dependencies )
        {
            String key = dependency.getManagementKey();

            if ( "import".equals( dependency.getScope() ) )
            {
                if ( !"pom".equals( dependency.getType() ) )
                {
                    addViolation( problems, Severity.WARNING, Version.V20, prefix + prefix2 + "type", key,
                                  "must be 'pom' to import the managed dependencies.", dependency );
                }
                else if ( StringUtils.isNotEmpty( dependency.getClassifier() ) )
                {
                    addViolation( problems, errOn30, Version.V20, prefix + prefix2 + "classifier", key,
                                  "must be empty, imported POM cannot have a classifier.", dependency );
                }
            }
            else if ( "system".equals( dependency.getScope() ) )
            {

                if ( request.getValidationLevel() >= ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_3_1 )
                {
                    addViolation( problems, Severity.WARNING, Version.V31, prefix + prefix2 + "scope", key,
                                  "declares usage of deprecated 'system' scope ", dependency );
                }

                String sysPath = dependency.getSystemPath();
                if ( StringUtils.isNotEmpty( sysPath ) )
                {
                    if ( !hasExpression( sysPath ) )
                    {
                        addViolation( problems, Severity.WARNING, Version.V20, prefix + prefix2 + "systemPath", key,
                                      "should use a variable instead of a hard-coded path " + sysPath, dependency );
                    }
                    else if ( sysPath.contains( "${basedir}" ) || sysPath.contains( "${project.basedir}" ) )
                    {
                        addViolation( problems, Severity.WARNING, Version.V20, prefix + prefix2 + "systemPath", key,
                                      "should not point at files within the project directory, " + sysPath
                                          + " will be unresolvable by dependent projects",
                                      dependency );
                    }
                }
            }

            if ( equals( "LATEST", dependency.getVersion() ) || equals( "RELEASE", dependency.getVersion() ) )
            {
                addViolation( problems, Severity.WARNING, Version.BASE, prefix + prefix2 + "version", key,
                              "is either LATEST or RELEASE (both of them are being deprecated)", dependency );
            }

            Dependency existing = index.get( key );

            if ( existing != null )
            {
                String msg;
                if ( equals( existing.getVersion(), dependency.getVersion() ) )
                {
                    msg = "duplicate declaration of version "
                        + Objects.toString( dependency.getVersion(), "(?)" );
                }
                else
                {
                    msg = "version " + Objects.toString( existing.getVersion(), "(?)" ) + " vs "
                        + Objects.toString( dependency.getVersion(), "(?)" );
                }

                addViolation( problems, errOn31, Version.V20, prefix + prefix2 + "(groupId:artifactId:type:classifier)",
                              null, "must be unique: " + key + " -> " + msg, dependency );
            }
            else
            {
                index.put( key, dependency );
            }
        }
    }

    private void validate20RawDependenciesSelfReferencing( ModelProblemCollector problems, Model m,
                                                           List<Dependency> dependencies, String prefix,
                                                           ModelBuildingRequest request )
    {
        // We only check for groupId/artifactId/version/classifier cause if there is another
        // module with the same groupId/artifactId/version/classifier this will fail the build
        // earlier like "Project '...' is duplicated in the reactor.
        // So it is sufficient to check only groupId/artifactId/version/classifier and not the
        // packaging type.
        for ( Dependency dependency : dependencies )
        {
            String key = dependency.getGroupId() + ":" + dependency.getArtifactId() + ":" + dependency.getVersion()
                    + ( dependency.getClassifier() != null ? ":" + dependency.getClassifier() : EMPTY  );
            String mKey = m.getGroupId() + ":" + m.getArtifactId() + ":" + m.getVersion();
            if ( key.equals( mKey ) )
            {
                // This means a module which is build has a dependency which has the same
                // groupId, artifactId, version and classifier coordinates. This is in consequence
                // a self reference or in other words a circular reference which can not being resolved.
                addViolation( problems, Severity.FATAL, Version.V31, prefix + "[" + key + "]", key,
                              "is referencing itself.", dependency );

            }
        }
    }

    private void validateEffectiveDependencies( ModelProblemCollector problems, Model m, List<Dependency> dependencies,
                                                boolean management, ModelBuildingRequest request )
    {
        Severity errOn30 = getSeverity( request, ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_3_0 );

        String prefix = management ? "dependencyManagement.dependencies.dependency." : "dependencies.dependency.";

        for ( Dependency d : dependencies )
        {
            validateEffectiveDependency( problems, d, management, prefix, request );

            if ( request.getValidationLevel() >= ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_2_0 )
            {
                validateBoolean( prefix, "optional", problems, errOn30, Version.V20, d.getOptional(),
                                 d.getManagementKey(), d );

                if ( !management )
                {
                    validateVersion( prefix, "version", problems, errOn30, Version.V20, d.getVersion(),
                                     d.getManagementKey(), d );

                    /*
                     * TODO Extensions like Flex Mojos use custom scopes like "merged", "internal", "external", etc. In
                     * order to don't break backward-compat with those, only warn but don't error out.
                     */
                    validateEnum( prefix, "scope", problems, Severity.WARNING, Version.V20, d.getScope(),
                                  d.getManagementKey(), d, "provided", "compile", "runtime", "test", "system" );

                    validateEffectiveModelAgainstDependency( prefix, problems, m, d, request );
                }
                else
                {
                    validateEnum( prefix, "scope", problems, Severity.WARNING, Version.V20, d.getScope(),
                                  d.getManagementKey(), d, "provided", "compile", "runtime", "test", "system",
                                  "import" );
                }
            }
        }
    }

    private void validateEffectiveModelAgainstDependency( String prefix, ModelProblemCollector problems, Model m,
                                                          Dependency d, ModelBuildingRequest request )
    {
        String key = d.getGroupId() + ":" + d.getArtifactId() + ":" + d.getVersion()
                + ( d.getClassifier() != null ? ":" + d.getClassifier() : EMPTY  );
        String mKey = m.getGroupId() + ":" + m.getArtifactId() + ":" + m.getVersion();
        if ( key.equals( mKey ) )
        {
            // This means a module which is build has a dependency which has the same
            // groupId, artifactId, version and classifier coordinates. This is in consequence
            // a self reference or in other words a circular reference which can not being resolved.
            addViolation( problems, Severity.FATAL, Version.V31, prefix + "[" + key + "]", key,
                          "is referencing itself.", d );

        }

    }

    private void validate20EffectivePluginDependencies( ModelProblemCollector problems, Plugin plugin,
                                                        ModelBuildingRequest request )
    {
        List<Dependency> dependencies = plugin.getDependencies();

        if ( !dependencies.isEmpty() )
        {
            String prefix = "build.plugins.plugin[" + plugin.getKey() + "].dependencies.dependency.";

            Severity errOn30 = getSeverity( request, ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_3_0 );

            for ( Dependency d : dependencies )
            {
                validateEffectiveDependency( problems, d, false, prefix, request );

                validateVersion( prefix, "version", problems, errOn30, Version.BASE, d.getVersion(),
                                 d.getManagementKey(), d );

                validateEnum( prefix, "scope", problems, errOn30, Version.BASE, d.getScope(), d.getManagementKey(), d,
                              "compile", "runtime", "system" );
            }
        }
    }

    private void validateEffectiveDependency( ModelProblemCollector problems, Dependency d, boolean management,
                                              String prefix, ModelBuildingRequest request )
    {
        validateId( prefix, "artifactId", problems, Severity.ERROR, Version.BASE, d.getArtifactId(),
                    d.getManagementKey(), d );

        validateId( prefix, "groupId", problems, Severity.ERROR, Version.BASE, d.getGroupId(),
                    d.getManagementKey(), d );

        if ( !management )
        {
            validateStringNotEmpty( prefix, "type", problems, Severity.ERROR, Version.BASE, d.getType(),
                                    d.getManagementKey(), d );

            validateDependencyVersion( problems, d, prefix );
        }

        if ( "system".equals( d.getScope() ) )
        {
            String systemPath = d.getSystemPath();

            if ( StringUtils.isEmpty( systemPath ) )
            {
                addViolation( problems, Severity.ERROR, Version.BASE, prefix + "systemPath", d.getManagementKey(),
                              "is missing.", d );
            }
            else
            {
                File sysFile = new File( systemPath );
                if ( !sysFile.isAbsolute() )
                {
                    addViolation( problems, Severity.ERROR, Version.BASE, prefix + "systemPath", d.getManagementKey(),
                                  "must specify an absolute path but is " + systemPath, d );
                }
                else if ( !sysFile.isFile() )
                {
                    String msg = "refers to a non-existing file " + sysFile.getAbsolutePath();
                    systemPath = systemPath.replace( '/', File.separatorChar ).replace( '\\', File.separatorChar );
                    String jdkHome =
                        request.getSystemProperties().getProperty( "java.home", EMPTY ) + File.separator + "..";
                    if ( systemPath.startsWith( jdkHome ) )
                    {
                        msg += ". Please verify that you run Maven using a JDK and not just a JRE.";
                    }
                    addViolation( problems, Severity.WARNING, Version.BASE, prefix + "systemPath", d.getManagementKey(),
                                  msg, d );
                }
            }
        }
        else if ( StringUtils.isNotEmpty( d.getSystemPath() ) )
        {
            addViolation( problems, Severity.ERROR, Version.BASE, prefix + "systemPath", d.getManagementKey(),
                          "must be omitted." + " This field may only be specified for a dependency with system scope.",
                          d );
        }

        if ( request.getValidationLevel() >= ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_2_0 )
        {
            for ( Exclusion exclusion : d.getExclusions() )
            {
                if ( request.getValidationLevel() < ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_3_0 )
                {
                    validateId( prefix, "exclusions.exclusion.groupId", problems, Severity.WARNING, Version.V20,
                                exclusion.getGroupId(), d.getManagementKey(), exclusion );

                    validateId( prefix, "exclusions.exclusion.artifactId", problems, Severity.WARNING, Version.V20,
                                exclusion.getArtifactId(), d.getManagementKey(), exclusion );
                }
                else
                {
                    validateIdWithWildcards( prefix, "exclusions.exclusion.groupId", problems, Severity.WARNING,
                                             Version.V30, exclusion.getGroupId(), d.getManagementKey(), exclusion );

                    validateIdWithWildcards( prefix, "exclusions.exclusion.artifactId", problems, Severity.WARNING,
                                             Version.V30, exclusion.getArtifactId(), d.getManagementKey(), exclusion );
                }
            }
        }
    }

    /**
     * @since 3.2.4
     */
    protected void validateDependencyVersion( ModelProblemCollector problems, Dependency d, String prefix )
    {
        validateStringNotEmpty( prefix, "version", problems, Severity.ERROR, Version.BASE, d.getVersion(),
                                d.getManagementKey(), d );
    }

    private void validateRawRepositories( ModelProblemCollector problems, List<Repository> repositories, String prefix,
                                          String prefix2, ModelBuildingRequest request )
    {
        Map<String, Repository> index = new HashMap<>();

        for ( Repository repository : repositories )
        {
            validateStringNotEmpty( prefix, prefix2, "id", problems, Severity.ERROR, Version.V20, repository.getId(),
                                    null, repository );

            validateStringNotEmpty( prefix, prefix2, "[" + repository.getId() + "].url", problems, Severity.ERROR,
                                    Version.V20, repository.getUrl(), null, repository );

            String key = repository.getId();

            Repository existing = index.get( key );

            if ( existing != null )
            {
                Severity errOn30 = getSeverity( request, ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_3_0 );

                addViolation( problems, errOn30, Version.V20, prefix + prefix2 + "id", null, "must be unique: "
                    + repository.getId() + " -> " + existing.getUrl() + " vs " + repository.getUrl(), repository );
            }
            else
            {
                index.put( key, repository );
            }
        }
    }

    private void validate20EffectiveRepository( ModelProblemCollector problems, Repository repository, String prefix,
                                                ModelBuildingRequest request )
    {
        if ( repository != null )
        {
            Severity errOn31 = getSeverity( request, ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_3_1 );

            validateBannedCharacters( prefix, "id", problems, errOn31, Version.V20, repository.getId(), null,
                                      repository, ILLEGAL_REPO_ID_CHARS );

            if ( "local".equals( repository.getId() ) )
            {
                addViolation( problems, errOn31, Version.V20, prefix + "id", null,
                              "must not be 'local'" + ", this identifier is reserved for the local repository"
                                  + ", using it for other repositories will corrupt your repository metadata.",
                              repository );
            }

            if ( "legacy".equals( repository.getLayout() ) )
            {
                addViolation( problems, Severity.WARNING, Version.V20, prefix + "layout", repository.getId(),
                              "uses the unsupported value 'legacy', artifact resolution might fail.", repository );
            }
        }
    }

    private void validate20RawResources( ModelProblemCollector problems, List<Resource> resources, String prefix,
                                         ModelBuildingRequest request )
    {
        Severity errOn30 = getSeverity( request, ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_3_0 );

        for ( Resource resource : resources )
        {
            validateStringNotEmpty( prefix, "directory", problems, Severity.ERROR, Version.V20,
                                    resource.getDirectory(), null, resource );

            validateBoolean( prefix, "filtering", problems, errOn30, Version.V20, resource.getFiltering(),
                             resource.getDirectory(), resource );
        }
    }

    // ----------------------------------------------------------------------
    // Field validation
    // ----------------------------------------------------------------------

    private boolean validateId( String fieldName, ModelProblemCollector problems, String id,
                                InputLocationTracker tracker )
    {
        return validateId( EMPTY, fieldName, problems, Severity.ERROR, Version.BASE, id, null, tracker );
    }

    @SuppressWarnings( "checkstyle:parameternumber" )
    private boolean validateId( String prefix, String fieldName, ModelProblemCollector problems, Severity severity,
                                Version version, String id, String sourceHint, InputLocationTracker tracker )
    {
        if ( validIds.contains( id ) )
        {
            return true;
        }
        if ( !validateStringNotEmpty( prefix, fieldName, problems, severity, version, id, sourceHint, tracker ) )
        {
            return false;
        }
        else
        {
            if ( !isValidId( id ) )
            {
                addViolation( problems, severity, version, prefix + fieldName, sourceHint,
                              "with value '" + id + "' does not match a valid id pattern.", tracker );
                return false;
            }
            validIds.add( id );
            return true;
        }
    }

    private boolean isValidId( String id )
    {
        for ( int i = 0; i < id.length(); i++ )
        {
            char c = id.charAt( i );
            if ( !isValidIdCharacter( c ) )
            {
                return false;
            }
        }
        return true;
    }


    private boolean isValidIdCharacter( char c )
    {
        return c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z' || c >= '0' && c <= '9' || c == '-' || c == '_' || c == '.';
    }

    @SuppressWarnings( "checkstyle:parameternumber" )
    private boolean validateIdWithWildcards( String prefix, String fieldName, ModelProblemCollector problems,
                                             Severity severity, Version version, String id, String sourceHint,
                                             InputLocationTracker tracker )
    {
        if ( !validateStringNotEmpty( prefix, fieldName, problems, severity, version, id, sourceHint, tracker ) )
        {
            return false;
        }
        else
        {
            if ( !isValidIdWithWildCards( id ) )
            {
                addViolation( problems, severity, version, prefix + fieldName, sourceHint,
                              "with value '" + id + "' does not match a valid id pattern.", tracker );
                return false;
            }
            return true;
        }
    }

    private boolean isValidIdWithWildCards( String id )
    {
        for ( int i = 0; i < id.length(); i++ )
        {
            char c = id.charAt( i );
            if ( !isValidIdWithWildCardCharacter( c ) )
            {
                return false;
            }
        }
        return true;
    }

    private boolean isValidIdWithWildCardCharacter( char c )
    {
        return isValidIdCharacter( c ) || c == '?' || c == '*';
    }

    private boolean validateStringNoExpression( String fieldName, ModelProblemCollector problems, Severity severity,
                                                Version version, String string, InputLocationTracker tracker )
    {
        if ( !hasExpression( string ) )
        {
            return true;
        }

        addViolation( problems, severity, version, fieldName, null, "contains an expression but should be a constant.",
                      tracker );

        return false;
    }

    private boolean validateVersionNoExpression( String fieldName, ModelProblemCollector problems, Severity severity,
                                                 Version version, String string, InputLocationTracker tracker )
    {
        if ( !hasExpression( string ) )
        {
            return true;
        }

        //
        // Acceptable versions for continuous delivery
        //
        // changelist
        // revision
        // sha1
        //
        Matcher m = CI_FRIENDLY_EXPRESSION.matcher( string.trim() );
        while ( m.find() )
        {
            if ( !CI_FRIENDLY_POSSIBLE_PROPERTY_NAMES.contains( m.group( 1 ) ) )
            {
                addViolation( problems, severity, version, fieldName, null,
                              "contains an expression but should be a constant.", tracker );

                return false;
            }
        }

        return true;
    }

    private boolean hasExpression( String value )
    {
        return value != null && value.contains( "${" );
    }

    private boolean hasProjectExpression( String value )
    {
        return value != null && value.contains( "${project." );
    }

    private boolean validateStringNotEmpty( String fieldName, ModelProblemCollector problems, Severity severity,
                                            Version version, String string, InputLocationTracker tracker )
    {
        return validateStringNotEmpty( EMPTY, fieldName, problems, severity, version, string, null, tracker );
    }

    /**
     * Asserts:
     * <p/>
     * <ul>
     * <li><code>string != null</code>
     * <li><code>string.length > 0</code>
     * </ul>
     */
    @SuppressWarnings( "checkstyle:parameternumber" )
    private boolean validateStringNotEmpty( String prefix, String prefix2, String fieldName,
                                            ModelProblemCollector problems, Severity severity, Version version,
                                            String string, String sourceHint, InputLocationTracker tracker )
    {
        if ( !validateNotNull( prefix, prefix2, fieldName, problems, severity, version, string, sourceHint, tracker ) )
        {
            return false;
        }

        if ( !string.isEmpty() )
        {
            return true;
        }

        addViolation( problems, severity, version, prefix + prefix2 + fieldName, sourceHint, "is missing.", tracker );

        return false;
    }

    /**
     * Asserts:
     * <p/>
     * <ul>
     * <li><code>string != null</code>
     * <li><code>string.length > 0</code>
     * </ul>
     */
    @SuppressWarnings( "checkstyle:parameternumber" )
    private boolean validateStringNotEmpty( String prefix, String fieldName, ModelProblemCollector problems,
                                            Severity severity, Version version, String string, String sourceHint,
                                            InputLocationTracker tracker )
    {
        if ( !validateNotNull( prefix, fieldName, problems, severity, version, string, sourceHint, tracker ) )
        {
            return false;
        }

        if ( string.length() > 0 )
        {
            return true;
        }

        addViolation( problems, severity, version, prefix + fieldName, sourceHint, "is missing.", tracker );

        return false;
    }

    /**
     * Asserts:
     * <p/>
     * <ul>
     * <li><code>string != null</code>
     * </ul>
     */
    @SuppressWarnings( "checkstyle:parameternumber" )
    private boolean validateNotNull( String prefix, String fieldName, ModelProblemCollector problems, Severity severity,
                                     Version version, Object object, String sourceHint, InputLocationTracker tracker )
    {
        if ( object != null )
        {
            return true;
        }

        addViolation( problems, severity, version, prefix + fieldName, sourceHint, "is missing.", tracker );

        return false;
    }

    /**
     * Asserts:
     * <p/>
     * <ul>
     * <li><code>string != null</code>
     * </ul>
     */
    @SuppressWarnings( "checkstyle:parameternumber" )
    private boolean validateNotNull( String prefix, String prefix2, String fieldName,
                                     ModelProblemCollector problems, Severity severity, Version version,
                                     Object object, String sourceHint, InputLocationTracker tracker )
    {
        if ( object != null )
        {
            return true;
        }

        addViolation( problems, severity, version, prefix + prefix2 + fieldName, sourceHint, "is missing.", tracker );

        return false;
    }

    @SuppressWarnings( "checkstyle:parameternumber" )
    private boolean validateBoolean( String prefix, String fieldName, ModelProblemCollector problems, Severity severity,
                                     Version version, String string, String sourceHint, InputLocationTracker tracker )
    {
        if ( string == null || string.length() <= 0 )
        {
            return true;
        }

        if ( "true".equalsIgnoreCase( string ) || "false".equalsIgnoreCase( string ) )
        {
            return true;
        }

        addViolation( problems, severity, version, prefix + fieldName, sourceHint,
                      "must be 'true' or 'false' but is '" + string + "'.", tracker );

        return false;
    }

    @SuppressWarnings( "checkstyle:parameternumber" )
    private boolean validateEnum( String prefix, String fieldName, ModelProblemCollector problems, Severity severity,
                                  Version version, String string, String sourceHint, InputLocationTracker tracker,
                                  String... validValues )
    {
        if ( string == null || string.length() <= 0 )
        {
            return true;
        }

        List<String> values = Arrays.asList( validValues );

        if ( values.contains( string ) )
        {
            return true;
        }

        addViolation( problems, severity, version, prefix + fieldName, sourceHint,
                      "must be one of " + values + " but is '" + string + "'.", tracker );

        return false;
    }

    @SuppressWarnings( "checkstyle:parameternumber" )
    private boolean validateModelVersion( ModelProblemCollector problems, String string, InputLocationTracker tracker,
                                          String... validVersions )
    {
        if ( string == null || string.length() <= 0 )
        {
            return true;
        }

        List<String> values = Arrays.asList( validVersions );

        if ( values.contains( string ) )
        {
            return true;
        }

        boolean newerThanAll = true;
        boolean olderThanAll = true;
        for ( String validValue : validVersions )
        {
            final int comparison = compareModelVersions( validValue, string );
            newerThanAll = newerThanAll && comparison < 0;
            olderThanAll = olderThanAll && comparison > 0;
        }

        if ( newerThanAll )
        {
            addViolation( problems, Severity.FATAL, Version.V20, "modelVersion", null,
                          "of '" + string + "' is newer than the versions supported by this version of Maven: " + values
                              + ". Building this project requires a newer version of Maven.", tracker );

        }
        else if ( olderThanAll )
        {
            // note this will not be hit for Maven 1.x project.xml as it is an incompatible schema
            addViolation( problems, Severity.FATAL, Version.V20, "modelVersion", null,
                          "of '" + string + "' is older than the versions supported by this version of Maven: " + values
                              + ". Building this project requires an older version of Maven.", tracker );

        }
        else
        {
            addViolation( problems, Severity.ERROR, Version.V20, "modelVersion", null,
                          "must be one of " + values + " but is '" + string + "'.", tracker );
        }

        return false;
    }

    /**
     * Compares two model versions.
     *
     * @param first the first version.
     * @param second the second version.
     * @return negative if the first version is newer than the second version, zero if they are the same or positive if
     * the second version is the newer.
     */
    private static int compareModelVersions( String first, String second )
    {
        // we use a dedicated comparator because we control our model version scheme.
        String[] firstSegments = StringUtils.split( first, "." );
        String[] secondSegments = StringUtils.split( second, "." );
        for ( int i = 0; i < Math.min( firstSegments.length, secondSegments.length ); i++ )
        {
            int result = Long.valueOf( firstSegments[i] ).compareTo( Long.valueOf( secondSegments[i] ) );
            if ( result != 0 )
            {
                return result;
            }
        }
        if ( firstSegments.length == secondSegments.length )
        {
            return 0;
        }
        return firstSegments.length > secondSegments.length ? -1 : 1;
    }

    @SuppressWarnings( "checkstyle:parameternumber" )
    private boolean validateBannedCharacters( String prefix, String fieldName, ModelProblemCollector problems,
                                              Severity severity, Version version, String string, String sourceHint,
                                              InputLocationTracker tracker, String banned )
    {
        if ( string != null )
        {
            for ( int i = string.length() - 1; i >= 0; i-- )
            {
                if ( banned.indexOf( string.charAt( i ) ) >= 0 )
                {
                    addViolation( problems, severity, version, prefix + fieldName, sourceHint,
                                  "must not contain any of these characters " + banned + " but found "
                                      + string.charAt( i ),
                                  tracker );
                    return false;
                }
            }
        }

        return true;
    }

    @SuppressWarnings( "checkstyle:parameternumber" )
    private boolean validateVersion( String prefix, String fieldName, ModelProblemCollector problems, Severity severity,
                                     Version version, String string, String sourceHint, InputLocationTracker tracker )
    {
        if ( string == null || string.length() <= 0 )
        {
            return true;
        }

        if ( hasExpression( string ) )
        {
            addViolation( problems, severity, version, prefix + fieldName, sourceHint,
                          "must be a valid version but is '" + string + "'.", tracker );
            return false;
        }

        return validateBannedCharacters( prefix, fieldName, problems, severity, version, string, sourceHint, tracker,
                                         ILLEGAL_VERSION_CHARS );

    }

    private boolean validate20ProperSnapshotVersion( String fieldName, ModelProblemCollector problems,
                                                     Severity severity, Version version, String string,
                                                     String sourceHint, InputLocationTracker tracker )
    {
        if ( string == null || string.length() <= 0 )
        {
            return true;
        }

        if ( string.endsWith( "SNAPSHOT" ) && !string.endsWith( "-SNAPSHOT" ) )
        {
            addViolation( problems, severity, version, fieldName, sourceHint,
                          "uses an unsupported snapshot version format, should be '*-SNAPSHOT' instead.", tracker );
            return false;
        }

        return true;
    }

    private boolean validate20PluginVersion( String fieldName, ModelProblemCollector problems, String string,
                                             String sourceHint, InputLocationTracker tracker,
                                             ModelBuildingRequest request )
    {
        if ( string == null )
        {
            // NOTE: The check for missing plugin versions is handled directly by the model builder
            return true;
        }

        Severity errOn30 = getSeverity( request, ModelBuildingRequest.VALIDATION_LEVEL_MAVEN_3_0 );

        if ( !validateVersion( EMPTY, fieldName, problems, errOn30, Version.V20, string, sourceHint, tracker ) )
        {
            return false;
        }

        if ( string.length() <= 0 || "RELEASE".equals( string ) || "LATEST".equals( string ) )
        {
            addViolation( problems, errOn30, Version.V20, fieldName, sourceHint,
                          "must be a valid version but is '" + string + "'.", tracker );
            return false;
        }

        return true;
    }

    private static void addViolation( ModelProblemCollector problems, Severity severity, Version version,
                                      String fieldName, String sourceHint, String message,
                                      InputLocationTracker tracker )
    {
        StringBuilder buffer = new StringBuilder( 256 );
        buffer.append( '\'' ).append( fieldName ).append( '\'' );

        if ( sourceHint != null )
        {
            buffer.append( " for " ).append( sourceHint );
        }

        buffer.append( ' ' ).append( message );

        // CHECKSTYLE_OFF: LineLength
        problems.add( new ModelProblemCollectorRequest( severity, version ).setMessage(
                                                                                        buffer.toString() ).setLocation( getLocation( fieldName, tracker ) ) );
        // CHECKSTYLE_ON: LineLength
    }

    private static InputLocation getLocation( String fieldName, InputLocationTracker tracker )
    {
        InputLocation location = null;

        if ( tracker != null )
        {
            if ( fieldName != null )
            {
                Object key = fieldName;

                int idx = fieldName.lastIndexOf( '.' );
                if ( idx >= 0 )
                {
                    fieldName = fieldName.substring( idx + 1 );
                    key = fieldName;
                }

                if ( fieldName.endsWith( "]" ) )
                {
                    key = fieldName.substring( fieldName.lastIndexOf( '[' ) + 1, fieldName.length() - 1 );
                    try
                    {
                        key = Integer.valueOf( key.toString() );
                    }
                    catch ( NumberFormatException e )
                    {
                        // use key as is
                    }
                }

                location = tracker.getLocation( key );
            }

            if ( location == null )
            {
                location = tracker.getLocation( EMPTY );
            }
        }

        return location;
    }

    private static boolean equals( String s1, String s2 )
    {
        return StringUtils.clean( s1 ).equals( StringUtils.clean( s2 ) );
    }

    private static Severity getSeverity( ModelBuildingRequest request, int errorThreshold )
    {
        return getSeverity( request.getValidationLevel(), errorThreshold );
    }

    private static Severity getSeverity( int validationLevel, int errorThreshold )
    {
        if ( validationLevel < errorThreshold )
        {
            return Severity.WARNING;
        }
        else
        {
            return Severity.ERROR;
        }
    }

}