/*
 * 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.apache.sling.servlets.resolver.internal.helper;

import java.util.HashSet;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Set;

import org.apache.sling.api.resource.Resource;
import org.apache.sling.api.resource.ResourceResolver;
import org.apache.sling.api.resource.ResourceUtil;
import org.slf4j.LoggerFactory;

/**
 * The <code>LocationIterator</code> provides access to an ordered collection
 * of absolute paths containing potential request handling. The primary order of
 * the collection is the resource type hierarchy with the base resource type at
 * the top. The secondary order is the search path retrieved from the resource
 * resolver.
 * <p>
 * Example: For a node type hierarchy "sample" > "super" > "default" and a
 * search path of [ "/apps", "/libs" ], the iterator would provide access to the
 * following list of paths:
 * <ol>
 * <li><code>/apps/sample</code></li>
 * <li><code>/libs/sample</code></li>
 * <li><code>/apps/super</code></li>
 * <li><code>/libs/super</code></li>
 * <li><code>/apps/default</code></li>
 * <li><code>/libs/default</code></li>
 * </ol>
 */
public class LocationIterator implements Iterator<String> {

    // The resource for which this iterator is created. This resource
    // gives the initial resource type and the first resource super type
    //private final Resource resource;

    // The resource resolver used to find resource super types of
    // resource types
    private final ResourceResolver resolver;

    // The base resource type to be used as a final entry if there is
    // no more resource super type. This is kind of the java.lang.Object
    // the resource type hierarchy.
    private final String baseResourceType;

    // The search path of the resource resolver
    private final String[] searchPath;

    // counter into the search path array
    private int pathCounter;

    // The resource type to use for the next iteration loop
    private String resourceType;

    // The first resource type to use
    private final String firstResourceType;

    // The first resource super type to use
    private final String firstResourceSuperType;

    // The current relative path generated from the resource type
    private String relPath;

    // the next absolute path to return from next(). This is null
    // if there is no more location to return
    private String nextLocation;

    /** Set of used resource types to detect a circular resource type hierarchy. */
    private final Set<String> usedResourceTypes = new HashSet<String>();

    /**
     * Creates an instance of this iterator starting with a location built from
     * the resource type of the <code>resource</code> and ending with the
     * given <code>baseResourceType</code>.
     *
     * @param resourceType the initial resource type.
     * @param resourceSuperType the initial resource super type.
     * @param baseResourceType The base resource type.
     * @param resolver The resource resolver
     */
    public LocationIterator(String resourceType, String resourceSuperType, String baseResourceType,
            ResourceResolver resolver) {
        this.resolver = resolver;
        this.baseResourceType = baseResourceType;

        String[] tmpPath = resolver.getSearchPath();
        if (tmpPath == null || tmpPath.length == 0) {
            tmpPath = new String[] { "/" };
        }
        searchPath = tmpPath;

        this.firstResourceType = resourceType;
        this.firstResourceSuperType = resourceSuperType;
        // we start with the first resource type
        this.resourceType = firstResourceType;

        this.usedResourceTypes.add(this.resourceType);

        nextLocation = seek();
    }

    /**
     * Returns <code>true</code> if there is another entry
     */
    public boolean hasNext() {
        return nextLocation != null;
    }

    /**
     * Returns the next entry of this iterator.
     *
     * @throws NoSuchElementException if {@link #hasNext()} returns
     *             <code>false</code>.
     */
    public String next() {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }

        String result = nextLocation;
        nextLocation = seek();
        return result;
    }

    /**
     * @throws UnsupportedOperationException
     */
    public void remove() {
        throw new UnsupportedOperationException();
    }

    private String seek() {

        if (relPath == null) {

            if (resourceType == null) {
                return null;
            }

            String typePath = ResourceUtil.resourceTypeToPath(resourceType);
            if (typePath.startsWith("/")) {
                resourceType = getResourceSuperType(resourceType);
                return typePath;
            }

            relPath = typePath;
        }

        String result = searchPath[pathCounter].concat(relPath);

        pathCounter++;
        if (pathCounter >= searchPath.length) {
            relPath = null;
            resourceType = getResourceSuperType(resourceType);
            pathCounter = 0;
        }

        return result;
    }

    /**
     * Returns the resource super type of the given resource type:
     * <ol>
     * <li>If the resource type is the base resource type <code>null</code>
     * is returned.</li>
     * <li>If the resource type is the resource type of the resource the
     * resource super type from the resource is returned.</li>
     * <li>Otherwise the resource super type is tried to be found in the
     * resource tree. If one is found, it is returned.</li>
     * <li>Otherwise the base resource type is returned.</li>
     * </ol>
     */
    private String getResourceSuperType(String resourceType) {

        // if the current resource type is the default value, there are no more
        if (resourceType.equals(baseResourceType)) {
            return null;
        }

        // get the super type of the current resource type
        String superType;
        if (resourceType.equals(this.firstResourceType)
                && this.firstResourceSuperType != null ) {
            superType = this.firstResourceSuperType;
        } else {
            superType = getResourceSuperType(resolver, resourceType);
        }

        // detect circular dependency
        if ( superType != null ) {
            if ( this.usedResourceTypes.contains(superType) ) {
                LoggerFactory.getLogger(this.getClass()).error("Circular dependency in resource type hierarchy detected! Check super types of {}", superType);
                superType = null;
            } else {
                this.usedResourceTypes.add(superType);
            }
        }
        // use default resource type if there is no super type any more
        if (superType == null) {
            superType = baseResourceType;
        }

        return superType;
    }

    // this method is largely duplicated from ResourceUtil
    private String getResourceSuperType(final ResourceResolver resourceResolver,
                                        final String resourceType) {
        // normalize resource type to a path string
        final String rtPath = ResourceUtil.resourceTypeToPath(resourceType);
        // get the resource type resource and check its super type
        String resourceSuperType = null;
        // if the path is absolute, use it directly
        if ( rtPath != null && rtPath.startsWith("/") ) {
            final String candidatePath = rtPath;

            final Resource rtResource = resourceResolver.getResource(candidatePath);
            if ( rtResource != null ) {
                resourceSuperType = rtResource.getResourceSuperType();
            }

        } else {
            // if the path is relative we use the search paths
            for(final String searchPath : this.searchPath) {
                final String candidatePath = searchPath + rtPath;
                final Resource rtResource = resourceResolver.getResource(candidatePath);
                if ( rtResource != null && rtResource.getResourceSuperType() != null ) {
                    resourceSuperType = rtResource.getResourceSuperType();
                    break;
                }
            }
        }
        return resourceSuperType;
    }

}