PagingUtil.java

/*******************************************************************************
 * Copyright (c) 2012-2017 Codenvy, S.A.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *   Codenvy, S.A. - initial API and implementation
 *******************************************************************************/
package org.eclipse.che.api.core.util;

import org.eclipse.che.api.core.Page;
import org.eclipse.che.commons.lang.Pair;

import javax.ws.rs.core.UriBuilder;
import java.net.URI;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import static com.google.common.base.Strings.isNullOrEmpty;
import static java.lang.String.format;
import static java.util.Collections.emptyMap;
import static java.util.Objects.requireNonNull;
import static java.util.stream.Collectors.joining;

/**
 * Provides useful methods for working with {@link Page} instances
 * and pageable uris management.
 *
 * @author Yevhenii Voevodin
 */
public final class PagingUtil {

    private static final String  LINK_HEADER_SEPARATOR = ", ";
    /**
     * Helps to retrieve href along with rel from link header value part.
     * Value format is {@literal <http://host:port/path?query=value>; rel="next"},
     * so the first group is href while the second group is rel.
     */
    private static final Pattern LINK_HEADER_REGEX     = Pattern.compile("<(?<href>.+)>;.*rel=\"(?<rel>.+)\".*");

    /**
     * Generates link header value from the page object and base uri.
     * <a href="https://tools.ietf.org/html/rfc5988">The Link header spec</a>
     *
     * @param page
     *         the page used to generate link
     * @param uri
     *         the uri which is used for adding {@code skipCount} & {@code maxItems} query parameters
     * @return 'Link' header value
     * @throws NullPointerException
     *         when either {@code page} or {@code uri} is null
     */
    public static String createLinkHeader(Page<?> page, URI uri) {
        requireNonNull(page, "Required non-null page");
        requireNonNull(uri, "Required non-null uri");
        final ArrayList<Pair<String, Page.PageRef>> pageRefs = new ArrayList<>(4);
        pageRefs.add(Pair.of("first", page.getFirstPageRef()));
        pageRefs.add(Pair.of("last", page.getLastPageRef()));
        if (page.hasPreviousPage()) {
            pageRefs.add(Pair.of("prev", page.getPreviousPageRef()));
        }
        if (page.hasNextPage()) {
            pageRefs.add(Pair.of("next", page.getNextPageRef()));
        }
        final UriBuilder ub = UriBuilder.fromUri(uri);
        return pageRefs.stream()
                       .map(refPair -> format("<%s>; rel=\"%s\"",
                                              ub.clone()
                                                .replaceQueryParam("skipCount", refPair.second.getItemsBefore())
                                                .replaceQueryParam("maxItems", refPair.second.getPageSize())
                                                .build()
                                                .toString(),
                                              refPair.first))
                       .collect(joining(LINK_HEADER_SEPARATOR));
    }

    /**
     * Returns REL to URI map based on the given {@code linkHeader} value.
     * If the {@code linkHeader} is null or empty then an empty map will be returned.
     *
     * <p>Note that link header is parsed due to the {@link #createLinkHeader(Page, URI)} method strategy.
     *
     * @param linkHeader
     *         link header value
     */
    public static Map<String, String> parseLinkHeader(String linkHeader) {
        if (isNullOrEmpty(linkHeader)) {
            return emptyMap();
        }
        final Map<String, String> res = new HashMap<>();
        for (String part : linkHeader.split(LINK_HEADER_SEPARATOR)) {
            final Matcher matcher = LINK_HEADER_REGEX.matcher(part);
            if (matcher.matches()) {
                res.put(matcher.group("rel"), matcher.group("href"));
            }
        }
        return res;
    }

    private PagingUtil() {}
}