szurubooru/public_html/js/Services/PostsAroundCalculator.js

var App = App || {};
App.Services = App.Services || {};

App.Services.PostsAroundCalculator = function(_, promise, util, pager) {

    pager.init({url: '/posts'});

    function resetCache() {
        pager.resetCache();
    }

    function getLinksToPostsAround(query, postId) {
        return promise.make(function(resolve, reject) {
            pager.setSearchParams(query);
            pager.setPage(query.page);
            promise.wait(pager.retrieveCached())
                .then(function(response) {
                    var postIds = _.pluck(response.entities, 'id');
                    var position = _.indexOf(postIds, postId);

                    if (position === -1) {
                        resolve(null, null);
                    }

                    promise.wait(
                            getLinkToPostAround(postIds, position, query.page, -1),
                            getLinkToPostAround(postIds, position, query.page, 1))
                        .then(function(nextPostUrl, prevPostUrl) {
                            resolve(nextPostUrl, prevPostUrl);
                        }).fail(function() {
                            reject();
                        });
                }).fail(function() {
                    reject();
                });
        });
    }

    function getLinkToPostAround(postIds, position, page, direction) {
        return promise.make(function(resolve, reject) {
            if (position + direction >= 0 && position + direction < postIds.length) {
                var url = util.appendComplexRouteParam(
                    '#/post/' + postIds[position + direction],
                    util.simplifySearchQuery(
                        _.extend(
                            {page: page},
                            pager.getSearchParams())));

                resolve(url);
            } else if (page + direction >= 1) {
                pager.setPage(page + direction);
                promise.wait(pager.retrieveCached())
                    .then(function(response) {
                        if (response.entities.length) {
                            var post = direction === - 1 ?
                                _.last(response.entities) :
                                _.first(response.entities);

                            var url = util.appendComplexRouteParam(
                                '#/post/' + post.id,
                                util.simplifySearchQuery(
                                    _.extend(
                                        {page: page + direction},
                                        pager.getSearchParams())));

                            resolve(url);
                        } else {
                            resolve(null);
                        }
                    }).fail(function() {
                        reject();
                    });
            } else {
                resolve(null);
            }
        });
    }

    return {
        resetCache: resetCache,
        getLinksToPostsAround: getLinksToPostsAround,
    };
};

App.DI.register('postsAroundCalculator', ['_', 'promise', 'util', 'pager'], App.Services.PostsAroundCalculator);