dhis2/d2

import { isDefined } from '../lib/check'

/**
 * @description
 * Pager object that can be used to navigate pages within a `Modelcollection`
 *
 * @memberof module:model
 */

/**
 * @memberof module:model
 */
// TODO: Move to the model map
class Pager {
    /**
     * @constructor
     *
     * @param {Object} [pager={page: 1, pageCount: 1}] Paging information object.
     * @param {Object} [pagingHandler={list: () => Promise.reject('No handler available')}] Paging handler object. The requirement for this object is that it has a list method.
     *
     * @description
     * Returns a newly created pager object with methods to navigate pages.
     */
    constructor(
        pager = { page: 1, pageCount: 1, query: {} },
        pagingHandler = { list: () => Promise.reject('No handler available') }
    ) {
        /**
         * @property {number} page Current page number
         */
        this.page = pager.page

        /**
         * @property {number} pageCount The total number of pages available
         */
        this.pageCount = pager.pageCount

        /**
         * @property {number} total The total number of items available.
         *
         * @description
         * This represents the total number of items available in the system. Note it is not the number of items
         * on the current page.
         */
        this.total = pager.total

        /**
         * @property {string} nextPage The url to the next page.
         *
         * @description
         * If there is no next page then this will be undefined.
         */
        this.nextPage = pager.nextPage

        /**
         * @property {string} prevPage The url to the previous page
         *
         * @description
         * If there is no previous page then this will be undefined.
         */
        this.prevPage = pager.prevPage

        /**
         * @property {object} query Query parameters
         *
         * @description
         * Query parameters are used for things like filtering and field selection. Used to guarantee that pages are
         * from the same collection.
         */
        this.query = pager.query

        this.pagingHandler = pagingHandler
    }

    /**
     * @returns {Boolean} Result is true when there is a next page, false when there is not.
     *
     * @description
     * Check whether there is a next page.
     */
    hasNextPage() {
        return isDefined(this.nextPage)
    }

    /**
     * Check whether there is a previous page.
     *
     * @returns {Boolean} Result is true when there is a previous page, false when there is not.
     */
    hasPreviousPage() {
        return isDefined(this.prevPage)
    }

    /**
     * @description
     * Loads the next page in the collection if there is one. If no additional pages are available the Promise will reject.
     *
     * @returns {Promise} Promise that resolves with a new `ModelCollection` containing the next page's data. Or rejects with
     * a string when there is no next page for this collection or when the request for the next page failed.
     *
     * @example
     * d2.models.organisationUnit
     *   .list()
     *   .then(collection => {
     *     collection.pager.getNextPage()
     *       .then(secondPageCollection => {
     *         console.log(secondPageCollection.toArray());
     *       });
     *   });
     */
    getNextPage() {
        if (this.hasNextPage()) {
            return this.goToPage(this.page + 1)
        }
        return Promise.reject('There is no next page for this collection')
    }

    /**
     * @description
     * Loads the previous page in the collection if there is one. If no previous pages are available the Promise will reject.
     *
     * @returns {Promise} Promise that resolves with a new `ModelCollection` containing the previous page's data. Or rejects with
     * a string when there is no previous page for this collection or when the request for the previous page failed.
     *
     * @example
     * d2.models.organisationUnit
     *   .list()
     *   .then(collection => {
     *     collection.pager.goToPage(3)
     *       .then(collection => collection.pager.getPreviousPage())
     *       .then(secondPageCollection => {
     *         console.log(secondPageCollection.toArray());
     *       });
     *   });
     */
    getPreviousPage() {
        if (this.hasPreviousPage()) {
            return this.goToPage(this.page - 1)
        }
        return Promise.reject('There is no previous page for this collection')
    }

    /**
     * Loads a specific page for the collection. If the requested page is out of the range of available pages (e.g < 0 or > page count)
     * the Promise will reject with an error.
     *
     * @param {Number} pageNr The number of the page you wish to navigate to.
     * @returns {Promise} Promise that resolves with a new `ModelCollection` containing the data for the requested page.
     *
     * @example
     * d2.models.organisationUnit
     *   .list()
     *   .then(collection => {
     *     collection.pager.goToPage(4)
     *       .then(fourthPageCollection => {
     *         console.log(fourthPageCollection.toArray());
     *       });
     *   });
     */
    // TODO: Throwing the errors here is not really consistent with the rejection of promises for the getNextPage and getPreviousPage
    goToPage(pageNr) {
        if (pageNr < 1) {
            throw new Error('PageNr can not be less than 1')
        }
        if (pageNr > this.pageCount) {
            throw new Error(
                `PageNr can not be larger than the total page count of ${this.pageCount}`
            )
        }

        return this.pagingHandler.list(
            Object.assign({}, this.query, { page: pageNr })
        )
    }
}

export default Pager