import pickBy from 'lodash/pickBy';
import { Identifier } from '../types';

const defaultCacheDuration = 10 * 60 * 1000; // ten minutes

/**
 * The dates each record was fetched at, index by record identifier
 *
 * @example
 * {
 *      12: new Date('2019-02-06T21:23:07.049Z'),
 *      34: new Date('2019-02-06T21:23:07.049Z'),
 * }
 */
interface FetchedOutDates {
    // FIXME: use [key: Identifier] once typeScript accepts any type as index (see https://github.com/Microsoft/TypeScript/pull/26797)
    [key: string]: Date;
    [key: number]: Date;
}

/**
 * Returns a list of fetch dates by record id
 *
 * Given a list of new record ids and a previous list of fetch dates by record id,
 * add the new record ids at the current date,
 * and removes those among the old record ids that are stale.
 *
 * @param newRecordIds an array of record identifiers, e.g. [34, 56]
 * @param oldRecordFetchedAt the fetch dates of old records, e.g. { 12: new Date('12 minutes ago), 34: new Date('5 minutes ago') }
 * @param now Current time (useful for tests)
 * @param cacheDuration How long until an old record is removed from the list
 */
const getFetchedAt = (
    newRecordIds: Identifier[] = [],
    oldRecordFetchedAt: FetchedOutDates = {},
    now = new Date(),
    cacheDuration = defaultCacheDuration
): FetchedOutDates => {
    // prepare new records and timestamp them
    const newFetchedAt = {};
    newRecordIds.forEach(recordId => (newFetchedAt[recordId] = now));

    // remove outdated entry
    const latestValidDate = new Date();
    latestValidDate.setTime(latestValidDate.getTime() - cacheDuration);

    const stillValidFetchedAt = pickBy(
        oldRecordFetchedAt,
        date => date > latestValidDate
    );

    return {
        ...stillValidFetchedAt,
        ...newFetchedAt,
    };
};

export default getFetchedAt;