index.ts

/*
 * Main class that handles scoring and sorting a feed made of Toot objects.
 */
import 'reflect-metadata';  // Required for class-transformer
import { Buffer } from 'buffer';  // Maybe Required for class-transformer though seems to be required in client?
import { mastodon } from "masto";
import { Mutex } from 'async-mutex';

import Account from './api/objects/account';
import AlreadyShownScorer from './scorer/toot/already_shown_scorer';
import AuthorFollowersScorer from './scorer/toot/author_followers_scorer';
import BooleanFilter from "./filters/boolean_filter";
import ChaosScorer from "./scorer/toot/chaos_scorer";
import DiversityFeedScorer from "./scorer/feed/diversity_feed_scorer";
import FavouritedTagsScorer from './scorer/toot/favourited_tags_scorer';
import FollowedAccountsScorer from './scorer/toot/followed_accounts_scorer';
import FollowedTagsScorer from "./scorer/toot/followed_tags_scorer";
import FollowersScorer from './scorer/toot/followers_scorer';
import HashtagParticipationScorer from "./scorer/toot/hashtag_participation_scorer";
import ImageAttachmentScorer from "./scorer/toot/image_attachment_scorer";
import InteractionsScorer from "./scorer/toot/interactions_scorer";
import MastoApi, { FULL_HISTORY_PARAMS } from "./api/api";
import MastodonServer from './api/mastodon_server';
import MentionsFollowedScorer from './scorer/toot/mentions_followed_scorer';
import MostFavouritedAccountsScorer from "./scorer/toot/most_favourited_accounts_scorer";
import MostRepliedAccountsScorer from "./scorer/toot/most_replied_accounts_scorer";
import MostRetootedAccountsScorer from "./scorer/toot/most_retooted_accounts_scorer";
import NumericFilter from './filters/numeric_filter';
import NumFavouritesScorer from "./scorer/toot/num_favourites_scorer";
import NumRepliesScorer from "./scorer/toot/num_replies_scorer";
import NumRetootsScorer from "./scorer/toot/num_retoots_scorer";
import RetootsInFeedScorer from "./scorer/toot/retoots_in_feed_scorer";
import Scorer from "./scorer/scorer";
import ScorerCache from './scorer/scorer_cache';
import Storage from "./Storage";
import TagList from './api/tag_list';
import TagsForFetchingToots from "./api/tags_for_fetching_toots";
import Toot, { earliestTootedAt, mostRecentTootedAt } from './api/objects/toot';
import TrendingLinksScorer from './scorer/toot/trending_links_scorer';
import TrendingTagsScorer from "./scorer/toot/trending_tags_scorer";
import TrendingTootScorer from "./scorer/toot/trending_toots_scorer";
import UserData from "./api/user_data";
import UserDataPoller from './api/user_data_poller';
import VideoAttachmentScorer from "./scorer/toot/video_attachment_scorer";
import type FeedScorer from './scorer/feed_scorer';
import type TootScorer from './scorer/toot_scorer';
import { AgeIn, ageString, sleep, timeString, toISOFormatIfExists } from './helpers/time_helpers';
import { buildNewFilterSettings, updateBooleanFilterOptions } from "./filters/feed_filters";
import { DEFAULT_FONT_SIZE, FEDIALGO, GIFV, VIDEO_TYPES, extractDomain, optionalSuffix } from './helpers/string_helpers';
import { isAccessTokenRevokedError, throwIfAccessTokenRevoked, throwSanitizedRateLimitError } from './api/errors';
import { isDebugMode, isDeepDebug, isLoadTest, isQuickMode } from './helpers/environment_helpers';
import { lockExecution } from './helpers/mutex_helpers';
import { Logger } from './helpers/logger';
import { MAX_ENDPOINT_RECORDS_TO_PULL, config } from './config';
import { rechartsDataPoints } from "./helpers/stats_helper";
import { WEIGHT_PRESETS, WeightPresetLabel, isWeightPresetLabel, type WeightPresets } from './scorer/weight_presets';
import { type ObjList } from "./api/counted_list";
import {
    AlgorithmStorageKey,
    BooleanFilterName,
    CacheKey,
    FediverseCacheKey,
    LoadAction,
    LogAction,
    MediaCategory,
    NonScoreWeightName,
    ScoreName,
    TagTootsCategory,
    TrendingType,
    TypeFilterName,
    ALL_ACTIONS,
    buildCacheKeyDict,
    isValueInStringEnum,
    type Action,
    type ApiCacheKey,
} from "./enums";
import {
    computeMinMax,
    makeChunks,
    makePercentileChunks,
    sortKeysByValue,
    truncateToLength,
} from "./helpers/collection_helpers";
import {
    FILTER_OPTION_DATA_SOURCES,
    type BooleanFilterOption,
    type ConcurrencyLockRelease,
    type FeedFilterSettings,
    type FilterOptionDataSource,
    type Hashtag,
    type KeysOfValueType,
    type MastodonInstance,
    type MinMaxAvgScore,
    type ScoreStats,
    type StringNumberDict,
    type TagWithUsageCounts,
    type TrendingData,
    type TrendingLink,
    type TrendingObj,
    type TrendingWithHistory,
    type WeightInfoDict,
    type WeightName,
    type Weights,
} from "./types";

const EMPTY_TRENDING_DATA: Readonly<TrendingData> = {
    links: [],
    tags: new TagList([], TagTootsCategory.TRENDING),
    servers: {},
    toots: []
};

const DEFAULT_SET_TIMELINE_IN_APP = (_feed: Toot[]) => console.debug(`Default setTimelineInApp() called`);

const logger = new Logger(`TheAlgorithm`);
const loadCacheLogger = logger.tempLogger(`loadCachedData()`);
const saveTimelineToCacheLogger = logger.tempLogger(`saveTimelineToCache`);

const loggers: Record<Action | ApiCacheKey, Logger> = buildCacheKeyDict<Action, Logger, Record<Action, Logger>>(
    (key) => new Logger(key as string),
    ALL_ACTIONS.reduce(
        (_loggers, action) => {
            _loggers[action] = logger.tempLogger(action);
            return _loggers;
        },
        {} as Record<Action, Logger>
    )
);

interface AlgorithmArgs {
    api: mastodon.rest.Client;
    user: mastodon.v1.Account;
    locale?: string;  // Optional locale to use for date formatting
    setTimelineInApp?: (feed: Toot[]) => void;  // Optional callback to set the feed in the code using this package
};


/**
 * Main class for scoring, sorting, and managing a Mastodon feed made of {@linkcode Toot} objects.
 *
 * {@linkcode TheAlgorithm} orchestrates fetching, scoring, filtering, and updating the user's timeline/feed.
 * It manages feature and feed scorers, trending data, filters, user weights, and background polling. Key
 * responsibilities:
 *
 *  1. Fetches and merges toots from multiple sources (home timeline, trending, hashtags, etc.).
 *  2. Applies scoring algorithms and user-defined weights to rank toots.
 *  3. Filters the feed based on user settings and filter options.
 *  4. Handles background polling for new data and saving state to storage.
 *  5. Provides methods for updating filters, weights, and retrieving current state.
 *  6. Exposes utility methods for stats, server info, and tag URLs.
 *
 * @property {string[]} apiErrorMsgs - API error messages
 * @property {FeedFilterSettings} filters - Current filter settings for the feed
 * @property {boolean} isLoading - Whether a feed load is in progress*
 * @property {number} [lastLoadTimeInSeconds] - Duration of the last load in seconds
 * @property {string | null} loadingStatus - String describing load activity
 * @property {Toot[]} timeline - The current filtered timeline
 * @property {TrendingData} trendingData - Trending data (links, tags, servers, toots)
 * @property {UserData} userData - User data for scoring and filtering
 * @property {WeightInfoDict} weightsInfo - Info about all scoring weights
 */
export default class TheAlgorithm {
    filters: FeedFilterSettings = buildNewFilterSettings();
    lastLoadTimeInSeconds?: number;
    loadingStatus: string | null = config.locale.messages[LogAction.INITIAL_LOADING_STATUS];
    trendingData: TrendingData = EMPTY_TRENDING_DATA;

    get apiErrorMsgs(): string[] { return MastoApi.instance.apiErrorMsgs() };
    get isLoading(): boolean { return this.loadingMutex.isLocked() };
    get timeline(): Toot[] { return [...this.feed] };
    get userData(): UserData { return MastoApi.instance.userData || new UserData() };

    // Constructor arguments
    private setTimelineInApp: (feed: Toot[]) => void;  // Optional callback to set the feed in the app using this package
    // Other private variables
    private feed: Toot[] = [];
    private homeFeed: Toot[] = [];  // Just the toots pulled from the home timeline
    private hasProvidedAnyTootsToClient = false;  // Flag to indicate if the feed has been set in the app
    private loadStartedAt: Date | undefined = new Date();  // Timestamp of when the feed started loading
    private totalNumTimesShown = 0;  // Sum of timeline toots' numTimesShown
    // Utility
    private loadingMutex = new Mutex();
    private mergeMutex = new Mutex();
    private numUnscannedToots = 0;  // Keep track of how many new toots were merged into the feed but not into the filter options
    private numTriggers = 0;  // How many times has a load been triggered, only matters for QUICK_LOAD mode
    private _releaseLoadingMutex?: ConcurrencyLockRelease;  // Mutex release function for loading state
    // Background tasks
    private cacheUpdater?: ReturnType<typeof setInterval>;
    private userDataPoller = new UserDataPoller();

    // These scorers require the complete feed to work properly
    private feedScorers: FeedScorer[] = [
        new DiversityFeedScorer(),
    ];

    // These can score a toot without knowing about the rest of the toots in the feed
    private tootScorers: TootScorer[] = [
        new AlreadyShownScorer(),
        new AuthorFollowersScorer(),
        new ChaosScorer(),
        new FavouritedTagsScorer(),
        new FollowedAccountsScorer(),
        new FollowedTagsScorer(),
        new FollowersScorer(),
        new HashtagParticipationScorer(),
        new ImageAttachmentScorer(),
        new InteractionsScorer(),
        new MentionsFollowedScorer(),
        new MostFavouritedAccountsScorer(),
        new MostRepliedAccountsScorer(),
        new MostRetootedAccountsScorer(),
        new NumFavouritesScorer(),
        new NumRepliesScorer(),
        new NumRetootsScorer(),
        new RetootsInFeedScorer(),
        new TrendingLinksScorer(),
        new TrendingTagsScorer(),
        new TrendingTootScorer(),
        new VideoAttachmentScorer(),
    ];

    private weightedScorers: Scorer[] = [
        ...this.tootScorers,
        ...this.feedScorers,
    ];

    weightsInfo: WeightInfoDict = this.weightedScorers.reduce(
        (scorerInfos, scorer) => {
            scorerInfos[scorer.name] = scorer.getInfo();
            return scorerInfos;
        },
        Object.values(NonScoreWeightName).reduce(
            (nonScoreWeights, weightName) => {
                nonScoreWeights[weightName] = Object.assign({}, config.scoring.nonScoreWeightsConfig[weightName]);
                nonScoreWeights[weightName].minValue = config.scoring.nonScoreWeightMinValue;
                return nonScoreWeights;
            },
            {} as WeightInfoDict
        )
    );

    /**
     * Publicly callable constructor that instantiates the class and loads the feed from storage.
     * @param {AlgorithmArgs} params - The parameters for algorithm creation.
     * @param {mastodon.rest.Client} params.api - The Mastodon REST API client instance.
     * @param {mastodon.v1.Account} params.user - The Mastodon user account for which to build the feed.
     * @param {string} [params.locale] - Optional locale string for date formatting.
     * @param {(feed: Toot[]) => void} [params.setTimelineInApp] - Optional callback to set the feed in the consuming app.
     * @returns {Promise<TheAlgorithm>} TheAlgorithm instance.
     */
    static async create(params: AlgorithmArgs): Promise<TheAlgorithm> {
        config.setLocale(params.locale);
        const user = Account.build(params.user);
        await MastoApi.init(params.api, user);
        await Storage.logAppOpen(user);

        // Construct the algorithm object, set the default weights, load feed and filters
        const algo = new TheAlgorithm(params);
        ScorerCache.addScorers(algo.tootScorers, algo.feedScorers);
        await algo.loadCachedData();
        return algo;
    }

    /**
     * Private constructor. Use {@linkcode TheAlgorithm.create} to instantiate.
     * @param {AlgorithmArgs} params - Constructor params (API client, user, and optional timeline callback/locale).
     */
    private constructor(params: AlgorithmArgs) {
        this.setTimelineInApp = params.setTimelineInApp ?? DEFAULT_SET_TIMELINE_IN_APP;
    }

    /**
     * Trigger the retrieval of the user's timeline from all the sources.
     * @returns {Promise<void>}
     */
    async triggerFeedUpdate(): Promise<void> {
        if (this.shouldSkip()) return;
        const action = LoadAction.FEED_UPDATE;
        const hereLogger = loggers[action];
        await this.startAction(action);

        try {
            const tootsForHashtags = async (key: TagTootsCategory): Promise<Toot[]> => {
                hereLogger.trace(`Fetching toots for hashtags with key: ${key}`);
                const tagList = await TagsForFetchingToots.create(key);
                return await this.fetchAndMergeToots(tagList.getToots(), tagList.logger);
            };

            const dataLoads: Promise<unknown>[] = [
                // Toot fetchers
                this.getHomeTimeline().then((toots) => this.homeFeed = toots),
                this.fetchAndMergeToots(MastoApi.instance.getHomeserverToots(), loggers[CacheKey.HOMESERVER_TOOTS]),
                this.fetchAndMergeToots(MastodonServer.fediverseTrendingToots(), loggers[FediverseCacheKey.TRENDING_TOOTS]),
                ...Object.values(TagTootsCategory).map(async (key) => await tootsForHashtags(key)),
                // Other data fetchers
                MastodonServer.getTrendingData().then((trendingData) => this.trendingData = trendingData),
                MastoApi.instance.getUserData(),
                ScorerCache.prepareScorers(),
            ];

            const allResults = await Promise.allSettled(dataLoads);
            hereLogger.deep(`FINISHED promises, allResults:`, allResults);
            await this.finishFeedUpdate();
        } finally {
            this.releaseLoadingMutex(action);
        }
    }

    /**
     * Trigger the fetching of additional earlier {@linkcode Toot}s from the server.
     * @returns {Promise<void>}
     */
    async triggerHomeTimelineBackFill(): Promise<void> {
        await this.startAction(LoadAction.TIMELINE_BACKFILL);

        try {
            this.homeFeed = await this.getHomeTimeline(true);
            await this.finishFeedUpdate();
        } finally {
            this.releaseLoadingMutex(LoadAction.TIMELINE_BACKFILL);
        }
    }

    /**
     * Manually trigger the loading of "moar" user data (recent toots, favourites, notifications, etc).
     * Usually done by a background task on a set interval.
     * @returns {Promise<void>}
     */
    async triggerMoarData(): Promise<void> {
        const shouldReenablePoller = this.userDataPoller.stop();
        await this.startAction(LoadAction.GET_MOAR_DATA);

        try {
            await this.userDataPoller.getMoarData();
            await this.recomputeScores();
        } catch (error) {
            throwSanitizedRateLimitError(error, `triggerMoarData() Error pulling user data:`);
        } finally {
            if (shouldReenablePoller) this.userDataPoller.start();
            this.releaseLoadingMutex(LoadAction.GET_MOAR_DATA);
        }
    }

    /**
     * Collect **ALL** the user's history data from the server - past toots, favourites, etc.
     * Use with caution!
     * @returns {Promise<void>}
     */
    async triggerPullAllUserData(): Promise<void> {
        const action = LoadAction.PULL_ALL_USER_DATA;
        const hereLogger = loggers[action];
        this.startAction(action);

        try {
            this.userDataPoller.stop();   // Stop the dataPoller if it's running

            const _allResults = await Promise.allSettled([
                MastoApi.instance.getFavouritedToots(FULL_HISTORY_PARAMS),
                // TODO: there's just too many notifications to pull all of them
                MastoApi.instance.getNotifications({maxRecords: MAX_ENDPOINT_RECORDS_TO_PULL, moar: true}),
                MastoApi.instance.getRecentUserToots(FULL_HISTORY_PARAMS),
            ]);

            await this.recomputeScores();
        } catch (error) {
            throwSanitizedRateLimitError(error, hereLogger.line(`Error pulling user data:`));
        } finally {
            this.releaseLoadingMutex(action);  // TODO: should we restart data poller?
        }
    }

    /**
     * Return an object describing the state of the world. Mostly for debugging.
     * @returns {Promise<Record<string, any>>} State object.
     */
    async getCurrentState(): Promise<Record<string, unknown>> {
        return {
            Algorithm: this.statusDict(),
            Api: MastoApi.instance.currentState(),
            Config: config,
            Filters: this.filters,
            Homeserver: await this.serverInfo(),
            Storage: await Storage.storedObjsInfo(),
            Trending: this.trendingData,
            UserData: await MastoApi.instance.getUserData(),
        };
    }

    /**
     * Build array of objects suitable for charting timeline scoring data by quintile/decile/etc.
     * with {@link https://recharts.org/ Recharts}.
     * @param {number} numPercentiles - Number of percentiles for stats.
     * @returns {object[]} Recharts data points.
     */
    getRechartsStatsData(numPercentiles: number): object[] {
        return rechartsDataPoints(this.feed, numPercentiles);
    }

    /**
     * Return the user's current weightings for each score category.
     * @returns {Promise<Weights>} The user's weights.
     */
    async getUserWeights(): Promise<Weights> {
        return await Storage.getWeights();
    }

    /**
     * Return the timestamp of the most recent toot from followed accounts + hashtags ONLY.
     * @returns {Date | null} The most recent toot date or null.
     */
    mostRecentHomeTootAt(): Date | null {
        // TODO: this.homeFeed is only set when fetchHomeFeed() is *finished*
        if (this.homeFeed.length == 0 && this.numTriggers > 1) {
            logger.warn(`mostRecentHomeTootAt() homeFeed is empty, falling back to full feed`);
            return mostRecentTootedAt(this.feed);
        }

        return mostRecentTootedAt(this.homeFeed);
    }

    /**
     * Return the number of seconds since the most recent home timeline {@linkcode Toot}.
     * @returns {number | null} Age in seconds or null.
     */
    mostRecentHomeTootAgeInSeconds(): number | null {
        const mostRecentAt = this.mostRecentHomeTootAt();
        if (!mostRecentAt) return null;
        logger.trace(`feed is ${AgeIn.minutes(mostRecentAt).toFixed(2)} min old, most recent home toot: ${timeString(mostRecentAt)}`);
        return AgeIn.seconds(mostRecentAt);
    }

    /**
     * Pull the latest list of muted accounts from the server and use that to filter any newly muted
     * accounts out of the timeline.
     * @returns {Promise<void>}
     */
    async refreshMutedAccounts(): Promise<void> {
        const hereLogger = loggers[LoadAction.REFRESH_MUTED_ACCOUNTS];
        hereLogger.log(`called (${Object.keys(this.userData.mutedAccounts).length} current muted accounts)...`);
        // TODO: move refreshMutedAccounts() to UserData class?
        const mutedAccounts = await MastoApi.instance.getMutedAccounts({bustCache: true});
        hereLogger.log(`Found ${mutedAccounts.length} muted accounts after refresh...`);
        this.userData.mutedAccounts = Account.buildAccountNames(mutedAccounts);
        await Toot.completeToots(this.feed, hereLogger, LoadAction.REFRESH_MUTED_ACCOUNTS);
        await this.finishFeedUpdate();
    }

    /**
     * Clear everything from browser storage except the user's identity and weightings (unless complete is true).
     * @param {boolean} [complete=false] - If true, remove user data as well.
     * @returns {Promise<void>}
     */
    async reset(complete: boolean = false): Promise<void> {
        await this.startAction(LoadAction.RESET);

        try {
            this.userDataPoller.stop();
            this.cacheUpdater && clearInterval(this.cacheUpdater!);
            this.cacheUpdater = undefined;
            this.hasProvidedAnyTootsToClient = false;
            this.loadingStatus = config.locale.messages[LogAction.INITIAL_LOADING_STATUS];
            this.loadStartedAt = new Date();
            this.numTriggers = 0;
            this.trendingData = EMPTY_TRENDING_DATA;
            this.feed = [];
            this.setTimelineInApp([]);

            // Call other classes' reset methods
            MastoApi.instance.reset();
            ScorerCache.resetScorers();
            await Storage.clearAll();

            if (complete) {
                await Storage.remove(AlgorithmStorageKey.USER);  // Remove user data so it gets reloaded
            } else {
                await this.loadCachedData();
            }
        } finally {
            this.releaseLoadingMutex(LoadAction.RESET);
        }
    }

    /**
     * Save the current timeline to the browser storage. Used to save the state of {@linkcode Toot.numTimesShown}.
     * @returns {Promise<void>}
     */
    async saveTimelineToCache(): Promise<void> {
        const newTotalNumTimesShown = this.feed.reduce((sum, toot) => sum + (toot.numTimesShown ?? 0), 0);
        if (this.isLoading || (this.totalNumTimesShown == newTotalNumTimesShown)) return;

        try {
            const numShownToots = this.feed.filter(toot => toot.numTimesShown).length;
            const msg = `Saving ${this.feed.length} toots with ${newTotalNumTimesShown} times shown` +
                ` on ${numShownToots} toots (previous totalNumTimesShown: ${this.totalNumTimesShown})`;
            saveTimelineToCacheLogger.debug(msg);
            await Storage.set(AlgorithmStorageKey.TIMELINE_TOOTS, this.feed);
            this.totalNumTimesShown = newTotalNumTimesShown;
        } catch (error) {
            saveTimelineToCacheLogger.error(`Error saving toots:`, error);
        }
    }

    /**
     * True if FediAlgo user is on a GoToSocial instance instead of plain vanilla Mastodon.
     * @returns {boolean}
     */
    async isGoToSocialUser(): Promise<boolean> {
        return await MastoApi.instance.isGoToSocialUser();
    }

    /**
     * Update {@linkcode this.trendingData} with latest available data.
     * // TODO: this shouldn't be necessary but there's weirdness on initial load
     * @returns {Promise<TrendingData>}
     */
    async refreshTrendingData(): Promise<TrendingData> {
        this.trendingData = await MastodonServer.getTrendingData();
        return this.trendingData;
    }

    /**
     * Returns info about the Fedialgo user's home Mastodon instance.
     * @returns {Promise<mastodon.v2.Instance>} Instance info.
     */
    async serverInfo(): Promise<mastodon.v2.Instance> {
        return await MastoApi.instance.instanceInfo();
    }

    /**
     * Get the URL for a tag on the user's home instance (aka "server").
     * @param {string | Hashtag} tag - The tag or tag object.
     * @returns {string} The tag URL.
     */
    tagUrl(tag: string | Hashtag): string {
        return MastoApi.instance.tagUrl(tag);
    }

    /**
     * Update the feed filters and return the newly filtered feed.
     * @param {FeedFilterSettings} newFilters - The new filter settings.
     * @returns {Toot[]} The filtered feed.
     */
    updateFilters(newFilters: FeedFilterSettings): Toot[] {
        logger.info(`updateFilters() called with newFilters:`, newFilters);
        this.filters = newFilters;
        Storage.setFilters(newFilters);
        return this.filterFeedAndSetInApp();
    }

    /**
     * Update user weightings and rescore / resort the feed.
     * @param {Weights} userWeights - The new user weights.
     * @returns {Promise<Toot[]>} The filtered and rescored feed.
     */
    async updateUserWeights(userWeights: Weights): Promise<Toot[]> {
        logger.info("updateUserWeights() called with weights:", userWeights);
        Scorer.validateWeights(userWeights);
        await Storage.setWeightings(userWeights);
        return this.scoreAndFilterFeed();
    }

    /**
     * Update user weightings to one of the preset values and rescore / resort the feed.
     * @param {WeightPresetLabel | string} presetName - The preset name.
     * @returns {Promise<Toot[]>} The filtered and rescored feed.
     */
    async updateUserWeightsToPreset(presetName: WeightPresetLabel | string): Promise<Toot[]> {
        logger.info("updateUserWeightsToPreset() called with presetName:", presetName);

        if (!isWeightPresetLabel(presetName)) {
            logger.logAndThrowError(`Invalid weight preset: "${presetName}"`);
        }

        return await this.updateUserWeights(WEIGHT_PRESETS[presetName as WeightPresetLabel]);
    }

    ///////////////////////////////
    //      Private Methods      //
    ///////////////////////////////

    /**
     * Merge a new batch of {@linkcode Toot}s into the feed. Mutates {@linkcode this.feed}
     * and returns whatever {@linkcode newToots} are retrieved by {@linkcode tootFetcher} argument.
     * @private
     * @param {Promise<Toot[]>} tootFetcher - Promise that resolves to an array of Toots.
     * @param {Logger} logger Logger to use.
     * @returns {Promise<Toot[]>} The new toots that were fetched and merged.
     */
    private async fetchAndMergeToots(tootFetcher: Promise<Toot[]>, logger: Logger): Promise<Toot[]> {
        const startedAt = new Date();
        let newToots: Toot[] = [];

        try {
            newToots = await tootFetcher;
            logger.logTelemetry(`Got ${newToots.length} toots for ${CacheKey.HOME_TIMELINE_TOOTS}`, startedAt);
        } catch (e) {
            throwIfAccessTokenRevoked(logger, e, `Error fetching toots ${ageString(startedAt)}`);
        }

        await this.lockedMergeToFeed(newToots, logger);
        return newToots;
    }

    /**
     * Filter the feed based on the user's settings. Has the side effect of calling the
     * {@linkcode TheAlgorithm.setTimelineInApp} callback (if it exists) to send the client
     * using this library the filtered subset of {@linkcode Toot} objects.
     * ({@linkcode TheAlgorithm.feed} will always maintain the master unfiltered set of {@linkcode Toot}s).
     * @private
     * @returns {Toot[]} The filtered feed.
     */
    private filterFeedAndSetInApp(): Toot[] {
        const filteredFeed = this.feed.filter(toot => toot.isInTimeline(this.filters));
        this.setTimelineInApp(filteredFeed);

        if (!this.hasProvidedAnyTootsToClient && this.feed.length > 0) {
            this.hasProvidedAnyTootsToClient = true;
            logger.logTelemetry(`First ${filteredFeed.length} toots sent to client`, this.loadStartedAt);
        }

        return filteredFeed;
    }

    /**
     * Do some final cleanup and scoring operations on the feed.
     * @private
     * @returns {Promise<void>}
     */
    private async finishFeedUpdate(): Promise<void> {
        const action = LogAction.FINISH_FEED_UPDATE;
        const hereLogger = loggers[action];
        this.loadingStatus = config.locale.messages[action];

        // Now that all data has arrived go back over the feed and do the slow calculations of trendingLinks etc.
        hereLogger.debug(`${this.loadingStatus}...`);
        await Toot.completeToots(this.feed, hereLogger);
        this.feed = await Toot.removeInvalidToots(this.feed, hereLogger);
        await updateBooleanFilterOptions(this.filters, this.feed, true);
        await this.scoreAndFilterFeed();

        if (this.loadStartedAt) {
            hereLogger.logTelemetry(`finished home TL load w/ ${this.feed.length} toots`, this.loadStartedAt);
            this.lastLoadTimeInSeconds = AgeIn.seconds(this.loadStartedAt);
        } else {
            hereLogger.warn(`finished but loadStartedAt is null!`);
        }

        this.loadStartedAt = undefined;
        this.loadingStatus = null;
        this.launchBackgroundPollers();
    }

    /**
     * Simple wrapper for triggering {@linkcode MastoApi.fetchHomeFeed}.
     * @private
     * @returns {Promise<Toot[]>}
     */
    private async getHomeTimeline(moreOldToots?: boolean): Promise<Toot[]> {
        return await MastoApi.instance.fetchHomeFeed({
            mergeTootsToFeed: this.lockedMergeToFeed.bind(this),
            moar: moreOldToots
        });
    }

    /**
     * Kick off the MOAR data poller to collect more user history data if it doesn't already exist
     * as well as the cache updater that saves the current state of the timeline toots'
     * {@linkcode alreadyShown} properties to storage.
     * @private
     */
    private launchBackgroundPollers(): void {
        this.userDataPoller.start();

        // The cache updater writes the current state of the feed to storage every few seconds
        // to capture changes to the alreadyShown state of toots.
        if (this.cacheUpdater) {
            logger.trace(`cacheUpdater already exists, not starting another one`);
        } else {
            this.cacheUpdater = setInterval(
                async () => await this.saveTimelineToCache(),
                config.toots.saveChangesIntervalSeconds * 1000
            );
        }
    }

    /**
     * Load cached data from {@linkcode Storage}. Called when the app is first opened and when
     * {@linkcode TheAlgorithm.reset} is invoked.
     * @private
     * @returns {Promise<void>}
     */
    private async loadCachedData(): Promise<void> {
        this.homeFeed = await Storage.getCoerced<Toot>(CacheKey.HOME_TIMELINE_TOOTS);
        this.feed = await Storage.getCoerced<Toot>(AlgorithmStorageKey.TIMELINE_TOOTS);

        if (this.feed.length == config.toots.maxTimelineLength) {
            const numToClear = config.toots.maxTimelineLength - config.toots.truncateFullTimelineToLength;
            loadCacheLogger.info(`Timeline cache is full (${this.feed.length}), discarding ${numToClear} old toots`);
            this.feed = truncateToLength(this.feed, config.toots.truncateFullTimelineToLength, logger);
            await Storage.set(AlgorithmStorageKey.TIMELINE_TOOTS, this.feed);
        }

        this.trendingData = await Storage.getTrendingData();
        this.filters = await Storage.getFilters() ?? buildNewFilterSettings();
        await updateBooleanFilterOptions(this.filters, this.feed);
        this.setTimelineInApp(this.feed);
        loadCacheLogger.debugWithTraceObjs(`Loaded ${this.feed.length} cached toots + trendingData`, this.trendingData);
    }

    /**
     * Apparently if the mutex lock is inside mergeTootsToFeed() then the state of {@linkcode TheAlgorithm.feed}
     * is not consistent which can result in toots getting lost as threads try to merge {@linkcode newToots}
     * into different {@linkcode TheAlgorithm.feed} states.
     * Wrapping the entire function in a mutex seems to fix this (though i'm not sure why).
     * @private
     * @param {Toot[]} newToots - New toots to merge into this.feed
     * @param {Logger} logger - Logger to use
     * @returns {Promise<void>}
     */
    private async lockedMergeToFeed(newToots: Toot[], logger: Logger): Promise<void> {
        const hereLogger = logger.tempLogger('lockedMergeToFeed');
        const releaseMutex = await lockExecution(this.mergeMutex, hereLogger);

        try {
            await this.mergeTootsToFeed(newToots, logger);
            hereLogger.trace(`Merged ${newToots.length} newToots, released mutex`);
        } finally {
            releaseMutex();
        }
    };

    /**
     * Merge newToots into {@linkcode TheAlgorithm.feed}, score, and filter the feed.
     * NOTE: Don't call this directly! Use {@linkcode TheAlgorithm.lockedMergeTootsToFeed} instead.
     * @private
     * @param {Toot[]} newToots - New toots to merge into this.feed
     * @param {Logger} inLogger - Logger to use
     * @returns {Promise<void>}
     */
    private async mergeTootsToFeed(newToots: Toot[], inLogger: Logger): Promise<void> {
        const hereLogger = inLogger.tempLogger('mergeTootsToFeed');
        const numTootsBefore = this.feed.length;
        const startedAt = new Date();

        // Merge new Toots
        this.feed = Toot.dedupeToots([...this.feed, ...newToots], hereLogger);
        this.numUnscannedToots += newToots.length;

        // Building filter options is expensive so we only do it when it's justifiable
        if ((this.feed.length < config.toots.minToSkipFilterUpdates) || (this.numUnscannedToots > config.toots.filterUpdateBatchSize)) {
            await updateBooleanFilterOptions(this.filters, this.feed);
            this.numUnscannedToots = 0;
        } else {
            logger.trace(`Skipping filter update, feed length: ${this.feed.length}, unscanned toots: ${this.numUnscannedToots}`);
        }

        await this.scoreAndFilterFeed();
        // Update loadingStatus and log telemetry
        const statusMsgFxn = config.locale.messages[LoadAction.FEED_UPDATE];
        this.loadingStatus = statusMsgFxn(this.feed, this.mostRecentHomeTootAt());
        hereLogger.logTelemetry(`Merged ${newToots.length} new toots into ${numTootsBefore} timeline toots`, startedAt);
    }

    /**
     * Recompute the scorers' computations based on user history etc. and trigger a rescore of the feed.
     * @private
     * @returns {Promise<void>}
     */
    private async recomputeScores(): Promise<void> {
        await ScorerCache.prepareScorers(true);
        await this.scoreAndFilterFeed();
    }

    /**
     * Release the loading mutex and reset the loading state variables.
     * @private
     * @param {LoadAction} logPrefix - Action for logging context.
     * @returns {void}
     */
    private releaseLoadingMutex(logPrefix: LoadAction): void {
        this.loadingStatus = null;

        if (this._releaseLoadingMutex) {
            loggers[logPrefix].info(`Finished, releasing mutex...`);
            this._releaseLoadingMutex();
        } else {
            loggers[logPrefix].warn(`releaseLoadingMutex() called but no mutex to release!`);
        }
    }

    /**
     * Score the feed, sort it, save it to storage, and call {@linkcode TheAlgorithm.filterFeedAndSetInApp}
     * to update the feed in the app.
     * @private
     * @returns {Promise<Toot[]>} The filtered set of Toots (NOT the entire feed).
     */
    private async scoreAndFilterFeed(): Promise<Toot[]> {
        this.feed = await Scorer.scoreToots(this.feed, true);

        this.feed = truncateToLength(
            this.feed,
            config.toots.maxTimelineLength,
            logger.tempLogger('scoreAndFilterFeed()')
        );

        await Storage.set(AlgorithmStorageKey.TIMELINE_TOOTS, this.feed);
        return this.filterFeedAndSetInApp();
    }

    /**
     * Return true if we're in {@linkcode QUICK_MODE} and the feed is fresh enough that we don't
     * need to retrieve any new data. Useful for testing UI changes without waiting
     * for the full feed load every time.
     * @private
     * @returns {boolean} True if we should skip the feed update.
     */
    private shouldSkip(): boolean {
        const hereLogger = loggers[LoadAction.FEED_UPDATE];
        hereLogger.debugWithTraceObjs(`${++this.numTriggers} triggers so far, state:`, this.statusDict());
        let feedAgeInMinutes = this.mostRecentHomeTootAgeInSeconds();
        if (feedAgeInMinutes) feedAgeInMinutes /= 60;
        const maxAgeMinutes = config.minTrendingMinutesUntilStale();

        if (isQuickMode && feedAgeInMinutes && feedAgeInMinutes < maxAgeMinutes && this.numTriggers <= 1) {
            hereLogger.debug(`isQuickMode=${isQuickMode}, feed's ${feedAgeInMinutes.toFixed(0)}s old, skipping`);
            // Needs to be called to update the feed in the app
            ScorerCache.prepareScorers().then((_t) => this.filterFeedAndSetInApp());
            return true;
        } else {
            return false;
        }
    }

    /**
     * Lock the mutex and set the {@linkcode TheAlgorithm.loadStartedAt} timestamp.
     * @private
     * @param {LoadAction} logPrefix - Action for logging context.
     * @returns {Promise<void>}
     * @throws {Error} If a load is already in progress.
     */
    private async startAction(logPrefix: LoadAction): Promise<void> {
        const hereLogger = loggers[logPrefix];
        const status = config.locale.messages[logPrefix];
        hereLogger.debugWithTraceObjs(`called`, this.statusDict());

        if (this.isLoading) {
            hereLogger.warn(`Load in progress already!`, this.statusDict());
            throw new Error(config.locale.messages.isBusy);
        }

        this.loadStartedAt = new Date();
        this._releaseLoadingMutex = await lockExecution(this.loadingMutex, logger);
        this.loadingStatus = (typeof status === 'string') ? status : status(this.feed, this.mostRecentHomeTootAt());
    }

    /**
     * Returns info about the state of this {@linkcode TheAlgorithm} instance.
     * @private
     * @returns {Record<string, unknown>} Status dictionary.
     */
    private statusDict(): Record<string, unknown> {
        const mostRecentTootAt = this.mostRecentHomeTootAt();
        const oldestTootAt = earliestTootedAt(this.homeFeed);
        let numHoursInHomeFeed: number | null = null;

        if (mostRecentTootAt && oldestTootAt) {
            numHoursInHomeFeed = AgeIn.hours(oldestTootAt, mostRecentTootAt);
        }

        return {
            feedNumToots: this.feed.length,
            homeFeedNumToots: this.homeFeed.length,
            homeFeedMostRecentAt: toISOFormatIfExists(mostRecentTootAt),
            homeFeedOldestAt: toISOFormatIfExists(oldestTootAt),
            homeFeedTimespanHours: numHoursInHomeFeed ? Number(numHoursInHomeFeed.toPrecision(2)) : null,
            isLoading: this.isLoading,
            loadingStatus: this.loadingStatus,
            loadStartedAt: toISOFormatIfExists(this.loadStartedAt),
            minMaxScores: computeMinMax(this.feed, (toot) => toot.score),
        };
    }

    ///////////////////////////////
    //      Static Methods       //
    ///////////////////////////////

    /** True if {@linkcode FEDIALGO_DEBUG} environment var was set at run time. */
    static get isDebugMode(): boolean { return isDebugMode };
    /** True if {@linkcode FEDIALGO_DEEP_DEBUG} environment var was set at run time. */
    static get isDeepDebug(): boolean { return isDeepDebug };
    /** True if {@linkcode LOAD_TEST} environment var was set at run time. */
    static get isLoadTest(): boolean { return isLoadTest };
    /** True if {@linkcode QUICK_MODE} environment var was set at run time. */
    static get isQuickMode(): boolean { return isQuickMode };

    /**
     * Dictionary of preset weight configurations that can be selected from to set weights.
     * @returns {WeightPresets}
     */
    static get weightPresets(): WeightPresets { return WEIGHT_PRESETS };
};


// Some strings we want to export from the config
const GET_FEED_BUSY_MSG = config.locale.messages[LoadAction.IS_BUSY];
const READY_TO_LOAD_MSG = config.locale.messages[LogAction.INITIAL_LOADING_STATUS];

// Export types and constants needed by apps using this package
export {
    // Constants
    DEFAULT_FONT_SIZE,
    FILTER_OPTION_DATA_SOURCES,
    FEDIALGO,
    GET_FEED_BUSY_MSG,
    GIFV,
    READY_TO_LOAD_MSG,
    VIDEO_TYPES,
    // Classes
    Account,
    BooleanFilter,
    Logger,
    NumericFilter,
    TagList,
    Toot,
    // Enums
    BooleanFilterName,
    MediaCategory,
    NonScoreWeightName,
    ScoreName,
    TagTootsCategory,
    TrendingType,
    TypeFilterName,
    WeightName,
    // Helpers
    AgeIn,
    extractDomain,
    isAccessTokenRevokedError,
    isValueInStringEnum,
    makeChunks,
    makePercentileChunks,  // TODO: unused in demo app (for now)
    optionalSuffix,
    sleep,
    sortKeysByValue,
    timeString,
    // Types
    type BooleanFilterOption,
    type FeedFilterSettings,
    type FilterOptionDataSource,
    type KeysOfValueType,
    type MastodonInstance,
    type MinMaxAvgScore,
    type ObjList,
    type ScoreStats,
    type StringNumberDict,
    type TagWithUsageCounts,
    type TrendingData,
    type TrendingLink,
    type TrendingObj,
    type TrendingWithHistory,
    type Weights,
};