// @flow

import md5 from 'js-md5';

const logger = require('jitsi-meet-logger').getLogger(__filename);

/**
 * The name of the {@code localStorage} store where the app persists its values.
 */
const PERSISTED_STATE_NAME = 'jitsi-state';

/**
 * Mixed type of the element (subtree) config. If it's a {@code boolean} (and is
 * {@code true}), we persist the entire subtree. If it's an {@code Object}, we
 * perist a filtered subtree based on the properties of the config object.
 */
declare type ElementConfig = boolean | Object;

/**
 * The type of the name-config pairs stored in {@code PersistenceRegistry}.
 */
declare type PersistencyConfigMap = { [name: string]: ElementConfig };

/**
 * A registry to allow features to register their redux store subtree to be
 * persisted and also handles the persistency calls too.
 */
class PersistenceRegistry {
    _checksum: string;
    _defaultStates: { [name: string ]: ?Object} = {};
    _elements: PersistencyConfigMap = {};

    /**
     * Returns the persisted redux state. Takes the {@link #_elements} into
     * account as we may have persisted something in the past that we don't want
     * to retreive anymore. The next {@link #persistState} will remove such
     * values.
     *
     * @returns {Object}
     */
    getPersistedState() {
        let filteredPersistedState = {};

        // localStorage key per feature
        for (const subtreeName of Object.keys(this._elements)) {
            // Assumes that the persisted value is stored under the same key as
            // the feature's redux state name.
            // TODO We'll need to introduce functions later that can control the
            // persist key's name. Similar to control serialization and
            // deserialization. But that should be a straightforward change.
            const persistedSubtree
                = this._getPersistedSubtree(
                    subtreeName,
                    this._elements[subtreeName],
                    this._defaultStates[subtreeName]);

            if (persistedSubtree !== undefined) {
                filteredPersistedState[subtreeName] = persistedSubtree;
            }
        }

        // legacy
        if (Object.keys(filteredPersistedState).length === 0) {
            const { localStorage } = window;
            let persistedState = localStorage.getItem(PERSISTED_STATE_NAME);

            if (persistedState) {
                try {
                    persistedState = JSON.parse(persistedState);
                } catch (error) {
                    logger.error(
                        'Error parsing persisted state',
                        persistedState,
                        error);
                    persistedState = {};
                }

                filteredPersistedState = this._getFilteredState(persistedState);

                // Store into the new format and delete the old format so that
                // it's not used again.
                this.persistState(filteredPersistedState);
                localStorage.removeItem(PERSISTED_STATE_NAME);
            }
        }

        // Initialize the checksum.
        this._checksum = this._calculateChecksum(filteredPersistedState);

        logger.info('redux state rehydrated as', filteredPersistedState);

        return filteredPersistedState;
    }

    /**
     * Initiates a persist operation, but its execution will depend on the
     * current checksums (checks changes).
     *
     * @param {Object} state - The redux state.
     * @returns {void}
     */
    persistState(state: Object) {
        const filteredState = this._getFilteredState(state);
        const checksum = this._calculateChecksum(filteredState);

        if (checksum !== this._checksum) {
            for (const subtreeName of Object.keys(filteredState)) {
                try {
                    window.localStorage.setItem(
                        subtreeName,
                        JSON.stringify(filteredState[subtreeName]));
                } catch (error) {
                    logger.error(
                        'Error persisting redux subtree',
                        subtreeName,
                        filteredState[subtreeName],
                        error);
                }
            }
            logger.info(
                `redux state persisted. ${this._checksum} -> ${checksum}`);
            this._checksum = checksum;
        }
    }

    /**
     * Registers a new subtree config to be used for the persistency.
     *
     * @param {string} name - The name of the subtree the config belongs to.
     * @param {ElementConfig} config - The config {@code Object}, or
     * {@code boolean} if the entire subtree needs to be persisted.
     * @param {Object} defaultState - The default state of the component. If
     * it's provided, the rehydrated state will be merged with it before it gets
     * pushed into Redux.
     * @returns {void}
     */
    register(
            name: string,
            config?: ElementConfig = true,
            defaultState?: Object) {
        this._elements[name] = config;
        this._defaultStates[name] = defaultState;
    }

    /**
     * Calculates the checksum of a specific state.
     *
     * @param {Object} state - The redux state to calculate the checksum of.
     * @private
     * @returns {string} The checksum of the specified {@code state}.
     */
    _calculateChecksum(state: Object) {
        try {
            return md5.hex(JSON.stringify(state) || '');
        } catch (error) {
            logger.error('Error calculating checksum for state', state, error);

            return '';
        }
    }

    /**
     * Prepares a filtered state from the actual or the persisted redux state,
     * based on this registry.
     *
     * @param {Object} state - The actual or persisted redux state.
     * @private
     * @returns {Object}
     */
    _getFilteredState(state: Object) {
        const filteredState = {};

        for (const name of Object.keys(this._elements)) {
            if (state[name]) {
                filteredState[name]
                    = this._getFilteredSubtree(
                        state[name],
                        this._elements[name]);
            }
        }

        return filteredState;
    }

    /**
     * Prepares a filtered subtree based on the config for persisting or for
     * retrieval.
     *
     * @param {Object} subtree - The redux state subtree.
     * @param {ElementConfig} subtreeConfig - The related config.
     * @private
     * @returns {Object}
     */
    _getFilteredSubtree(subtree, subtreeConfig) {
        let filteredSubtree;

        if (typeof subtreeConfig === 'object') {
            // Only a filtered subtree gets persisted as specified by
            // subtreeConfig.
            filteredSubtree = {};
            for (const persistedKey of Object.keys(subtree)) {
                if (subtreeConfig[persistedKey]) {
                    filteredSubtree[persistedKey] = subtree[persistedKey];
                }
            }
        } else if (subtreeConfig) {
            // Persist the entire subtree.
            filteredSubtree = subtree;
        }

        return filteredSubtree;
    }

    /**
     * Retreives a persisted subtree from the storage.
     *
     * @param {string} subtreeName - The name of the subtree.
     * @param {Object} subtreeConfig - The config of the subtree from
     * {@link #_elements}.
     * @param {Object} subtreeDefaults - The defaults of the persisted subtree.
     * @private
     * @returns {Object}
     */
    _getPersistedSubtree(subtreeName, subtreeConfig, subtreeDefaults) {
        let persistedSubtree = window.localStorage.getItem(subtreeName);

        if (persistedSubtree) {
            try {
                persistedSubtree = JSON.parse(persistedSubtree);

                const filteredSubtree
                    = this._getFilteredSubtree(persistedSubtree, subtreeConfig);

                if (filteredSubtree !== undefined) {
                    return this._mergeDefaults(
                        filteredSubtree, subtreeDefaults);
                }
            } catch (error) {
                logger.error(
                    'Error parsing persisted subtree',
                    subtreeName,
                    persistedSubtree,
                    error);
            }
        }

        return undefined;
    }

    /**
     * Merges the persisted subtree with its defaults before rehydrating the
     * values.
     *
     * @private
     * @param {Object} subtree - The Redux subtree.
     * @param {?Object} defaults - The defaults, if any.
     * @returns {Object}
     */
    _mergeDefaults(subtree: Object, defaults: ?Object) {
        if (!defaults) {
            return subtree;
        }

        // If the subtree is an array, we don't need to merge it with the
        // defaults, because if it has a value, it will overwrite it, and if
        // it's undefined, it won't be even returned, and Redux will natively
        // use the default values instead.
        if (!Array.isArray(subtree)) {
            return {
                ...defaults,
                ...subtree
            };
        }
    }
}

export default new PersistenceRegistry();