/**
 * Configuration loader
 *
 *  Copyright (C) 2010-2019 R-T Specialty, LLC.
 *
 *  This file is part of the Liza Data Collection Framework.
 *
 *  liza is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

import { readFile } from "fs";
import { Store } from "../store/Store";


/**
 * Load system configuration from JSON
 *
 * @example
 *   ConfLoader( require( 'fs' ), SomeStore )
 *     .fromFile( 'conf/vanilla-server.json' )
 *     .then( conf => conf.get( 'foo' ) );
 *
 * TODO: Merging multiple configuration files would be convenient for
 * modular configuration.
 */
export class ConfLoader
{
    /**
     * Initialize with provided filesystem module and Store constructor
     *
     * The module should implement `#readFile` compatible with
     * Node.js'.  The Store constructor `store_ctor` is used to instantiate
     * new stores to be populated with configuration data.
     *
     * @param fs         - filesystem module
     * @param store_ctor - Store object constructor
     */
    constructor(
        private _fs:        { readFile: typeof readFile },
        private _storeCtor: () => Store,
    ) {}


    /**
     * Produce configuration from file
     *
     * A Store will be produced, populated with the configuration data.
     *
     * @param filename - path to configuration JSON
     *
     * @return a promise of a populated Store
     */
    fromFile( filename: string ): Promise<Store>
    {
        return new Promise( ( resolve, reject ) =>
        {
            this._fs.readFile( filename, 'utf8', ( err, data ) =>
            {
                if ( err )
                {
                    reject( err );
                    return;
                }

                try
                {
                    const store = this._storeCtor();

                    resolve(
                        this.parseConfData( data )
                            .then( parsed => store.populate( parsed ) )
                            .then( _ => store )
                    );
                }
                catch ( e )
                {
                    reject( e );
                }
            } );
        } );
    }


    /**
     * Parse raw configuration string
     *
     * Parses configuration string as JSON.
     *
     * @param data raw configuration data
     *
     * @return `data` parsed as JSON
     */
    protected parseConfData( data: string ): Promise<any>
    {
        return Promise.resolve( JSON.parse( data ) );
    }
}