"use strict";

const Promise = require("bluebird");

function withProperty(dataSource, id, property) {
	return withData(dataSource, id, (value) => {
		return value[property];
	});
}

function withData(dataSource, id, callback) {
	return function (args, context) {
		let {data} = context;

		return Promise.try(() => {
			if (data[dataSource] != null) {
				return data[dataSource].load(id);
			} else {
				throw new Error(`Specified data source '${dataSource}' does not exist`);
			}
		}).then((value) => {
			if (value != null) {
				return callback(value, args, context);
			} else {
				throw new Error(`Got a null value from data source '${dataSource}' for ID '${id}'`);
			}
		});
	};
}

let ID = Symbol("ID");
let LocalProperties = Symbol("localProperties");

module.exports = {
	ID: ID,
	LocalProperties: LocalProperties,
	createDataObject: function createDataObject(mappings) {
		let object = {};
	
		if (mappings[LocalProperties] != null) {
			Object.assign(object, mappings[LocalProperties]);
		}
	
		for (let [dataSource, items] of Object.entries(mappings)) {
			if (items[ID] != null) {
				let id = items[ID];
	
				for (let [property, source] of Object.entries(items)) {
					if (typeof source === "string") {
						object[property] = withProperty(dataSource, id, source);
					} else if (typeof source === "function") {
						object[property] = withData(dataSource, id, source);
					}
				}
			} else {
				throw new Error(`No object ID was provided for the '${dataSource}' data source`);
			}
		}
	
		return object;
	}
};