"use strict";

const Promise = require("bluebird");
const objectFromEntries = require("object.fromentries");
const util = require("util");

function resolveFromDataSource(dataContext, dataSource, id) {
	if (dataContext[dataSource] != null) {
		return dataContext[dataSource].load(id);
	} else {
		throw new Error(`Specified data source '${dataSource}' does not exist`);
	}
}

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(() => {
			return resolveFromDataSource(data, dataSource, id);
		}).then((value) => {
			if (value != null) {
				// FIXME: Inject 'properties'
				return callback(value, args, context);
			} else {
				// QUESTION: Why do we disallow this again?
				throw new Error(`Got a null-ish value from data source '${dataSource}' for ID '${util.inspect(id)}'`);
			}
		});
	};
}

function withDynamicHandler(handler, object) {
	return function (args, context) {
		let { data } = context;

		function resolveProperty(property, fromObject = object) {
			if (typeof fromObject[property] !== "function") {
				throw new Error(`FIXME: Properties can apparently be non-functions`);
			}

			return fromObject[property](args, context);
		}

		let extendedContext = {
			... context,
			resolveProperty: resolveProperty,
			resolveProperties: function (properties, fromObject) {
				return Promise.map(properties, (property) => {
					return Promise.try(() => {
						return resolveProperty(property, fromObject);
					}).then((value) => {
						return [ property, value ];
					});
				}).then((entries) => {
					return objectFromEntries(entries);
				});
			},
			resolvePropertyPath: function (propertyPath, fromObject) {
				let initialObject = fromObject ?? object;

				return Promise.reduce(propertyPath, (last, property) => {
					if (last != null) {
						return resolveProperty(property, last);
					}
				}, initialObject);	
			},
			resolveDataSource: function (dataSource, id) {
				return resolveFromDataSource(data, dataSource, id);
			}
		};

		return handler(args, extendedContext);
	};
}

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

module.exports = {
	ID: ID,
	Dynamic: Dynamic,
	LocalProperties: LocalProperties,
	createDataObject: function createDataObject(mappings) {
		let object = {};
		
		if (mappings[LocalProperties] != null) {
			Object.assign(object, mappings[LocalProperties]);
		}

		if (mappings[Dynamic] != null) {
			for (let [property, handler] of Object.entries(mappings[Dynamic])) {
				object[property] = withDynamicHandler(handler, object);
			}
		}
	
		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 (object[property] == null) {
						if (typeof source === "string") {
							object[property] = withProperty(dataSource, id, source);
						} else if (typeof source === "function") {
							object[property] = withData(dataSource, id, source);
						} /* FIXME: else */
					} else {
						throw new Error(`Handler already defined for property '${property}' - maybe you specified it twice for different data sources?`);
					}
				}
			} else {
				throw new Error(`No object ID was provided for the '${dataSource}' data source`);
			}
		}

		return object;
	}
};