const ajax = (function() { "use strict";

class AjaxError extends Error {
	constructor (message, response, ...rest) {
		super(message, ...rest);
		this.response = response;
	}
}
class NetworkError extends AjaxError {}
class NotOkError extends AjaxError {}
class UnexpectedMimeError extends AjaxError {}
class InvalidJsonError extends AjaxError {}

const MIME_JSON = "application/json";


/**
 * Wrap a function so that it receives `this` as first argument.
 */
function thisToArg(f) {
	return function(...rest) {
		return f(this, ...rest);
	}
}

function xhrInitForPromise(resolve, reject, url, method, headers) {
	const t_resolve = thisToArg(resolve),
		t_reject = thisToArg(reject);

	let xhr = new XMLHttpRequest();
	xhr.addEventListener("load", t_resolve);
	xhr.addEventListener("error", t_resolve);
	xhr.addEventListener("abort", t_reject);
	xhr.addEventListener("timeout", t_reject);
	xhr.open(method, url);
	for (let header of Object.getOwnPropertyNames(headers)) {
		xhr.setRequestHeader(header, headers[header]);
	}
	return xhr;
}

function xhrParseJsonResponse(xhr) {
	xhr.responseJson = null;
	xhr.ok = false;

	if (xhr.responseText) {
		const contentType = xhr.getResponseHeader("Content-Type");
		if (contentType != MIME_JSON) {
			throw new UnexpectedMimeError(`Server did not reply with JSON but with ${contentType}`, xhr);
		}
		try {
			xhr.responseJson = JSON.parse(xhr.responseText);
		} catch(e) {
			throw new InvalidJsonError("Server replied with JSON that we couldn't parse", xhr);
		}
	}

	xhr.ok = 200 <= xhr.status && xhr.status < 300;
	if (!xhr.ok) {
		console.error(xhr);
		if (xhr.status === 0) {
			throw new NetworkError("Failed to connect to server. Developer console may have more information", xhr);
		} else {
			throw new NotOkError(xhr.statusText, xhr);
		}
	}

	return xhr;
}

function getJson(url, options={}) {
	if (!options.headers) options.headers = {};
	options.headers["Accept"] = MIME_JSON;

	return new Promise((resolve, reject) => {
		let xhr = xhrInitForPromise(resolve, reject, url, "GET", options.headers);
		xhr.send();
	}).then(xhrParseJsonResponse);
}

function postJson(url, data=undefined, options={}) {
	if (!options.headers) options.headers = {};
	options.headers["Content-Type"] = MIME_JSON;
	options.headers["Accept"] = MIME_JSON;

	return new Promise((resolve, reject) => {
		let xhr = xhrInitForPromise(resolve, reject, url, "POST", options.headers);
		if (data === undefined) {
			xhr.send();
		} else {
			xhr.send(JSON.stringify(data));
		}
	}).then(xhrParseJsonResponse);
}

return {
	NetworkError, NotOkError, UnexpectedMimeError, InvalidJsonError,
	getJson, postJson
};

})();