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

class NetworkError extends Error {}
class UnexpectedMimeError extends Error {}
class InvalidJsonError extends Error {}

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) {
	if (xhr.status == 0) {
		console.error(xhr);
		throw new NetworkError("Failed to connect to server");
	}

	if (!xhr.responseText) {
		const json = null;
	} else {
		const contentType = xhr.getResponseHeader("Content-Type");
		if (contentType != MIME_JSON) {
			throw new UnexpectedMimeError(`Server did not reply with JSON but with ${contentType}`);
		}
		try {
			const json = JSON.parse(xhr.responseText);
		} catch(e) {
			throw new InvalidJsonError();
		}
	}

	return {
		ok: 200 <= xhr.status && xhr.status < 300,
		status: xhr.status,
		statusText: xhr.statusText,
		getHeader: xhr.getResponseHeader,
		json,
		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={}, 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);
		xhr.send(JSON.stringify(data));
	}).then(xhrParseJsonResponse);
}

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

})();