// @flow
function monkeyPatch(name: string, parentObj: Object, patches: Object): ()=>void {
	const injId   = Symbol()
	const before  = patches["before"];
	const instead = patches["instead"];
	const after   = patches["after"];
	const handler = {
		apply: (target, thisArg, [ctx, args]) => {
			if (before !== undefined) before.apply(ctx, args);
			const res = (patches["instead"] !== undefined) ? instead.apply(ctx, [target.bind(ctx), ...args]) : target.apply(ctx, args)
			if (after === undefined) return res
			return after.apply(ctx, [res].concat(args));
		}
	};
	const prox = new Proxy(parentObj[name], handler);
	const orig = parentObj[name];
	parentObj[name] = function() {
		return prox(this, arguments);
	};
	const unpatch = () => {
		parentObj[name] = orig;
	}
	parentObj[injId] = {
		name:    name,
		orig:    orig,
		unpatch: unpatch
	};
	return unpatch;
}

function before(name: string, parentObj: Object, func: (...args: mixed[])=>void): ()=>void {
	return monkeyPatch(name, parentObj, {before: func})
}

function instead(name: string, parentObj: Object, func: (orig: (any)=>any, ...args: mixed[])=>any): ()=>void {
	return monkeyPatch(name, parentObj, {instead: func})
}

function after(name: string, parentObj: Object, func: (res: any, ...args: mixed[])=>any): ()=>void {
	return monkeyPatch(name, parentObj, {after: func})
}

module.exports = {
	monkeyPatch,
	before,
	instead,
	after
}