export interface IAttributeMap {
	[key: string]: string | null;
}

export function compose(
	a: IAttributeMap = {},
	b: IAttributeMap = {},
	keepNull: boolean
): IAttributeMap | undefined {
	if (typeof a !== 'object') {
		a = {};
	}
	if (typeof b !== 'object') {
		b = {};
	}
	let attributes = { ...b };
	if (!keepNull) {
		attributes = Object.keys(attributes).reduce<IAttributeMap>((copy, key) => {
			if (attributes[key] != null) {
				copy[key] = attributes[key];
			}
			return copy;
		}, {});
	}
	for (const key in a) {
		if (a[key] !== undefined && b[key] === undefined) {
			attributes[key] = a[key];
		}
	}
	return Object.keys(attributes).length > 0 ? attributes : undefined;
}

export function diff(
	a: IAttributeMap = {},
	b: IAttributeMap = {}
): IAttributeMap | undefined {
	if (typeof a !== 'object') {
		a = {};
	}
	if (typeof b !== 'object') {
		b = {};
	}
	const attributes = Object.keys(a)
		.concat(Object.keys(b))
		.reduce<IAttributeMap>((attrs, key) => {
			if (a[key] !== b[key]) {
				attrs[key] = b[key] === undefined ? null : b[key];
			}
			return attrs;
		}, {});
	return Object.keys(attributes).length > 0 ? attributes : undefined;
}

export function invert(attr: IAttributeMap = {}, base: IAttributeMap = {}) {
	attr = attr || {};
	const baseInverted = Object.keys(base).reduce<IAttributeMap>((memo, key) => {
		if (base[key] !== attr[key] && attr[key] !== undefined) {
			memo[key] = base[key];
		}
		return memo;
	}, {});
	return Object.keys(attr).reduce<IAttributeMap>((memo, key) => {
		if (attr[key] !== base[key] && base[key] === undefined) {
			memo[key] = null;
		}
		return memo;
	}, baseInverted);
}

export function transform(
	a: IAttributeMap | undefined,
	b: IAttributeMap | undefined,
	priority: boolean = false
): IAttributeMap | undefined {
	if (typeof a !== 'object') {
		return b;
	}
	if (typeof b !== 'object') {
		return undefined;
	}
	if (!priority) {
		return b; // b simply overwrites us without priority
	}
	const attributes = Object.keys(b).reduce<IAttributeMap>((attrs, key) => {
		if (a[key] === undefined) {
			attrs[key] = b[key]; // null is a valid value
		}
		return attrs;
	}, {});
	return Object.keys(attributes).length > 0 ? attributes : undefined;
}