/**
 * Map an IntersectionHandler callback to an element. We only ever make one handler for one
 * element, so even though these handlers might all be triggered by different
 * observers, we can keep them in the same map.
 */
const observerCallbacks = new WeakMap();
/**
 * Multiple observers can be created for multiple element/document roots. Each with
 * different settings. So here we store dictionaries of observers to each root,
 * using serialised settings (threshold/margin) as lookup keys.
 */
const observers = new WeakMap();
const fireObserverCallback = (entry) => {
    const callback = observerCallbacks.get(entry.target);
    callback && callback(entry);
};
const fireAllObserverCallbacks = (entries) => {
    entries.forEach(fireObserverCallback);
};
function initIntersectionObserver({ root, ...options }) {
    const lookupRoot = root || document;
    /**
     * If we don't have an observer lookup map for this root, create one.
     */
    if (!observers.has(lookupRoot)) {
        observers.set(lookupRoot, {});
    }
    const rootObservers = observers.get(lookupRoot);
    const key = JSON.stringify(options);
    /**
     * If we don't have an observer for this combination of root and settings,
     * create one.
     */
    if (!rootObservers[key]) {
        rootObservers[key] = new IntersectionObserver(fireAllObserverCallbacks, { root, ...options });
    }
    return rootObservers[key];
}
function observeIntersection(element, options, callback) {
    const rootInteresectionObserver = initIntersectionObserver(options);
    observerCallbacks.set(element, callback);
    rootInteresectionObserver.observe(element);
    return () => {
        observerCallbacks.delete(element);
        rootInteresectionObserver.unobserve(element);
    };
}

export { observeIntersection };