import { ref, watch, onUnmounted, type Ref } from 'vue';

export function useAutoScroll(elementRef: Ref<HTMLElement | null>) {
    const userIsScrollingUp = ref(false);
    const THRESHOLD = 50;

    const isAtBottom = () => {
        const el = elementRef.value;
        if (!el) return false;
        const distanceToBottom = el.scrollHeight - el.scrollTop - el.clientHeight;
        return distanceToBottom <= THRESHOLD;
    };

    const scrollToBottom = (behavior: ScrollBehavior = 'auto') => {
        const el = elementRef.value;
        if (!el) return;
        el.scrollTo({
            top: el.scrollHeight,
            behavior,
        });
    };

    const handleScroll = () => {
        const el = elementRef.value;
        if (!el) return;

        userIsScrollingUp.value = !isAtBottom();
    };

    let observer: MutationObserver | null = null;

    watch(elementRef, (newEl, oldEl) => {
        if (oldEl) {
            oldEl.removeEventListener('scroll', handleScroll);
            observer?.disconnect();
        }

        if (newEl) {
            newEl.addEventListener('scroll', handleScroll, { passive: true });

            observer = new MutationObserver(() => {
                if (!userIsScrollingUp.value) {
                    scrollToBottom();
                }
            });

            observer.observe(newEl, {
                childList: true,
                subtree: true,
                characterData: true
            });
        }
    });

    onUnmounted(() => {
        elementRef.value?.removeEventListener('scroll', handleScroll);
        observer?.disconnect();
    });

    return {
        userIsScrollingUp,
        scrollToBottom,
    };
}