import { VirtualScroll } from '@web/base';

export const createScrollManager = (container, state, onRenderedListChange) => {
    const vs = VirtualScroll();
    let scrollEl = null;

    const refresh = () => {
        if (!scrollEl) return;
        const virtualContainer = { 
            clientHeight: scrollEl.clientHeight * 1.6, 
            scrollTop: scrollEl.scrollTop 
        };
        const res = vs.calc(virtualContainer, state.list);
        if (res) {
            Object.assign(state, {
                prevHeight: res.prevHeight,
                postHeight: res.postHeight,
                _listStartIndex: res.listStartIndex,
                _renderedList: res.renderedList
            });
            onRenderedListChange?.(res.renderedList.length);
        }
    };

    return {
        init: () => {
            scrollEl = container.querySelector('.dt-main');
        },
        reset: (list) => {
            state._listStartIndex = 0;
            state._renderedList = vs.reset(list, scrollEl || container) || [];
            
            // Performance Fix: Seed the VirtualScroll average height to 40px BEFORE init().
            // This entirely eliminates the need for onItemUpdate (which causes layout thrashing
            // during fast scrolls) while still perfectly fixing the "row 70 jump" bug caused
            // by VirtualScroll's 32px default assumption.
            const mockNode = document.createElement('div');
            mockNode.style.marginTop = '0px';
            mockNode.style.marginBottom = '0px';
            Object.defineProperty(mockNode, 'offsetHeight', { value: 40 });
            document.body.appendChild(mockNode);
            vs.update(0, mockNode);
            document.body.removeChild(mockNode);

            if (state.list === list) {
                vs.init(list, refresh);
            }
        },
        refresh,
        onScroll: refresh
    };
};