import React, { useRef, useEffect, useState, useCallback } from 'react';
import { motion } from 'framer-motion';

interface VirtualListProps<T> {
  items: T[];
  renderItem: (item: T, index: number) => React.ReactNode;
  itemHeight: number;
  containerHeight: number;
  overscan?: number;
  className?: string;
  onEndReached?: () => void;
  endReachedThreshold?: number;
}

export function VirtualList<T>({
  items,
  renderItem,
  itemHeight,
  containerHeight,
  overscan = 5,
  className = '',
  onEndReached,
  endReachedThreshold = 200,
}: VirtualListProps<T>) {
  const containerRef = useRef<HTMLDivElement>(null);
  const [scrollTop, setScrollTop] = useState(0);
  const [isScrolling, setIsScrolling] = useState(false);
  const scrollTimeoutRef = useRef<ReturnType<typeof setTimeout> | null>(null);

  const totalHeight = items.length * itemHeight;
  const visibleCount = Math.ceil(containerHeight / itemHeight);
  const startIndex = Math.max(0, Math.floor(scrollTop / itemHeight) - overscan);
  const endIndex = Math.min(
    items.length,
    Math.ceil((scrollTop + containerHeight) / itemHeight) + overscan
  );

  const visibleItems = items.slice(startIndex, endIndex);
  const offsetY = startIndex * itemHeight;

  const handleScroll = useCallback(
    (e: React.UIEvent<HTMLDivElement>) => {
      const newScrollTop = e.currentTarget.scrollTop;
      setScrollTop(newScrollTop);
      setIsScrolling(true);

      // Clear previous timeout
      if (scrollTimeoutRef.current) {
        clearTimeout(scrollTimeoutRef.current);
      }

      // Set new timeout to detect scroll end
      scrollTimeoutRef.current = setTimeout(() => {
        setIsScrolling(false);
      }, 150);

      // Check if end is reached
      if (onEndReached) {
        const scrollHeight = e.currentTarget.scrollHeight;
        const clientHeight = e.currentTarget.clientHeight;
        const scrollBottom = scrollHeight - newScrollTop - clientHeight;

        if (scrollBottom < endReachedThreshold) {
          onEndReached();
        }
      }
    },
    [onEndReached, endReachedThreshold]
  );

  useEffect(() => {
    return () => {
      if (scrollTimeoutRef.current) {
        clearTimeout(scrollTimeoutRef.current);
      }
    };
  }, []);

  return (
    <div
      ref={containerRef}
      className={`overflow-auto ${className}`}
      style={{ height: containerHeight }}
      onScroll={handleScroll}
    >
      <div style={{ height: totalHeight, position: 'relative' }}>
        <div
          style={{
            position: 'absolute',
            top: offsetY,
            left: 0,
            right: 0,
          }}
        >
          {visibleItems.map((item, index) => {
            const actualIndex = startIndex + index;
            return (
              <motion.div
                key={actualIndex}
                initial={isScrolling ? false : { opacity: 0, y: 10 }}
                animate={{ opacity: 1, y: 0 }}
                transition={{ duration: 0.2, delay: index * 0.02 }}
                style={{ height: itemHeight }}
              >
                {renderItem(item, actualIndex)}
              </motion.div>
            );
          })}
        </div>
      </div>
    </div>
  );
}

// 简化版虚拟列表（用于卡片布局）
interface VirtualGridProps<T> {
  items: T[];
  renderItem: (item: T, index: number) => React.ReactNode;
  itemHeight: number;
  containerHeight: number;
  columns: number;
  gap?: number;
  className?: string;
}

export function VirtualGrid<T>({
  items,
  renderItem,
  itemHeight,
  containerHeight,
  columns,
  gap = 16,
  className = '',
}: VirtualGridProps<T>) {
  const containerRef = useRef<HTMLDivElement>(null);
  const [scrollTop, setScrollTop] = useState(0);

  const rowHeight = itemHeight + gap;
  const totalRows = Math.ceil(items.length / columns);
  const totalHeight = totalRows * rowHeight;
  const visibleCount = Math.ceil(containerHeight / rowHeight);
  const startRow = Math.max(0, Math.floor(scrollTop / rowHeight) - 2);
  const endRow = Math.min(
    totalRows,
    Math.ceil((scrollTop + containerHeight) / rowHeight) + 2
  );

  const startIndex = startRow * columns;
  const endIndex = Math.min(items.length, endRow * columns);
  const visibleItems = items.slice(startIndex, endIndex);
  const offsetY = startRow * rowHeight;

  const handleScroll = useCallback((e: React.UIEvent<HTMLDivElement>) => {
    setScrollTop(e.currentTarget.scrollTop);
  }, []);

  return (
    <div
      ref={containerRef}
      className={`overflow-auto ${className}`}
      style={{ height: containerHeight }}
      onScroll={handleScroll}
    >
      <div style={{ height: totalHeight, position: 'relative' }}>
        <div
          style={{
            position: 'absolute',
            top: offsetY,
            left: 0,
            right: 0,
            display: 'grid',
            gridTemplateColumns: `repeat(${columns}, 1fr)`,
            gap: `${gap}px`,
          }}
        >
          {visibleItems.map((item, index) => {
            const actualIndex = startIndex + index;
            return (
              <div key={actualIndex} style={{ height: itemHeight }}>
                {renderItem(item, actualIndex)}
              </div>
            );
          })}
        </div>
      </div>
    </div>
  );
}

// 懒加载图片组件
interface LazyImageProps {
  src: string;
  alt: string;
  className?: string;
  placeholder?: React.ReactNode;
  onLoad?: () => void;
  onError?: () => void;
}

export function LazyImage({
  src,
  alt,
  className = '',
  placeholder,
  onLoad,
  onError,
}: LazyImageProps) {
  const [isLoaded, setIsLoaded] = useState(false);
  const [isInView, setIsInView] = useState(false);
  const imgRef = useRef<HTMLImageElement>(null);

  useEffect(() => {
    const observer = new IntersectionObserver(
      ([entry]) => {
        if (entry.isIntersecting) {
          setIsInView(true);
          observer.disconnect();
        }
      },
      { rootMargin: '50px' }
    );

    if (imgRef.current) {
      observer.observe(imgRef.current);
    }

    return () => observer.disconnect();
  }, []);

  const handleLoad = () => {
    setIsLoaded(true);
    onLoad?.();
  };

  const handleError = () => {
    onError?.();
  };

  return (
    <div ref={imgRef} className={`relative ${className}`}>
      {!isLoaded && placeholder}
      {isInView && (
        <img
          src={src}
          alt={alt}
          className={`${className} ${isLoaded ? 'opacity-100' : 'opacity-0'}`}
          style={{ transition: 'opacity 0.3s' }}
          onLoad={handleLoad}
          onError={handleError}
        />
      )}
    </div>
  );
}

// 懒加载组件包装器
interface LazyComponentProps {
  children: React.ReactNode;
  fallback?: React.ReactNode;
  threshold?: number;
}

export function LazyComponent({
  children,
  fallback = null,
  threshold = 0,
}: LazyComponentProps) {
  const [isVisible, setIsVisible] = useState(false);
  const ref = useRef<HTMLDivElement>(null);

  useEffect(() => {
    const observer = new IntersectionObserver(
      ([entry]) => {
        if (entry.isIntersecting) {
          setIsVisible(true);
          observer.disconnect();
        }
      },
      { threshold }
    );

    if (ref.current) {
      observer.observe(ref.current);
    }

    return () => observer.disconnect();
  }, [threshold]);

  return (
    <div ref={ref}>
      {isVisible ? children : fallback}
    </div>
  );
}