import { useEffect, useRef, type CSSProperties } from 'react';

export interface ScrollProgressProps {
  /** Element to track. Defaults to `document.documentElement`. */
  target?: HTMLElement | null;
  /** Bar position. Default 'top'. */
  position?: 'top' | 'bottom';
  /** Bar height in pixels. Default 3. */
  thickness?: number;
  /** Bar fill color. Default `var(--bl-accent)`. */
  color?: string;
  className?: string;
  style?: CSSProperties;
}

/**
 * `<ScrollProgress>` — fixed progress bar reflecting scroll position
 * of a target (defaults to the document). Common chrome element on
 * long-form pages and onboarding flows.
 *
 * Updates the bar's `scaleX` transform directly via a ref so React
 * doesn't re-render on every scroll event.
 */
export function ScrollProgress({
  target,
  position = 'top',
  thickness = 3,
  color = 'var(--bl-accent, #6366f1)',
  className,
  style,
}: ScrollProgressProps) {
  const ref = useRef<HTMLDivElement>(null);

  useEffect(() => {
    const root =
      target ??
      (typeof document !== 'undefined' ? document.documentElement : null);
    if (!root) return;
    const update = () => {
      const el = ref.current;
      if (!el) return;
      const max = root.scrollHeight - root.clientHeight;
      const pct = max > 0 ? Math.min(1, root.scrollTop / max) : 0;
      el.style.transform = `scaleX(${pct})`;
    };
    update();
    const scrollEl = root === document.documentElement ? window : root;
    scrollEl.addEventListener('scroll', update, { passive: true });
    window.addEventListener('resize', update);
    return () => {
      scrollEl.removeEventListener('scroll', update);
      window.removeEventListener('resize', update);
    };
  }, [target]);

  return (
    <div
      data-testid="bl-scroll-progress"
      aria-hidden
      className={className}
      style={{
        position: 'fixed',
        left: 0,
        right: 0,
        height: thickness,
        zIndex: 100,
        background: 'transparent',
        pointerEvents: 'none',
        [position]: 0,
        ...style,
      }}
    >
      <div
        ref={ref}
        style={{
          height: '100%',
          width: '100%',
          background: color,
          transformOrigin: 'left center',
          transform: 'scaleX(0)',
          willChange: 'transform',
        }}
      />
    </div>
  );
}