"use client";

import { Children, useEffect, useRef, type ReactNode, type RefObject } from "react";
import { useReducedMotion } from "motion/react";

type ScrollRevealProps = {
  /** Each direct child reveals on its own. */
  children: ReactNode;
  /** Side each child slides in from. */
  from?: "bottom" | "top" | "left" | "right";
  /** Pixels each child travels. */
  distance?: number;
  /** Seconds one child takes. */
  duration?: number;
  /** Seconds between children that come into view together. */
  stagger?: number;
  /** Seconds before the first child starts. */
  delay?: number;
  /** How much of a child must show before it starts, 0 to 1. */
  amount?: number;
  /** False hides children again when they leave, so they replay. */
  once?: boolean;
  /** Scrolling box that holds this. Leave out to use the page. */
  root?: RefObject<HTMLElement | null>;
  /** On the wrapper, so a grid or flex layout goes here. */
  className?: string;
};

const SHIFT = { bottom: [0, 1], top: [0, -1], left: [-1, 0], right: [1, 0] };

/* ScrollReveal: children slide and fade in as they scroll into view. Those arriving together go one after another. */
export function ScrollReveal({
  children,
  from = "bottom",
  distance = 28,
  duration = 0.6,
  stagger = 0.08,
  delay = 0,
  amount = 0.25,
  once = true,
  root,
  className = "",
}: ScrollRevealProps) {
  const reduce = useReducedMotion();
  const box = useRef<HTMLDivElement>(null);
  const count = Children.count(children);
  const [dx, dy] = SHIFT[from];
  const hidden = `translate3d(${dx * distance}px, ${dy * distance}px, 0)`;

  // The outer slot is watched and the inner layer moves, so the shift cannot change what is seen.
  useEffect(() => {
    if (!box.current) return;
    const inner = (slot: Element) => slot.firstElementChild as HTMLElement;
    const slots = [...box.current.children];
    if (reduce) slots.forEach((slot) => (inner(slot).style.transform = "none"));

    const observer = new IntersectionObserver(
      (entries) => {
        let order = 0;
        for (const { target, isIntersecting } of entries) {
          const el = inner(target);
          if (isIntersecting) {
            el.style.transitionDelay = `${delay + order++ * stagger}s`;
            el.style.opacity = "1";
            el.style.transform = "none";
            if (once) observer.unobserve(target);
          } else if (!once) {
            el.style.transitionDelay = "0s";
            el.style.opacity = "0";
            if (!reduce) el.style.transform = hidden;
          }
        }
      },
      { root: root?.current, threshold: amount },
    );
    slots.forEach((slot) => observer.observe(slot));
    return () => observer.disconnect();
  }, [reduce, hidden, stagger, delay, amount, once, root, count]);

  return (
    <div ref={box} className={className}>
      {Children.map(children, (child) => (
        <div>
          <div
            className="h-full"
            style={{
              opacity: 0,
              transform: hidden,
              transition: `opacity ${duration}s ease-out, transform ${duration}s cubic-bezier(0.22, 1, 0.36, 1)`,
            }}
          >
            {child}
          </div>
        </div>
      ))}
    </div>
  );
}