"use client";

import { useEffect, useId, useRef, useState, type ElementType, type KeyboardEvent, type ReactNode } from "react";
import { AnimatePresence, motion, useReducedMotion } from "motion/react";

export type FAQMorphItem = {
  question: string;
  answer: ReactNode;
};

type FAQMorphProps = {
  items: FAQMorphItem[];
  title?: string;
  /** Small line above the title. */
  eyebrow?: string;
  subtitle?: string;
  /** Which question starts open. */
  defaultOpen?: number;
  /** Colour of the marker and the answer panel. */
  tile?: string;
  /** Colour of the number in the answer panel. */
  color?: string;
  /** h2 by default. */
  headingAs?: ElementType;
  /** Set padding and colours here. */
  className?: string;
};

const SPRING = { type: "spring", stiffness: 380, damping: 34 } as const;

/* FAQMorph: questions on one side, one answer panel on the other. The marker slides to the question and the panel reshapes to fit the answer. Narrow, it turns into a plain accordion. */
export function FAQMorph({
  items,
  title,
  eyebrow,
  subtitle,
  defaultOpen = 0,
  tile = "#15122a",
  color = "#a78bfa",
  headingAs: Heading = "h2",
  className = "bg-[#0b0a14] px-5 py-16 text-white",
}: FAQMorphProps) {
  const reduce = useReducedMotion();
  const id = useId();
  const side = useRef<HTMLDivElement>(null);
  const inner = useRef<HTMLDivElement>(null);
  const [open, setOpen] = useState<number | null>(defaultOpen);
  const [shown, setShown] = useState(defaultOpen);
  const [height, setHeight] = useState<number>();
  const answer = items[shown];

  useEffect(() => {
    if (!inner.current) return;
    const watcher = new ResizeObserver(([entry]) => {
      const next = entry.target.getBoundingClientRect().height;
      if (next) setHeight(next);
    });
    watcher.observe(inner.current);
    return () => watcher.disconnect();
  }, []);

  const pick = (i: number) => {
    // With the side panel showing there is always one answer; only the narrow accordion can close.
    if (open === i && !side.current?.offsetWidth) return setOpen(null);
    setOpen(i);
    setShown(i);
  };

  const step = (e: KeyboardEvent<HTMLUListElement>) => {
    if (e.key !== "ArrowDown" && e.key !== "ArrowUp") return;
    const buttons = Array.from(e.currentTarget.querySelectorAll("button"));
    const at = buttons.indexOf(e.target as HTMLButtonElement);
    if (at < 0) return;
    e.preventDefault();
    buttons[(at + (e.key === "ArrowDown" ? 1 : -1) + buttons.length) % buttons.length].focus();
  };

  return (
    <section aria-labelledby={title ? id : undefined} className={`@container ${className}`}>
      {title && (
        <header className="mx-auto mb-8 max-w-2xl text-center">
          {eyebrow && <p className="mb-2 text-xs font-medium tracking-[0.2em] uppercase opacity-70">{eyebrow}</p>}
          <Heading id={id} className="text-[clamp(1.5rem,6cqw,2.5rem)] leading-tight font-semibold tracking-tight text-balance">
            {title}
          </Heading>
          {subtitle && <p className="mt-3 text-pretty opacity-70">{subtitle}</p>}
        </header>
      )}
      <div className="mx-auto grid max-w-5xl items-start gap-4 @3xl:grid-cols-[minmax(0,2fr)_minmax(0,3fr)] @3xl:gap-8">
        <ul className="grid gap-1" onKeyDown={step}>
          {items.map((item, i) => {
            const on = open === i;
            return (
              <li key={item.question}>
                <div className="relative">
                  {on && (
                    <motion.span
                      layoutId={`${id}-mark`}
                      className="absolute inset-0 rounded-xl"
                      style={{ backgroundColor: tile }}
                      transition={reduce ? { duration: 0 } : SPRING}
                    />
                  )}
                  <button
                    type="button"
                    id={`${id}-q${i}`}
                    aria-expanded={on}
                    aria-controls={`${id}-a${i}`}
                    onClick={() => pick(i)}
                    className={`relative flex w-full items-center justify-between gap-4 rounded-xl px-4 py-3 text-left text-sm font-medium transition-opacity ${on ? "" : "opacity-70 hover:opacity-100"}`}
                  >
                    <span>{item.question}</span>
                    <span aria-hidden className="relative size-3 shrink-0">
                      <span className="absolute inset-x-0 top-[5px] h-0.5 rounded bg-current" />
                      <span className={`absolute inset-x-0 top-[5px] h-0.5 rounded bg-current transition-transform duration-300 motion-reduce:transition-none ${on ? "" : "rotate-90"}`} />
                    </span>
                  </button>
                </div>
                <AnimatePresence initial={false}>
                  {on && (
                    <motion.div
                      id={`${id}-a${i}`}
                      role="region"
                      aria-labelledby={`${id}-q${i}`}
                      className="overflow-hidden @3xl:hidden"
                      initial={reduce ? false : { height: 0, opacity: 0 }}
                      animate={{ height: "auto", opacity: 1 }}
                      exit={reduce ? { height: 0, transition: { duration: 0 } } : { height: 0, opacity: 0 }}
                      transition={{ duration: 0.3, ease: [0.16, 1, 0.3, 1] }}
                    >
                      <div className="px-4 pt-2 pb-4 text-sm text-pretty opacity-80">{item.answer}</div>
                    </motion.div>
                  )}
                </AnimatePresence>
              </li>
            );
          })}
        </ul>

        <motion.div
          ref={side}
          aria-live="polite"
          className="hidden overflow-hidden rounded-2xl @3xl:block"
          style={{ backgroundColor: tile }}
          initial={false}
          animate={height ? { height } : undefined}
          transition={reduce ? { duration: 0 } : SPRING}
        >
          <div ref={inner} className="p-6">
            <AnimatePresence mode="wait" initial={false}>
              {answer && (
                <motion.div
                  key={shown}
                  initial={reduce ? false : { opacity: 0, y: 8 }}
                  animate={{ opacity: 1, y: 0 }}
                  exit={reduce ? { opacity: 0, transition: { duration: 0 } } : { opacity: 0, y: -8 }}
                  transition={{ duration: 0.18 }}
                >
                  <p aria-hidden className="text-3xl font-semibold tabular-nums" style={{ color }}>
                    {String(shown + 1).padStart(2, "0")}
                  </p>
                  <p className="mt-2 font-semibold text-balance">{answer.question}</p>
                  <div className="mt-2 text-sm leading-relaxed text-pretty opacity-80">{answer.answer}</div>
                </motion.div>
              )}
            </AnimatePresence>
          </div>
        </motion.div>
      </div>
    </section>
  );
}