// Minimal drag-to-reorder for vertical lists. Works with mouse, touch and pen
// via pointer events; the handle also supports ArrowUp/ArrowDown.

/**
 * @param {HTMLElement} container  element whose children are sortable
 * @param {object} opts
 * @param {string} opts.handle      selector for the drag handle inside a child
 * @param {string} opts.placeholderClass  classes for the gap left while dragging
 * @param {(el: HTMLElement, from: number, to: number) => void} opts.onMove
 */
export function sortable(container, { handle = ".drag-handle", placeholderClass = "is-placeholder", onMove }) {
  const children = () => [...container.children];

  container.addEventListener("pointerdown", (e) => {
    const hdl = e.target.closest(handle);
    if (!hdl || !container.contains(hdl) || (e.pointerType === "mouse" && e.button !== 0)) return;
    const el = [...container.children].find((c) => c.contains(hdl));
    if (!el) return;
    e.preventDefault();

    const from = children().indexOf(el);
    const rect = el.getBoundingClientRect();
    const grabOffset = e.clientY - rect.top;
    let pointerY = e.clientY;

    const ph = el.cloneNode(true);
    ph.className = el.className + " " + placeholderClass;
    ph.removeAttribute("id");
    ph.setAttribute("aria-hidden", "true");
    ph.style.height = rect.height + "px";
    el.after(ph);

    Object.assign(el.style, {
      position: "fixed",
      left: rect.left + "px",
      top: rect.top + "px",
      width: rect.width + "px",
      zIndex: "50",
      pointerEvents: "none",
    });
    el.classList.add("is-dragging");
    document.documentElement.classList.add("dragging-active");
    hdl.setPointerCapture(e.pointerId);

    const place = () => {
      el.style.top = pointerY - grabOffset + "px";
      const others = children().filter((c) => c !== el && c !== ph);
      let before = null;
      for (const c of others) {
        const r = c.getBoundingClientRect();
        if (pointerY < r.top + r.height / 2) { before = c; break; }
      }
      if (before) {
        if (ph.nextElementSibling !== before) container.insertBefore(ph, before);
      } else if (container.lastElementChild !== ph) {
        container.append(ph);
      }
    };

    // Scroll the page when dragging near the top or bottom edge.
    let raf = 0;
    const autoscroll = () => {
      const edge = 70;
      let dy = 0;
      if (pointerY < edge + 60) dy = -Math.ceil((edge + 60 - pointerY) / 6);
      else if (pointerY > innerHeight - edge) dy = Math.ceil((pointerY - (innerHeight - edge)) / 6);
      if (dy) { scrollBy(0, dy); place(); }
      raf = requestAnimationFrame(autoscroll);
    };
    raf = requestAnimationFrame(autoscroll);

    const move = (ev) => { pointerY = ev.clientY; place(); };
    const end = () => {
      cancelAnimationFrame(raf);
      hdl.removeEventListener("pointermove", move);
      hdl.removeEventListener("pointerup", end);
      hdl.removeEventListener("pointercancel", end);
      ph.replaceWith(el);
      el.removeAttribute("style");
      el.classList.remove("is-dragging");
      document.documentElement.classList.remove("dragging-active");
      const to = children().indexOf(el);
      if (to !== from) onMove(el, from, to);
    };
    hdl.addEventListener("pointermove", move);
    hdl.addEventListener("pointerup", end);
    hdl.addEventListener("pointercancel", end);
  });

  container.addEventListener("keydown", (e) => {
    if (e.key !== "ArrowUp" && e.key !== "ArrowDown") return;
    const hdl = e.target.closest(handle);
    if (!hdl) return;
    const el = children().find((c) => c.contains(hdl));
    const from = children().indexOf(el);
    const to = e.key === "ArrowUp" ? from - 1 : from + 1;
    if (to < 0 || to >= children().length) return;
    e.preventDefault();
    if (e.key === "ArrowUp") el.previousElementSibling.before(el);
    else el.nextElementSibling.after(el);
    hdl.focus();
    onMove(el, from, to);
  });
}