import { useEffect, useRef, useState, type RefObject } from "react";

// The focus half of the dialog contract, in ONE place. `ui/Modal` owns it for every dialog that goes
// through the shell; the onboarding wizard renders its own shell (a bare `ui/Overlay`, because it is
// not a titled dialog with a header row) and used to have NONE of this — no trap, no initial focus,
// no restore — so a keyboard user tabbed straight out of the first-login wizard into the app behind
// the scrim, where the focus ring is invisible (deep-review round 5). A second hand-written copy is
// how two shells drift apart; this is the shared one.

// Tabbable elements inside the dialog, in DOM order. `:not([tabindex="-1"])` drops programmatically
// focusable-only nodes; the visibility filter (offsetParent) drops display:none ones (e.g. a
// collapsed disclosure) so the trap never lands focus on something the user can't see.
const FOCUSABLE =
  'a[href], button:not([disabled]), textarea:not([disabled]), input:not([disabled]), select:not([disabled]), [tabindex]:not([tabindex="-1"])';

const focusablesIn = (root: HTMLElement): HTMLElement[] =>
  [...root.querySelectorAll<HTMLElement>(FOCUSABLE)].filter((el) => el.offsetParent !== null);

/** Keep Tab / Shift+Tab cycling inside `scope` instead of reaching the inert page behind the
 *  backdrop. Call from a keydown handler that has already decided this dialog is the topmost layer —
 *  a nested dialog must defer to its child. `container` is the scope's own tabIndex=-1 panel, if it
 *  has one: focus can land there by clicking dialog dead space, and `Node.contains(self)` is true, so
 *  without naming it a Shift+Tab from the panel would leak to the background. */
export function trapTab(
  e: KeyboardEvent,
  scope: HTMLElement,
  container?: HTMLElement | null,
): void {
  const items = focusablesIn(scope);
  const first = items[0];
  const last = items[items.length - 1];
  if (!first || !last) {
    e.preventDefault();
    return;
  }
  const active = document.activeElement;
  if (!scope.contains(active) || (container != null && active === container)) {
    e.preventDefault();
    (e.shiftKey ? last : first).focus();
  } else if (e.shiftKey && active === first) {
    e.preventDefault();
    last.focus();
  } else if (!e.shiftKey && active === last) {
    e.preventDefault();
    first.focus();
  }
}

/** The rest of the contract: lock the page behind, open focused, hand the focus back on close.
 *  `scopeRef` is the panel the focus should land in. */
export function useDialogChrome(scopeRef: RefObject<HTMLElement | null>): void {
  // Capture the trigger to restore focus to, during THIS render — i.e. before commit runs the
  // dialog's autoFocus (or our own initial focus) and moves activeElement into the dialog. Reading it
  // later (in the effect) would capture a now-inside element that unmounts on close, stranding focus
  // on <body>. useState initializer runs exactly once, on first render.
  const [restoreTo] = useState(() => document.activeElement as HTMLElement | null);
  // What the dialog is focused ON, so a RE-mount can put the focus back where it was rather than on
  // the first control. React 18's StrictMode runs every effect mount→cleanup→mount in development:
  // the cleanup hands focus back to the trigger, and the second mount then found the focus outside
  // the dialog and moved it to the first focusable — taking it off the `autoFocus` field a beat
  // after the browser had put it there. Measured in Chrome: the command palette opened with the
  // focus on its ✕ in dev and on its search field in prod, from the same code (round 5).
  const focusedRef = useRef<HTMLElement | null>(null);
  // Whether the dialog still holds the focus. Read at cleanup rather than computed there: by the
  // time a passive cleanup runs on a real unmount the panel is already out of the document and the
  // focus has fallen to <body>, so "does the panel contain activeElement" answers false for both a
  // close and a user who clicked away — and those two want opposite things.
  const heldRef = useRef(true);
  useEffect(() => {
    const prev = document.body.style.overflow;
    document.body.style.overflow = "hidden";
    // Initial focus: unless an autoFocus child already placed it inside (e.g. a rename input), move
    // focus to where this dialog was focused before — or, the first time, to the first control, so
    // the dialog opens focused rather than stranded on the background.
    const scope = scopeRef.current;
    if (scope && !scope.contains(document.activeElement)) {
      const back = focusedRef.current;
      const target =
        (back?.isConnected && scope.contains(back) ? back : null) ??
        focusablesIn(scope)[0] ??
        scope;
      target.focus();
    }
    const onFocusIn = () => {
      const inside = scopeRef.current?.contains(document.activeElement) ?? false;
      heldRef.current = inside;
      if (inside) focusedRef.current = document.activeElement as HTMLElement;
    };
    onFocusIn();
    document.addEventListener("focusin", onFocusIn);
    return () => {
      document.removeEventListener("focusin", onFocusIn);
      document.body.style.overflow = prev;
      // Restore focus to the trigger, if it's still in the document and focusable — and only if the
      // dialog still held the focus. Once the user has moved it elsewhere, taking it back is the
      // rudest thing a closing dialog can do.
      if (heldRef.current && restoreTo && restoreTo.isConnected) restoreTo.focus();
    };
  }, [restoreTo, scopeRef]);
}