/**
 * iOS Safari scrolls `window` to keep a focused input in view. `html, body
 * { overflow: hidden }` then blocks the user from dragging back, and reload
 * restores that offset. Reset window scroll only after the keyboard is gone.
 */

export const KEYBOARD_OPEN_GAP_PX = 100;
export const KEYBOARD_CLOSED_SLACK_PX = 1;

export function isKeyboardOpen(visualHeight: number, innerHeight: number): boolean {
  return visualHeight < innerHeight - KEYBOARD_OPEN_GAP_PX;
}

export function shouldResetWindowScroll(input: {
  scrollY: number;
  innerHeight: number;
  visualViewportHeight: number | null;
}): boolean {
  if (input.scrollY <= 0) return false;
  if (input.visualViewportHeight == null) return true;
  if (isKeyboardOpen(input.visualViewportHeight, input.innerHeight)) return false;
  return input.visualViewportHeight >= input.innerHeight - KEYBOARD_CLOSED_SLACK_PX;
}

type ViewportLike = {
  height: number;
  addEventListener: (type: "resize" | "scroll", listener: () => void) => void;
  removeEventListener: (type: "resize" | "scroll", listener: () => void) => void;
};

export type ViewportScrollLockHost = {
  matchMedia?: (query: string) => { matches: boolean };
  visualViewport?: ViewportLike | null;
  innerHeight: number;
  scrollY: number;
  scrollTo: (x: number, y: number) => void;
  history: { scrollRestoration: string };
  addEventListener: (type: string, listener: () => void) => void;
  removeEventListener: (type: string, listener: () => void) => void;
  document: {
    addEventListener: (type: string, listener: () => void) => void;
    removeEventListener: (type: string, listener: () => void) => void;
  };
  setTimeout?: (fn: () => void, ms: number) => unknown;
};

export function installViewportScrollLock(host: ViewportScrollLockHost): () => void {
  const coarse = host.matchMedia?.("(pointer: coarse)").matches === true;
  const viewport = host.visualViewport ?? null;
  if (!coarse && !viewport) return () => undefined;

  host.history.scrollRestoration = "manual";
  host.scrollTo(0, 0);

  const resetIfSettled = () => {
    if (!shouldResetWindowScroll({
      scrollY: host.scrollY,
      innerHeight: host.innerHeight,
      visualViewportHeight: viewport?.height ?? null,
    })) return;
    host.scrollTo(0, 0);
  };

  const onPageShow = () => {
    host.scrollTo(0, 0);
  };
  const onFocusOut = () => {
    const later = host.setTimeout ?? ((fn: () => void) => fn());
    later(resetIfSettled, 300);
  };

  viewport?.addEventListener("resize", resetIfSettled);
  viewport?.addEventListener("scroll", resetIfSettled);
  host.addEventListener("pageshow", onPageShow);
  host.addEventListener("orientationchange", resetIfSettled);
  host.document.addEventListener("focusout", onFocusOut);

  return () => {
    viewport?.removeEventListener("resize", resetIfSettled);
    viewport?.removeEventListener("scroll", resetIfSettled);
    host.removeEventListener("pageshow", onPageShow);
    host.removeEventListener("orientationchange", resetIfSettled);
    host.document.removeEventListener("focusout", onFocusOut);
  };
}