/**
 * The admin's own chrome, drawn INSIDE the rendered page.
 *
 * ── What this is, and where it deliberately does not live ──
 *
 * Nothing here exists in the website's source. The site renders a page; this puts the builder's
 * controls on top of it, from the plugin, at runtime. So the site never carries a line of code that
 * only an editor would ever see, and the builder can change without the site being rebuilt.
 *
 * It is possible because the preview frame is SAME ORIGIN — it comes through the platform's dev proxy
 * on Officer's own host — so `iframe.contentDocument` is readable and writable. That single fact is
 * what the whole page builder rests on, and it is why swapping this plugin's hand-rolled cross-origin
 * proxy for the platform's mattered beyond the bugs it fixed.
 *
 * ── Plain DOM, not React ──
 *
 * React could portal into another document, and it would mean running a reconciler against a document
 * that reloads under it whenever the dev server rebuilds. The controls here are a handful of elements
 * with click handlers; the cost of building them by hand is a few lines, and the benefit is that a
 * reload is just a document that no longer exists rather than a tree React still thinks it owns.
 *
 * ── Styles are inline, on purpose ──
 *
 * The frame has the SITE's stylesheet, which is Tailwind built by scanning the site's own templates.
 * A class this file invents was never scanned and does not exist there, so it would silently do
 * nothing. Inline styles cannot be purged by a build that has never heard of this file.
 */

/** Everything injected carries this, so a re-injection can find and remove what the last one left. */
const MARK = 'data-fofos-admin-canvas';

export type Canvas = {
  /** Somebody pressed Add Section. */
  onAddSection: () => void;
};

/**
 * Where the page's content goes.
 *
 * `<main>` because it is what the site's Layout renders and what the word means — the page minus the
 * furniture around it. Not a class or a data attribute, which would be a contract the SITE has to keep,
 * and the point of this file is that the site does not have to know it exists.
 */
const contentRoot = (document: Document): HTMLElement | null => document.querySelector('main');

/**
 * Draw the builder's controls into a rendered page.
 *
 * Returns a cleanup. It runs before the next injection and on unmount — see `onDocument` in
 * SitePreview — so a reload never leaves two sets of controls behind.
 */
export function paintCanvas(document: Document, canvas: Canvas): () => void {
  const root = contentRoot(document);
  if (!root) return () => {};

  // Anything a previous injection left. Cheap insurance: the cleanup should have run, and if a reload
  // raced it, a second Add Section button is exactly the sort of thing nobody reports as a bug.
  root.querySelectorAll(`[${MARK}]`).forEach((node) => node.remove());

  /*
    The site's EMPTY-PAGE placeholder, hidden while the builder draws its own controls in that space.

    Found by its marker rather than by position. It was `firstElementChild`, which was right for exactly
    as long as an empty page was the only page — the moment a page has a section, the first child IS
    that section and hiding it would hide the thing being edited.

    Hidden rather than removed, so the cleanup can simply put it back. A page whose last section is
    deleted then shows it again without a reload.
  */
  const placeholder = root.querySelector<HTMLElement>('[data-page-empty]');
  if (placeholder) placeholder.style.display = 'none';

  const zone = document.createElement('div');
  zone.setAttribute(MARK, 'add-section');
  zone.style.cssText = [
    'display:flex',
    'align-items:center',
    'justify-content:center',
    'min-height:40vh',
    'margin:2rem 0',
    // A dashed outline reads as "a space waiting for something" in every editor anybody has used.
    'border:2px dashed color-mix(in srgb, currentColor 25%, transparent)',
    'border-radius:0.75rem',
  ].join(';');

  const button = document.createElement('button');
  button.type = 'button';
  button.textContent = '+  Add section';
  button.style.cssText = [
    'cursor:pointer',
    'padding:0.75rem 1.5rem',
    'border-radius:0.5rem',
    // The page's own colours, inverted — so the control belongs to whatever theme the site is wearing
    // rather than being a grey box sitting on it.
    'background:var(--site-fg, #000)',
    'color:var(--site-bg, #fff)',
    'border:0',
    'font:inherit',
    'font-size:0.875rem',
  ].join(';');
  button.addEventListener('click', canvas.onAddSection);

  zone.append(button);
  root.append(zone);

  return () => {
    button.removeEventListener('click', canvas.onAddSection);
    zone.remove();
    if (placeholder) placeholder.style.display = '';
  };
}