import { existsSync, readdirSync, readFileSync, statSync } from 'node:fs';
import { join } from 'node:path';
import { PLATFORM_DIR } from '../data-path';

/**
 * Themes installed on this machine, as opposed to the ones the platform ships.
 *
 * ── Why this is simpler than plugin discovery ──
 *
 * A plugin is code: it has a manifest module, routers, tables, a sidecar, panels, and the platform has to
 * import and mount it. A theme is a block of CSS custom properties. Nothing to import, nothing to mount,
 * nothing to bundle — which is why an installed theme takes effect without a frontend rebuild, and why
 * this file is eighty lines rather than four hundred.
 *
 * Discovery is by CONVENTION, the same as plugins. Presence is the declaration:
 *
 *   themes/<id>/theme.css     required — `[data-theme='<id>'] { … }`
 *   themes/<id>/theme.json    required — the name a person sees, and how to group it
 *   themes/<id>/assets/       optional — a wallpaper, a logo; served at /themes/<id>/assets/…
 *
 * The id is the DIRECTORY NAME and is not declared in the manifest, for the reason `appName` is not
 * declared in a plugin's: an id that can disagree with where the files sit eventually will.
 *
 * ── What this does not do ──
 *
 * It answers "what is on disk". It does not read the database, validate colour values, or care whether a
 * theme is any good. A theme whose CSS is nonsense will simply look wrong, which is the author's business
 * and is visible immediately.
 */

/** Where installed themes live. Sibling of `plugins/`, and gitignored the same way. */
export const THEMES_DIR = join(PLATFORM_DIR, 'themes');

export type InstalledTheme = {
  /** The directory name. Also the `data-theme` value its CSS must target. */
  id: string;
  name: string;
  family: string;
  /**
   * Whether to put `.dark` on the document alongside `data-theme`.
   *
   * A compatibility flag, not a description: several hundred `dark:` utilities are still written into
   * components and resolve against that class. A dark theme wants it.
   */
  dark: boolean;
};

export type ThemeProblem = { id: string; problem: string };

const isDir = (path: string): boolean => {
  try {
    return statSync(path).isDirectory();
  } catch {
    return false;
  }
};

/**
 * Read one directory as a theme, or say why it is not one.
 *
 * Returns problems rather than throwing, and never stops the scan: one malformed theme must not hide the
 * eleven good ones next to it. The problems are reported so the app store can render them, which is the
 * same posture `discoverPlugins` takes.
 */
function readTheme(id: string): InstalledTheme | ThemeProblem {
  const dir = join(THEMES_DIR, id);
  const cssPath = join(dir, 'theme.css');
  const manifestPath = join(dir, 'theme.json');

  if (!existsSync(cssPath)) return { id, problem: 'no theme.css' };
  if (!existsSync(manifestPath)) return { id, problem: 'no theme.json' };

  let parsed: unknown;
  try {
    parsed = JSON.parse(readFileSync(manifestPath, 'utf-8'));
  } catch (err) {
    return { id, problem: `theme.json is not valid JSON: ${err instanceof Error ? err.message : String(err)}` };
  }

  const manifest = parsed as { name?: unknown; family?: unknown; dark?: unknown };
  if (typeof manifest.name !== 'string' || !manifest.name.trim()) return { id, problem: 'theme.json needs a "name"' };

  /*
    The CSS must target its own directory name.

    Checked because the failure is otherwise silent and baffling: the theme appears in the picker, is
    selected, and nothing changes — because its rules are scoped to a `data-theme` value nobody sets.
  */
  const css = readFileSync(cssPath, 'utf-8');
  if (!css.includes(`[data-theme='${id}']`) && !css.includes(`[data-theme="${id}"]`)) {
    return { id, problem: `theme.css does not target [data-theme='${id}'] — it must match the directory name` };
  }

  return {
    id,
    name: manifest.name,
    family: typeof manifest.family === 'string' && manifest.family.trim() ? manifest.family : manifest.name,
    dark: manifest.dark === true,
  };
}

export function discoverThemes(): { themes: InstalledTheme[]; problems: ThemeProblem[] } {
  if (!existsSync(THEMES_DIR)) return { themes: [], problems: [] };

  const themes: InstalledTheme[] = [];
  const problems: ThemeProblem[] = [];

  for (const id of readdirSync(THEMES_DIR).sort()) {
    if (id.startsWith('.') || !isDir(join(THEMES_DIR, id))) continue;
    const result = readTheme(id);
    if ('problem' in result) problems.push(result);
    else themes.push(result);
  }

  return { themes, problems };
}

/** The CSS of one installed theme, or null. `id` is validated, so it cannot climb out of the directory. */
export function readThemeCss(id: string): string | null {
  if (!/^[a-z0-9][a-z0-9-]*$/i.test(id)) return null;
  const path = join(THEMES_DIR, id, 'theme.css');
  if (!existsSync(path)) return null;
  try {
    return readFileSync(path, 'utf-8');
  } catch {
    return null;
  }
}