// User-preference cookies — the canonical names the framework's pre-paint
// theme script + auto-wired i18n resolver agree on (innovation/00). The kit's
// ProfileMenu / ThemeToggle write them; the server parses them at SSR time.
//
// Why cookies (and not localStorage)? Cookies travel on every HTTP request,
// so the SERVER can read them and bake the correct HTML (dark class,
// translated strings) BEFORE first paint — no theme/locale flash.
// localStorage is browser-only and would force a post-hydration flicker.
//
// Every helper is isomorphic: pass the request `Cookie` header on the server,
// omit it in the browser (falls back to `document.cookie`).

export const THEME_COOKIE = 'voltro:theme'
export const LOCALE_COOKIE = 'voltro:locale'

export type ThemePreference = 'system' | 'light' | 'dark'

const isThemePreference = (v: string): v is ThemePreference =>
  v === 'system' || v === 'light' || v === 'dark'

// 400 days — Chrome caps cookie lifetime at 400d (RFC 6265bis). Picking the
// max so the preference survives long absences without being silently
// re-defaulted.
const DEFAULT_MAX_AGE_SEC = 60 * 60 * 24 * 400

/** Read a cookie by name from a header string (SSR: the request `Cookie`
 *  header) or, when omitted, `document.cookie` (browser). Returns undefined
 *  when absent or off-document. */
export const getCookie = (name: string, cookieHeader?: string): string | undefined => {
  const header = cookieHeader ?? (typeof document !== 'undefined' ? document.cookie : '')
  if (!header) return undefined
  for (const part of header.split(';')) {
    const eq = part.indexOf('=')
    if (eq === -1) continue
    if (part.slice(0, eq).trim() === name) {
      try { return decodeURIComponent(part.slice(eq + 1).trim()) } catch { return part.slice(eq + 1).trim() }
    }
  }
  return undefined
}

export interface SetCookieOptions {
  /** Default: 400 days (Chrome's cap). Pass 0 to delete the cookie. */
  readonly maxAgeSeconds?: number
  /** Default: '/' — preference applies app-wide. */
  readonly path?: string
  /** Default: 'lax' — allows top-level navigation but blocks cross-site
   *  XHR, which is what preference cookies want. 'none' implies `Secure`
   *  (modern browsers reject a SameSite=None cookie without it). */
  readonly sameSite?: 'lax' | 'strict' | 'none'
}

/** Build a `Set-Cookie`-style string AND, in the browser, write it to
 *  `document.cookie`. Returns the string so SSR callers can emit a header. */
export const setCookie = (name: string, value: string, options: SetCookieOptions = {}): string => {
  const { maxAgeSeconds = DEFAULT_MAX_AGE_SEC, path = '/', sameSite = 'lax' } = options
  const parts = [
    `${name}=${encodeURIComponent(value)}`,
    `Path=${path}`,
    `Max-Age=${maxAgeSeconds}`,
    `SameSite=${sameSite}`,
  ]
  if (sameSite === 'none') parts.push('Secure')
  const cookie = parts.join('; ')
  if (typeof document !== 'undefined') document.cookie = cookie
  return cookie
}

/** Expire a cookie (Max-Age=0). Same isomorphic contract as `setCookie`:
 *  writes `document.cookie` in the browser, returns the header string. */
export const deleteCookie = (name: string, path = '/'): string =>
  setCookie(name, '', { maxAgeSeconds: 0, path })

export interface PreferenceCookies {
  /** undefined → server should fall back to its default (typically the
   *  app config's theme). 'system' means the client's pre-paint script
   *  decides against `prefers-color-scheme`. */
  readonly theme?: ThemePreference
  /** Locale tag as written by the client (lowercase IETF: `en`, `de`,
   *  `fr-CA`). Consumer decides which tags are valid + how to fall back if
   *  the value is unknown. */
  readonly locale?: string
}

/** Extract both preference cookies (theme + locale) from a cookie header (or
 *  `document.cookie` when omitted). Tolerates `null`/`undefined` headers.
 *  An invalid theme value is dropped (not coerced); an empty locale is
 *  dropped. */
export const parsePreferenceCookies = (cookieHeader?: string | null): PreferenceCookies => {
  const header = cookieHeader ?? undefined
  const themeRaw = getCookie(THEME_COOKIE, header)
  const locale = getCookie(LOCALE_COOKIE, header)
  return {
    ...(themeRaw !== undefined && isThemePreference(themeRaw) ? { theme: themeRaw } : {}),
    ...(locale !== undefined && locale !== '' ? { locale } : {}),
  }
}

/** Apply a theme preference to `<html>` (browser only) — adds/removes the
 *  `dark` class, resolving `system` against `prefers-color-scheme`. The kit's
 *  ProfileMenu / ThemeToggle call this after writing the cookie so the change
 *  is immediate (the pre-paint `themeBootScript` handles the first load). */
export const applyTheme = (theme: ThemePreference): void => {
  if (typeof document === 'undefined') return
  const dark =
    theme === 'dark' ||
    (theme === 'system' &&
      typeof window !== 'undefined' &&
      window.matchMedia?.('(prefers-color-scheme: dark)').matches === true)
  document.documentElement.classList.toggle('dark', dark)
}