// Creating the offscreen document at most once, without betting sign-in on an
// API whose availability we cannot pin down.
//
// `chrome.offscreen.hasDocument()` has existed since the offscreen API shipped
// in Chrome 109, but it was marked `[nodoc]` in the Chromium IDL and only
// appeared in the published reference with Chrome 150 — so the "Chrome 150+"
// annotation is the date it was documented, not the date it became callable.
// Verified directly: on Chrome 149 it is a function, returns false before
// creation and true after. The reason it stayed undocumented is that the team
// was not committed to it (a per-extension existence check does not survive
// multiple offscreen documents, crbug.com/1339382), and removal has been
// discussed on chromium-extensions.
//
// So it is unsafe in both directions: unverifiable below 149, and a candidate
// for removal above it. This module therefore prefers the documented
// `chrome.runtime.getContexts()` (Chrome 116), falls back to `hasDocument()`
// for the 110-115 window the manifest still supports, and treats
// `createDocument`'s own "already exists" failure as the final authority when
// neither is usable. Sign-in is the one path that must not break on a version
// difference — it already did once, in v1.2.1.

/** Chrome's message when a second offscreen document is requested. */
const ALREADY_EXISTS = /single offscreen document/i

/**
 * Whether an offscreen document exists. `undefined` means neither existence
 * API was available, so the caller must not treat the answer as "no".
 */
export async function hasOffscreenDocument(): Promise<boolean | undefined> {
  if (typeof chrome.runtime.getContexts === 'function') {
    const contexts = await chrome.runtime.getContexts({
      contextTypes: [chrome.runtime.ContextType.OFFSCREEN_DOCUMENT],
    })
    return contexts.length > 0
  }
  if (typeof chrome.offscreen.hasDocument === 'function') {
    return await chrome.offscreen.hasDocument()
  }
  return undefined
}

/**
 * Create the offscreen document unless one is already there. A lost race, or
 * an existence check we could not run, both surface as the same
 * createDocument error — which means the document exists, which is what the
 * caller wanted. Any other failure is real and propagates.
 */
export async function ensureOffscreenDocument(
  parameters: chrome.offscreen.CreateParameters,
): Promise<void> {
  if (await hasOffscreenDocument()) return
  try {
    await chrome.offscreen.createDocument(parameters)
  } catch (err) {
    if (!ALREADY_EXISTS.test(String(err))) throw err
    console.debug('[substandard] offscreen document already existed', err)
  }
}