/**
 * Vary Params Decoding
 *
 * This module is shared between server and client.
 */

export type VaryParams = Set<string>

type FulfilledVaryParamsThenable = {
  status: 'fulfilled'
  value: VaryParams
} & PromiseLike<VaryParams>

type PendingVaryParamsThenable = {
  // 'resolved_model' is an internal React Flight state: the underlying model
  // data has arrived but the thenable hasn't been "unwrapped" yet. Calling
  // .then() triggers Flight to synchronously transition to 'fulfilled'.
  status: 'pending' | 'resolved_model'
  value: unknown
} & PromiseLike<VaryParams>

export type VaryParamsThenable =
  | FulfilledVaryParamsThenable
  | PendingVaryParamsThenable

/**
 * Synchronously reads vary params from a thenable.
 *
 * By the time this is called (client-side or in collectSegmentData), the
 * thenable should already be fulfilled because the Flight stream has been
 * fully received. We check the status synchronously to avoid unnecessary
 * microtasks.
 *
 * Returns null if the thenable is still pending (which shouldn't happen in
 * normal operation - it indicates the server failed to track vary params).
 */
export function readVaryParams(
  thenable: VaryParamsThenable
): VaryParams | null {
  // Attach a no-op listener to force Flight to synchronously resolve the
  // thenable. When a thenable arrives from the Flight stream, it may be in an
  // intermediate 'resolved_model' state (data received but not unwrapped).
  // Calling .then() triggers Flight to transition it to 'fulfilled', making
  // the value available synchronously. React uses this same optimization
  // internally to avoid unnecessary microtasks.
  thenable.then(noop)
  // If the thenable is still not 'fulfilled' after calling .then(), the server
  // failed to resolve it before the stream ended. Treat as unknown.
  if (thenable.status !== 'fulfilled') {
    return null
  }
  return thenable.value
}

const noop = () => {}