import { createCapability } from './capabilities'

/**
 * Namespaced key/value store for app and middleware metadata.
 *
 * `(namespace, key)` is the composite identity. Keep both values separate;
 * joining them with a delimiter can create collisions.
 */
export interface MetadataStore {
  /** Return the value for `(namespace, key)`, or `null` when it is absent. */
  get: (namespace: string, key: string) => Promise<unknown | null>
  /** Insert or replace the value for `(namespace, key)`. */
  set: (namespace: string, key: string, value: unknown) => Promise<void>
  /** Delete `(namespace, key)`. Do nothing when it is absent. */
  delete: (namespace: string, key: string) => Promise<void>
  /**
   * Optional. The value with a revision, or `null` when it is absent. Pair it
   * with `setIf` for compare-and-set writes: implement both or neither.
   * Every write (also a plain `set`) gives the key a new revision, and a
   * revision does not come back after a `delete`.
   */
  getVersioned?: (
    namespace: string,
    key: string,
  ) => Promise<{ value: unknown; revision: string } | null>
  /**
   * Optional. Write only when the current revision is `expectedRevision`
   * (`null` means the key is absent). Success is `{ ok: true, revision }`.
   * A different revision returns `{ ok: false, reason: 'conflict' }`.
   */
  setIf?: (
    namespace: string,
    key: string,
    value: unknown,
    expectedRevision: string | null,
  ) => Promise<
    { ok: true; revision: string } | { ok: false; reason: 'conflict' }
  >
}

export const MetadataCapability = createCapability<MetadataStore>()('metadata')

export const [getMetadata, provideMetadata] = MetadataCapability