/**
 * Read, save, and remove one value by string id.
 * `get` returns the value, or `null` when the id is absent.
 */
interface KeyedStore<Value> {
  /** Returns the value for `id`, or `null` when `id` is absent. */
  get: (id: string) => Promise<Value | null>
  /** Saves `value` for `id`. A later save for the same `id` replaces it. */
  set: (id: string, value: Value) => Promise<void>
  /** Removes `id`. An absent `id` stays absent. */
  delete: (id: string) => Promise<void>
}

/**
 * Keeps one JSON value for each task id.
 * `inMemoryTaskStore` keeps those values in one process.
 * When the server runs on more than one instance, pass a store that you own.
 */
export type TaskStore<Value = unknown> = KeyedStore<Value>

// Map.get adds undefined. The wrapper keeps a saved value, including null.
function createInMemoryStore<Value = unknown>() {
  const values = new Map<string, { value: Value }>()

  return {
    async get(id: string) {
      const entry = values.get(id)
      if (entry === undefined) return null
      return entry.value
    },
    async set(id: string, value: Value) {
      values.set(id, { value })
    },
    async delete(id: string) {
      values.delete(id)
    },
  }
}

/**
 * Creates a {@link TaskStore} that keeps values in memory.
 * Each call has its own map. The map lives in this process only.
 * A saved task stays in the map until `delete`, or until the process exits.
 * A long-lived server passes its own store.
 *
 * ```ts
 * const tasks = inMemoryTaskStore()
 * await tasks.set('task-1', { status: 'working' })
 * await tasks.get('task-1')
 * ```
 */
export function inMemoryTaskStore() {
  return createInMemoryStore()
}