import { getHeads, init, merge, type Doc } from '@automerge/automerge'
import type { DocumentId } from '../../document-id.js'
import type { Source } from '../source.js'

type InboundListener = (documentId: DocumentId, doc: Doc<unknown>) => void

/**
 * A Fake {@link Source} with real in-memory state — for testing the Engine without
 * any transport. Accumulates pushed docs (via `merge`), serves them on `pull`, and
 * can simulate an inbound delivery from a peer via {@link deliver}. Assertions read
 * {@link current} / {@link pushes}.
 */
export class InMemorySource implements Source {
  readonly id: string
  readonly #docs = new Map<DocumentId, Doc<unknown>>()
  readonly #listeners = new Set<InboundListener>()

  /** One entry per `push`, for asserting call counts (e.g. debounce coalescing). */
  readonly pushes: Array<{ documentId: DocumentId; heads: readonly string[] }> = []

  constructor(id: string) {
    this.id = id
  }

  async push(documentId: DocumentId, doc: Doc<unknown>): Promise<void> {
    const next = merge(this.#docs.get(documentId) ?? init<unknown>(), doc)
    this.#docs.set(documentId, next)
    this.pushes.push({ documentId, heads: getHeads(next) })
  }

  async pull(documentId: DocumentId): Promise<Doc<unknown> | null> {
    return this.#docs.get(documentId) ?? null
  }

  subscribe(onInbound: InboundListener): () => void {
    this.#listeners.add(onInbound)
    return () => this.#listeners.delete(onInbound)
  }

  // --- test helpers ---

  /** Simulate this source receiving a change from its transport and delivering it. */
  deliver(documentId: DocumentId, doc: Doc<unknown>): void {
    const next = merge(this.#docs.get(documentId) ?? init<unknown>(), doc)
    this.#docs.set(documentId, next)
    for (const listener of this.#listeners) listener(documentId, next)
  }

  /** Seed a doc as if it already existed here (drives a later `pull`). */
  seed(documentId: DocumentId, doc: Doc<unknown>): void {
    this.#docs.set(documentId, doc)
  }

  current(documentId: DocumentId): Doc<unknown> | null {
    return this.#docs.get(documentId) ?? null
  }
}