/**
 * Durable session-persistence Service Definition (`ctx.sessionPersistence`). Backends store
 * {@link SessionEvent}s as the event-sourced log and carry non-replayable
 * {@link SessionHeader} metadata separately.
 * @module @deepseek-ai/dsh-session-persistence
 */

import { Context, Service } from '@deepseek-ai/cordis'
import { SessionPreparation } from '@deepseek-ai/dsh-session'
import type { SessionEvent, SessionId, SessionHeader } from '@deepseek-ai/dsh-session'
import type { SessionPersistenceRevision } from './revision.ts'

// Re-export the metadata vocabulary so Consumers import it from the Service Definition.
export type { SessionHeader } from '@deepseek-ai/dsh-session'
export { SessionPersistenceRevision } from './revision.ts'

/** Lightweight immutable source identity returned without loading a full log. */
export interface SessionPersistenceSnapshot {
  /** Detached metadata for one materialized session. */
  header: SessionHeader
  /** Opaque source-qualified token that changes whenever this stored log changes. */
  revision: SessionPersistenceRevision
}

/** Immutable logical session prepared from persistence or a live owner. */
export interface SessionInspection {
  /** Validated immutable session metadata. */
  readonly meta: SessionHeader
  /** Validated contiguous logical event log. */
  readonly events: readonly SessionEvent[]
}

// The backend-agnostic write-path orchestration first-party backends compose.
export {
  DEFAULT_PREPARED_SESSION_CACHE_SIZE,
  DEFAULT_WRITE_BATCH_MAX_DELAY_MS,
  MAX_WRITE_BATCH_DELAY_MS,
  PersistenceCoordinator,
  SessionPersistenceCorruptionError,
} from './coordinator.ts'
export type {
  PersistenceBackend,
  PersistenceCoordinatorOptions,
  StoredPrefix,
  StoredSuffix,
} from './coordinator.ts'

declare module '@deepseek-ai/cordis' {
  interface Context {
    sessionPersistence: SessionPersistence
  }
}

/**
 * A backend-resolved, per-session local artifact location. The path is an
 * absolute target path and can name an artifact that has not materialized yet.
 * Consumers must treat it as a location hint, never as an authorization token.
 */
export interface SessionLocation {
  /** Backend-specific artifact kind, for example `jsonl`. */
  readonly kind: string
  /** Absolute path to this session's backend-owned artifact. */
  readonly path: string
}

/**
 * Durable append-only session storage. Implementations preserve contiguous,
 * losslessly JSON-serializable events; {@link append} resolves only after
 * durability, and {@link load} balances a complete interrupted tail without
 * rewriting committed events.
 */
export abstract class SessionPersistence extends Service {
  constructor(ctx: Context) {
    super(ctx, 'sessionPersistence')
  }

  /**
   * Resolve this backend's independent local artifact for a session without
   * reading, creating, flushing, or otherwise materializing it. Backends such
   * as SQLite that do not own one artifact per session return `undefined`.
   * @param meta - the immutable session header whose artifact is requested.
   * @returns the backend-specific absolute location, when one exists.
   */
  abstract locate(meta: SessionHeader): SessionLocation | undefined

  /**
   * Register a new session's metadata. A backend MAY defer the physical write
   * until the first {@link append} (lazy materialization), in which case a
   * created-but-never-appended session is absent from {@link list}
   * — abandoned sessions leave nothing behind.
   * @param meta - the immutable header (id, version, cwd, lineage) to record.
   */
  abstract create(meta: SessionHeader): Promise<void>

  /**
   * Durably persist a batch of events. Honors the append-only and contiguous-
   * seq contracts: the first event's `seq` MUST equal the stored next-seq
   * (after `load` has durably closed any interrupted turn). Rejects non-JSON-
   * serializable `event.data` with an error naming the offending event type.
   * @param id - the session the batch belongs to.
   * @param events - the contiguous batch to persist, in seq order.
   */
  abstract append(id: SessionId, events: readonly SessionEvent[]): Promise<void>

  /**
   * Prepare the exact unpublished Session used by resume. Implementations may
   * reuse object graphs retained by an earlier {@link inspect} after confirming
   * their durable revision is still current; disposal releases an unpublished
   * reservation. Revision retries require the durable log to remain unchanged
   * for one read/check round trip; continuous external writers may delay completion.
   * @param id - persisted session to prepare.
   * @param signal - optional cancellation for preparation work.
   * @returns one owned unpublished Session preparation.
   */
  async prepare(id: SessionId, signal?: AbortSignal): Promise<SessionPreparation> {
    signal?.throwIfAborted()
    const loaded = await this.load(id)
    signal?.throwIfAborted()
    const sessions = this.ctx.get('sessions')
    if (sessions === undefined) {
      throw new Error('cannot prepare a session: SessionStore is not configured')
    }
    return SessionPreparation.create(sessions.prepare(id, {
      seed: loaded.events.map(event => structuredClone(event)),
      meta: structuredClone(loaded.meta),
      seedSource: 'persistence',
    }))
  }

  /**
   * Load an immutable balanced logical view and commit any required cold
   * recovery. A complete interrupted final turn is preserved and durably
   * closed with missing tool errors plus any open step and turn boundaries;
   * only a torn final record is discarded. Unknown versions and corruption in
   * the committed prefix reject. Implementations MUST NOT crash-repair an
   * identity still bound to a live Session: a balanced live log may return as a
   * durable snapshot, while an open live turn rejects. Returned values may be
   * shared with immutable live or prepared state and must not be mutated.
   * Revision-based implementations may wait for one stable read/check round trip.
   * @param id - the persisted session to reload.
   * @returns the header and a log ending on a balanced `turn/end`.
   */
  abstract load(id: SessionId): Promise<SessionInspection>

  /**
   * Inspect an immutable logical session without committing recovery or
   * publishing it. A cold complete interrupted turn receives synthetic closers
   * in memory and a torn physical tail remains untouched. An already-live
   * Session instead yields its current immutable snapshot, which may contain an
   * open turn and its `session/end-seed` boundary. Coordinator-backed
   * implementations retain the exact cold unpublished Session for bounded
   * reuse by a later {@link prepare}. A stale ready source is reloaded; a source
   * already committing or reserved for resume remains exclusive, and inspection
   * may borrow its immutable view. Callers borrow only the immutable header and
   * log. Continuous external writers may delay revision convergence.
   * @param id - the persisted session to inspect.
   * @param signal - optional cancellation for queued and backend read work.
   * @returns the validated header and current logical event log.
   */
  abstract inspect(id: SessionId, signal?: AbortSignal): Promise<SessionInspection>

  /**
   * Read the stored events from `fromSeq` onward — the read-from-seq
   * primitive for read models that resume from a watermark (e.g. a persisted
   * projection cache folding only the tail past its checkpoint). Unlike
   * {@link inspect}, it is a detached physical suffix read: no preparation
   * cache, torn-tail truncation, synthetic closers, or coordinator-state
   * publication. Only events from the valid contiguous stored prefix are
   * returned, so a torn fragment never reaches the caller. `fromSeq` at or
   * beyond the stored prefix returns an empty event list (never an error).
   * Backends whose medium can seek by seq
   * (SQLite) read only the suffix; sequential media (JSONL, both encodings)
   * still parse the whole artifact and skip forward — the primitive bounds
   * what is RETURNED and refolded, not every backend's physical read.
   * @param id - the persisted session to read.
   * @param fromSeq - first event seq to include; a non-negative safe integer.
   * @param signal - optional cancellation for queued and backend read work.
   * @returns the header and the stored events with `seq >= fromSeq`.
   */
  abstract readFrom(id: SessionId, fromSeq: number, signal?: AbortSignal):
  Promise<{ meta: SessionHeader; events: SessionEvent[] }>

  /**
   * Lightweight listing from metadata, without a full-log parse.
   * @param signal - optional cancellation for backend listing work.
   * @returns one header per materialized session.
   */
  abstract list(signal?: AbortSignal): Promise<SessionHeader[]>

  /**
   * List materialized sessions with cheap per-log change tokens.
   *
   * Repeated observations of an unchanged log return the same revision. A
   * successful mutating {@link load} repair changes the next listed revision.
   * Revisions also distinguish independently backed stores so backend-local
   * counters cannot compare equal across different persistence sources.
   * @param signal - optional cancellation for backend snapshot-listing work.
   * @returns one header and opaque revision per materialized session without loading full logs.
   */
  abstract listSnapshots(signal?: AbortSignal): Promise<SessionPersistenceSnapshot[]>
}

export default SessionPersistence