/**
 * @module db/errors
 * @description Error types for the 8.0 generational-MVCC Db API.
 *
 * Three failure modes have dedicated classes so callers can branch on them
 * with `instanceof` instead of string matching:
 *
 * - {@link GenerationConflictError} — the store's current generation differs
 *   from what the caller's operation requires: a `transact()`
 *   compare-and-swap (`ifAtGeneration`) observed a different generation than
 *   expected, or `db.persist()` was called on a view that history has moved
 *   past (snapshots capture current bytes, so persisting requires the view
 *   to still be the latest generation). The standard retry pattern is:
 *   re-read via `brain.now()`, re-derive the operation, and re-submit.
 * - {@link SpeculativeOverlayError} — an index-accelerated query (vector /
 *   hybrid / graph-traversal search, cursors, aggregation) or `persist()`
 *   was issued against a speculative `db.with()` overlay. Overlays are pure
 *   in-memory values whose entities carry no embeddings (`with()` never
 *   invokes the embedder), so a "full" index query over one would silently
 *   miss the overlay's own entities — an honest error beats silently-wrong
 *   results. Commit the operations with `brain.transact()` to get the full
 *   query surface. Historical (non-overlay) views do NOT throw this: they
 *   serve the full query surface via at-generation index materialization.
 * - {@link GenerationCompactedError} — `asOf()` asked for a generation whose
 *   immutable records were reclaimed by `compactHistory()`.
 *
 * All three are exported from the package root (`@soulcraft/brainy`).
 */

/**
 * @description Thrown when an operation requires the store to be at a
 * specific generation and it is not:
 *
 * - `brain.transact(ops, { ifAtGeneration })` — the whole-store
 *   compare-and-swap counterpart to the per-entity `ifRev` /
 *   `RevisionConflictError` pair: it guarantees that *nothing* was committed
 *   between the caller's read (`brain.now()`) and this transaction. The
 *   transaction is rejected before any record is staged — the store is
 *   untouched and the generation counter is unchanged.
 * - `db.persist(path)` — snapshots capture current bytes, so persisting
 *   requires the view to still be the store's **latest** generation. A view
 *   that history has moved past throws this error: pin with `brain.now()`
 *   and persist before further writes.
 *
 * @example
 * const db = brain.now()
 * try {
 *   await brain.transact(ops, { ifAtGeneration: db.generation })
 * } catch (err) {
 *   if (err instanceof GenerationConflictError) {
 *     // Someone committed since we pinned — re-read and retry.
 *     console.log(`expected ${err.expected}, store is at ${err.actual}`)
 *   }
 * }
 */
export class GenerationConflictError extends Error {
  /** The generation the operation required the store to be at. */
  public readonly expected: number
  /** The generation the store was actually at. */
  public readonly actual: number

  /**
   * @param expected - The required generation (`ifAtGeneration`, or the
   *   pinned generation of the view being persisted).
   * @param actual - The store's current generation.
   */
  constructor(expected: number, actual: number) {
    super(
      `Generation conflict: the operation requires the store at generation ${expected}, ` +
        `but it is at generation ${actual}. Another write committed in between. ` +
        `Re-read with brain.now(), rebuild the operation, and retry.`
    )
    this.name = 'GenerationConflictError'
    this.expected = expected
    this.actual = actual
  }
}

/**
 * @description Thrown when an operation on a speculative `db.with()` overlay
 * requires index machinery (vector / hybrid / graph-traversal search, cursor
 * pagination, aggregation) or durability (`persist()`).
 *
 * Why this single boundary exists: overlays are pure in-memory values —
 * `with()` never touches disk, the generation counter, or the embedder, so
 * overlay entities carry **no embeddings**. A "full" vector or hybrid query
 * over an overlay would silently exclude the overlay's own entities, and a
 * persisted overlay would be a store whose entities cannot be semantically
 * searched. An honest error beats silently-wrong results. Commit the
 * operations with `brain.transact()` to get the full query surface, or query
 * the overlay's base view (historical views serve the complete surface via
 * at-generation index materialization).
 *
 * `get()`, metadata-filter `find()`, and filter-based `related()` remain
 * fully supported on overlays.
 *
 * @example
 * const spec = await brain.now().with([{ op: 'add', type, data: 'draft' }])
 * await spec.find({ where: { draft: true } })  // ✅ metadata find — supported
 * await spec.search('semantic query')          // ❌ throws this error
 * await brain.transact([{ op: 'add', type, data: 'draft' }]) // → full surface
 */
export class SpeculativeOverlayError extends Error {
  /** The capability that is not supported on speculative overlays. */
  public readonly capability: string
  /** The overlay's pinned base generation. */
  public readonly generation: number

  /**
   * @param capability - Human-readable name of the unsupported capability
   *   (e.g. `'vector search'`, `'persist'`).
   * @param generation - The overlay's pinned base generation.
   */
  constructor(capability: string, generation: number) {
    super(
      `${capability} is not supported on a speculative with() overlay ` +
        `(pinned at base generation ${generation}). Overlays are pure in-memory ` +
        `values whose entities carry no embeddings, so index-accelerated reads ` +
        `over them would be silently incomplete. Supported on overlays: get(), ` +
        `metadata-filter find(), and filter-based related(). Commit the operations ` +
        `with brain.transact() for the full query surface, or query the base view.`
    )
    this.name = 'SpeculativeOverlayError'
    this.capability = capability
    this.generation = generation
  }
}

/**
 * @description Thrown by `brain.asOf(generationOrTimestamp)` when the
 * requested generation's history was reclaimed by `brain.compactHistory()`.
 * The store records the compaction horizon (the highest reclaimed
 * generation) in its manifest; any generation BELOW the horizon is
 * unreachable — its reads would need the reclaimed before-images. The
 * horizon itself stays reachable, resolved from the record-sets above it.
 *
 * To keep a generation readable forever, `persist()` it to a snapshot
 * directory before compacting — snapshots are self-contained and unaffected
 * by compaction of the source store.
 */
export class GenerationCompactedError extends Error {
  /** The generation that was requested. */
  public readonly requested: number
  /** The compaction horizon — generations < this value are unreachable. */
  public readonly horizon: number

  /**
   * @param requested - The generation the caller asked for.
   * @param horizon - The store's current compaction horizon.
   */
  constructor(requested: number, horizon: number) {
    super(
      `Generation ${requested} has been compacted away (compaction horizon: ${horizon}). ` +
        `Only generations at or above the horizon are reachable via asOf(). ` +
        `Use db.persist(path) before compactHistory() to keep a generation readable.`
    )
    this.name = 'GenerationCompactedError'
    this.requested = requested
    this.horizon = horizon
  }
}