/**
 * @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} — a `transact()` compare-and-swap
 *   (`ifAtGeneration`) observed a different current generation than the
 *   caller expected. The standard retry pattern is: re-read via
 *   `brain.now()`, re-derive the transaction, and re-submit.
 * - {@link NotYetSupportedAtHistoricalGenerationError} — an index-accelerated
 *   query (vector / hybrid / graph-traversal search) was issued against a
 *   `Db` pinned at a generation the live indexes no longer represent.
 *   `get()` and metadata-filter `find()` remain fully supported at pinned
 *   generations; index-accelerated queries at historical generations require
 *   an index rebuild from the pinned record set, which ships as a follow-up
 *   (`asOf` rebuild). The error message names the unsupported capability and
 *   points at the supported alternatives.
 * - {@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 by `brain.transact(ops, { ifAtGeneration })` when the
 * store's current generation does not equal the caller-supplied expectation.
 * This is 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.
 *
 * @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 caller expected the store to be at. */
  public readonly expected: number
  /** The generation the store was actually at when the transaction arrived. */
  public readonly actual: number

  /**
   * @param expected - The generation supplied via `ifAtGeneration`.
   * @param actual - The store's current generation at submission time.
   */
  constructor(expected: number, actual: number) {
    super(
      `Generation conflict: transact() was submitted with ifAtGeneration: ${expected}, ` +
        `but the store is at generation ${actual}. Another write committed in between. ` +
        `Re-read with brain.now(), rebuild the transaction, and retry.`
    )
    this.name = 'GenerationConflictError'
    this.expected = expected
    this.actual = actual
  }
}

/**
 * @description Thrown when an operation on a pinned `Db` requires
 * index-accelerated machinery (vector search, hybrid search, graph
 * traversal, cursor pagination, aggregation) that the live indexes cannot
 * answer for a *historical* generation. Storage-record reads (`get()`) and
 * metadata-filter `find()` are always supported at pinned generations.
 *
 * Rebuilding indexes from a pinned record set (the standard LSM answer for
 * historical index queries) is the documented `asOf`-rebuild follow-up; this
 * error is the honest boundary until it ships.
 *
 * @example
 * const db = brain.now()
 * await brain.transact([{ op: 'update', id, metadata: { v: 2 } }])
 * await db.get(id)                      // ✅ pinned read — supported
 * await db.find({ where: { v: 1 } })    // ✅ metadata find — supported
 * await db.search('semantic query')     // ❌ throws this error
 */
export class NotYetSupportedAtHistoricalGenerationError extends Error {
  /** The pinned generation the operation was attempted against. */
  public readonly generation: number
  /** The capability that is not supported at historical generations. */
  public readonly capability: string

  /**
   * @param capability - Human-readable name of the unsupported capability
   *   (e.g. `'vector search'`, `'graph traversal (connected)'`).
   * @param generation - The Db's pinned generation.
   * @param currentGeneration - The store's current generation, for context.
   */
  constructor(capability: string, generation: number, currentGeneration: number) {
    super(
      `${capability} is not yet supported at a historical generation ` +
        `(Db pinned at generation ${generation}; store is at ${currentGeneration}). ` +
        `Supported at pinned generations: get() and metadata-filter find(). ` +
        `For index-accelerated queries over historical state, persist() the generation ` +
        `you need and open it with Brainy.load(path) — the loaded snapshot rebuilds ` +
        `its indexes and supports the full query surface.`
    )
    this.name = 'NotYetSupportedAtHistoricalGenerationError'
    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
  }
}