/**
 * @module tests/integration/db-mvcc
 * @description Credibility-bar proof suite for Brainy 8.0's generational MVCC
 * storage core + Datomic-style Db API. Each test PROVES a stated guarantee
 * (not merely exercises the API):
 *
 *  1. Isolation — a pinned Db reads its pinned state exactly, across 200
 *     subsequent transact mutations (updates + removes).
 *  2. Atomicity — a batch with a failing later operation applies NOTHING
 *     (generation unchanged, no partial records), for both plan-time and
 *     execution-phase failures (the latter via real fault injection into the
 *     storage adapter, exercising the production rollback path).
 *  3. CAS — `ifAtGeneration` commits when fresh, throws
 *     `GenerationConflictError` with correct expected/actual when stale.
 *  4. Snapshot integrity — persist → mutate source heavily → `Brainy.load()`
 *     is byte-identical to the pre-mutation state (hard-link safety), and an
 *     in-memory brain persists to a loadable directory.
 *  5. Compaction safety — pinned reads stay correct across `compactHistory()`;
 *     after release + compact the superseded record-sets are reclaimed.
 *  6. with() — speculative overlays are visible on the overlay Db only; the
 *     underlying brain and the original Db are untouched.
 *  7. Generation monotonicity across close/reopen.
 *  8. Crash consistency — a simulated crash between record staging and the
 *     manifest rename (the commit point) recovers to the exact pre-transaction
 *     state on reopen, through the REAL production recovery path.
 *  9. Versioned provider wiring — a provider implementing the 4-method
 *     `VersionedIndexProvider` contract receives balanced pin/release (and a
 *     visibility consult per pin) as Db values are created and released.
 * 10. Historical full query surface — vector search, graph traversal, and
 *     aggregation at a past pinned generation return at-generation results
 *     via ephemeral index materialization (for `now()` and `asOf()` pins
 *     alike, built once per Db and cached); `release()` frees the
 *     materialization by closing the ephemeral reader; speculative overlays
 *     keep the documented `SpeculativeOverlayError` boundary.
 *
 * All entities carry explicit vectors so the suite never invokes the
 * embedding engine (vector queries use find({ vector }); semantic string
 * search on overlays is exercised only for its documented error).
 */

import { describe, it, expect, afterEach, vi } from 'vitest'
import * as fs from 'node:fs'
import * as os from 'node:os'
import * as path from 'node:path'
import { Brainy } from '../../src/brainy.js'
import { Db, type HistoricalQueryHandle } from '../../src/db/db.js'
import {
  GenerationCompactedError,
  GenerationConflictError,
  SpeculativeOverlayError
} from '../../src/db/errors.js'
import type { GenerationStore } from '../../src/db/generationStore.js'
import { GraphAdjacencyIndex } from '../../src/graph/graphAdjacencyIndex.js'
import { RevisionConflictError } from '../../src/transaction/RevisionConflictError.js'
import type { StorageAdapter } from '../../src/coreTypes.js'
import { NounType, VerbType } from '../../src/types/graphTypes.js'

/** Deterministic 384-dim vector so no test ever invokes the embedder. */
function vec(seed: number): number[] {
  return Array.from({ length: 384 }, (_, i) => ((seed * 31 + i * 7) % 100) / 100)
}

/**
 * Map a readable label to a deterministic UUID-shaped id (entity ids must be
 * UUIDs — the sharded storage layout derives the shard from the UUID hex).
 * Two independent FNV-style mixes keep 7+ distinct labels collision-free.
 */
function uid(label: string): string {
  let h1 = 0x811c9dc5
  for (let i = 0; i < label.length; i++) {
    h1 = Math.imul(h1 ^ label.charCodeAt(i), 0x01000193) >>> 0
  }
  let h2 = 0xdeadbeef
  for (let i = label.length - 1; i >= 0; i--) {
    h2 = Math.imul(h2 ^ label.charCodeAt(i), 0x85ebca6b) >>> 0
  }
  const hex = h1.toString(16).padStart(8, '0') + h2.toString(16).padStart(8, '0')
  return `00000000-0000-4000-8000-${hex.slice(0, 12)}`
}

/** Typed access to the brain's private generation store (test injection point). */
function generationStoreOf(brain: Brainy): GenerationStore {
  return (brain as unknown as { generationStore: GenerationStore }).generationStore
}

describe('8.0 Db API — generational MVCC', () => {
  const brains: Brainy[] = []
  const tempDirs: string[] = []

  /** Track a brain for afterEach teardown. */
  async function openMemoryBrain(): Promise<Brainy> {
    const brain = new Brainy({ requireSubtype: false, storage: { type: 'memory' } })
    await brain.init()
    brains.push(brain)
    return brain
  }

  /** Open (and track) a filesystem brain rooted at a fresh temp directory. */
  async function openFsBrain(dir?: string): Promise<{ brain: Brainy; dir: string }> {
    const rootDirectory = dir ?? makeTempDir()
    const brain = new Brainy({
      requireSubtype: false,
      storage: { type: 'filesystem', path: rootDirectory }
    })
    await brain.init()
    brains.push(brain)
    return { brain, dir: rootDirectory }
  }

  function makeTempDir(): string {
    const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'brainy-db-mvcc-'))
    tempDirs.push(dir)
    return dir
  }

  afterEach(async () => {
    for (const brain of brains.splice(0)) {
      try {
        await brain.close()
      } catch {
        // already closed by the test
      }
    }
    for (const dir of tempDirs.splice(0)) {
      await fs.promises.rm(dir, { recursive: true, force: true })
    }
  })

  // ==========================================================================
  // 1. Isolation
  // ==========================================================================
  it('proof 1 — a pinned Db reads its pinned state exactly across 200 transact mutations', async () => {
    const brain = await openMemoryBrain()

    const ids = Array.from({ length: 20 }, (_, i) => uid(`iso-e${i}`))
    await brain.transact(
      ids.map((id, i) => ({
        op: 'add' as const,
        id,
        type: NounType.Document,
        data: `doc-${i}`,
        vector: vec(i),
        metadata: { v: 0, slot: i }
      }))
    )

    const db = brain.now()
    const pinned = new Map<string, unknown>()
    for (const id of ids) {
      pinned.set(id, await db.get(id))
      expect(pinned.get(id)).not.toBeNull()
    }
    const pinnedFind = await db.find({ type: NounType.Document, limit: 100 })
    expect(pinnedFind.length).toBe(20)

    // 200 mutations: 198 updates over the first 18 ids + 2 removes.
    for (let i = 0; i < 200; i++) {
      if (i === 100) {
        await brain.transact([{ op: 'remove', id: uid('iso-e18') }])
      } else if (i === 150) {
        await brain.transact([{ op: 'remove', id: uid('iso-e19') }])
      } else {
        await brain.transact([
          { op: 'update', id: ids[i % 18], metadata: { v: i + 1, touchedAt: i } }
        ])
      }
    }

    // Live state moved…
    const liveUpdated = await brain.get(ids[0])
    expect((liveUpdated?.metadata as { v: number }).v).toBeGreaterThan(0)
    expect(await brain.get(uid('iso-e18'))).toBeNull()
    expect(await brain.get(uid('iso-e19'))).toBeNull()

    // …but EVERY read through the pinned Db is exactly the pinned state,
    // including the two entities removed after the pin.
    for (const id of ids) {
      expect(await db.get(id)).toEqual(pinned.get(id))
    }

    // Set-level isolation: metadata find() at the pin still sees all 20.
    const histFind = await db.find({ type: NounType.Document, limit: 100 })
    expect(histFind.length).toBe(20)
    for (const row of histFind) {
      expect((row.metadata as { v: number }).v).toBe(0)
    }

    await db.release()
  })

  // ==========================================================================
  // 2. Atomicity
  // ==========================================================================
  it('proof 2a — a plan-time failure in a later op applies nothing', async () => {
    const brain = await openMemoryBrain()
    const g0 = brain.generation()

    await expect(
      brain.transact([
        {
          op: 'add',
          id: uid('atomic-a'),
          type: NounType.Document,
          data: 'a',
          vector: vec(1),
          metadata: { ok: true }
        },
        { op: 'update', id: uid('never-created'), metadata: { boom: true } }
      ])
    ).rejects.toThrow(`Entity ${uid('never-created')} not found`)

    expect(brain.generation()).toBe(g0)
    expect(await brain.get(uid('atomic-a'))).toBeNull()
    expect((await brain.find({ type: NounType.Document, limit: 10 })).length).toBe(0)
  })

  it('proof 2b — an execution-phase failure rolls back every applied op (generation unchanged, zero partial records)', async () => {
    const brain = await openMemoryBrain()

    await brain.transact([
      {
        op: 'add',
        id: uid('atomic-base'),
        type: NounType.Document,
        data: 'base',
        vector: vec(0),
        metadata: { base: true }
      }
    ])
    const g0 = brain.generation()
    const storage = brain.storageAdapter

    // Real fault injection: the verb-metadata write of the LAST operation
    // fails once, after the earlier add operations have already executed —
    // exercising the production TransactionManager rollback path.
    const spy = vi
      .spyOn(storage, 'saveVerbMetadata')
      .mockImplementationOnce(async () => {
        throw new Error('disk full (injected)')
      })

    await expect(
      brain.transact([
        {
          op: 'add',
          id: uid('atomic-b'),
          type: NounType.Person,
          data: 'b',
          vector: vec(2),
          metadata: { name: 'b' }
        },
        {
          op: 'relate',
          from: uid('atomic-b'),
          to: uid('atomic-base'),
          type: VerbType.RelatedTo
        }
      ])
    ).rejects.toThrow('disk full (injected)')
    spy.mockRestore()

    // ZERO ops applied: generation unchanged, the add rolled back, no edge.
    expect(brain.generation()).toBe(g0)
    expect(await brain.get(uid('atomic-b'))).toBeNull()
    expect((await brain.related({ from: uid('atomic-b') })).length).toBe(0)
    expect((await brain.find({ type: NounType.Person, limit: 10 })).length).toBe(0)

    // No partial generation records: the staging directory was removed.
    const records = await storage.listRawObjects(`_generations/${g0 + 1}`)
    expect(records).toEqual([])

    // The store remains fully writable and consistent after the rollback.
    const db = await brain.transact([
      {
        op: 'add',
        id: uid('atomic-c'),
        type: NounType.Person,
        data: 'c',
        vector: vec(3),
        metadata: { name: 'c' }
      }
    ])
    expect(db.generation).toBe(g0 + 1)
    await db.release()
  })

  it('proof 2c — an ifRev conflict on a later update rejects the whole batch', async () => {
    const brain = await openMemoryBrain()
    await brain.transact([
      {
        op: 'add',
        id: uid('rev-e'),
        type: NounType.Document,
        data: 'rev',
        vector: vec(4),
        metadata: { v: 1 }
      }
    ])
    await brain.update({ id: uid('rev-e'), metadata: { v: 2 } }) // _rev is now 2
    const g0 = brain.generation()

    await expect(
      brain.transact([
        {
          op: 'add',
          id: uid('rev-new'),
          type: NounType.Document,
          data: 'new',
          vector: vec(5),
          metadata: {}
        },
        { op: 'update', id: uid('rev-e'), metadata: { v: 3 }, ifRev: 1 } // stale
      ])
    ).rejects.toThrow(RevisionConflictError)

    expect(brain.generation()).toBe(g0)
    expect(await brain.get(uid('rev-new'))).toBeNull()
    expect(((await brain.get(uid('rev-e')))?.metadata as { v: number }).v).toBe(2)
  })

  // ==========================================================================
  // 3. CAS (ifAtGeneration)
  // ==========================================================================
  it('proof 3 — ifAtGeneration commits when fresh and conflicts when stale', async () => {
    const brain = await openMemoryBrain()
    const db = brain.now()

    const committed = await brain.transact(
      [
        {
          op: 'add',
          id: uid('cas-e'),
          type: NounType.Document,
          data: 'cas',
          vector: vec(6),
          metadata: { n: 1 }
        }
      ],
      { ifAtGeneration: db.generation }
    )
    expect(committed.generation).toBe(db.generation + 1)

    try {
      await brain.transact(
        [{ op: 'update', id: uid('cas-e'), metadata: { n: 2 } }],
        { ifAtGeneration: db.generation } // stale — committed moved past it
      )
      expect.unreachable('should have thrown GenerationConflictError')
    } catch (err) {
      expect(err).toBeInstanceOf(GenerationConflictError)
      expect((err as GenerationConflictError).expected).toBe(db.generation)
      expect((err as GenerationConflictError).actual).toBe(committed.generation)
    }

    // Nothing committed by the conflicting attempt.
    expect(((await brain.get(uid('cas-e')))?.metadata as { n: number }).n).toBe(1)
    expect(brain.generation()).toBe(committed.generation)

    await db.release()
    await committed.release()
  })

  // ==========================================================================
  // 4. Snapshot integrity (persist → mutate → load)
  // ==========================================================================
  it('proof 4a — a persisted snapshot is immune to heavy source mutation (hard-link safety)', async () => {
    const { brain } = await openFsBrain()

    const ids = Array.from({ length: 10 }, (_, i) => uid(`snap-e${i}`))
    await brain.transact(
      ids.map((id, i) => ({
        op: 'add' as const,
        id,
        type: NounType.Document,
        data: `snapshot-doc-${i}`,
        vector: vec(i + 10),
        metadata: { generation: 'pre', slot: i }
      }))
    )
    const relDb = await brain.transact([
      { op: 'relate', from: ids[0], to: ids[1], type: VerbType.References },
      { op: 'relate', from: ids[1], to: ids[2], type: VerbType.References }
    ])
    await relDb.release()

    // Capture pre-mutation state, then snapshot.
    const preState = new Map<string, unknown>()
    for (const id of ids) {
      preState.set(id, await brain.get(id))
    }
    const preRelations = await brain.related({ from: ids[0] })
    expect(preRelations.length).toBe(1)

    const snapDir = path.join(makeTempDir(), 'snapshot')
    const db = brain.now()
    await db.persist(snapDir)
    await db.release()

    // Mutate the source heavily: rewrite every entity, delete some, add new.
    for (const [i, id] of ids.entries()) {
      await brain.transact([
        { op: 'update', id, data: `mutated-${i}`, metadata: { generation: 'post' } }
      ])
    }
    await brain.transact([{ op: 'remove', id: ids[0] }])
    await brain.transact([
      {
        op: 'add',
        id: uid('snap-after'),
        type: NounType.Document,
        data: 'added after snapshot',
        vector: vec(99),
        metadata: { generation: 'post' }
      }
    ])
    await brain.flush()

    // The snapshot opens as a self-contained store with the PRE state.
    const loaded = await Brainy.load(snapDir)
    for (const id of ids) {
      const entity = await loaded.get(id)
      expect(entity).toEqual(preState.get(id))
    }
    expect(await loaded.get(uid('snap-after'))).toBeNull()
    const loadedFind = await loaded.find({ type: NounType.Document, limit: 100 })
    expect(loadedFind.length).toBe(10)
    for (const row of loadedFind) {
      expect((row.metadata as { generation: string }).generation).toBe('pre')
    }
    const loadedRelations = await loaded.related({ from: ids[0] })
    expect(loadedRelations.length).toBe(1)
    expect(loadedRelations[0].to).toBe(ids[1])
    await loaded.release()

    // And asOf(path) is the instance-method route to the same snapshot.
    const viaAsOf = await brain.asOf(snapDir)
    expect(((await viaAsOf.get(ids[3])) as { metadata: { generation: string } }).metadata.generation).toBe('pre')
    await viaAsOf.release()
  })

  it('proof 4b — an in-memory brain persists to a directory loadable as a real store', async () => {
    const brain = await openMemoryBrain()
    await brain.transact([
      {
        op: 'add',
        id: uid('mem-e1'),
        type: NounType.Document,
        data: 'memory-persisted',
        vector: vec(21),
        metadata: { from: 'memory' }
      }
    ])

    const snapDir = path.join(makeTempDir(), 'memory-snapshot')
    const db = brain.now()
    await db.persist(snapDir)
    await db.release()

    const loaded = await Brainy.load(snapDir)
    const entity = await loaded.get(uid('mem-e1'))
    expect(entity).not.toBeNull()
    expect((entity?.metadata as { from: string }).from).toBe('memory')
    await loaded.release()
  })

  it('proof 4c — persist() refuses a view that history has moved past (honest boundary)', async () => {
    const brain = await openMemoryBrain()
    const db = brain.now()
    await brain.transact([
      {
        op: 'add',
        id: uid('stale-persist'),
        type: NounType.Document,
        data: 'x',
        vector: vec(30),
        metadata: {}
      }
    ])

    const snapDir = path.join(makeTempDir(), 'stale-snapshot')
    await expect(db.persist(snapDir)).rejects.toThrow(GenerationConflictError)
    await db.release()
  })

  // ==========================================================================
  // 5. Compaction safety
  // ==========================================================================
  it('proof 5 — compaction never breaks pinned reads; after release the records are reclaimed', async () => {
    const { brain } = await openFsBrain()
    const storage = brain.storageAdapter

    // Every transact() returns a PINNED Db — release the ones this test
    // does not keep, so compaction eligibility is determined solely by `db`.
    await (
      await brain.transact([
        {
          op: 'add',
          id: uid('compact-e'),
          type: NounType.Document,
          data: 'v1',
          vector: vec(40),
          metadata: { v: 1 }
        }
      ])
    ).release()
    await (
      await brain.transact([{ op: 'update', id: uid('compact-e'), metadata: { v: 2 } }])
    ).release()
    const db = brain.now() // pinned at v2
    const pinnedEntity = await db.get(uid('compact-e'))
    await (
      await brain.transact([{ op: 'update', id: uid('compact-e'), metadata: { v: 3 } }])
    ).release()
    await (
      await brain.transact([{ op: 'update', id: uid('compact-e'), metadata: { v: 4 } }])
    ).release()

    const recordsBefore = (await storage.listRawObjects('_generations')).length
    expect(recordsBefore).toBeGreaterThan(0)

    // Compact while pinned: record-sets above the pin survive, pinned reads stay correct.
    const first = await brain.compactHistory()
    expect(await db.get(uid('compact-e'))).toEqual(pinnedEntity)
    expect(((await db.get(uid('compact-e')))?.metadata as { v: number }).v).toBe(2)

    // After release, everything is reclaimable.
    await db.release()
    const second = await brain.compactHistory()
    expect(first.removedGenerations + second.removedGenerations).toBeGreaterThan(0)

    const recordsAfter = (await storage.listRawObjects('_generations')).length
    expect(recordsAfter).toBeLessThan(recordsBefore)
    expect(recordsAfter).toBe(0)

    // Compacted generations are honestly unreachable.
    await expect(brain.asOf(1)).rejects.toThrow(GenerationCompactedError)
  })

  // ==========================================================================
  // 6. with() — speculative overlays
  // ==========================================================================
  it('proof 6 — with() overlays are visible only on the overlay Db; brain and base Db untouched', async () => {
    const brain = await openMemoryBrain()
    await brain.transact([
      {
        op: 'add',
        id: uid('with-a'),
        type: NounType.Person,
        data: 'Ada',
        vector: vec(50),
        metadata: { v: 1 }
      },
      {
        op: 'add',
        id: uid('with-b'),
        type: NounType.Person,
        data: 'Grace',
        vector: vec(51),
        metadata: { v: 1 }
      }
    ])
    const g0 = brain.generation()
    const db = brain.now()

    const spec = await db.with([
      { op: 'update', id: uid('with-a'), metadata: { v: 2 } },
      {
        op: 'add',
        id: uid('with-new'),
        type: NounType.Project,
        data: 'Apollo',
        metadata: { speculative: true }
      },
      { op: 'remove', id: uid('with-b') },
      { op: 'relate', from: uid('with-a'), to: uid('with-new'), type: VerbType.WorksOn }
    ])

    // The overlay view sees everything…
    expect(spec.speculative).toBe(true)
    expect(((await spec.get(uid('with-a')))?.metadata as { v: number }).v).toBe(2)
    expect(await spec.get(uid('with-new'))).not.toBeNull()
    expect(await spec.get(uid('with-b'))).toBeNull()
    const specEdges = await spec.related({ from: uid('with-a') })
    expect(specEdges.length).toBe(1)
    expect(specEdges[0].to).toBe(uid('with-new'))
    const specFind = await spec.find({ type: NounType.Project, limit: 10 })
    expect(specFind.length).toBe(1)

    // …the base Db sees none of it…
    expect(db.speculative).toBe(false)
    expect(((await db.get(uid('with-a')))?.metadata as { v: number }).v).toBe(1)
    expect(await db.get(uid('with-new'))).toBeNull()
    expect(await db.get(uid('with-b'))).not.toBeNull()

    // …and the brain (disk, indexes, generation counter) is untouched.
    expect(brain.generation()).toBe(g0)
    expect(((await brain.get(uid('with-a')))?.metadata as { v: number }).v).toBe(1)
    expect(await brain.get(uid('with-new'))).toBeNull()
    expect((await brain.related({ from: uid('with-a') })).length).toBe(0)

    await spec.release()
    await db.release()
  })

  // ==========================================================================
  // 7. Generation monotonicity across close/reopen
  // ==========================================================================
  it('proof 7 — the generation counter never moves backwards across close/reopen', async () => {
    const dir = makeTempDir()
    const { brain: first } = await openFsBrain(dir)

    await first.transact([
      {
        op: 'add',
        id: uid('mono-e'),
        type: NounType.Document,
        data: 'v1',
        vector: vec(60),
        metadata: { v: 1 }
      }
    ])
    await first.transact([{ op: 'update', id: uid('mono-e'), metadata: { v: 2 } }])
    // Single-operation write — bumps the counter outside transact.
    await first.update({ id: uid('mono-e'), metadata: { v: 3 } })
    const beforeClose = first.generation()
    expect(beforeClose).toBeGreaterThanOrEqual(3)
    await first.close()

    const { brain: second } = await openFsBrain(dir)
    expect(second.generation()).toBeGreaterThanOrEqual(beforeClose)

    const db = await second.transact([{ op: 'update', id: uid('mono-e'), metadata: { v: 4 } }])
    expect(db.generation).toBeGreaterThan(beforeClose)
    await db.release()
  })

  // ==========================================================================
  // 8. Crash consistency
  // ==========================================================================
  it('proof 8 — a crash before the manifest rename recovers to the exact pre-transaction state', async () => {
    const dir = makeTempDir()
    const { brain: first } = await openFsBrain(dir)

    await first.transact([
      {
        op: 'add',
        id: uid('crash-e'),
        type: NounType.Document,
        data: 'stable',
        vector: vec(70),
        metadata: { v: 1 }
      }
    ])
    const committedGen = first.generation()

    // Simulated crash at the worst point of the REAL commit protocol: every
    // record is staged and the batch has executed, but the atomic manifest
    // rename — the commit point — never happens. The throwing injector skips
    // the abort cleanup exactly as a dead process would.
    generationStoreOf(first).setCommitFaultInjector((phase) => {
      if (phase === 'before-manifest-rename') {
        throw new Error('simulated process crash')
      }
    })
    await expect(
      first.transact([
        { op: 'update', id: uid('crash-e'), metadata: { v: 2 } },
        {
          op: 'add',
          id: uid('crash-new'),
          type: NounType.Document,
          data: 'uncommitted',
          vector: vec(71),
          metadata: {}
        }
      ])
    ).rejects.toThrow('simulated process crash')
    generationStoreOf(first).setCommitFaultInjector(undefined)

    // The in-process state is dirty (batch executed) — close flushes it all,
    // the realistic worst case for the recovery path.
    await first.close()

    // Reopen: recovery rolls the uncommitted generation back and rebuilds
    // the indexes from the repaired records.
    const { brain: second } = await openFsBrain(dir)
    const recovered = await second.get(uid('crash-e'))
    expect((recovered?.metadata as { v: number }).v).toBe(1)
    expect(await second.get(uid('crash-new'))).toBeNull()
    const found = await second.find({ type: NounType.Document, limit: 10 })
    expect(found.length).toBe(1)
    expect((found[0].metadata as { v: number }).v).toBe(1)

    // The crashed generation number is never reissued.
    expect(second.generation()).toBeGreaterThanOrEqual(committedGen + 1)
    const db = await second.transact([{ op: 'update', id: uid('crash-e'), metadata: { v: 5 } }])
    expect(db.generation).toBeGreaterThan(committedGen + 1)
    await db.release()
  })

  // ==========================================================================
  // 9. Versioned provider wiring
  // ==========================================================================
  it('proof 9 — a VersionedIndexProvider receives balanced pin/release as Db values live and die', async () => {
    const calls = {
      pins: [] as bigint[],
      releases: [] as bigint[],
      visibility: [] as bigint[]
    }

    const brain = new Brainy({ requireSubtype: false, storage: { type: 'memory' } })
    brain.use({
      name: 'test-versioned-graph-index',
      activate: async (ctx) => {
        ctx.registerProvider('graphIndex', (storage: StorageAdapter) =>
          Object.assign(new GraphAdjacencyIndex(storage), {
            generation: () => 0n,
            isGenerationVisible: (g: bigint) => {
              calls.visibility.push(g)
              return true
            },
            pin: (g: bigint) => {
              calls.pins.push(g)
            },
            release: (g: bigint) => {
              calls.releases.push(g)
            }
          })
        )
        return true
      }
    })
    await brain.init()
    brains.push(brain)

    const base = brain.generation()
    const db1 = brain.now()
    expect(calls.pins).toEqual([BigInt(base)])
    expect(calls.visibility).toEqual([BigInt(base)]) // consulted at pin time

    const db2 = await brain.transact([
      {
        op: 'add',
        id: uid('vp-e'),
        type: NounType.Document,
        data: 'pinned',
        vector: vec(80),
        metadata: {}
      }
    ])
    expect(calls.pins).toEqual([BigInt(base), BigInt(base + 1)])

    const db3 = await brain.asOf(base)
    expect(calls.pins).toEqual([BigInt(base), BigInt(base + 1), BigInt(base)])

    // A with() overlay takes its own pin on the same generation.
    const spec = await db2.with([{ op: 'update', id: uid('vp-e'), metadata: { spec: true } }])
    expect(calls.pins.length).toBe(4)

    await db1.release()
    await db2.release()
    await db3.release()
    await spec.release()
    await spec.release() // idempotent — must NOT double-release the pin

    expect(calls.releases.length).toBe(calls.pins.length)
    expect([...calls.releases].sort()).toEqual([...calls.pins].sort())
  })

  // ==========================================================================
  // Supporting guarantees: asOf, since, receipts, tx-log, honest errors
  // ==========================================================================
  it('asOf(generation) and asOf(Date) resolve historical state; future/negative are rejected', async () => {
    const brain = await openMemoryBrain()

    const tx1 = await brain.transact(
      [
        {
          op: 'add',
          id: uid('asof-e'),
          type: NounType.Document,
          data: 'v1',
          vector: vec(90),
          metadata: { v: 1 }
        }
      ],
      { meta: { reason: 'first commit' } }
    )
    await new Promise((resolve) => setTimeout(resolve, 5))
    const betweenCommits = new Date()
    await new Promise((resolve) => setTimeout(resolve, 5))
    await brain.transact([{ op: 'update', id: uid('asof-e'), metadata: { v: 2 } }])

    // By generation.
    const atFirst = await brain.asOf(tx1.generation)
    expect(((await atFirst.get(uid('asof-e')))?.metadata as { v: number }).v).toBe(1)
    expect(atFirst.timestamp).toBe(tx1.timestamp)
    await atFirst.release()

    // By timestamp: resolves to the newest commit at-or-before the instant.
    const atInstant = await brain.asOf(betweenCommits)
    expect(atInstant.generation).toBe(tx1.generation)
    expect(((await atInstant.get(uid('asof-e')))?.metadata as { v: number }).v).toBe(1)
    await atInstant.release()

    // Honest range errors.
    await expect(brain.asOf(brain.generation() + 100)).rejects.toThrow(RangeError)
    await expect(brain.asOf(-1)).rejects.toThrow(RangeError)
    await expect(brain.asOf('/no/such/snapshot/dir')).rejects.toThrow('not a snapshot directory')

    // The transact meta was reified into the tx-log.
    const lines = await brain.storageAdapter.readTxLogLines()
    const entries = lines.map((line) => JSON.parse(line))
    const first = entries.find((entry) => entry.generation === tx1.generation)
    expect(first?.meta).toEqual({ reason: 'first commit' })

    await tx1.release()
  })

  it('receipts resolve ids in input order, including intra-batch references and relate dedupe', async () => {
    const brain = await openMemoryBrain()

    const db = await brain.transact([
      { op: 'add', id: uid('rcpt-aa'), type: NounType.Person, data: 'a', vector: vec(91), metadata: {} },
      { op: 'add', id: uid('rcpt-bb'), type: NounType.Person, data: 'b', vector: vec(92), metadata: {} },
      { op: 'relate', from: uid('rcpt-aa'), to: uid('rcpt-bb'), type: VerbType.Knows },
      { op: 'update', id: uid('rcpt-bb'), metadata: { seen: true } }
    ])
    expect(db.receipt).toBeDefined()
    expect(db.receipt!.generation).toBe(db.generation)
    expect(db.receipt!.ids.length).toBe(4)
    expect(db.receipt!.ids[0]).toBe(uid('rcpt-aa'))
    expect(db.receipt!.ids[1]).toBe(uid('rcpt-bb'))
    expect(db.receipt!.ids[3]).toBe(uid('rcpt-bb'))
    const relationId = db.receipt!.ids[2]
    expect((await brain.related({ from: uid('rcpt-aa') }))[0].id).toBe(relationId)

    // Duplicate relate (same from/to/type) dedupes to the SAME id — both
    // within a batch and against committed state, mirroring relate().
    const dup = await brain.transact([
      { op: 'relate', from: uid('rcpt-aa'), to: uid('rcpt-bb'), type: VerbType.Knows }
    ])
    expect(dup.receipt!.ids[0]).toBe(relationId)
    expect((await brain.related({ from: uid('rcpt-aa') })).length).toBe(1)

    await db.release()
    await dup.release()
  })

  it('since() reports the ids changed between two pinned views', async () => {
    const brain = await openMemoryBrain()
    await brain.transact([
      { op: 'add', id: uid('since-a'), type: NounType.Document, data: 'a', vector: vec(93), metadata: {} }
    ])
    const before = brain.now()

    await brain.transact([
      { op: 'add', id: uid('since-b'), type: NounType.Document, data: 'b', vector: vec(94), metadata: {} },
      { op: 'relate', from: uid('since-a'), to: uid('since-b'), type: VerbType.References }
    ])
    await brain.transact([{ op: 'update', id: uid('since-a'), metadata: { touched: true } }])
    const after = brain.now()

    const changed = await after.since(before)
    expect([...changed.nouns].sort()).toEqual([uid('since-a'), uid('since-b')].sort())
    expect(changed.verbs.length).toBe(1)
    expect(changed.fromGeneration).toBe(before.generation)
    expect(changed.toGeneration).toBe(after.generation)

    // Direction is enforced.
    await expect(before.since(after)).rejects.toThrow(RangeError)

    await before.release()
    await after.release()
  })

  it('historical vector search is served at the pinned generation via index materialization', async () => {
    const brain = await openMemoryBrain()

    // Two one-hot vectors: orthogonal under cosine, so nearest-neighbor
    // results are fully deterministic.
    const axisA = Array.from({ length: 384 }, (_, i) => (i === 0 ? 1 : 0))
    const axisB = Array.from({ length: 384 }, (_, i) => (i === 1 ? 1 : 0))
    const axisC = Array.from({ length: 384 }, (_, i) => (i === 2 ? 1 : 0))

    await brain.transact([
      { op: 'add', id: uid('mat-a'), type: NounType.Document, data: 'a', vector: axisA, metadata: { who: 'a' } },
      { op: 'add', id: uid('mat-b'), type: NounType.Document, data: 'b', vector: axisB, metadata: { who: 'b' } },
      { op: 'add', id: uid('mat-del'), type: NounType.Document, data: 'del', vector: axisC, metadata: { who: 'del' } }
    ])
    const db = brain.now()

    // Mutate distinctively after the pin: swap a/b's vectors, delete 'del'.
    await brain.transact([
      { op: 'update', id: uid('mat-a'), vector: axisB },
      { op: 'update', id: uid('mat-b'), vector: axisA },
      { op: 'remove', id: uid('mat-del') }
    ])

    // Live index reflects the swap…
    const liveNearA = await brain.find({ vector: axisA, limit: 1 })
    expect(liveNearA[0].id).toBe(uid('mat-b'))

    // …the pinned view finds the OLD vector placement, including the
    // since-deleted entity, via the at-generation materialization.
    const pinnedNearA = await db.find({ vector: axisA, limit: 1 })
    expect(pinnedNearA[0].id).toBe(uid('mat-a'))
    const pinnedNearC = await db.find({ vector: axisC, limit: 1 })
    expect(pinnedNearC[0].id).toBe(uid('mat-del'))

    // Record-path reads on the same Db stay consistent with the materialization.
    expect(((await db.get(uid('mat-del')))?.metadata as { who: string }).who).toBe('del')

    // Released views refuse all reads (and free the materialization).
    await db.release()
    await expect(db.find({ vector: axisA, limit: 1 })).rejects.toThrow('released Db')
  })

  it('historical graph traversal is served at the pinned generation via index materialization', async () => {
    const brain = await openMemoryBrain()
    await brain.transact([
      { op: 'add', id: uid('trav-a'), type: NounType.Person, data: 'a', vector: vec(110), metadata: {} },
      { op: 'add', id: uid('trav-b'), type: NounType.Document, data: 'b', vector: vec(111), metadata: {} },
      { op: 'add', id: uid('trav-c'), type: NounType.Document, data: 'c', vector: vec(112), metadata: {} },
      { op: 'relate', from: uid('trav-a'), to: uid('trav-b'), type: VerbType.References }
    ])
    const db = brain.now()

    // Rewire the graph after the pin: a→b becomes a→c.
    const oldEdge = (await brain.related({ from: uid('trav-a') }))[0]
    await brain.transact([
      { op: 'unrelate', id: oldEdge.id },
      { op: 'relate', from: uid('trav-a'), to: uid('trav-c'), type: VerbType.References }
    ])

    // Live traversal sees the new wiring…
    const liveConnected = await brain.find({ connected: { from: uid('trav-a') }, limit: 10 })
    expect(liveConnected.map((row) => row.id)).toContain(uid('trav-c'))
    expect(liveConnected.map((row) => row.id)).not.toContain(uid('trav-b'))

    // …the pinned view traverses the OLD graph.
    const pinnedConnected = await db.find({ connected: { from: uid('trav-a') }, limit: 10 })
    expect(pinnedConnected.map((row) => row.id)).toContain(uid('trav-b'))
    expect(pinnedConnected.map((row) => row.id)).not.toContain(uid('trav-c'))

    // The record-path related() agrees with the materialized traversal.
    const pinnedEdges = await db.related({ from: uid('trav-a') })
    expect(pinnedEdges.length).toBe(1)
    expect(pinnedEdges[0].to).toBe(uid('trav-b'))

    await db.release()
  })

  it('historical aggregation computes at-generation values via index materialization', async () => {
    const brain = await openMemoryBrain()
    brain.defineAggregate({
      name: 'order_totals',
      source: { type: NounType.Document },
      groupBy: ['category'],
      metrics: {
        total: { op: 'sum', field: 'amount' },
        orders: { op: 'count' }
      }
    })

    await brain.transact([
      { op: 'add', id: uid('agg-1'), type: NounType.Document, data: 'o1', vector: vec(120), metadata: { category: 'invoice', amount: 10 } },
      { op: 'add', id: uid('agg-2'), type: NounType.Document, data: 'o2', vector: vec(121), metadata: { category: 'invoice', amount: 20 } }
    ])
    const db = brain.now()

    // Mutate after the pin: add an order and inflate an existing one.
    await brain.transact([
      { op: 'add', id: uid('agg-3'), type: NounType.Document, data: 'o3', vector: vec(122), metadata: { category: 'invoice', amount: 70 } },
      { op: 'update', id: uid('agg-1'), metadata: { amount: 100 } }
    ])

    // Live aggregate reflects the mutations…
    const liveRows = await brain.find({ aggregate: 'order_totals' })
    const liveInvoice = liveRows.find((row) => (row.metadata as { category: string }).category === 'invoice')
    expect((liveInvoice?.metadata as { total: number }).total).toBe(190)
    expect((liveInvoice?.metadata as { orders: number }).orders).toBe(3)

    // …the pinned view computes the aggregate over the at-G record set.
    const pinnedRows = await db.find({ aggregate: 'order_totals' })
    const pinnedInvoice = pinnedRows.find((row) => (row.metadata as { category: string }).category === 'invoice')
    expect((pinnedInvoice?.metadata as { total: number }).total).toBe(30)
    expect((pinnedInvoice?.metadata as { orders: number }).orders).toBe(2)

    await db.release()
  })

  it('asOf() pins serve the materialized surface; the build is cached per Db; release() closes the ephemeral reader', async () => {
    const brain = await openMemoryBrain()

    // Two one-hot vectors — orthogonal, fully deterministic neighbors.
    const axisA = Array.from({ length: 384 }, (_, i) => (i === 0 ? 1 : 0))
    const axisB = Array.from({ length: 384 }, (_, i) => (i === 1 ? 1 : 0))

    const tx = await brain.transact([
      { op: 'add', id: uid('asof-mat-a'), type: NounType.Document, data: 'a', vector: axisA, metadata: {} },
      { op: 'add', id: uid('asof-mat-b'), type: NounType.Document, data: 'b', vector: axisB, metadata: {} }
    ])
    const pinnedGeneration = tx.generation
    await tx.release()

    // Swap the vectors after the pin point, then open the past via asOf().
    await brain.transact([
      { op: 'update', id: uid('asof-mat-a'), vector: axisB },
      { op: 'update', id: uid('asof-mat-b'), vector: axisA }
    ])
    const db = await brain.asOf(pinnedGeneration)

    // The asOf() view serves index-accelerated queries at its pinned
    // generation — same materialized surface as a now() pin.
    expect((await brain.find({ vector: axisA, limit: 1 }))[0].id).toBe(uid('asof-mat-b'))
    expect((await db.find({ vector: axisA, limit: 1 }))[0].id).toBe(uid('asof-mat-a'))

    // The O(n at G) build runs ONCE per Db: the second index query reuses
    // the cached handle (typed access to the private cache, mirroring
    // generationStoreOf above).
    const internals = db as unknown as { materializedHandle?: Promise<HistoricalQueryHandle> }
    expect(internals.materializedHandle).toBeDefined()
    const handle = await internals.materializedHandle!
    expect((await db.find({ vector: axisB, limit: 1 }))[0].id).toBe(uid('asof-mat-b'))
    expect(await internals.materializedHandle).toBe(handle)

    // release() frees the materialization by CLOSING the ephemeral reader —
    // the reader itself refuses reads afterwards, proving its indexes and
    // timers were torn down (not merely hidden behind the released-Db guard).
    await db.release()
    await db.release() // idempotent
    await expect(db.find({ vector: axisA, limit: 1 })).rejects.toThrow('released Db')
    await expect(handle.find({ vector: axisA, limit: 1 })).rejects.toThrow('not initialized')
  })

  it('speculative overlays keep the one honest boundary: index queries and persist throw SpeculativeOverlayError', async () => {
    const brain = await openMemoryBrain()
    await brain.transact([
      { op: 'add', id: uid('spec-e'), type: NounType.Document, data: 'x', vector: vec(95), metadata: { v: 1 } }
    ])
    const db = brain.now()
    const spec = await db.with([{ op: 'update', id: uid('spec-e'), metadata: { v: 2 } }])

    // Metadata reads on the overlay are fully supported…
    expect(((await spec.get(uid('spec-e')))?.metadata as { v: number }).v).toBe(2)
    expect((await spec.find({ type: NounType.Document, where: { v: 2 } })).length).toBe(1)

    // …index-accelerated dimensions and persist throw the named error.
    await expect(spec.find({ query: 'anything' })).rejects.toThrow(SpeculativeOverlayError)
    await expect(spec.find({ vector: vec(95) })).rejects.toThrow(SpeculativeOverlayError)
    await expect(spec.find({ connected: { from: uid('spec-e') } })).rejects.toThrow(
      SpeculativeOverlayError
    )
    await expect(spec.persist(path.join(makeTempDir(), 'overlay-snap'))).rejects.toThrow(
      SpeculativeOverlayError
    )

    // The non-speculative base view is unaffected by the overlay boundary.
    expect(((await db.get(uid('spec-e')))?.metadata as { v: number }).v).toBe(1)

    await spec.release()
    await db.release()
  })

  it('restore() requires confirmation and replaces state from a snapshot (generation floor preserved)', async () => {
    const { brain } = await openFsBrain()
    await brain.transact([
      { op: 'add', id: uid('restore-e'), type: NounType.Document, data: 'keep', vector: vec(96), metadata: { v: 1 } }
    ])

    const snapDir = path.join(makeTempDir(), 'restore-snapshot')
    const db = brain.now()
    await db.persist(snapDir)
    await db.release()

    await brain.transact([{ op: 'update', id: uid('restore-e'), metadata: { v: 2 } }])
    await brain.transact([
      { op: 'add', id: uid('restore-extra'), type: NounType.Document, data: 'extra', vector: vec(97), metadata: {} }
    ])
    const beforeRestore = brain.generation()

    await expect(
      brain.restore(snapDir, { confirm: false })
    ).rejects.toThrow('confirm')

    await brain.restore(snapDir, { confirm: true })
    expect(((await brain.get(uid('restore-e')))?.metadata as { v: number }).v).toBe(1)
    expect(await brain.get(uid('restore-extra'))).toBeNull()
    expect((await brain.find({ type: NounType.Document, limit: 10 })).length).toBe(1)

    // The counter never moves backwards — pre-restore generations are not reissued.
    expect(brain.generation()).toBeGreaterThanOrEqual(beforeRestore)
    const after = await brain.transact([{ op: 'update', id: uid('restore-e'), metadata: { v: 9 } }])
    expect(after.generation).toBeGreaterThan(beforeRestore)
    await after.release()
  })

  it('restore() rebuilds graph adjacency — relationships survive a snapshot round-trip (id-mapper reloaded before graph)', async () => {
    const { brain } = await openFsBrain()
    const a = uid('rr-a')
    const b = uid('rr-b')
    const c = uid('rr-c')
    await brain.transact([
      { op: 'add', id: a, type: NounType.Person, data: 'a', vector: vec(1), metadata: {} },
      { op: 'add', id: b, type: NounType.Person, data: 'b', vector: vec(2), metadata: {} },
      { op: 'add', id: c, type: NounType.Document, data: 'c', vector: vec(3), metadata: {} },
      { op: 'relate', from: a, to: b, type: VerbType.RelatedTo },
      { op: 'relate', from: a, to: c, type: VerbType.References }
    ])
    expect((await brain.related(a)).map((r) => r.to).sort()).toEqual([b, c].sort())

    const snapDir = path.join(makeTempDir(), 'rel-snapshot')
    const db = brain.now()
    await db.persist(snapDir)
    await db.release()

    // Drop a + its edges live so the restore must genuinely RECONSTRUCT them.
    await brain.remove(a)
    expect(await brain.related(a)).toEqual([])

    await brain.restore(snapDir, { confirm: true })

    // a's relationships are back: the graph adjacency rebuilt by resolving each
    // verb's endpoints (sourceId/targetId → ints) through the id-mapper that
    // restore() reloaded from the snapshot BEFORE graphIndex.rebuild(). Without
    // that reload, a native adjacency keyed on those ints loses the edges.
    expect((await brain.related(a)).map((r) => r.to).sort()).toEqual([b, c].sort())
  })

  it('Db values are first-class: Brainy.open() + chained with() overlays compose', async () => {
    const brain = await Brainy.open({ requireSubtype: false, storage: { type: 'memory' } })
    brains.push(brain)
    expect(brain.isInitialized).toBe(true)

    await brain.transact([
      { op: 'add', id: uid('chain-e'), type: NounType.Document, data: 'x', vector: vec(98), metadata: { v: 1 } }
    ])
    const db = brain.now()
    const spec1 = await db.with([{ op: 'update', id: uid('chain-e'), metadata: { v: 2 } }])
    const spec2 = await spec1.with([{ op: 'update', id: uid('chain-e'), metadata: { v: 3 } }])

    expect(((await db.get(uid('chain-e')))?.metadata as { v: number }).v).toBe(1)
    expect(((await spec1.get(uid('chain-e')))?.metadata as { v: number }).v).toBe(2)
    expect(((await spec2.get(uid('chain-e')))?.metadata as { v: number }).v).toBe(3)
    expect(spec2).toBeInstanceOf(Db)

    await spec2.release()
    await spec1.release()
    await db.release()
  })

  it('transactionLog() includes single-op AND transact generations, newest first, with meta and limit', async () => {
    const brain = await openMemoryBrain()

    // Model-B: a single-op write is its OWN generation and IS logged (no meta —
    // tx metadata is a transact()-only concept). It is generation 1 on a fresh
    // brain (init-time infrastructure writes are the un-versioned gen-0 baseline).
    await brain.add({ id: uid('txlog-solo'), type: NounType.Document, data: 'solo', vector: vec(99), subtype: 'note' })
    const soloLog = await brain.transactionLog()
    expect(soloLog.map((entry) => entry.generation)).toEqual([1])
    expect(soloLog[0].meta).toBeUndefined()
    const soloGen = 1

    const first = await brain.transact(
      [{ op: 'add', id: uid('txlog-a'), type: NounType.Document, data: 'a', vector: vec(100), metadata: {} }],
      { meta: { author: 'job-1' } }
    )
    const second = await brain.transact(
      [{ op: 'update', id: uid('txlog-a'), metadata: { v: 2 } }],
      { meta: { author: 'job-2' } }
    )
    const third = await brain.transact([{ op: 'update', id: uid('txlog-a'), metadata: { v: 3 } }])

    const entries = await brain.transactionLog()
    // Newest first: the three transacts, then the single-op solo write (gen 1).
    expect(entries.map((entry) => entry.generation)).toEqual([
      third.generation,
      second.generation,
      first.generation,
      soloGen
    ])
    expect(entries[1].meta).toEqual({ author: 'job-2' })
    expect(entries[2].meta).toEqual({ author: 'job-1' })
    expect(entries[0].meta).toBeUndefined() // third() had no meta
    expect(entries[3].meta).toBeUndefined() // the single-op solo write carries no meta
    for (const entry of entries) {
      expect(entry.timestamp).toBeGreaterThan(0)
    }

    const limited = await brain.transactionLog({ limit: 2 })
    expect(limited.map((entry) => entry.generation)).toEqual([third.generation, second.generation])

    await first.release()
    await second.release()
    await third.release()
  })

  // ==========================================================================
  // 8b. Model-B single-op generation-stamping (every write versioned)
  // ==========================================================================
  it('Model-B — a now() pin freezes against single-op add/update/remove (the Model-A hole is closed)', async () => {
    const brain = await openMemoryBrain()
    const a = uid('mb-a')
    const b = uid('mb-b')
    // Seed `a` with a single-op add, then pin the current generation.
    await brain.add({ id: a, type: NounType.Document, data: 'a', vector: vec(1), metadata: { v: 1 } })
    const pin = brain.now()

    // Single-op mutations AFTER the pin: update `a`, add `b`.
    await brain.update({ id: a, metadata: { v: 2 } })
    await brain.add({ id: b, type: NounType.Document, data: 'b', vector: vec(2), metadata: { v: 1 } })

    // The pin is frozen at its generation — single-op writes do not leak through
    // it (pre-Model-B, the pin tracked the live single-op mutation).
    expect((await pin.get(a))?.metadata?.v).toBe(1)
    expect(await pin.get(b)).toBeNull()
    expect((await brain.get(a))?.metadata?.v).toBe(2)
    expect((await brain.get(b))?.metadata?.v).toBe(1)
    expect(pin.isHistorical()).toBe(true)

    // A single-op REMOVE also leaves the pin untouched.
    await brain.remove(a)
    expect((await pin.get(a))?.metadata?.v).toBe(1)
    expect(await brain.get(a)).toBeNull()
    await pin.release()
  })

  it('Model-B — historical find() overlays an un-flushed single-op write (overlay bound is generation, not committed)', async () => {
    const brain = await openMemoryBrain()
    const a = uid('ov-a')
    const b = uid('ov-b')
    await (
      await brain.transact([
        { op: 'add', id: a, type: NounType.Document, data: 'a', vector: vec(1), metadata: { v: 1 } },
        { op: 'add', id: b, type: NounType.Document, data: 'b', vector: vec(2), metadata: { v: 1 } }
      ])
    ).release()
    const at1 = await brain.asOf(1)

    // A single-op REMOVE of `b` lands AFTER the pin and is NOT flushed (pending).
    await brain.remove(b)

    const liveIds = (await brain.find({})).map((r) => r.id)
    const pastIds = (await at1.find({})).map((r) => r.id)
    // Live: `b` is gone. Historical (pinned at gen 1): the un-flushed removal is
    // overlaid out, so `b` is still present at its pinned state.
    expect(liveIds).toContain(a)
    expect(liveIds).not.toContain(b)
    expect(pastIds).toContain(a)
    expect(pastIds).toContain(b)
    await at1.release()
  })

  it('Model-B retention — explicit caps reclaim single-op history; committed history survives reopen', async () => {
    const { brain, dir } = await openFsBrain()
    const a = uid('ret-a')
    await brain.add({ id: a, type: NounType.Document, data: 'a', vector: vec(1), metadata: { v: 1 } })
    for (let v = 2; v <= 6; v++) await brain.update({ id: a, metadata: { v } })
    await brain.flush() // persist the per-write generations to disk
    expect(brain.generation()).toBe(6)

    // Cap to the 2 most recent generations — older single-op history is reclaimed.
    const res = await brain.compactHistory({ maxGenerations: 2 })
    expect(res.removedGenerations).toBeGreaterThan(0)
    expect(res.horizon).toBeGreaterThan(0)
    await brain.close()

    // Reopen: the survivors + live value are intact; below-horizon asOf throws.
    const { brain: reopened } = await openFsBrain(dir)
    expect((await reopened.get(a))?.metadata?.v).toBe(6)
    await expect(reopened.asOf(1)).rejects.toBeInstanceOf(GenerationCompactedError)
  })

  it('Model-B retention — setRetentionBudget drives adaptive reclaim on flush; live data intact', async () => {
    // Default brain → ADAPTIVE retention. A coordinator (e.g. cor's ResourceManager)
    // pushes a byte budget via setRetentionBudget(); auto-compaction on flush() reclaims
    // oldest history down toward it. Each update's before-image carries the full prior
    // 384-dim vector (~KBs), so ~13 generations far exceed a few-KB budget.
    const { brain } = await openFsBrain()
    const a = uid('ret-budget')
    await brain.add({ id: a, type: NounType.Document, data: 'v0', vector: vec(1), metadata: { v: 0 } })
    for (let v = 1; v <= 12; v++) await brain.update({ id: a, metadata: { v } })

    brain.setRetentionBudget(6000) // ~6 KB — well below the accumulated history
    await brain.flush() // group-commit + adaptive auto-compaction under the budget

    // History was reclaimed (the horizon advanced past the oldest generations)…
    expect(generationStoreOf(brain).horizon()).toBeGreaterThan(0)
    await expect(brain.asOf(1)).rejects.toBeInstanceOf(GenerationCompactedError)
    // …but the budget reclaims HISTORY only — the live record is untouched.
    expect((await brain.get(a))?.metadata?.v).toBe(12)
  })

  // ==========================================================================
  // 9. asOf() / released-Db error paths (Y.15 spot-checks)
  // ==========================================================================
  it('proof 9 — asOf() rejects out-of-range targets and a released Db throws on every read', async () => {
    const brain = await openMemoryBrain()
    await (
      await brain.transact([
        { op: 'add', id: uid('asof-err'), type: NounType.Document, data: 'x', metadata: { v: 1 } }
      ])
    ).release()
    const current = brain.generation()

    // Negative, non-integer, and future generations are honest RangeErrors —
    // never a silent clamp to a neighbouring generation.
    await expect(brain.asOf(-1)).rejects.toThrow(RangeError)
    await expect(brain.asOf(1.5)).rejects.toThrow(RangeError)
    await expect(brain.asOf(current + 999)).rejects.toThrow(RangeError)

    // A string that is not a snapshot directory is a descriptive error, not a crash.
    await expect(brain.asOf('/no/such/snapshot/dir')).rejects.toThrow(/not a snapshot directory/)

    // Use-after-release is rejected on every read method, naming the released generation.
    const db = brain.now()
    await db.release()
    await expect(db.get(uid('asof-err'))).rejects.toThrow(/released Db/)
    await expect(db.find({ where: { v: 1 } })).rejects.toThrow(/released Db/)
    await expect(db.related(uid('asof-err'))).rejects.toThrow(/released Db/)
  })
})