/**
 * @module db/factLog
 * @description The generation FACT LOG — an append-only, CRC-framed record of
 * every committed generation as an AFTER-IMAGE "fact": what each touched
 * entity/relationship BECAME (or a body-less tombstone when it was removed).
 * This is the dual-write half of the log-canonical transition: today the
 * before-image history + canonical tree remain authoritative; the fact log is
 * appended at the same commit points and reconciled to committed truth at
 * open, so consumers (index heals, replays, scans) can read one sequential,
 * self-verifying stream instead of walking the entity tree.
 *
 * ## Wire format (frozen; additive-only within a major)
 *
 * Fact (msgpack, POSITIONAL array — the segment header's formatVersion
 * governs the schema):
 *
 *   fact := [ generation:u64, timestamp:u64, ops, meta|nil, blobHashes|nil ]
 *   op   := [ kind:u8 (0=noun, 1=verb), id:bin16 (raw uuid bytes),
 *             record:[metaLeg, vecLeg] | nil ]        // nil = TOMBSTONE
 *
 * Segment file (`_generations/facts/seg-<firstGeneration, zero-padded 20>.bfl`):
 *
 *   header := magic "BFACTS\0\0" (8B) | formatVersion:u32 LE |
 *             firstGeneration:u64 LE | reserved 12B (ZEROED, verified)
 *   frame  := length:u32 LE | crc32c:u32 LE (of payload) | payload
 *
 * A fact is never split across segments; a torn tail (length overrun or CRC
 * mismatch) terminates that segment's scan — everything before it is intact.
 * Zero-padded names make lexicographic order == generation order.
 *
 * ## Invariant
 *
 * After {@link FactLog.open}, the log contains EXACTLY the committed prefix:
 * facts are appended BEFORE the commit point (inside the same durability
 * window), so a crash can only leave the log AHEAD of committed truth — open
 * truncates any fact beyond the committed generation. Absent generation =
 * never committed; present = committed. A scan can never see an uncommitted
 * fact.
 *
 * The manifest (`_generations/facts/manifest.json`, JSON — forensics stay
 * terminal-readable) is the single source of truth for the segment SET;
 * rotation flips it atomically (write-new → fsync → rename) BEFORE the new
 * tail's first byte exists, so no segment file is ever unaccounted for.
 *
 * ## Mixed-version logs (the v2 live-write cutover)
 *
 * The segment header's `formatVersion` selects the decoder PER SEGMENT:
 * v1 segments (ops-shaped facts, the format above) stay readable forever and
 * are NEVER rewritten; a NEW tail segment writes the v2 format
 * (`src/db/factLogFormat.ts` — record envelope, minted dense ints, genesis,
 * sector seals) whenever the int minter is installed ({@link FactLog.setIntMinter} —
 * the brain wires it from the metadata index's id mapper right after init).
 * A bare `FactLog` with no minter keeps writing v1 (there is no authority
 * that could reproduce int assignments, and 0 is never written). Cutover
 * mechanics on an existing v1 log: an EMPTY v1 tail is re-headed to v2 in
 * place; a non-empty v1 tail is sealed by an immediate rotation and the new
 * tail is v2. Decoded v2 facts map back to the SAME {@link CommitFact} shape
 * v1 consumers read (noun/verb ops with `{metadata, vector} | null` records) —
 * the vector wrapper object is reconstructed from the record's metadata leg
 * through the reserved-field hydration law (see `commitFactFromV2`).
 *
 * V2 tails additionally: write the `log.genesis` record (id-space width 64 +
 * the brain id, minted once into the manifest's additive `brainId` field) as
 * the first record of the FIRST fact of a brand-new log, and seal every
 * `sync()` to the header-declared sector size with pad frames that are
 * invisible to readers (torn-page defense at group-commit boundaries).
 */
import { encode as defaultEncode, decode as defaultDecode } from '@msgpack/msgpack'
import { crc32c } from '../utils/crc32c.js'
import { prodLog } from '../utils/logger.js'
import {
  FACT_LOG_FORMAT_V1,
  FACT_LOG_FORMAT_V2,
  DEFAULT_SEAL_SIZE,
  parseSegmentHeader,
  encodeSegmentHeaderV2,
  encodeFactV2,
  decodeFact as decodeFormatFact,
  decodeGroupV2,
  encodePadFrame,
  minPadFrameBytes,
  type CommitFactV2,
  type LogRecord,
  type EmbedPendingRecord,
  type EmbedLandedRecord,
  type BlobManifestRecord,
  type BootstrapBaselineRecord,
  type ProjectionNoteRecord
} from './factLogFormat.js'
import {
  splitNounMetadataRecord
} from '../types/reservedFields.js'
import { NounType } from '../types/graphTypes.js'
import { v4 as uuidv4 } from '../universal/uuid.js'

// Swappable msgpack implementation — defaults to the JS codec; a native
// provider (registered via the plugin registry's 'msgpack' key) may replace
// it. Byte-compatibility is the contract (positional arrays, bin16 ids).
let msgpackEncode: (value: unknown) => Uint8Array = defaultEncode
let msgpackDecode: (bytes: Uint8Array) => unknown = defaultDecode

/** Replace the msgpack encode/decode implementation at runtime. */
export function setFactCodec(impl: {
  encode: (value: unknown) => Uint8Array
  decode: (bytes: Uint8Array) => unknown
}): void {
  msgpackEncode = impl.encode
  msgpackDecode = impl.decode
}

/** Storage-root-relative home of the fact log. */
export const FACTS_PREFIX = '_generations/facts'
/** The facts manifest path (JSON). */
export const FACTS_MANIFEST_PATH = `${FACTS_PREFIX}/manifest.json`
/**
 * The v1 segment format version — the MANIFEST's formatVersion gate and the
 * header value of v1 (minter-less) tails. NOT the live-write ceiling: new
 * tails write `FACT_LOG_FORMAT_V2` (src/db/factLogFormat.ts) whenever the
 * int minter is installed; both versions are read forever, per segment.
 */
export const FACTS_FORMAT_VERSION = 1
/** Rotation threshold: seal the tail segment once it exceeds this many bytes. */
const SEGMENT_ROTATE_BYTES = 8 * 1024 * 1024
/** Segment header: magic(8) + formatVersion(4) + firstGeneration(8) + reserved(12). */
const HEADER_BYTES = 32
const MAGIC = new Uint8Array([0x42, 0x46, 0x41, 0x43, 0x54, 0x53, 0x00, 0x00]) // "BFACTS\0\0"
/** Frame prefix: length(4) + crc32c(4). */
const FRAME_PREFIX_BYTES = 8

/** One write inside a fact: what the id became (or a tombstone). */
export interface FactOp {
  kind: 'noun' | 'verb'
  id: string
  /** The AFTER-IMAGE legs, or `null` for a tombstone (the id was removed). */
  record: { metadata: unknown | null; vector: unknown | null } | null
}

/**
 * V2-native records beyond noun/verb ops that a fact may carry through the
 * ENCODER (types 6/7/8/9/10 of the v2 registry: embed markers, blob
 * manifests, projection notes, bootstrap baselines). The deferred-embedding
 * lifecycle PRODUCES types 6/7 today: `embed.pending` rides the deferred
 * write's own commit fact and `embed.landed` rides the background worker's
 * landing commit (recovery folds the pair back out of the log at open). The
 * blob lifecycle remodels onto type 8 in a later leg.
 */
export type FactMarkerRecord =
  | EmbedPendingRecord
  | EmbedLandedRecord
  | BlobManifestRecord
  | ProjectionNoteRecord
  | BootstrapBaselineRecord

/**
 * Mints the dense integer handle for an entity/verb id at fact-append time —
 * REQUIRED to be reproducible: a rebuilt id mapper must reproduce the same
 * assignments exactly, so the only legal implementation delegates to the
 * metadata index's id mapper (`getOrAssign`). Returns a POSITIVE bigint; a
 * minter that cannot resolve its mapper throws — an int of 0 is never written.
 */
export type FactIntMinter = (kind: 'noun' | 'verb', id: string) => bigint

/** One committed generation, as scanned back out of the log. */
export interface CommitFact {
  generation: number
  timestamp: number
  ops: FactOp[]
  meta?: Record<string, unknown>
  blobHashes?: string[]
  /**
   * V2-native marker records riding this fact (see {@link FactMarkerRecord}).
   * Optional and additive: absent on every v1 fact and on every fact the
   * current writers produce; requires a v2 tail to encode.
   */
  records?: FactMarkerRecord[]
}

/** The telemetry a scan batch carries (frozen shape). */
export interface FactScanBatch {
  facts: CommitFact[]
  firstGeneration: number
  lastGeneration: number
  factCount: number
  byteSize: number
  segmentId: string
}

/**
 * Liveness bound on a scan's FIRST batch (Stage-2 co-freeze, D1 contract):
 * `batches()` must yield its first batch — or fail loudly — within this many
 * ms of the first pull. A backlogged or damaged store may be SLOW, but it may
 * never be SILENT: a consumer awaiting the first batch is otherwise
 * indistinguishable from a wedge (the exact failure shape a production heal
 * hit against a generations-backlogged brain).
 */
export const SCANFACTS_FIRST_BATCH_MS = 10_000

/** The telemetry a scan OPEN returns (frozen shape). */
export interface FactScanHandle {
  headGeneration: number
  segmentCount: number
  approxFactCount: number
  /**
   * Ordered batches; a detected gap aborts LOUDLY, never a silent skip.
   * Liveness contract: the FIRST batch resolves or rejects within
   * {@link SCANFACTS_FIRST_BATCH_MS} of the first pull — never a silent hang.
   */
  batches: () => AsyncGenerator<FactScanBatch>
  /** Close telemetry — the invariant cross-check, valid after iteration ends. */
  summary: () => { factsYielded: number; segmentsRead: number }
}

/** Manifest entry for a sealed segment. */
interface SegmentEntry {
  file: string
  firstGeneration: number
  lastGeneration: number
  facts: number
  bytes: number
}

/** The facts manifest (JSON on disk). */
interface FactsManifest {
  formatVersion: number
  segments: SegmentEntry[]
  /** The append target. Its true content is established by scanning (crash tolerance). */
  tailSegment: string | null
  updatedAt: string
  /**
   * This brain's stable id (additive, v2 cutover): minted as a uuid at the
   * first v2 tail creation and never changed; the `log.genesis` record
   * carries it. Absent on logs that have never had a v2 tail.
   */
  brainId?: string
}

/** The narrow byte-level storage surface the fact log rides. */
export interface FactLogStorage {
  appendRawBytes(path: string, bytes: Uint8Array): Promise<void>
  readRawBytes(path: string): Promise<Uint8Array | null>
  writeRawBytes(path: string, bytes: Uint8Array): Promise<void>
  rawByteSize(path: string): Promise<number | null>
  readRawObject(path: string): Promise<any | null>
  writeRawObject(path: string, data: any): Promise<void>
  syncRawObjects(paths: string[]): Promise<void>
  deleteRawObject(path: string): Promise<void>
}

/** True when the storage adapter exposes every primitive the fact log needs. */
export function storageSupportsFactLog(storage: unknown): storage is FactLogStorage {
  const s = storage as Record<string, unknown>
  return (
    typeof s.appendRawBytes === 'function' &&
    typeof s.readRawBytes === 'function' &&
    typeof s.writeRawBytes === 'function' &&
    typeof s.rawByteSize === 'function'
  )
}

/** uuid string → 16 raw bytes (bin16 on the wire). */
function uuidToBytes(id: string): Uint8Array {
  const hex = id.replace(/-/g, '')
  if (hex.length !== 32) {
    // Non-uuid ids (legacy/natural keys) ride as UTF-8 with a length prefix
    // marker impossible for uuids: we refuse instead — the write API has
    // guaranteed uuid ids since 8.0, so anything else is a corruption signal.
    throw new Error(`fact log: id is not a uuid: ${id}`)
  }
  const bytes = new Uint8Array(16)
  for (let i = 0; i < 16; i++) {
    bytes[i] = parseInt(hex.slice(i * 2, i * 2 + 2), 16)
  }
  return bytes
}

/** 16 raw bytes → canonical lowercase uuid string. */
function bytesToUuid(bytes: Uint8Array): string {
  let hex = ''
  for (let i = 0; i < 16; i++) hex += bytes[i].toString(16).padStart(2, '0')
  return `${hex.slice(0, 8)}-${hex.slice(8, 12)}-${hex.slice(12, 16)}-${hex.slice(16, 20)}-${hex.slice(20)}`
}

/** Zero-padded segment filename: lexicographic order == generation order. */
function segmentFileName(firstGeneration: number): string {
  return `seg-${String(firstGeneration).padStart(20, '0')}.bfl`
}

/** Build a segment header. Reserved bytes are ZEROED (and verified on open). */
function buildHeader(firstGeneration: number): Uint8Array {
  const header = new Uint8Array(HEADER_BYTES)
  header.set(MAGIC, 0)
  const view = new DataView(header.buffer)
  view.setUint32(8, FACTS_FORMAT_VERSION, true)
  view.setBigUint64(12, BigInt(firstGeneration), true)
  // bytes 20..31 stay zero (reserved)
  return header
}

/** Encode one fact into a framed record (length + crc32c + msgpack payload). */
function encodeFrame(fact: CommitFact): Uint8Array {
  const payload = msgpackEncode([
    fact.generation,
    fact.timestamp,
    fact.ops.map((op) => [
      op.kind === 'noun' ? 0 : 1,
      uuidToBytes(op.id),
      op.record === null ? null : [op.record.metadata, op.record.vector]
    ]),
    fact.meta ?? null,
    fact.blobHashes && fact.blobHashes.length > 0 ? fact.blobHashes : null
  ])
  const frame = new Uint8Array(FRAME_PREFIX_BYTES + payload.length)
  const view = new DataView(frame.buffer)
  view.setUint32(0, payload.length, true)
  view.setUint32(4, crc32c(payload), true)
  frame.set(payload, FRAME_PREFIX_BYTES)
  return frame
}

/** Decode one msgpack payload back into a CommitFact. */
function decodeFact(payload: Uint8Array): CommitFact {
  const raw = msgpackDecode(payload) as unknown[]
  const [generation, timestamp, ops, meta, blobHashes] = raw as [
    number,
    number,
    Array<[number, Uint8Array, [unknown, unknown] | null]>,
    Record<string, unknown> | null,
    string[] | null
  ]
  return {
    generation: Number(generation),
    timestamp: Number(timestamp),
    ops: ops.map(([kind, idBytes, record]) => ({
      kind: kind === 0 ? ('noun' as const) : ('verb' as const),
      id: bytesToUuid(idBytes),
      record: record === null ? null : { metadata: record[0] ?? null, vector: record[1] ?? null }
    })),
    ...(meta ? { meta } : {}),
    ...(blobHashes && blobHashes.length > 0 ? { blobHashes } : {})
  }
}

/**
 * Deep-normalize a decoded v2 JSON position (metadata legs, meta maps,
 * notes) back to plain-JSON values: the v2 codec decodes msgpack int64/uint64
 * as `bigint` (its u64 wire discipline), but canonical records are JSON — a
 * metadata timestamp like `createdAt: 1786…` must come back as the NUMBER it
 * was encoded from. Safe-range bigints narrow exactly; anything beyond the
 * safe-integer range in a JSON position refuses loudly (it cannot have come
 * from a JSON write).
 */
function normalizeWireJson(value: unknown): unknown {
  if (typeof value === 'bigint') {
    if (value > BigInt(Number.MAX_SAFE_INTEGER) || value < -BigInt(Number.MAX_SAFE_INTEGER)) {
      throw new Error(
        `fact log v2: decoded integer ${value} exceeds the JS safe-integer range in a JSON position`
      )
    }
    return Number(value)
  }
  if (Array.isArray(value)) return value.map(normalizeWireJson)
  if (value && typeof value === 'object' && !(value instanceof Uint8Array)) {
    const out: Record<string, unknown> = {}
    for (const [k, v] of Object.entries(value)) out[k] = normalizeWireJson(v)
    return out
  }
  return value
}

/**
 * JSON-serialization equivalence for a v2 ENCODE-side JSON position: drop
 * undefined-valued object keys and map undefined array elements to null —
 * exactly what `JSON.stringify` does when canonical records are persisted.
 * Commit facts are built from write-cache-WARM objects that may still carry
 * undefined-valued engine keys (`service: undefined`, …) which the durable
 * JSON never had; msgpack would preserve them as nil (the v1 capture's known
 * wart), so the v2 capture — the future storage authority — sanitizes to the
 * DURABLE truth instead.
 */
function toJsonSafe(value: unknown): unknown {
  if (value === undefined) return null
  if (Array.isArray(value)) return value.map((v) => (v === undefined ? null : toJsonSafe(v)))
  if (value && typeof value === 'object' && !(value instanceof Uint8Array)) {
    const out: Record<string, unknown> = {}
    for (const [k, v] of Object.entries(value)) {
      if (v === undefined) continue
      out[k] = toJsonSafe(v)
    }
    return out
  }
  return value
}

/** Mirror of the storage layer's stored-timestamp normalization, minus its
 *  `Date.now()` fallback (a DECODER must be deterministic — an unreadable
 *  timestamp is omitted, and the divergence surfaces via the oracle). */
function reconstructTimestamp(value: unknown): number | undefined {
  if (typeof value === 'number' && value > 0) return value
  if (
    value !== null &&
    typeof value === 'object' &&
    typeof (value as { seconds?: unknown }).seconds === 'number'
  ) {
    return (value as { seconds: number }).seconds * 1000
  }
  return undefined
}

/**
 * Rebuild a noun's canonical VECTOR-FILE wrapper from a v2 after-image —
 * the read-side of the hydration law. Canonical noun vector files hold the
 * denormalized enumerable entity (`{id, vector, connections, level, type,
 * …reserved fields…, metadata}` — the write path's composition); the v2
 * record deliberately carries only the ENTITY state (metadata leg + embedding
 * floats), because connections/level are derived HNSW residue with their own
 * rebuild paths (empty in every 8.x write) and the denormalized top-level
 * fields are projections of the metadata leg. This reconstruction applies
 * the SAME split/hydrate law the storage layer uses
 * (`splitNounMetadataRecord` — the single source of truth in
 * src/types/reservedFields.ts; field map mirrors
 * `BaseStorage.hydrateNounWithMetadata`, undefined keys omitted exactly as
 * JSON serialization omits them), so in the no-drift case the reconstructed
 * wrapper digests byte-equal to canonical. A drifted denormalized copy
 * surfaces as an oracle `state-differs` — named, never silently absorbed.
 */
export function reconstructNounWrapper(
  id: string,
  metadataLeg: unknown,
  floats: number[]
): Record<string, unknown> {
  const { reserved, custom } = splitNounMetadataRecord(
    (metadataLeg ?? null) as Record<string, unknown> | null
  )
  const wrapper: Record<string, unknown> = {
    id,
    vector: floats,
    connections: {},
    level: 0,
    type: (reserved.noun as string) || NounType.Thing
  }
  if (reserved.subtype !== undefined) wrapper.subtype = reserved.subtype
  if (reserved.visibility !== undefined) wrapper.visibility = reserved.visibility
  const createdAt = reconstructTimestamp(reserved.createdAt)
  if (createdAt !== undefined) wrapper.createdAt = createdAt
  const updatedAt = reconstructTimestamp(reserved.updatedAt)
  if (updatedAt !== undefined) wrapper.updatedAt = updatedAt
  if (reserved.confidence !== undefined) wrapper.confidence = reserved.confidence
  if (reserved.weight !== undefined) wrapper.weight = reserved.weight
  if (reserved.service !== undefined) wrapper.service = reserved.service
  if (reserved.data !== undefined) wrapper.data = reserved.data
  if (reserved.createdBy !== undefined) wrapper.createdBy = reserved.createdBy
  wrapper._rev = typeof reserved._rev === 'number' ? reserved._rev : 1
  wrapper.metadata = custom
  return wrapper
}

/** Coerce a candidate embedding to `number[]`: plain arrays pass through
 *  (element-checked); numeric typed arrays (the JS HNSW rebuild path stores
 *  `Float32Array` vectors on the memory adapter) widen via `Array.from`. */
function floatsOf(candidate: unknown, context: string): number[] | undefined {
  if (Array.isArray(candidate)) {
    for (const el of candidate) {
      if (typeof el !== 'number') {
        throw new Error(`fact log v2: ${context} vector carries a non-number element`)
      }
    }
    return candidate as number[]
  }
  if (ArrayBuffer.isView(candidate) && !(candidate instanceof DataView)) {
    return Array.from(candidate as unknown as ArrayLike<number>)
  }
  return undefined
}

/** Extract the embedding float array from a canonical vector value: a bare
 *  float array (or numeric typed array) passes through; a wrapper object
 *  yields its `vector` floats; `null` stays `null`; anything else refuses
 *  loudly. */
function embeddingLegOf(value: unknown, context: string): number[] | null {
  if (value === null || value === undefined) return null
  const direct = floatsOf(value, context)
  if (direct !== undefined) return direct
  if (typeof value === 'object') {
    const nested = floatsOf((value as { vector?: unknown }).vector, context)
    if (nested !== undefined) return nested
  }
  throw new Error(
    `fact log v2: ${context} has a canonical vector record with no float vector — ` +
      `cannot encode its after-image`
  )
}

/**
 * Map one decoded v2 fact to the {@link CommitFact} shape every consumer
 * already reads: noun/verb after-images and tombstones become ops (vector
 * wrappers reconstructed — see {@link reconstructNounWrapper}); a
 * `batch.meta` record becomes `meta` when the fact position carries none;
 * `log.genesis` is log-level metadata (its width was verified at decode) and
 * is not an op; marker records surface on the additive `records` field so
 * nothing is silently dropped. Decoded JSON positions are normalized back
 * from the codec's bigint discipline ({@link normalizeWireJson}).
 */
function commitFactFromV2(f: CommitFactV2): CommitFact {
  const ops: FactOp[] = []
  const markers: FactMarkerRecord[] = []
  let batchMeta: Record<string, unknown> | undefined
  for (const r of f.records) {
    switch (r.type) {
      case 'noun.afterImage': {
        const metadata = normalizeWireJson(r.metadata) ?? null
        let vector: unknown | null = null
        if (r.vectorLeg !== null) {
          if (!Array.isArray(r.vectorLeg)) {
            throw new Error(
              `fact log v2: noun.afterImage ${r.id} carries a vector ref — this reader ` +
                `resolves inline vectors only (refs are a later leg); refusing`
            )
          }
          vector = reconstructNounWrapper(r.id, metadata, r.vectorLeg)
        }
        ops.push({ kind: 'noun', id: r.id, record: { metadata, vector } })
        break
      }
      case 'noun.tombstone':
        ops.push({ kind: 'noun', id: r.id, record: null })
        break
      case 'verb.afterImage': {
        const metadata = normalizeWireJson(r.metadata) ?? null
        if (r.vectorLeg !== null && !Array.isArray(r.vectorLeg)) {
          throw new Error(
            `fact log v2: verb.afterImage ${r.id} carries a vector ref — this reader ` +
              `resolves inline vectors only (refs are a later leg); refusing`
          )
        }
        // The canonical verb vector-file wrapper: endpoints + verb name ride
        // as first-class v2 wire fields precisely so this reconstruction is
        // exact ({id, vector, connections:{}, verb, sourceId, targetId} —
        // verbs carry no `level`).
        const vector: Record<string, unknown> = {
          id: r.id,
          vector: r.vectorLeg ?? [],
          connections: {},
          verb: r.verb,
          sourceId: r.sourceId,
          targetId: r.targetId
        }
        ops.push({ kind: 'verb', id: r.id, record: { metadata, vector } })
        break
      }
      case 'verb.tombstone':
        ops.push({ kind: 'verb', id: r.id, record: null })
        break
      case 'batch.meta':
        batchMeta = normalizeWireJson(r.meta) as Record<string, unknown>
        break
      case 'log.genesis':
        break // the log's birth certificate — log-level metadata, not an op
      case 'projection.note':
        markers.push({ ...r, note: normalizeWireJson(r.note) as Record<string, unknown> })
        break
      case 'bootstrap.baseline':
        markers.push({ ...r, metadata: normalizeWireJson(r.metadata) })
        break
      default:
        // embed.pending / embed.landed / blob.manifest carry no loose JSON maps.
        markers.push(r)
        break
    }
  }
  const meta = f.meta ? (normalizeWireJson(f.meta) as Record<string, unknown>) : batchMeta
  return {
    generation: f.generation,
    timestamp: f.timestamp,
    ops,
    ...(meta ? { meta } : {}),
    ...(f.blobHashes && f.blobHashes.length > 0 ? { blobHashes: f.blobHashes } : {}),
    ...(markers.length > 0 ? { records: markers } : {})
  }
}

/** One intact v2 frame's extent inside a segment (byte-slicing support). */
interface V2FrameExtent {
  /** Byte offset just past this frame. */
  end: number
  /** The frame's generation (0 for pad filler). */
  generation: number
  /** True when the frame is a pad (invisible filler). */
  isPad: boolean
}

/** Walk a v2 segment's intact frames (torn-tail terminated), returning each
 *  frame's extent — the byte-level view truncation slices against, so kept
 *  frames are never re-encoded (byte-immutability of CRC-covered frames). */
function walkV2Frames(bytes: Uint8Array): V2FrameExtent[] {
  const view = new DataView(bytes.buffer, bytes.byteOffset, bytes.byteLength)
  const extents: V2FrameExtent[] = []
  let offset = HEADER_BYTES
  while (offset + FRAME_PREFIX_BYTES <= bytes.length) {
    const length = view.getUint32(offset, true)
    const expectedCrc = view.getUint32(offset + 4, true)
    const start = offset + FRAME_PREFIX_BYTES
    const end = start + length
    if (end > bytes.length) break // torn tail
    const payload = bytes.subarray(start, end)
    if (crc32c(payload) !== expectedCrc) break // torn tail
    const fact = decodeFormatFact(payload, FACT_LOG_FORMAT_V2, {
      expectedIdSpaceWidth: 64
    }) as CommitFactV2
    extents.push({ end, generation: fact.generation, isPad: fact.records.length === 0 })
    offset = end
  }
  return extents
}

/**
 * The byte offset a v2 segment is cut at to keep exactly the facts with
 * `generation ≤ keepThrough`: the end of the last kept FACT frame (pads
 * between kept facts sit inside the retained span; pads after the cut are
 * dropped and re-sealed at the next sync). When nothing is dropped the cut
 * lands after the last intact frame — trailing pads retained, only a torn
 * suffix (if any) removed.
 */
function v2CutOffset(extents: V2FrameExtent[], keepThrough: number): number {
  let cut = HEADER_BYTES
  let lastIntactEnd = HEADER_BYTES
  for (const e of extents) {
    lastIntactEnd = e.end
    if (e.isPad) continue
    if (e.generation <= keepThrough) {
      cut = e.end
    } else {
      return cut // first beyond-keep fact: everything from here (pads included) goes
    }
  }
  return lastIntactEnd
}

/**
 * Parse a segment's bytes: verify the header, then walk frames until the end
 * or a torn tail (length overrun / CRC mismatch), which terminates the walk —
 * everything before it is intact. The header's formatVersion selects the
 * decoder: the v1 walk below is byte-identical to the original v1 reader;
 * v2 segments decode through the reference codec (`decodeGroupV2`, pads
 * invisible, id-space width verified at 64 — a disagreeing genesis throws
 * the codec's typed `GenesisWidthMismatchError`). Returns the decoded facts
 * plus the byte length of the VALID prefix (header + intact frames), which
 * reconciliation uses to cut a torn tail without re-encoding.
 */
function parseSegment(
  file: string,
  bytes: Uint8Array
): { facts: CommitFact[]; validBytes: number; formatVersion: number; sealSize?: number } {
  if (bytes.length < HEADER_BYTES) {
    prodLog.warn(`[FactLog] segment ${file} shorter than its header — treating as empty`)
    return { facts: [], validBytes: 0, formatVersion: 0 }
  }
  let header: { formatVersion: number; sealSize?: number }
  try {
    header = parseSegmentHeader(bytes.subarray(0, HEADER_BYTES))
  } catch (err) {
    throw new Error(`fact log: segment ${file}: ${(err as Error).message}`)
  }

  if (header.formatVersion === FACT_LOG_FORMAT_V2) {
    const group = decodeGroupV2(bytes.subarray(HEADER_BYTES), { expectedIdSpaceWidth: 64 })
    return {
      facts: group.facts.map(commitFactFromV2),
      validBytes: HEADER_BYTES + group.validBytes,
      formatVersion: FACT_LOG_FORMAT_V2,
      sealSize: header.sealSize
    }
  }

  // v1 walk — byte-identical to the original reader.
  const view = new DataView(bytes.buffer, bytes.byteOffset, bytes.byteLength)
  const facts: CommitFact[] = []
  let offset = HEADER_BYTES
  while (offset + FRAME_PREFIX_BYTES <= bytes.length) {
    const length = view.getUint32(offset, true)
    const expectedCrc = view.getUint32(offset + 4, true)
    const start = offset + FRAME_PREFIX_BYTES
    const end = start + length
    if (end > bytes.length) break // torn tail: frame length overruns the file
    const payload = bytes.subarray(start, end)
    if (crc32c(payload) !== expectedCrc) break // torn tail: payload CRC mismatch
    facts.push(decodeFact(payload))
    offset = end
  }
  return { facts, validBytes: offset, formatVersion: FACT_LOG_FORMAT_V1 }
}

/**
 * The generation fact log. One instance per open store; every method assumes
 * the single-writer discipline the generation store already enforces (calls
 * arrive under its commit mutex).
 */
export class FactLog {
  private readonly storage: FactLogStorage
  /** Rotation threshold (bytes); tests may lower it to exercise rotation. */
  private readonly rotateBytes: number
  private manifest: FactsManifest = {
    formatVersion: FACTS_FORMAT_VERSION,
    segments: [],
    tailSegment: null,
    updatedAt: new Date(0).toISOString()
  }
  /** Decoded facts of the TAIL segment (bounded by the rotation threshold). */
  private tailFacts: CommitFact[] = []
  /** Byte size of the tail segment file (valid prefix, pads included —
   *  pads count toward bytes but NEVER toward facts). */
  private tailBytes = 0
  /** Highest generation in the log (0 = empty). */
  private head = 0
  /** Segment paths appended since the last sync (the fsync batch). */
  private readonly dirtySegments = new Set<string>()
  /** The TAIL segment's on-disk format version (selects the live encoder). */
  private tailVersion: number = FACT_LOG_FORMAT_V1
  /** The tail's sector-seal size (v2 tails; from its header on reopen). */
  private tailSealSize: number = DEFAULT_SEAL_SIZE
  /** The v2 int minter (see {@link FactIntMinter}); null = v1 live writes. */
  private intMinter: FactIntMinter | null = null

  constructor(storage: FactLogStorage, options?: { rotateBytes?: number }) {
    this.storage = storage
    this.rotateBytes = options?.rotateBytes ?? SEGMENT_ROTATE_BYTES
  }

  /**
   * Install the v2 int minter — the capability gate for v2 LIVE WRITES.
   * With a minter installed, every NEW tail segment writes the v2 format and
   * after-image records carry minted dense ints; without one, live writes
   * stay v1 (no authority could reproduce int assignments, and 0 is never
   * written). The brain wires this from the metadata index's id mapper right
   * after the index is ready; an existing v1 tail cuts over on the next
   * append (empty tail: re-headed in place; non-empty: sealed by rotation).
   */
  setIntMinter(mint: FactIntMinter): void {
    this.intMinter = mint
  }

  /** The highest committed generation the log holds (0 = empty). */
  headGeneration(): number {
    return this.head
  }

  /**
   * True when this log has EVER had a v2 tail — the manifest's `brainId` is
   * minted at every v2 tail creation seam and never removed (the tail-version
   * check is a belt-and-braces second signal). Only v2 facts can carry marker
   * records, so marker folds (e.g. the deferred-embed recovery scan) skip
   * v1-only logs WHOLESALE on this one cheap check — no segment is read.
   */
  hasV2History(): boolean {
    return this.manifest.brainId !== undefined || this.tailVersion === FACT_LOG_FORMAT_V2
  }

  /**
   * Open the log and reconcile it to committed truth: read the manifest,
   * establish the tail's intact content (torn-tail scan), then TRUNCATE any
   * fact with `generation > committedGeneration` — those never committed (a
   * crash between fact-append and the commit point). After open, the log is
   * exactly the committed prefix.
   */
  /**
   * Read (without truncating) every intact fact ABOVE a generation — the
   * log-authority recovery surface: after a crash, facts beyond the
   * manifest watermark that survived with valid CRCs are ACKED writes in
   * durable-at-ack mode, and the owner REPLAYS them instead of letting
   * open() truncate them. Must be called BEFORE open() (it reads the raw
   * segments directly; the torn tail's invalid suffix is ignored exactly
   * like open() would).
   */
  /**
   * STREAMING twin of {@link FactLog.peekFactsAbove} for the recovery fold:
   * yields facts above the bound one SEGMENT at a time, ascending, without
   * ever materializing the whole log (a production first-boot fold OOM-class
   * allocation storm came from exactly that — GBs of decoded after-images in
   * one array while the process looked hung). Memory is one segment's worth.
   * Works manifest-direct (safe before {@link FactLog.open}). Ordering is
   * structural (segments rotate in order; appends are ordered within one) and
   * ASSERTED — a violation aborts loudly, never a silent misordered replay.
   */
  async *streamFactsAbove(committedGeneration: number): AsyncGenerator<CommitFact[], void> {
    const stored = (await this.storage.readRawObject(FACTS_MANIFEST_PATH)) as FactsManifest | null
    if (!stored || typeof stored !== 'object' || !Array.isArray(stored.segments)) return
    if (stored.formatVersion !== FACTS_FORMAT_VERSION) return
    const files = [...stored.segments.map((s) => s.file)]
    if (stored.tailSegment) files.push(stored.tailSegment)
    let lastGen = committedGeneration
    for (const file of files) {
      const bytes = await this.storage.readRawBytes(`${FACTS_PREFIX}/${file}`)
      if (bytes === null) continue
      const { facts } = parseSegment(file, bytes)
      const batch: CommitFact[] = []
      for (const f of facts) {
        if (f.generation <= committedGeneration) continue
        if (f.generation <= lastGen) {
          throw new Error(
            `fact log: streamFactsAbove found non-ascending generations ` +
              `(${f.generation} after ${lastGen} in ${file}) — refusing to replay out of order`
          )
        }
        lastGen = f.generation
        batch.push(f)
      }
      if (batch.length > 0) yield batch
    }
  }

  async peekFactsAbove(committedGeneration: number): Promise<CommitFact[]> {
    const stored = (await this.storage.readRawObject(FACTS_MANIFEST_PATH)) as FactsManifest | null
    if (!stored || typeof stored !== 'object' || !Array.isArray(stored.segments)) return []
    if (stored.formatVersion !== FACTS_FORMAT_VERSION) return []
    const out: CommitFact[] = []
    const files = [...stored.segments.map((s) => s.file)]
    if (stored.tailSegment) files.push(stored.tailSegment)
    for (const file of files) {
      const bytes = await this.storage.readRawBytes(`${FACTS_PREFIX}/${file}`)
      if (bytes === null) continue
      const { facts } = parseSegment(file, bytes)
      for (const f of facts) {
        if (f.generation > committedGeneration) out.push(f)
      }
    }
    out.sort((a, b) => a.generation - b.generation)
    return out
  }

  async open(committedGeneration: number): Promise<void> {
    const stored = (await this.storage.readRawObject(FACTS_MANIFEST_PATH)) as FactsManifest | null
    if (stored && typeof stored === 'object' && Array.isArray(stored.segments)) {
      if (stored.formatVersion !== FACTS_FORMAT_VERSION) {
        throw new Error(
          `fact log: manifest formatVersion ${stored.formatVersion}; this build reads ${FACTS_FORMAT_VERSION}`
        )
      }
      this.manifest = stored
    }

    // Drop sealed segments that sit ENTIRELY beyond committed truth (a crash
    // right after a rotation whose facts never committed), newest first.
    while (this.manifest.segments.length > 0) {
      const last = this.manifest.segments[this.manifest.segments.length - 1]
      if (last.firstGeneration > committedGeneration) {
        prodLog.warn(
          `[FactLog] dropping sealed segment ${last.file} (generations ${last.firstGeneration}..` +
            `${last.lastGeneration} never committed)`
        )
        await this.storage.deleteRawObject(`${FACTS_PREFIX}/${last.file}`)
        this.manifest.segments.pop()
        await this.persistManifest()
      } else if (last.lastGeneration > committedGeneration) {
        // A sealed segment STRADDLING committed truth: cut it back.
        await this.truncateSegmentTo(last.file, committedGeneration)
        const cut = await this.reloadSegmentEntry(last.file)
        this.manifest.segments[this.manifest.segments.length - 1] = cut
        await this.persistManifest()
        break
      } else {
        break
      }
    }

    // Establish the tail: scan its intact prefix, then truncate beyond
    // committed truth (the common crash shape: buffered single-op facts whose
    // counter never went durable).
    if (this.manifest.tailSegment) {
      const tailPath = `${FACTS_PREFIX}/${this.manifest.tailSegment}`
      const bytes = await this.storage.readRawBytes(tailPath)
      if (bytes === null) {
        // Manifest named a tail whose first byte never landed — an empty
        // tail. Its header (and format version) is established at the next
        // append (see the tail-provisioning ladder there).
        this.tailFacts = []
        this.tailBytes = 0
      } else {
        const parsed = parseSegment(this.manifest.tailSegment, bytes)
        const { facts, validBytes } = parsed
        this.tailVersion =
          parsed.formatVersion === FACT_LOG_FORMAT_V2 ? FACT_LOG_FORMAT_V2 : FACT_LOG_FORMAT_V1
        this.tailSealSize = parsed.sealSize ?? DEFAULT_SEAL_SIZE
        const kept = facts.filter((f) => f.generation <= committedGeneration)
        if (kept.length !== facts.length || validBytes !== bytes.length) {
          const dropped = facts.length - kept.length
          if (dropped > 0) {
            prodLog.warn(
              `[FactLog] truncating ${dropped} uncommitted fact(s) beyond generation ` +
                `${committedGeneration} from the tail (never committed)`
            )
          }
          if (this.tailVersion === FACT_LOG_FORMAT_V2) {
            // V2: byte-slice at frame boundaries — CRC-covered frames are
            // byte-immutable; a truncation never re-encodes what it keeps.
            const cut = v2CutOffset(walkV2Frames(bytes), committedGeneration)
            await this.storage.writeRawBytes(tailPath, bytes.subarray(0, cut))
            this.tailFacts = kept
            this.tailBytes = cut
          } else {
            await this.rewriteTail(kept)
          }
        } else {
          this.tailFacts = facts
          this.tailBytes = validBytes
        }
      }
    }

    this.head = this.computeHead()
  }

  /**
   * Append one committed generation's fact. NOT durable until {@link sync} —
   * the caller batches durability at its commit barrier (transact syncs in
   * the same call; Model-B group-commit syncs at flush).
   *
   * Tail provisioning (in order): a missing tail starts one; a named tail
   * whose header never landed (manifest-first crash) gets its header now; an
   * existing V1 tail cuts over to v2 once the minter is installed (empty:
   * re-headed in place, non-empty: sealed by rotation — v1 segments are never
   * rewritten); a full tail rotates. The frame then encodes in the TAIL's
   * format: v2 tails carry after-image records with minted ints (and the
   * genesis record on the very first fact of a brand-new log); v1 tails keep
   * the v1 wire format byte-identically.
   */
  async append(fact: CommitFact): Promise<void> {
    if (fact.generation <= this.head) {
      throw new Error(
        `fact log: non-monotonic append (generation ${fact.generation} ≤ head ${this.head})`
      )
    }
    if (this.manifest.tailSegment === null) {
      await this.startTail(fact.generation)
    } else if (this.tailBytes === 0) {
      await this.reinitializeTailHeader()
    } else if (this.intMinter !== null && this.tailVersion === FACT_LOG_FORMAT_V1) {
      if (this.tailFacts.length === 0 && this.tailBytes <= HEADER_BYTES) {
        await this.upgradeEmptyTailToV2()
      } else {
        await this.rotate(fact.generation)
      }
    } else if (this.tailBytes >= this.rotateBytes) {
      await this.rotate(fact.generation)
    }

    let frame: Uint8Array
    if (this.tailVersion === FACT_LOG_FORMAT_V2) {
      const records = this.buildV2Records(fact)
      if (this.needsGenesis()) {
        if (this.ensureBrainId()) await this.persistManifest()
        records.unshift(this.genesisRecord())
      }
      frame = encodeFactV2({
        generation: fact.generation,
        timestamp: fact.timestamp,
        records,
        ...(fact.meta ? { meta: toJsonSafe(fact.meta) as Record<string, unknown> } : {}),
        ...(fact.blobHashes && fact.blobHashes.length > 0 ? { blobHashes: fact.blobHashes } : {})
      })
    } else {
      if (fact.records && fact.records.length > 0) {
        throw new Error(
          `fact log: marker records (${fact.records.map((r) => r.type).join(', ')}) require a ` +
            `v2 tail segment — this log's tail is v1 (no int minter installed); refusing rather ` +
            `than silently dropping them`
        )
      }
      frame = encodeFrame(fact)
    }
    const tailPath = `${FACTS_PREFIX}/${this.manifest.tailSegment}`
    await this.storage.appendRawBytes(tailPath, frame)
    this.tailFacts.push(fact)
    this.tailBytes += frame.length
    this.head = fact.generation
    this.dirtySegments.add(tailPath)
  }

  /**
   * Fsync every segment appended since the last sync. SEALS AT SYNC: a v2
   * tail is first padded to its sector-seal boundary (one pad frame,
   * invisible to readers; a gap smaller than the smallest constructible pad
   * frame pads through one extra sector — the codec's rule), so every
   * durability barrier leaves the tail sector-aligned: a torn page can only
   * tear INSIDE the group being written, never a previously-sealed one.
   */
  async sync(): Promise<void> {
    await this.padTailToSealBoundary()
    if (this.dirtySegments.size === 0) return
    const paths = [...this.dirtySegments]
    this.dirtySegments.clear()
    await this.storage.syncRawObjects(paths)
  }

  // --- GROUP COMMIT ON THE LOG (durable-at-ack mode) ------------------------
  // Classic group commit: concurrent writers append, then join ONE fsync
  // whose completion releases every covered ack. Two slots — the running
  // sync and at most one queued behind it — give the covering guarantee:
  // an append followed by ensureSynced() is always covered, because the
  // sync it awaits STARTS after the append landed (a running sync that
  // may have snapshotted earlier is never joined; the queued one is).
  private syncRunning: Promise<void> | null = null
  private syncQueued: Promise<void> | null = null

  /**
   * Await a sync that covers every byte appended before this call. Many
   * concurrent callers share one fsync (solo caller = immediate sync). The
   * durability contract of an acked write in log-durable mode: this promise
   * resolving means the caller's frames survive power loss.
   */
  async ensureSynced(): Promise<void> {
    if (this.syncQueued) {
      // A sync that has NOT started yet exists — it will snapshot after our
      // append, so it covers us.
      return this.syncQueued
    }
    if (this.syncRunning) {
      // The running sync may have snapshotted before our append — queue the
      // next one behind it and join that.
      const queued = this.syncRunning
        .catch(() => {})
        .then(() => {
          // Promote: the queued sync becomes the running one.
          this.syncQueued = null
          this.syncRunning = this.sync().finally(() => {
            this.syncRunning = null
          })
          return this.syncRunning
        })
      this.syncQueued = queued
      return queued
    }
    this.syncRunning = this.sync().finally(() => {
      this.syncRunning = null
    })
    return this.syncRunning
  }

  /**
   * Open a scan over committed facts. The scan runs against a MANIFEST
   * SNAPSHOT (sealed segments + the tail's decoded facts at open) — exactly-
   * once per fact, inclusive bounds, stable under concurrent appends. Gaps
   * abort LOUDLY: a missing generation inside a segment's declared range is
   * corruption, never silently skipped.
   */
  scanFacts(options?: {
    fromGeneration?: number
    toGeneration?: number
    kinds?: Array<'noun' | 'verb'>
    batchSize?: number
    /** Test override for the first-batch liveness bound (default {@link SCANFACTS_FIRST_BATCH_MS}). */
    firstBatchTimeoutMs?: number
  }): FactScanHandle {
    const from = options?.fromGeneration ?? 1
    const to = options?.toGeneration ?? this.head
    const kinds = options?.kinds
    const batchSize = Math.max(1, options?.batchSize ?? 256)

    // Snapshot: the segment list + tail content as of NOW.
    const segments = this.manifest.segments.filter(
      (s) => s.lastGeneration >= from && s.firstGeneration <= to
    )
    const tailSnapshot = this.tailFacts.filter((f) => f.generation >= from && f.generation <= to)
    const tailId = this.manifest.tailSegment ?? 'tail'
    const approxFactCount =
      segments.reduce((sum, s) => sum + s.facts, 0) + tailSnapshot.length

    let factsYielded = 0
    let segmentsRead = 0
    const storage = this.storage

    async function* batches(this: void): AsyncGenerator<FactScanBatch> {
      let expectedNext = 0 // gap detection: generations are monotonic, not necessarily dense
      const emit = (facts: CommitFact[], segmentId: string, byteSize: number): FactScanBatch => ({
        facts,
        firstGeneration: facts[0].generation,
        lastGeneration: facts[facts.length - 1].generation,
        factCount: facts.length,
        byteSize,
        segmentId
      })
      const filterOps = (fact: CommitFact): CommitFact =>
        kinds
          ? { ...fact, ops: fact.ops.filter((op) => kinds.includes(op.kind)) }
          : fact

      for (const entry of segments) {
        const bytes = await storage.readRawBytes(`${FACTS_PREFIX}/${entry.file}`)
        if (bytes === null) {
          throw new Error(
            `fact log: sealed segment ${entry.file} is MISSING — the log is damaged; aborting scan`
          )
        }
        const { facts } = parseSegment(entry.file, bytes)
        segmentsRead++
        const inRange = facts.filter((f) => f.generation >= from && f.generation <= to)
        for (const f of inRange) {
          if (f.generation <= expectedNext - 1) {
            throw new Error(`fact log: out-of-order fact ${f.generation} in ${entry.file} — aborting scan`)
          }
          expectedNext = f.generation + 1
        }
        for (let i = 0; i < inRange.length; i += batchSize) {
          const slice = inRange.slice(i, i + batchSize).map(filterOps)
          if (slice.length === 0) continue
          factsYielded += slice.length
          yield emit(slice, entry.file, slice.reduce((n, f) => n + encodeFrame(f).length, 0))
        }
      }

      if (tailSnapshot.length > 0) {
        segmentsRead++
        for (const f of tailSnapshot) {
          if (f.generation <= expectedNext - 1) {
            throw new Error(`fact log: out-of-order fact ${f.generation} in the tail — aborting scan`)
          }
          expectedNext = f.generation + 1
        }
        for (let i = 0; i < tailSnapshot.length; i += batchSize) {
          const slice = tailSnapshot.slice(i, i + batchSize).map(filterOps)
          factsYielded += slice.length
          yield emit(slice, tailId, slice.reduce((n, f) => n + encodeFrame(f).length, 0))
        }
      }
    }

    // Liveness wrapper: the FIRST pull races the contract deadline. Only the
    // first — the bound is time-to-first-batch (proof the producer is alive),
    // not per-batch pacing; and it runs only while a pull is actually pending,
    // so consumer think-time between pulls never counts against the producer.
    const firstBatchTimeoutMs = options?.firstBatchTimeoutMs ?? SCANFACTS_FIRST_BATCH_MS
    async function* batchesWithLiveness(this: void): AsyncGenerator<FactScanBatch> {
      const inner = batches()
      let timer: NodeJS.Timeout | undefined
      try {
        const deadline = new Promise<never>((_, reject) => {
          timer = setTimeout(
            () =>
              reject(
                new Error(
                  `fact log: scanFacts produced no first batch within ${firstBatchTimeoutMs}ms ` +
                    `(liveness contract) — the store is wedged or unreadably slow; aborting scan LOUDLY ` +
                    `instead of hanging the consumer.`
                )
              ),
            firstBatchTimeoutMs
          )
          timer.unref?.()
        })
        const first = await Promise.race([inner.next(), deadline])
        if (first.done) return
        yield first.value
      } finally {
        clearTimeout(timer)
      }
      yield* inner
    }

    return {
      headGeneration: this.head,
      segmentCount: segments.length + (tailSnapshot.length > 0 ? 1 : 0),
      approxFactCount,
      batches: batchesWithLiveness,
      summary: () => ({ factsYielded, segmentsRead })
    }
  }

  /**
   * The mmap fast path (capability handoff): the immutable sealed segment
   * files covering `fromGeneration`, in order. The TAIL is deliberately NOT
   * included — it is append-mutable; consumers read it via {@link scanFacts}.
   */
  segmentPaths(options?: { fromGeneration?: number }): string[] {
    const from = options?.fromGeneration ?? 1
    return this.manifest.segments
      .filter((s) => s.lastGeneration >= from)
      .map((s) => `${FACTS_PREFIX}/${s.file}`)
  }

  /**
   * Drop every fact with `generation > keepThrough` — the in-session abort
   * compensation: a transact appends its fact BEFORE the commit point, so a
   * real (non-crash) abort after the append must take the fact back out. The
   * dropped facts can only live in the TAIL (they were just appended); the
   * rewrite is atomic and bounded by the rotation threshold.
   */
  async dropAbove(keepThrough: number): Promise<void> {
    if (this.head <= keepThrough) return
    const kept = this.tailFacts.filter((f) => f.generation <= keepThrough)
    if (kept.length === this.tailFacts.length) {
      throw new Error(
        `fact log: dropAbove(${keepThrough}) found no droppable facts in the tail ` +
          `(head ${this.head}) — the fact to drop was already sealed; the log needs reopen`
      )
    }
    if (this.tailVersion === FACT_LOG_FORMAT_V2) {
      // V2: byte-slice at frame boundaries (kept frames stay byte-identical;
      // pads between kept facts are retained inside the prefix, trailing pads
      // go and the next sync re-seals). The dropped frames may be unsynced —
      // readRawBytes is read-after-write coherent over the append path.
      const file = this.manifest.tailSegment
      if (!file) return
      const tailPath = `${FACTS_PREFIX}/${file}`
      const bytes = await this.storage.readRawBytes(tailPath)
      if (bytes === null) {
        throw new Error(`fact log: dropAbove(${keepThrough}) cannot read the tail segment ${file}`)
      }
      const cut = v2CutOffset(walkV2Frames(bytes), keepThrough)
      await this.storage.writeRawBytes(tailPath, bytes.subarray(0, cut))
      this.tailFacts = kept
      this.tailBytes = cut
    } else {
      await this.rewriteTail(kept)
    }
    this.head = this.computeHead()
  }

  // -- internals -------------------------------------------------------------

  private computeHead(): number {
    if (this.tailFacts.length > 0) return this.tailFacts[this.tailFacts.length - 1].generation
    const sealed = this.manifest.segments
    if (sealed.length > 0) return sealed[sealed.length - 1].lastGeneration
    return 0
  }

  /** The header bytes for a NEW tail: v2 whenever the minter is installed. */
  private newTailHeader(firstGeneration: number): Uint8Array {
    return this.intMinter !== null
      ? encodeSegmentHeaderV2(firstGeneration, DEFAULT_SEAL_SIZE)
      : buildHeader(firstGeneration)
  }

  /** Record the just-created tail's format in memory (mirrors its header). */
  private noteFreshTail(): void {
    this.tailVersion = this.intMinter !== null ? FACT_LOG_FORMAT_V2 : FACT_LOG_FORMAT_V1
    this.tailSealSize = DEFAULT_SEAL_SIZE
  }

  /** Create the very first tail segment (manifest-first, then header bytes). */
  private async startTail(firstGeneration: number): Promise<void> {
    const file = segmentFileName(firstGeneration)
    this.manifest.tailSegment = file
    if (this.intMinter !== null) this.ensureBrainId()
    await this.persistManifest()
    await this.storage.appendRawBytes(`${FACTS_PREFIX}/${file}`, this.newTailHeader(firstGeneration))
    this.tailFacts = []
    this.tailBytes = HEADER_BYTES
    this.noteFreshTail()
  }

  /**
   * Seal the tail into the manifest and start a new one. Manifest-first: the
   * flip both seals the old tail AND names the new one atomically, so no
   * segment file ever exists unaccounted for. The NEW tail's format follows
   * the minter gate ({@link newTailHeader}) — this is also the v1→v2 cutover
   * seam for a non-empty v1 tail (sealed as-is, never rewritten).
   */
  private async rotate(nextGeneration: number): Promise<void> {
    const sealedFile = this.manifest.tailSegment
    if (!sealedFile) return
    // Seal what the tail actually holds (sync() also sector-seals a v2 tail).
    await this.sync() // sealed segments are always fully durable
    const entry: SegmentEntry = {
      file: sealedFile,
      firstGeneration: this.tailFacts[0]?.generation ?? nextGeneration,
      lastGeneration: this.tailFacts[this.tailFacts.length - 1]?.generation ?? nextGeneration - 1,
      facts: this.tailFacts.length,
      bytes: this.tailBytes
    }
    const newFile = segmentFileName(nextGeneration)
    this.manifest.segments.push(entry)
    this.manifest.tailSegment = newFile
    if (this.intMinter !== null) this.ensureBrainId()
    await this.persistManifest()
    await this.storage.appendRawBytes(`${FACTS_PREFIX}/${newFile}`, this.newTailHeader(nextGeneration))
    this.tailFacts = []
    this.tailBytes = HEADER_BYTES
    this.noteFreshTail()
  }

  /**
   * The v1→v2 cutover for an EMPTY v1 tail: re-head it in place (nothing but
   * the 32-byte header exists, so no v1 frame is ever rewritten). Also the
   * cheapest cutover shape: brand-new brains whose first tail predates the
   * minter installation converge here on their first post-install append.
   */
  private async upgradeEmptyTailToV2(): Promise<void> {
    const file = this.manifest.tailSegment
    if (!file) return
    if (this.ensureBrainId()) await this.persistManifest()
    const first = this.segmentFirstGenerationFromName(file)
    const path = `${FACTS_PREFIX}/${file}`
    await this.storage.writeRawBytes(path, encodeSegmentHeaderV2(first, DEFAULT_SEAL_SIZE))
    this.tailBytes = HEADER_BYTES
    this.tailVersion = FACT_LOG_FORMAT_V2
    this.tailSealSize = DEFAULT_SEAL_SIZE
    this.dirtySegments.add(path)
  }

  /**
   * A manifest-named tail whose header never landed (crash between the
   * manifest flip and the first header byte — previously this appended
   * frames into a headerless file the next open could not parse): write the
   * header now, in the CURRENT format gate.
   */
  private async reinitializeTailHeader(): Promise<void> {
    const file = this.manifest.tailSegment
    if (!file) return
    if (this.intMinter !== null && this.ensureBrainId()) await this.persistManifest()
    const first = this.segmentFirstGenerationFromName(file)
    const path = `${FACTS_PREFIX}/${file}`
    await this.storage.writeRawBytes(path, this.newTailHeader(first))
    this.tailBytes = HEADER_BYTES
    this.noteFreshTail()
    this.dirtySegments.add(path)
  }

  /** True when the NEXT appended fact is the first fact of a brand-new v2
   *  log — the one that must open with the log.genesis record. */
  private needsGenesis(): boolean {
    return (
      this.tailVersion === FACT_LOG_FORMAT_V2 &&
      this.manifest.segments.length === 0 &&
      this.tailFacts.length === 0
    )
  }

  /** Mint the brain id into the manifest if absent; true when it changed. */
  private ensureBrainId(): boolean {
    if (this.manifest.brainId) return false
    this.manifest.brainId = uuidv4()
    return true
  }

  /** The log's birth certificate (id-space width 64 — the only width this
   *  writer mints; a reader expecting another width refuses at decode). */
  private genesisRecord(): LogRecord {
    const brainId = this.manifest.brainId
    if (!brainId) {
      throw new Error(
        'fact log v2: genesis requires a brainId in the facts manifest — invariant violated'
      )
    }
    return { type: 'log.genesis', idSpaceWidth: 64, brainId, createdAt: Date.now() }
  }

  /**
   * Convert one CommitFact's ops (+ optional marker records) to v2 wire
   * records, MINTING ints at append time: entity/verb ints come from the
   * injected minter (the metadata index's id mapper — the one authority a
   * rebuild reproduces exactly). Verb endpoints and the verb name ride as
   * first-class wire fields, lifted from the canonical verb vector wrapper.
   * Every refusal here is loud — an after-image without a mintable int, a
   * verb without endpoints, or a vector record without floats fails the
   * WRITE, never writes a 0.
   */
  private buildV2Records(fact: CommitFact): LogRecord[] {
    const mint = (kind: 'noun' | 'verb', id: string): bigint => {
      if (this.intMinter === null) {
        throw new Error(
          `fact log v2: no int minter is installed — cannot mint the ${kind} int for ${id}; ` +
            `refusing to write a v2 after-image (an int of 0 is never written)`
        )
      }
      const minted = this.intMinter(kind, id)
      // Reserved-root exemption: int 0 is legitimate for exactly one id —
      // the all-zeros VFS root, minted 0 by construction at genesis on
      // existing brains. Zero anywhere else is a corrupt mint.
      const isReservedRoot =
        minted === 0n && id === '00000000-0000-0000-0000-000000000000'
      if (typeof minted !== 'bigint' || minted < 0n || (minted === 0n && !isReservedRoot)) {
        throw new Error(
          `fact log v2: the int minter returned ${String(minted)} for ${kind} ${id} — ` +
            `minted ints are positive bigints (int 0 reserved for the VFS root alone); ` +
            `refusing to write`
        )
      }
      return minted
    }

    const records: LogRecord[] = []
    for (const op of fact.ops) {
      if (op.kind === 'noun') {
        if (op.record === null) {
          records.push({ type: 'noun.tombstone', id: op.id })
          continue
        }
        records.push({
          type: 'noun.afterImage',
          id: op.id,
          entityInt: mint('noun', op.id),
          metadata: toJsonSafe(op.record.metadata ?? null),
          vectorLeg: embeddingLegOf(op.record.vector, `noun ${op.id}`)
        })
      } else {
        if (op.record === null) {
          records.push({ type: 'verb.tombstone', id: op.id })
          continue
        }
        const wrapper = op.record.vector as Record<string, unknown> | null
        const verbName = wrapper?.verb
        const sourceId = wrapper?.sourceId
        const targetId = wrapper?.targetId
        if (
          typeof verbName !== 'string' ||
          typeof sourceId !== 'string' ||
          typeof targetId !== 'string'
        ) {
          throw new Error(
            `fact log v2: verb ${op.id} has no canonical endpoints (verb/sourceId/targetId ` +
              `live in its vector record, which is missing or torn) — refusing to write an ` +
              `after-image that could not be replayed`
          )
        }
        const floats = floatsOf(wrapper?.vector, `verb ${op.id}`) ?? []
        records.push({
          type: 'verb.afterImage',
          id: op.id,
          verbInt: mint('verb', op.id),
          metadata: toJsonSafe(op.record.metadata ?? null),
          vectorLeg: floats,
          verb: verbName,
          sourceId,
          sourceInt: mint('noun', sourceId),
          targetId,
          targetInt: mint('noun', targetId)
        })
      }
    }
    for (const marker of fact.records ?? []) records.push(marker)
    return records
  }

  /**
   * Pad a v2 tail to its next sector-seal boundary with ONE pad frame —
   * called from {@link sync} so alignment holds at every durability barrier.
   * Pads count toward {@link tailBytes} but never toward facts (they are
   * invisible to every reader); a gap smaller than the smallest constructible
   * pad frame pads through one extra sector (the codec's rule). No-op for v1
   * tails, empty tails, and already-aligned tails.
   */
  private async padTailToSealBoundary(): Promise<void> {
    if (this.tailVersion !== FACT_LOG_FORMAT_V2) return
    const file = this.manifest.tailSegment
    if (!file || this.tailBytes <= HEADER_BYTES) return
    const remainder = this.tailBytes % this.tailSealSize
    if (remainder === 0) return
    let padBytes = this.tailSealSize - remainder
    if (padBytes < minPadFrameBytes()) padBytes += this.tailSealSize
    const tailPath = `${FACTS_PREFIX}/${file}`
    await this.storage.appendRawBytes(tailPath, encodePadFrame(padBytes))
    this.tailBytes += padBytes
    this.dirtySegments.add(tailPath)
  }

  /** Atomically persist the manifest (write-new → fsync → rename downstream). */
  private async persistManifest(): Promise<void> {
    this.manifest.updatedAt = new Date().toISOString()
    await this.storage.writeRawObject(FACTS_MANIFEST_PATH, this.manifest)
    await this.storage.syncRawObjects([FACTS_MANIFEST_PATH])
  }

  /** Rewrite the tail segment to hold exactly `facts` (atomic replace). */
  private async rewriteTail(facts: CommitFact[]): Promise<void> {
    const file = this.manifest.tailSegment
    if (!file) return
    const first = facts[0]?.generation ?? this.segmentFirstGenerationFromName(file)
    const parts: Uint8Array[] = [buildHeader(first)]
    for (const f of facts) parts.push(encodeFrame(f))
    const total = parts.reduce((n, p) => n + p.length, 0)
    const merged = new Uint8Array(total)
    let offset = 0
    for (const p of parts) {
      merged.set(p, offset)
      offset += p.length
    }
    await this.storage.writeRawBytes(`${FACTS_PREFIX}/${file}`, merged)
    this.tailFacts = facts
    this.tailBytes = total
  }

  /** Cut a SEALED segment back to `committedGeneration` (atomic replace).
   *  v2 segments byte-slice at frame boundaries (kept frames — pads
   *  included — are never re-encoded); the v1 re-encode path is unchanged. */
  private async truncateSegmentTo(file: string, committedGeneration: number): Promise<void> {
    const path = `${FACTS_PREFIX}/${file}`
    const bytes = await this.storage.readRawBytes(path)
    if (bytes === null) return
    const { facts, formatVersion } = parseSegment(file, bytes)
    const kept = facts.filter((f) => f.generation <= committedGeneration)
    prodLog.warn(
      `[FactLog] truncating sealed segment ${file} to generation ${committedGeneration} ` +
        `(${facts.length - kept.length} uncommitted fact(s) dropped)`
    )
    if (formatVersion === FACT_LOG_FORMAT_V2) {
      const cut = v2CutOffset(walkV2Frames(bytes), committedGeneration)
      await this.storage.writeRawBytes(path, bytes.subarray(0, cut))
      return
    }
    const first = kept[0]?.generation ?? this.segmentFirstGenerationFromName(file)
    const parts: Uint8Array[] = [buildHeader(first)]
    for (const f of kept) parts.push(encodeFrame(f))
    const total = parts.reduce((n, p) => n + p.length, 0)
    const merged = new Uint8Array(total)
    let offset = 0
    for (const p of parts) {
      merged.set(p, offset)
      offset += p.length
    }
    await this.storage.writeRawBytes(path, merged)
  }

  /** Re-derive a sealed segment's manifest entry from its actual bytes. */
  private async reloadSegmentEntry(file: string): Promise<SegmentEntry> {
    const bytes = await this.storage.readRawBytes(`${FACTS_PREFIX}/${file}`)
    const { facts, validBytes } = bytes
      ? parseSegment(file, bytes)
      : { facts: [] as CommitFact[], validBytes: 0 }
    return {
      file,
      firstGeneration: facts[0]?.generation ?? this.segmentFirstGenerationFromName(file),
      lastGeneration: facts[facts.length - 1]?.generation ?? 0,
      facts: facts.length,
      bytes: validBytes
    }
  }

  /** Parse the zero-padded firstGeneration back out of a segment filename. */
  private segmentFirstGenerationFromName(file: string): number {
    const match = /^seg-(\d{20})\.bfl$/.exec(file)
    return match ? Number(match[1]) : 0
  }
}