Function migrateStatsCache

src/utils/statsCache.ts:107–141

* Migrate an older cache to the current schema. * Returns null if the version is unknown or too old to migrate. * * Preserves historical aggregates that would otherwise be lost when * transcript files have already aged out past cleanupPeriodDays. * Pre-migration days may undercount (e.g. v2 lac

(
  parsed: Partial<PersistedStatsCache> & { version: number },
)

 * we accept that rather than drop the history.
 */
function migrateStatsCache(
  parsed: Partial<PersistedStatsCache> & { version: number },
): PersistedStatsCache | null {
  if (
    typeof parsed.version !== 'number' ||
    parsed.version < MIN_MIGRATABLE_VERSION ||
    parsed.version > STATS_CACHE_VERSION
  ) {
    return null
  }
  if (
    !Array.isArray(parsed.dailyActivity) ||
    !Array.isArray(parsed.dailyModelTokens) ||
    typeof parsed.totalSessions !== 'number' ||
    typeof parsed.totalMessages !== 'number'
  ) {
    return null
  }
  return {
    version: STATS_CACHE_VERSION,
    lastComputedDate: parsed.lastComputedDate ?? null,
    dailyActivity: parsed.dailyActivity,
    dailyModelTokens: parsed.dailyModelTokens,
    modelUsage: parsed.modelUsage ?? {},
    totalSessions: parsed.totalSessions,
    totalMessages: parsed.totalMessages,
    longestSession: parsed.longestSession ?? null,
    firstSessionDate: parsed.firstSessionDate ?? null,
    hourCounts: parsed.hourCounts ?? {},
    totalSpeculationTimeSavedMs: parsed.totalSpeculationTimeSavedMs ?? 0,
    // Preserve undefined (don't default to {}) so the SHOT_STATS recompute
    // check in loadStatsCache fires for v1/v2 caches that lacked this field.
    shotDistribution: parsed.shotDistribution,
  }
}

/**
 * Load the stats cache from disk.