/**
 * v2.6 Phase 3 — pure lifecycle decision helpers.
 *
 * The eviction / LRU-cap / busy-aware-restart / reaper-target logic, factored out
 * of AgentPool + the backends + the periodic sweeper so it's unit-testable with no
 * DB, no child processes, no timers (modeled on
 * apps/server/src/services/inference/prune.ts:selectPruneTargets — a pure decision
 * core the caller acts on).
 *
 * Three decisions live here:
 *   1. selectIdleEvictionTargets — which warm backends to evict for being idle.
 *   2. selectLruEvictionTargets — which warm backends to evict to honour a max-live
 *      cap (least-recently-used beyond the cap), NEVER a busy one.
 *   3. shouldRestartCrashedBackend (busy-aware) — openchamber's skip-while-busy +
 *      stale-grace state machine, re-implemented for BooCode's per-(chat,agent) pool.
 *
 * "Busy" = the backend has an in-flight turn. The hard rule (design §6, decisions):
 * never evict or force-restart a busy backend; defer with a stale-grace.
 */

// ─── Idle TTL eviction (3.1) ─────────────────────────────────────────────────

/** Default idle TTL before a warm backend/session is evicted (design §6 ~30 min). */
export const DEFAULT_IDLE_TTL_MS = 30 * 60 * 1000;

/** A pool entry as the decision helpers see it (no backend internals). */
export interface PoolEntrySnapshot {
  /** Pool key `${primary}:${agent}` — opaque to the decision, used for selection. */
  key: string;
  /** Epoch ms of the last turn activity (start or settle) on this backend. */
  lastActiveAt: number;
  /** True iff a turn is in flight right now. Busy entries are never evicted. */
  busy: boolean;
}

/**
 * Idle eviction: an entry is evictable when it has been idle (no turn) for longer
 * than `ttlMs` AND is not currently busy. Returns the keys to evict.
 *
 * Pure: `now` is injected so tests don't depend on wall-clock. Busy entries are
 * categorically excluded — a long-running turn that exceeds the TTL must NOT be
 * torn down mid-stream (the §6 / openchamber busy rule).
 */
export function selectIdleEvictionTargets(
  entries: ReadonlyArray<PoolEntrySnapshot>,
  now: number,
  ttlMs: number = DEFAULT_IDLE_TTL_MS,
): string[] {
  const out: string[] = [];
  for (const e of entries) {
    if (e.busy) continue;
    if (now - e.lastActiveAt >= ttlMs) out.push(e.key);
  }
  return out;
}

// ─── LRU cap (3.4) ───────────────────────────────────────────────────────────

/** Default max live warm backends/worktrees before the LRU cap evicts (env-overridable). */
export const DEFAULT_MAX_LIVE_BACKENDS = 10;

/**
 * LRU cap: when more than `cap` non-busy entries are live, evict the
 * least-recently-used ones (oldest `lastActiveAt` first) until at most `cap`
 * remain. Busy entries are never evicted AND are not counted toward the cap's
 * "kept" budget being freed — i.e. we only ever evict idle entries, so a burst of
 * concurrent busy turns can transiently exceed the cap rather than kill live work.
 *
 * Returns the keys to evict, least-recently-used first. Pure / deterministic:
 * ties broken by key for stable test output.
 */
export function selectLruEvictionTargets(
  entries: ReadonlyArray<PoolEntrySnapshot>,
  cap: number = DEFAULT_MAX_LIVE_BACKENDS,
): string[] {
  if (cap < 0) cap = 0;
  if (entries.length <= cap) return [];
  // Only idle entries are eligible to be evicted.
  const evictable = entries
    .filter((e) => !e.busy)
    .sort((a, b) => a.lastActiveAt - b.lastActiveAt || (a.key < b.key ? -1 : a.key > b.key ? 1 : 0));
  // We must shrink total live count down to `cap`. Busy entries can't be evicted,
  // so the number we CAN remove is bounded by the evictable pool; evict the oldest
  // (total - cap) of them, never more than exist.
  const overBy = entries.length - cap;
  const toEvict = evictable.slice(0, Math.max(0, overBy));
  return toEvict.map((e) => e.key);
}

// ─── Busy-aware crash restart (3.2) — openchamber lift ───────────────────────

/**
 * Default grace after which a backend that has stayed unhealthy WHILE busy is
 * force-restarted anyway (openchamber's STALE_BUSY_GRACE_MS = 2 min). Guards
 * against a permanently-stuck "busy" turn wedging recovery forever.
 */
export const DEFAULT_STALE_BUSY_GRACE_MS = 2 * 60 * 1000;

/** Default consecutive health-check failures before a restart is attempted. */
export const DEFAULT_HEALTH_FAILURE_THRESHOLD = 3;

export interface RestartDecisionInput {
  /** True iff the process is actually dead (exited). A dead process restarts
   *  immediately regardless of busy/threshold — there's nothing to protect. */
  processExited: boolean;
  /** Consecutive failed health probes so far (including the current one). */
  consecutiveFailures: number;
  /** Whether the backend currently has an in-flight turn. */
  busy: boolean;
  /** Epoch ms when the unhealthy-while-busy window started, or 0 if not in one. */
  unhealthyBusySince: number;
  /** Injected clock. */
  now: number;
  failureThreshold?: number;
  staleBusyGraceMs?: number;
}

export type RestartDecision =
  | { action: 'restart'; reason: 'process-exited' | 'threshold' | 'stale-busy-grace' }
  | { action: 'wait'; reason: 'below-threshold' | 'busy-grace' }
  | { action: 'none'; reason: 'healthy' };

/**
 * Decide whether to restart a backend after a health probe. Mirrors
 * openchamber's `runHealthCheckCycle` + `shouldSkipRestartForBusySessions`,
 * re-implemented as a pure function over injected state (the caller owns the
 * mutable counters + the actual restart side-effect).
 *
 * Order (matches openchamber):
 *   - process exited            → restart now (nothing live to protect).
 *   - below failure threshold   → wait (transient blip; the next probe re-checks).
 *   - threshold reached + idle   → restart now.
 *   - threshold reached + busy   → skip UNLESS the unhealthy-busy window exceeded
 *                                  the stale grace, then force restart.
 *
 * `healthy: true` callers don't reach here; included for completeness so the
 * caller can pass through and reset counters on a single code path.
 */
export function decideRestart(input: RestartDecisionInput & { healthy?: boolean }): RestartDecision {
  if (input.healthy) return { action: 'none', reason: 'healthy' };
  if (input.processExited) return { action: 'restart', reason: 'process-exited' };

  const threshold = input.failureThreshold ?? DEFAULT_HEALTH_FAILURE_THRESHOLD;
  if (input.consecutiveFailures < threshold) {
    return { action: 'wait', reason: 'below-threshold' };
  }

  if (!input.busy) {
    return { action: 'restart', reason: 'threshold' };
  }

  // Busy + unhealthy at/over threshold: defer, but not forever.
  const grace = input.staleBusyGraceMs ?? DEFAULT_STALE_BUSY_GRACE_MS;
  if (input.unhealthyBusySince > 0 && input.now - input.unhealthyBusySince >= grace) {
    return { action: 'restart', reason: 'stale-busy-grace' };
  }
  return { action: 'wait', reason: 'busy-grace' };
}

// ─── Orphan worktree reaper target selection (3.4) ───────────────────────────

/** Default TTL: an on-disk worktree dir with no live `worktrees` row is reaped
 *  only after it's been orphaned at least this long (mtime-based grace so a
 *  just-created dir mid-`ensureSessionWorktree` race is never swept). */
export const DEFAULT_ORPHAN_WORKTREE_GRACE_MS = 60 * 60 * 1000; // 1h

export interface OnDiskWorktree {
  /** Absolute path of the worktree dir on disk. */
  path: string;
  /** Last-modified epoch ms of the dir (newest of dir + contents, caller's choice). */
  mtimeMs: number;
}

/**
 * Reaper target selection: which on-disk worktree dirs are orphans safe to
 * inspect-and-reap. An orphan is a dir under the worktree base that has NO live
 * `worktrees` row (path not in `liveWorktreePaths`) AND whose mtime is older than
 * the grace window (so an in-flight create isn't swept).
 *
 * Pure — the caller (the sweeper) then runs the at-risk preflight (dirty/unpushed)
 * on each returned path and only physically removes the SAFE ones. This helper
 * never decides to remove work-at-risk; it only narrows the candidate set.
 */
export function selectOrphanWorktreeTargets(
  onDisk: ReadonlyArray<OnDiskWorktree>,
  liveWorktreePaths: ReadonlySet<string>,
  now: number,
  graceMs: number = DEFAULT_ORPHAN_WORKTREE_GRACE_MS,
): string[] {
  const out: string[] = [];
  for (const w of onDisk) {
    if (liveWorktreePaths.has(w.path)) continue; // tracked → not an orphan
    if (now - w.mtimeMs < graceMs) continue; // too fresh → could be mid-create
    out.push(w.path);
  }
  return out;
}