/**
 * Hold/release sweep — the generalized scheduler (U6, KTD-10, R3 behavior half).
 *
 * Flag-ON, the scheduler's poll becomes a *hold/release sweep*: for each
 * workflow in use by live tasks, it finds cards resting at `hold`-trait columns
 * and evaluates their release condition:
 *
 *   - `manual`         — released ONLY by an explicit {@link promoteHeldTask}
 *                        call (U9's promote endpoint / CLI). The sweep never
 *                        auto-releases a manual hold.
 *   - `external-event` — released ONLY by {@link releaseHeldTaskByEvent} (a
 *                        webhook/API release, same shape as manual + an event
 *                        tag).
 *   - `timer`          — released when the injected clock passes the hold's
 *                        deadline (`columnMovedAt + durationMs`, or an explicit
 *                        `deadlineAt`). Fake-timer friendly (FN-5048): the clock
 *                        is injected, never `Date.now()` baked in.
 *   - `capacity`       — released when a downstream capacity (`wip`) column has a
 *                        free slot (same counting rules as the in-txn check).
 *   - `dependency`     — released when the card's dependencies are satisfied
 *                        (KTD-5: dependency task's column has the `complete`
 *                        trait flag in ITS resolved workflow; FN-5719 dual-accept
 *                        also honors the legacy completion signal, logging an
 *                        audit-diff when the two disagree).
 *
 * Eligible cards move via `store.moveTask(..., { moveSource: "scheduler" })`.
 * A scheduler move bypasses trait guards (it is substrate-driven) but the in-txn
 * capacity check is NOT a guard — it still runs (KTD-10), so two holds racing
 * into one slot serialize: exactly one commits, the other rejects with
 * `capacity-exhausted` and retries next sweep.
 *
 * Reservation ordering (KTD-10): for releases into a processing (capacity)
 * column, the sweep reserves worktree + semaphore slots BEFORE issuing the move
 * and releases the reservation if the move rejects on capacity — a card is never
 * moved into a column it cannot actually start in, and a semaphore-exhausted
 * interleaving leaves the card held with no commit.
 */

import {
  isWorkflowColumnsEnabled,
  resolveColumnCapacity,
  resolveColumnFlags,
  resolveColumnAdjacency,
  DEFAULT_WORKFLOW_POOL_ID,
  TransitionRejectionError,
  resolveWorkflowIrForTask,
  type TaskStore,
  type Task,
  type WorkflowIr,
  type WorkflowIrV2,
  type WorkflowIrColumn,
} from "@fusion/core";
import { schedulerLog } from "./logger.js";

/** A reservation handle returned by {@link HoldReleaseDeps.reserveSlot}. The
 *  sweep calls `release()` if the subsequent move rejects on capacity. */
export interface SlotReservation {
  release(): void;
}

/** Injected dependencies so the sweep stays unit-testable with fake timers and
 *  without real worktree/session allocation. */
export interface HoldReleaseDeps {
  /** Monotonic clock (ms). Inject a fake-timer-driven clock in tests; production
   *  passes `() => Date.now()`. */
  now: () => number;
  /**
   * Reserve a worktree + semaphore slot for a card about to be released into a
   * processing column (KTD-10 reservation-first). Returns `null` when no slot
   * could be reserved (e.g. semaphore exhausted) — the sweep then leaves the
   * card held without issuing a move. Returns a {@link SlotReservation} whose
   * `release()` the sweep calls if the move rejects on capacity.
   *
   * Optional: when absent, releases into processing columns proceed without a
   * reservation (the in-txn capacity check still arbitrates), which is the
   * default-workflow legacy parity path where the scheduler dispatch loop owns
   * worktree allocation via `allocateWorktree`.
   */
  reserveSlot?: (task: Task, targetColumn: string) => SlotReservation | null | Promise<SlotReservation | null>;
  /** Allocate a worktree path for a release into a processing column (passed
   *  through to `moveTask`'s `allocateWorktree`). */
  allocateWorktree?: (task: Task, reservedNames: Set<string>) => string | null;
}

/** Outcome of one sweep pass (for tests + observability). */
export interface HoldReleaseResult {
  released: string[];
  /** taskId → reason it stayed held this pass. */
  held: Array<{ taskId: string; reason: string }>;
}

// ── Workflow IR resolution (read-only) ────────────────────────────────────────
// The selection → builtin/custom → default rule lives in @fusion/core's
// resolveWorkflowIrForTask (GitHub #1402); the optional per-sweep irCache Map is
// threaded straight through.

function effectiveWorkflowId(store: TaskStore, taskId: string): string {
  try {
    return store.getTaskWorkflowSelection(taskId)?.workflowId ?? DEFAULT_WORKFLOW_POOL_ID;
  } catch {
    return DEFAULT_WORKFLOW_POOL_ID;
  }
}

function findColumn(ir: WorkflowIr, columnId: string): WorkflowIrColumn | undefined {
  if (ir.version !== "v2") return undefined;
  return (ir as WorkflowIrV2).columns.find((c) => c.id === columnId);
}

/** The hold trait config on a column, if any. */
function resolveHoldConfig(column: WorkflowIrColumn): Record<string, unknown> | undefined {
  const flags = resolveColumnFlags(column);
  if (!flags.hold) return undefined;
  const ct = column.traits.find((t) => t.trait === "hold");
  return ct?.config ?? {};
}

/** True when the card currently rests at a hold column. */
function isHeldTask(ir: WorkflowIr, task: Task): boolean {
  const column = findColumn(ir, task.column);
  if (!column) return false;
  return resolveColumnFlags(column).hold === true;
}

/**
 * Resolve the release target column for a held card.
 *
 * For `capacity` holds, the target is the nearest downstream column (by the
 * workflow's column adjacency, breadth-first from the hold column) that carries
 * a capacity (`wip`) trait — for the default workflow this is `in-progress`.
 * For other release kinds the target is the first adjacency neighbor that is not
 * the hold column itself (the forward step out of the hold).
 */
function resolveReleaseTarget(ir: WorkflowIr, fromColumn: string, preferCapacity: boolean): string | undefined {
  const v2 = ir as WorkflowIrV2;
  const orderedIds = Array.isArray(v2.columns) ? v2.columns.map((c) => c.id) : [];
  const fromIdx = orderedIds.indexOf(fromColumn);
  const adjacency = resolveColumnAdjacency(ir);
  const neighbors = adjacency.get(fromColumn) ?? [];

  if (preferCapacity) {
    // Walk FORWARD in declared order for the nearest capacity-bearing column;
    // the hold releases downstream, never backward.
    for (let i = fromIdx + 1; i < orderedIds.length; i++) {
      const col = findColumn(ir, orderedIds[i]);
      if (col && resolveColumnFlags(col).countsTowardWip && neighbors.includes(orderedIds[i])) {
        return orderedIds[i];
      }
    }
    // No directly-adjacent capacity column: fall back to the nearest forward
    // capacity column reachable via adjacency BFS.
    const seen = new Set<string>([fromColumn]);
    const queue = [...neighbors];
    while (queue.length > 0) {
      const candidate = queue.shift()!;
      if (seen.has(candidate)) continue;
      seen.add(candidate);
      const col = findColumn(ir, candidate);
      if (col && resolveColumnFlags(col).countsTowardWip) return candidate;
      for (const next of adjacency.get(candidate) ?? []) {
        if (!seen.has(next)) queue.push(next);
      }
    }
  }

  // Forward neighbor (declared-order next) if it is adjacent; else any neighbor
  // that is forward in declared order; else the first neighbor.
  const forwardId = fromIdx >= 0 ? orderedIds[fromIdx + 1] : undefined;
  if (forwardId && neighbors.includes(forwardId)) return forwardId;
  const forwardNeighbor = neighbors.find((n) => orderedIds.indexOf(n) > fromIdx);
  if (forwardNeighbor) return forwardNeighbor;
  return neighbors.find((n) => n !== fromColumn);
}

// ── Dependency satisfaction (KTD-5 + FN-5719 dual-accept) ─────────────────────

/** Legacy completion signal: dependency's column is a terminal/handoff column. */
function legacyDependencySatisfied(dep: Task): boolean {
  return dep.column === "done" || dep.column === "in-review" || dep.column === "archived";
}

/**
 * KTD-5 dependency satisfaction: the dependency task's current column has the
 * `complete` trait flag in ITS resolved workflow. Dual-accept (FN-5719): the
 * legacy completion signal (done/in-review/archived column, or an accepted
 * completion-handoff marker) is also honored; when the two disagree an
 * audit-diff event is logged.
 */
async function dependencySatisfied(store: TaskStore, dep: Task): Promise<boolean> {
  const ir = await resolveWorkflowIrForTask(store, dep.id);
  const column = findColumn(ir, dep.column);
  const completeFlag = column ? resolveColumnFlags(column).complete === true : false;

  let markerAccepted = false;
  try {
    markerAccepted = store.getCompletionHandoffAcceptedMarker(dep.id) !== null;
  } catch {
    markerAccepted = false;
  }
  const legacy = legacyDependencySatisfied(dep) || markerAccepted;

  if (completeFlag !== legacy) {
    try {
      void store.recordRunAuditEvent?.({
        taskId: dep.id,
        agentId: "scheduler",
        runId: `hold-release:${dep.id}`,
        domain: "database",
        mutationType: "merge:dependency-parity-diff",
        target: dep.id,
        metadata: {
          depId: dep.id,
          completeFlagResult: completeFlag,
          legacyResult: legacy,
          source: "hold-release.dependency",
        },
      });
    } catch {
      // Audit is best-effort.
    }
  }
  // Dual-accept: satisfied if EITHER signal says so (the dual-accept window
  // closes at graduation per U12; until then both are accepted).
  return completeFlag || legacy;
}

async function allDependenciesSatisfied(store: TaskStore, task: Task, allTasks: Task[]): Promise<boolean> {
  for (const depId of task.dependencies ?? []) {
    const dep = allTasks.find((t) => t.id === depId);
    if (!dep) continue; // missing dep does not block (matches scheduler posture)
    if (!(await dependencySatisfied(store, dep))) return false;
  }
  return true;
}

// ── Timer release ─────────────────────────────────────────────────────────────

/** Resolve the timer deadline (ms epoch) for a timer hold, or `undefined` if not
 *  resolvable. Supports an explicit `deadlineAt` (ISO or ms) or a relative
 *  `durationMs`/`timerMs` measured from `columnMovedAt`. */
function resolveTimerDeadline(holdConfig: Record<string, unknown>, task: Task): number | undefined {
  const deadlineAt = holdConfig.deadlineAt;
  if (typeof deadlineAt === "number" && Number.isFinite(deadlineAt)) return deadlineAt;
  if (typeof deadlineAt === "string") {
    const parsed = Date.parse(deadlineAt);
    if (Number.isFinite(parsed)) return parsed;
  }
  const duration =
    (typeof holdConfig.durationMs === "number" ? holdConfig.durationMs : undefined) ??
    (typeof holdConfig.timerMs === "number" ? holdConfig.timerMs : undefined);
  if (typeof duration === "number" && Number.isFinite(duration)) {
    const base = Date.parse(task.columnMovedAt ?? task.createdAt);
    if (Number.isFinite(base)) return base + duration;
  }
  return undefined;
}

// ── Capacity availability (same counting rule as the in-txn check) ────────────

/**
 * Count cards occupying the (workflow, column) capacity slot from a task
 * snapshot, mirroring the store's in-txn count: cards in the column now, plus
 * (when countPending) cards mid-`transitionPending` targeting it, scoped to the
 * SAME effective workflow. This is the sweep's *pre-check* — the authoritative
 * arbitration is still the in-txn check, which rejects a losing racer.
 */
function countCapacitySlot(
  allTasks: Task[],
  // Pre-built taskId → effective workflowId map (one pass per sweep) so this
  // counting loop avoids a per-task `effectiveWorkflowId` DB call.
  effectiveWorkflowIdByTask: Map<string, string>,
  targetColumn: string,
  workflowId: string,
  countPending: boolean,
): number {
  let count = 0;
  for (const t of allTasks) {
    if ((effectiveWorkflowIdByTask.get(t.id) ?? DEFAULT_WORKFLOW_POOL_ID) !== workflowId) continue;
    if (t.column === targetColumn) {
      count += 1;
      continue;
    }
    if (!countPending) continue;
    const tp = (t as Task & { transitionPending?: { toColumn?: string } | null }).transitionPending;
    if (tp && typeof tp === "object" && tp.toColumn === targetColumn) count += 1;
  }
  return count;
}

// ── The sweep ─────────────────────────────────────────────────────────────────

/**
 * Run one hold/release sweep pass. No-op (returns empty) when the workflowColumns
 * flag is OFF — flag-OFF scheduler behavior is byte-identical (the legacy
 * pull-from-todo loop is untouched).
 */
export async function runHoldReleaseSweep(
  store: TaskStore,
  deps: HoldReleaseDeps,
): Promise<HoldReleaseResult> {
  const result: HoldReleaseResult = { released: [], held: [] };

  const settings = await store.getSettings();
  if (!isWorkflowColumnsEnabled(settings)) return result;

  const allTasks = await store.listTasks({ includeArchived: false });

  // Per-sweep caches. `allTasks` is a snapshot-stable read within a sweep, so we
  // resolve each workflow's IR at most once (irCache) and pre-build the
  // taskId → effective-workflowId map a single time rather than per-task DB
  // calls inside the capacity counting loop. The authoritative in-txn capacity
  // check is unaffected — this only trims the sweep pre-check cost.
  const irCache = new Map<string, WorkflowIr>();
  const effectiveWorkflowIdByTask = new Map<string, string>();
  for (const t of allTasks) {
    effectiveWorkflowIdByTask.set(t.id, effectiveWorkflowId(store, t.id));
  }

  for (const task of allTasks) {
    // Skip paused / recovery-backoff tasks exactly as the legacy scheduler does.
    if (task.paused || task.userPaused) {
      continue;
    }
    if (task.nextRecoveryAt && Date.parse(task.nextRecoveryAt) > deps.now()) {
      continue;
    }

    const ir = await resolveWorkflowIrForTask(store, task.id, irCache);
    if (!isHeldTask(ir, task)) continue;

    const column = findColumn(ir, task.column);
    const holdConfig = column ? resolveHoldConfig(column) : undefined;
    if (!column || !holdConfig) continue;
    const release = typeof holdConfig.release === "string" ? holdConfig.release : "manual";

    // manual / external-event are NEVER auto-released by the sweep.
    if (release === "manual" || release === "external-event") {
      result.held.push({ taskId: task.id, reason: `${release}-only` });
      continue;
    }

    let shouldRelease = false;
    if (release === "timer") {
      const deadline = resolveTimerDeadline(holdConfig, task);
      shouldRelease = deadline !== undefined && deps.now() >= deadline;
      if (!shouldRelease) {
        result.held.push({ taskId: task.id, reason: "timer-not-elapsed" });
        continue;
      }
    } else if (release === "dependency") {
      shouldRelease = await allDependenciesSatisfied(store, task, allTasks);
      if (!shouldRelease) {
        result.held.push({ taskId: task.id, reason: "deps-unsatisfied" });
        continue;
      }
    } else if (release === "capacity") {
      // Capacity holds release into the nearest downstream capacity column when a
      // slot is free (pre-check); the in-txn check is the authority.
      const target = resolveReleaseTarget(ir, task.column, true);
      if (!target) {
        result.held.push({ taskId: task.id, reason: "no-downstream-capacity-column" });
        continue;
      }
      const capacity = resolveColumnCapacity(ir, target, settings);
      if (capacity.hasCapacity && Number.isFinite(capacity.limit)) {
        const workflowId = effectiveWorkflowIdByTask.get(task.id) ?? DEFAULT_WORKFLOW_POOL_ID;
        const occupants = countCapacitySlot(allTasks, effectiveWorkflowIdByTask, target, workflowId, capacity.countPending);
        if (occupants >= capacity.limit) {
          result.held.push({ taskId: task.id, reason: "downstream-full" });
          continue;
        }
      }
      shouldRelease = true;
    }

    if (!shouldRelease) continue;

    const target = resolveReleaseTarget(ir, task.column, release === "capacity");
    if (!target) {
      result.held.push({ taskId: task.id, reason: "no-release-target" });
      continue;
    }

    const released = await issueRelease(store, deps, task, target, ir);
    if (released) {
      result.released.push(task.id);
    } else {
      result.held.push({ taskId: task.id, reason: "move-rejected-or-no-slot" });
    }
  }

  return result;
}

/**
 * Issue a single release move (`moveSource: "scheduler"`). For releases into a
 * processing (capacity) column the reservation-first ordering (KTD-10) reserves
 * worktree + semaphore before the move and releases the reservation if the move
 * rejects on capacity. Returns true on a committed move, false otherwise (the
 * card stays held).
 */
async function issueRelease(
  store: TaskStore,
  deps: HoldReleaseDeps,
  task: Task,
  target: string,
  ir: WorkflowIr,
): Promise<boolean> {
  const targetColumn = findColumn(ir, target);
  const targetIsProcessing = targetColumn ? resolveColumnFlags(targetColumn).countsTowardWip === true : false;

  let reservation: SlotReservation | null = null;
  if (targetIsProcessing && deps.reserveSlot) {
    reservation = await deps.reserveSlot(task, target);
    if (!reservation) {
      // Semaphore/worktree exhausted — reservation-first means no move at all.
      schedulerLog.log(`Hold release for ${task.id} deferred — no reservable slot for ${target}`);
      return false;
    }
  }

  // A concurrent sweep (or explicit promote) can win the move for this same card
  // while we hold a reservation. The store serializes the move under a per-task
  // lock and resolves a redundant same-column move to a silent no-op: it returns
  // the card already at the target WITHOUT re-allocating a slot or emitting a
  // `task:moved`. A snapshot/pre-read can't tell winner from loser (both reads
  // race ahead of either commit on the per-task lock). Instead we attribute the
  // transition by OBJECT IDENTITY: a real move emits `task:moved` with the very
  // Task object it then returns, whereas a no-op returns a freshly-read object
  // and emits nothing. So the call whose `moveTask` result IS the emitted task is
  // the real mover; any other call that reserved performed a redundant no-op and
  // must release the slot it grabbed (FN-1415).
  const movedTaskObjects = new Set<object>();
  const onMoved = (data: { task: object; to: string }): void => {
    if (data.to === target) movedTaskObjects.add(data.task);
  };
  store.on("task:moved", onMoved);

  try {
    const result = await store.moveTask(task.id, target, {
      moveSource: "scheduler",
      allocateWorktree:
        targetIsProcessing && deps.allocateWorktree
          ? (reservedNames) => deps.allocateWorktree!(task, reservedNames)
          : undefined,
    });
    if (reservation && !movedTaskObjects.has(result)) {
      // Same-column no-op: a racing sweep already moved this card to the target.
      reservation.release();
      schedulerLog.log(`Hold release for ${task.id} skipped — already at ${target} (racing sweep won)`);
      return false;
    }
    return true;
  } catch (error) {
    if (error instanceof TransitionRejectionError && error.rejection.code === "capacity-exhausted") {
      // Lost the in-txn race for the slot — release the reservation, stay held.
      reservation?.release();
      schedulerLog.log(`Hold release for ${task.id} rejected on capacity for ${target} — staying held`);
      return false;
    }
    // Any other failure: release the reservation and let the card stay held.
    reservation?.release();
    schedulerLog.warn(
      `Hold release for ${task.id} into ${target} failed: ${error instanceof Error ? error.message : String(error)}`,
    );
    return false;
  } finally {
    store.off("task:moved", onMoved);
  }
}

// ── Explicit (manual / external-event) releases ───────────────────────────────

/**
 * Manually promote a held card out of its hold column (U9's promote endpoint /
 * CLI calls this). Releases regardless of the hold's release kind — a manual
 * promote is the explicit operator action the `manual` release kind waits for,
 * and it is also accepted for other kinds as an operator override. The move
 * still serializes through the in-txn capacity check (KTD-10): a promote into a
 * full column rejects with `capacity-exhausted`, surfaced to the caller.
 */
export async function promoteHeldTask(
  store: TaskStore,
  taskId: string,
  deps: Pick<HoldReleaseDeps, "reserveSlot" | "allocateWorktree"> = {},
): Promise<{ released: boolean; toColumn?: string; rejection?: string }> {
  const task = await store.getTask(taskId);
  if (!task) return { released: false, rejection: "task-not-found" };

  const ir = await resolveWorkflowIrForTask(store, taskId);
  if (!isHeldTask(ir, task)) {
    return { released: false, rejection: "not-held" };
  }
  const target = resolveReleaseTarget(ir, task.column, true);
  if (!target) return { released: false, rejection: "no-release-target" };

  const released = await issueRelease(
    store,
    { now: () => Date.now(), reserveSlot: deps.reserveSlot, allocateWorktree: deps.allocateWorktree },
    task,
    target,
    ir,
  );
  return released ? { released: true, toColumn: target } : { released: false, rejection: "capacity-exhausted-or-no-slot" };
}

/**
 * Release a held card on an external event (webhook/API). Same shape as
 * {@link promoteHeldTask} plus an `eventTag` recorded in the audit; only acts on
 * `external-event` holds (a no-op otherwise so a stray webhook can't release a
 * manual/timer/capacity hold).
 */
export async function releaseHeldTaskByEvent(
  store: TaskStore,
  taskId: string,
  eventTag: string,
  deps: Pick<HoldReleaseDeps, "reserveSlot" | "allocateWorktree"> = {},
): Promise<{ released: boolean; toColumn?: string; rejection?: string }> {
  const task = await store.getTask(taskId);
  if (!task) return { released: false, rejection: "task-not-found" };

  const ir = await resolveWorkflowIrForTask(store, taskId);
  const column = findColumn(ir, task.column);
  const holdConfig = column ? resolveHoldConfig(column) : undefined;
  if (!column || !holdConfig || holdConfig.release !== "external-event") {
    return { released: false, rejection: "not-external-event-hold" };
  }
  try {
    void store.recordRunAuditEvent?.({
      taskId,
      agentId: "scheduler",
      runId: `hold-release:event:${taskId}`,
      domain: "database",
      mutationType: "task:hold-release-event",
      target: taskId,
      metadata: { eventTag, fromColumn: task.column },
    });
  } catch {
    // best-effort
  }
  const target = resolveReleaseTarget(ir, task.column, true);
  if (!target) return { released: false, rejection: "no-release-target" };

  const released = await issueRelease(
    store,
    { now: () => Date.now(), reserveSlot: deps.reserveSlot, allocateWorktree: deps.allocateWorktree },
    task,
    target,
    ir,
  );
  return released ? { released: true, toColumn: target } : { released: false, rejection: "capacity-exhausted-or-no-slot" };
}