/**
 * Arena battle-runner — DB-backed execution engine for Arena battles.
 *
 * Mirrors flow-runner.ts but implements the Arena's two-lane scheduler instead
 * of the Orchestrator's wave scheduler. Persists to battles/contestants tables
 * (not flow_runs/flow_steps). Each contestant is dispatched as a real tasks row
 * via an injected DispatchContestantFn (Phase 4 wires this to the dispatcher).
 * Advances on the dispatcher's onTaskTerminal hook.
 *
 * Scheduling:
 *   - Cloud lane: all contestants start immediately, in parallel.
 *   - Local lane: contestants run strictly one at a time (serial queue). Only
 *     the first local contestant runs at start; the next is dispatched when the
 *     current one terminates. Both lanes run concurrently with each other.
 *
 * Results:
 *   Written to <projectRoot>/Arena/<battleSlug>/<identity>-<model>/
 *   Coding: result.md + diff.patch (from the contestant's worktree).
 *   Q&A:    result.md with the text answer.
 *
 * Analyzer seam:
 *   onBattleComplete is called when all contestants are terminal. Phase 5 wires
 *   this to the two-stage digest→judge analyzer. A failed contestant does NOT
 *   abort the battle — others continue and the analyzer judges survivors.
 */
import type { Sql } from '../db.js';
import type { Broker } from '@boocode/server/broker';
import type { WsFrame } from '@boocode/contracts/ws-frames';
import type { FastifyBaseLogger } from 'fastify';
import type { BattleType, ContestantLane } from '@boocode/contracts/arena';
import { mkdir, writeFile } from 'node:fs/promises';
import { join } from 'node:path';
import { diffWorktree } from './worktrees.js';
import {
  buildBattleSlug,
  buildContestantDir,
  classifyLane,
  computeBenchmark,
  isBattleComplete,
  nextLocalContestant,
  reconcileContestants,
  type ContestantResumeAction,
  type ContestantSlot,
} from './arena-decisions.js';

// ─── Public types ─────────────────────────────────────────────────────────────

export interface ContestantSpec {
  /** Backend name (coding) or persona name (qa). */
  identity: string;
  model: string;
}

export interface BattleStartOpts {
  projectId: string;
  battleType: BattleType;
  prompt: string;
  /** 2–6 contestants. Duplicate (identity, model) pairs are rejected by the schema UNIQUE constraint. */
  contestants: ContestantSpec[];
}

/**
 * Injected dispatch function — Phase 4 wires this to the real task inserter.
 * Must INSERT a tasks row and return its id. The arena-runner sets the
 * contestant's task_id and status after this call.
 * `sessionId` is returned when already known (Q&A pre-creates the session);
 * null for coding contestants whose session is created lazily by the dispatcher.
 */
export type DispatchContestantFn = (opts: {
  projectId: string;
  contestantId: string;
  prompt: string;
  identity: string;
  model: string;
  battleType: BattleType;
}) => Promise<{ taskId: string; sessionId: string | null }>;

/**
 * Called once when every contestant in a battle has reached a terminal state.
 * Phase 5 wires this to the two-stage digest→judge analyzer.
 * Must never throw — the caller swallows errors.
 */
export type OnBattleComplete = (battleId: string) => void;

/**
 * Called after a cross_examinations row has been inserted, with its id.
 * Phase 5 wires this to the analyzer's cross-examination runner.
 * Must never throw — the caller swallows errors.
 */
export type OnCrossExamStart = (opts: {
  battleId: string;
  crossExamId: string;
  identity: string;
  model: string;
}) => void;

export interface BattleRunner {
  /** Start a battle: persist it + its contestants, classify lanes, dispatch initial wave. */
  startBattle(opts: BattleStartOpts): Promise<{ battleId: string }>;
  /**
   * Wire to createDispatcher({ onTaskTerminal }). Fires when ANY task settles;
   * the runner ignores tasks it doesn't own. Never throws.
   */
  handleTaskTerminal(taskId: string, state: string): void;
  /**
   * Re-advance any battles still marked 'running' after a coder restart.
   * Mirrors flow-runner's initResume (D-9). Never throws.
   */
  initResume(): Promise<void>;
  /**
   * Cancel a running battle. Marks it and all non-terminal contestants cancelled,
   * publishes frames, and returns the task_ids of in-flight contestants so the
   * route can abort them via the dispatcher's cancelExternalTask.
   */
  cancelBattle(battleId: string): Promise<{ cancelled: boolean; taskIds: string[] }>;
  /**
   * Trigger analysis for a completed (or manually re-analyzed) battle.
   * Phase 5 wires this to the two-stage digest→judge analyzer. For now, calls
   * the injected onBattleComplete seam directly.
   */
  triggerAnalysis(battleId: string): Promise<{ triggered: boolean }>;
  /**
   * Start a cross-examination on a battle. Inserts a cross_examinations row and
   * invokes the analyzer seam. Phase 5 fills the actual verdict logic.
   */
  startCrossExam(
    battleId: string,
    opts: { identity: string; model: string },
  ): Promise<{ crossExamId: string }>;
  /**
   * Manually set (or clear) the winner. Validates the contestant belongs to the
   * battle, updates battles.winner_contestant_id, and publishes a battle_updated
   * frame so the pane reflects the override immediately.
   */
  setWinner(battleId: string, winnerId: string | null): Promise<{
    ok: boolean;
    notFound?: boolean;
    invalidContestant?: boolean;
  }>;
}

// ─── Internal row shapes ──────────────────────────────────────────────────────

interface ContestantRow {
  id: string;
  battle_id: string;
  identity: string;
  model: string;
  lane: ContestantLane;
  task_id: string | null;
  worktree_id: string | null;
  status: string;
}

interface BattleRow {
  id: string;
  project_id: string;
  battle_type: BattleType;
  prompt: string;
  status: string;
  results_path: string | null;
  created_at: Date;
}

// ─── Deps / factory ───────────────────────────────────────────────────────────

interface Deps {
  sql: Sql;
  broker: Broker;
  log: FastifyBaseLogger;
  dispatch: DispatchContestantFn;
  onBattleComplete: OnBattleComplete;
  /**
   * Called after a cross_examinations row is inserted. Phase 5 wires this to
   * the analyzer's cross-examination runner. Optional: absent → no cross-exam
   * logic runs (stub behaviour for tests).
   */
  onCrossExamStart?: OnCrossExamStart;
  /**
   * Model IDs served by the local llama-swap server. Used for lane classification:
   * a contestant whose model is in this set runs in the local lane (serial, GPU-fair).
   * Q&A contestants are always local regardless of this set.
   * Defaults to an empty set → all coding contestants go to the cloud lane.
   */
  localModels?: ReadonlySet<string>;
}

const DEFAULT_LOCAL_MODELS: ReadonlySet<string> = new Set();

export function createBattleRunner(deps: Deps): BattleRunner {
  const { sql, broker, log, dispatch, onBattleComplete, onCrossExamStart } = deps;
  const localModels = deps.localModels ?? DEFAULT_LOCAL_MODELS;

  // Serialize local-lane advance per battle so two near-simultaneous terminal
  // callbacks don't double-dispatch the next local contestant.
  const advanceChain = new Map<string, Promise<void>>();

  // Delta bridge: per-contestant broker unsubscribe functions.
  // 'terminated' sentinel prevents a late-arriving setupDeltaBridge from
  // registering a subscription that would never be cleaned up.
  const deltaUnsubs = new Map<string, (() => void) | 'terminated'>();

  function publishUser(frame: Record<string, unknown>): void {
    broker.publishUserFrame('default', frame as unknown as WsFrame);
  }

  /**
   * Subscribe to the contestant's inference session and forward delta frames
   * to the user channel as contestant_updated{delta}. Polls for session_id
   * when not immediately known (coding contestants whose session is created
   * lazily by the dispatcher). Unsubscribes on termination or max retries.
   */
  async function setupDeltaBridge(
    battleId: string,
    contestantId: string,
    taskId: string,
    knownSessionId: string | null,
  ): Promise<void> {
    let sessionId = knownSessionId;
    if (!sessionId) {
      // Coding contestant: session_id is written by the dispatcher just before
      // inference starts. Poll until it appears or the contestant terminates.
      for (let i = 0; i < 50; i++) {
        if (deltaUnsubs.get(contestantId) === 'terminated') return;
        const [row] = await sql<{ session_id: string | null }[]>`
          SELECT session_id FROM tasks WHERE id = ${taskId}
        `.catch(() => []);
        if (row?.session_id) { sessionId = row.session_id; break; }
        await new Promise((r) => setTimeout(r, 200));
      }
    }
    if (!sessionId) return;
    if (deltaUnsubs.get(contestantId) === 'terminated') return;

    const unsub = broker.subscribe(sessionId, (frame) => {
      if (frame.type === 'delta') {
        const deltaContent = (frame as unknown as { content?: unknown }).content;
        if (typeof deltaContent === 'string') {
          publishUser({
            type: 'contestant_updated',
            battle_id: battleId,
            contestant_id: contestantId,
            delta: deltaContent,
          });
        }
      }
    });

    const existing = deltaUnsubs.get(contestantId);
    if (existing === 'terminated') {
      unsub();
    } else {
      deltaUnsubs.set(contestantId, unsub);
    }
  }

  function teardownDeltaBridge(contestantId: string): void {
    const entry = deltaUnsubs.get(contestantId);
    if (typeof entry === 'function') {
      entry();
      deltaUnsubs.delete(contestantId);
    } else {
      deltaUnsubs.set(contestantId, 'terminated');
    }
  }

  // ─── startBattle ────────────────────────────────────────────────────────────

  async function startBattle(opts: BattleStartOpts): Promise<{ battleId: string }> {
    if (opts.contestants.length < 2 || opts.contestants.length > 6) {
      throw new Error(`battle requires 2–6 contestants; got ${opts.contestants.length}`);
    }

    const [proj] = await sql<{ path: string }[]>`SELECT path FROM projects WHERE id = ${opts.projectId}`;
    if (!proj) throw new Error(`project not found: ${opts.projectId}`);

    // Insert the battle row as 'running'; update results_path once we have the id.
    const [battle] = await sql<{ id: string; created_at: Date }[]>`
      INSERT INTO battles (project_id, battle_type, prompt, status)
      VALUES (${opts.projectId}, ${opts.battleType}, ${opts.prompt}, 'running')
      RETURNING id, created_at
    `;
    const battleId = battle!.id;
    const battleSlug = buildBattleSlug(battleId, opts.battleType, battle!.created_at);
    const resultsPath = join(proj.path, 'Arena', battleSlug);

    await sql`
      UPDATE battles SET results_path = ${resultsPath}, updated_at = clock_timestamp()
      WHERE id = ${battleId}
    `;

    // Insert all contestant rows with lane classification.
    const contestantRows: Array<{ id: string; identity: string; model: string; lane: ContestantLane }> = [];
    for (const spec of opts.contestants) {
      const lane = classifyLane(opts.battleType, spec.identity, spec.model, localModels);
      const [row] = await sql<{ id: string }[]>`
        INSERT INTO contestants (battle_id, identity, model, lane, status)
        VALUES (${battleId}, ${spec.identity}, ${spec.model}, ${lane}, 'queued')
        RETURNING id
      `;
      contestantRows.push({ id: row!.id, identity: spec.identity, model: spec.model, lane });
    }

    // Write initial manifest so the results folder is always populated.
    await writeManifest(
      battleId, resultsPath, opts.battleType, opts.prompt, battle!.created_at,
      contestantRows.map((c) => ({ identity: c.identity, model: c.model, lane: c.lane })),
      null,
    ).catch((err) => {
      log.warn({ err: errMsg(err), battleId }, 'arena-runner: initial manifest write failed');
    });

    publishUser({
      type: 'battle_started',
      battle_id: battleId,
      battle_type: opts.battleType,
      prompt: opts.prompt,
      contestants: contestantRows.map((c) => ({
        id: c.id,
        identity: c.identity,
        model: c.model,
        lane: c.lane,
      })),
    });

    // Dispatch: cloud lane starts all contestants in parallel; local lane starts
    // only the first queued contestant (serial queue).
    let localStarted = false;
    for (const c of contestantRows) {
      if (c.lane === 'cloud') {
        await dispatchContestant(battleId, opts.projectId, opts.battleType, opts.prompt, c);
      } else if (!localStarted) {
        await dispatchContestant(battleId, opts.projectId, opts.battleType, opts.prompt, c);
        localStarted = true;
        // remaining local contestants stay 'queued' until this one finishes
      }
    }

    return { battleId };
  }

  async function dispatchContestant(
    battleId: string,
    projectId: string,
    battleType: BattleType,
    prompt: string,
    c: { id: string; identity: string; model: string; lane: ContestantLane },
  ): Promise<void> {
    const { taskId, sessionId } = await dispatch({
      projectId,
      contestantId: c.id,
      prompt,
      identity: c.identity,
      model: c.model,
      battleType,
    });
    await sql`
      UPDATE contestants
      SET task_id = ${taskId}, status = 'running', updated_at = clock_timestamp()
      WHERE id = ${c.id}
    `;
    publishContestantFrame(battleId, c.id, { status: 'running' });
    // Start the delta bridge in the background; unsubscribe when the contestant
    // terminates (teardownDeltaBridge called in handleTaskTerminal).
    void setupDeltaBridge(battleId, c.id, taskId, sessionId ?? null);
  }

  // ─── local-lane advance (serialized per battle) ───────────────────────────

  function advanceLocalLane(battleId: string): Promise<void> {
    const prev = advanceChain.get(battleId) ?? Promise.resolve();
    const next = prev
      .catch(() => {})
      .then(() =>
        advanceLocalLaneInner(battleId).catch((err) => {
          log.error({ err: errMsg(err), battleId }, 'arena-runner: advanceLocalLane failed');
        }),
      );
    advanceChain.set(battleId, next);
    void next.finally(() => {
      if (advanceChain.get(battleId) === next) advanceChain.delete(battleId);
    });
    return next;
  }

  async function advanceLocalLaneInner(battleId: string): Promise<void> {
    const battle = await loadBattle(battleId);
    if (!battle || battle.status !== 'running') return;

    const contestants = await loadContestants(battleId);
    const slots: ContestantSlot[] = contestants.map((c) => ({
      id: c.id,
      lane: c.lane,
      status: c.status,
    }));

    // Nothing to do if the local lane is still busy.
    const localRunning = slots.some((c) => c.lane === 'local' && c.status === 'running');
    if (localRunning) return;

    const nextId = nextLocalContestant(slots);
    if (!nextId) return; // local queue is exhausted

    const next = contestants.find((c) => c.id === nextId)!;
    await dispatchContestant(battleId, battle.project_id, battle.battle_type, battle.prompt, {
      id: next.id,
      identity: next.identity,
      model: next.model,
      lane: next.lane,
    });
  }

  // ─── handleTaskTerminal ───────────────────────────────────────────────────

  function handleTaskTerminal(taskId: string, state: string): void {
    void (async () => {
      // Look up which contestant owns this task (contestants_task_id_idx).
      const [row] = await sql<ContestantRow[]>`
        SELECT id, battle_id, identity, model, lane, task_id, worktree_id, status
        FROM contestants WHERE task_id = ${taskId}
      `;
      if (!row) return; // not an arena task — ignore
      if (row.status !== 'running') return; // already settled (idempotent)

      const battle = await loadBattle(row.battle_id);

      // Pull the task row for benchmark + output.
      const [task] = await sql<{
        chat_id: string | null;
        started_at: Date | null;
        ended_at: Date | null;
        cost_tokens: number | null;
      }[]>`SELECT chat_id, started_at, ended_at, cost_tokens FROM tasks WHERE id = ${taskId}`;

      const endedAt = task?.ended_at ?? new Date();

      if (state === 'completed') {
        const startedAt = task?.started_at ?? endedAt;
        const bench = computeBenchmark(startedAt, endedAt, task?.cost_tokens ?? null, row.lane);

        const output = task?.chat_id ? await readChatOutput(task.chat_id) : '';

        const resultPath = battle
          ? await writeContestantResults(battle, row, output, bench).catch((err) => {
              log.warn({ err: errMsg(err), contestantId: row.id }, 'arena-runner: result write failed');
              return null;
            })
          : null;

        await sql`
          UPDATE contestants
          SET status      = 'done',
              duration_ms = ${Math.round(bench.durationMs)},
              tokens_per_sec = ${bench.tokensPerSec},
              cost_tokens = ${task?.cost_tokens ?? null},
              result_path = ${resultPath},
              updated_at  = clock_timestamp()
          WHERE id = ${row.id} AND status = 'running'
        `;
        teardownDeltaBridge(row.id);

        // Check if this was the last contestant.
        const allContestants = await loadContestants(row.battle_id);
        const battleDone = isBattleComplete(allContestants);

        publishContestantFrame(row.battle_id, row.id, {
          status: 'done',
          duration_ms: Math.round(bench.durationMs),
          ...(bench.tokensPerSec !== null ? { tokens_per_sec: bench.tokensPerSec } : {}),
          ...(battleDone ? { battle_status: 'completed' } : {}),
        });

        if (battleDone) {
          await completeBattle(row.battle_id);
        } else if (row.lane === 'local') {
          void advanceLocalLane(row.battle_id);
        }
      } else {
        // failed or cancelled — the contest continues; this contestant is error.
        const errorMsg = state === 'cancelled' ? 'cancelled' : `task ${state}`;
        await sql`
          UPDATE contestants
          SET status = 'error', error = ${errorMsg}, updated_at = clock_timestamp()
          WHERE id = ${row.id} AND status = 'running'
        `;
        teardownDeltaBridge(row.id);

        const allContestants = await loadContestants(row.battle_id);
        const battleDone = isBattleComplete(allContestants);

        publishContestantFrame(row.battle_id, row.id, {
          status: 'error',
          error: errorMsg,
          ...(battleDone ? { battle_status: 'completed' } : {}),
        });

        if (battleDone) {
          await completeBattle(row.battle_id);
        } else if (row.lane === 'local') {
          void advanceLocalLane(row.battle_id);
        }
      }
    })().catch((err) => {
      log.error({ err: errMsg(err), taskId }, 'arena-runner: handleTaskTerminal failed');
    });
  }

  // ─── battle finalization ──────────────────────────────────────────────────

  async function completeBattle(battleId: string): Promise<void> {
    const updated = await sql`
      UPDATE battles SET status = 'completed', updated_at = clock_timestamp()
      WHERE id = ${battleId} AND status = 'running'
    `;
    if (updated.count === 0) return; // already terminal (race guard)
    log.info({ battleId }, 'arena-runner: battle completed');

    // Update manifest with finished_at timestamp.
    const completedBattle = await loadBattle(battleId);
    if (completedBattle?.results_path) {
      const contestants = await loadContestants(battleId);
      await writeManifest(
        battleId,
        completedBattle.results_path,
        completedBattle.battle_type,
        completedBattle.prompt,
        completedBattle.created_at,
        contestants.map((c) => ({ identity: c.identity, model: c.model, lane: c.lane })),
        new Date(),
      ).catch((err) => {
        log.warn({ err: errMsg(err), battleId }, 'arena-runner: manifest update failed');
      });
    }

    onBattleComplete(battleId);
  }

  // ─── manifest writer ─────────────────────────────────────────────────────

  async function writeManifest(
    battleId: string,
    resultsPath: string,
    battleType: BattleType,
    prompt: string,
    createdAt: Date,
    contestants: Array<{ identity: string; model: string; lane: ContestantLane }>,
    finishedAt: Date | null,
  ): Promise<void> {
    await mkdir(resultsPath, { recursive: true });
    const manifest = {
      id: battleId,
      battle_type: battleType,
      prompt,
      contestants,
      created_at: createdAt.toISOString(),
      finished_at: finishedAt?.toISOString() ?? null,
    };
    await writeFile(join(resultsPath, 'manifest.json'), JSON.stringify(manifest, null, 2), 'utf8');
  }

  // ─── results writer ───────────────────────────────────────────────────────

  async function writeContestantResults(
    battle: BattleRow,
    contestant: { identity: string; model: string; lane: ContestantLane; worktree_id: string | null },
    output: string,
    bench: { durationMs: number; tokensPerSec: number | null },
  ): Promise<string> {
    const resultsPath = await getOrBuildResultsPath(battle);
    if (!resultsPath) throw new Error('cannot resolve results path for battle ' + battle.id);

    const contestantDir = buildContestantDir(contestant.identity, contestant.model);
    const dir = join(resultsPath, contestantDir);
    await mkdir(dir, { recursive: true });

    const benchLines = [
      `duration: ${bench.durationMs}ms`,
      bench.tokensPerSec != null ? `tokens/sec: ${bench.tokensPerSec.toFixed(1)}` : null,
    ]
      .filter(Boolean)
      .join('\n');

    const resultMd =
      `# ${contestant.identity} / ${contestant.model}\n\n` +
      `## Benchmark\n\n${benchLines}\n\n` +
      `## Output\n\n${output}\n`;
    await writeFile(join(dir, 'result.md'), resultMd, 'utf8');

    if (battle.battle_type === 'coding' && contestant.worktree_id) {
      const [wt] = await sql<{ path: string; base_commit: string | null }[]>`
        SELECT path, base_commit FROM worktrees WHERE id = ${contestant.worktree_id}
      `;
      if (wt) {
        const [proj] = await sql<{ path: string }[]>`
          SELECT path FROM projects WHERE id = ${battle.project_id}
        `;
        if (proj) {
          const diff = await diffWorktree(wt.path, proj.path, {
            baseRef: wt.base_commit ?? undefined,
          }).catch(() => '');
          await writeFile(join(dir, 'diff.patch'), diff, 'utf8');
        }
      }
    }

    return dir;
  }

  /** Resolve or rebuild results_path for a battle (handles crash-before-UPDATE). */
  async function getOrBuildResultsPath(battle: BattleRow): Promise<string | null> {
    if (battle.results_path) return battle.results_path;
    const [proj] = await sql<{ path: string }[]>`SELECT path FROM projects WHERE id = ${battle.project_id}`;
    if (!proj) return null;
    const slug = buildBattleSlug(battle.id, battle.battle_type, battle.created_at);
    const resultsPath = join(proj.path, 'Arena', slug);
    await sql`
      UPDATE battles SET results_path = ${resultsPath}, updated_at = clock_timestamp()
      WHERE id = ${battle.id}
    `;
    return resultsPath;
  }

  // ─── helpers ──────────────────────────────────────────────────────────────

  async function readChatOutput(chatId: string): Promise<string> {
    const [m] = await sql<{ content: string | null }[]>`
      SELECT content FROM messages
      WHERE chat_id = ${chatId} AND role = 'assistant'
      ORDER BY created_at DESC LIMIT 1
    `;
    return m?.content ?? '';
  }

  async function loadBattle(battleId: string): Promise<BattleRow | null> {
    const [b] = await sql<BattleRow[]>`
      SELECT id, project_id, battle_type, prompt, status, results_path, created_at
      FROM battles WHERE id = ${battleId}
    `;
    return b ?? null;
  }

  async function loadContestants(battleId: string): Promise<ContestantRow[]> {
    return sql<ContestantRow[]>`
      SELECT id, battle_id, identity, model, lane, task_id, worktree_id, status
      FROM contestants WHERE battle_id = ${battleId}
      ORDER BY created_at ASC
    `;
  }

  function publishContestantFrame(
    battleId: string,
    contestantId: string,
    extra: Record<string, unknown>,
  ): void {
    publishUser({
      type: 'contestant_updated',
      battle_id: battleId,
      contestant_id: contestantId,
      ...extra,
    });
  }

  // ─── initResume ───────────────────────────────────────────────────────────

  async function initResume(): Promise<void> {
    const battles = await sql<BattleRow[]>`
      SELECT id, project_id, battle_type, prompt, status, results_path, created_at
      FROM battles WHERE status = 'running'
    `;
    if (battles.length === 0) return;
    log.info({ count: battles.length }, 'arena-runner: resuming in-flight battles on startup');
    for (const battle of battles) {
      await resumeBattle(battle).catch((err) => {
        log.error({ err: errMsg(err), battleId: battle.id }, 'arena-runner: initResume failed for battle');
      });
    }
  }

  async function resumeBattle(battle: BattleRow): Promise<void> {
    const contestants = await loadContestants(battle.id);

    const taskIds = contestants.map((c) => c.task_id).filter((id): id is string => id !== null);
    const taskStates = new Map<string, string>();
    if (taskIds.length > 0) {
      const tasks = await sql<{ id: string; state: string }[]>`
        SELECT id, state FROM tasks WHERE id = ANY(${taskIds})
      `;
      for (const t of tasks) taskStates.set(t.id, t.state);
    }

    const decisions = reconcileContestants(
      contestants.map((c) => ({ contestantId: c.id, taskId: c.task_id, status: c.status })),
      taskStates,
    );

    for (const decision of decisions) {
      if (decision.action === 'keep') continue;
      const contestant = contestants.find((c) => c.id === decision.contestantId)!;
      await applyResumeDecision(battle, contestant, decision.action);
    }

    // Re-check completion after applying decisions.
    const updated = await loadContestants(battle.id);
    if (isBattleComplete(updated)) {
      await completeBattle(battle.id);
    } else {
      // Advance local lane in case a slot opened up.
      void advanceLocalLane(battle.id);
    }

    log.info({ battleId: battle.id }, 'arena-runner: battle resumed');
  }

  async function applyResumeDecision(
    battle: BattleRow,
    contestant: ContestantRow,
    action: ContestantResumeAction,
  ): Promise<void> {
    switch (action) {
      case 'keep': break;

      case 'mark-done': {
        const taskRow = contestant.task_id
          ? (await sql<{ started_at: Date | null; ended_at: Date | null; cost_tokens: number | null; chat_id: string | null }[]>`
              SELECT started_at, ended_at, cost_tokens, chat_id FROM tasks WHERE id = ${contestant.task_id}`)[0]
          : null;
        const endedAt = taskRow?.ended_at ?? new Date();
        const startedAt = taskRow?.started_at ?? endedAt;
        const bench = computeBenchmark(startedAt, endedAt, taskRow?.cost_tokens ?? null, contestant.lane);
        const output = taskRow?.chat_id ? await readChatOutput(taskRow.chat_id) : '';
        const resultPath = battle
          ? await writeContestantResults(battle, contestant, output, bench).catch((err) => {
              log.warn({ err: errMsg(err), contestantId: contestant.id }, 'arena-runner: resume result write failed');
              return null;
            })
          : null;
        await sql`
          UPDATE contestants
          SET status = 'done',
              duration_ms = ${Math.round(bench.durationMs)},
              tokens_per_sec = ${bench.tokensPerSec},
              result_path = ${resultPath},
              updated_at = clock_timestamp()
          WHERE id = ${contestant.id}
        `;
        break;
      }

      case 'mark-error':
        await sql`
          UPDATE contestants
          SET status = 'error', error = 'task failed before callback',
              updated_at = clock_timestamp()
          WHERE id = ${contestant.id}
        `;
        break;

      case 'mark-cancelled':
        await sql`
          UPDATE contestants
          SET status = 'error', error = 'cancelled before callback',
              updated_at = clock_timestamp()
          WHERE id = ${contestant.id}
        `;
        break;

      case 're-dispatch': {
        const { taskId } = await dispatch({
          projectId: battle.project_id,
          contestantId: contestant.id,
          prompt: battle.prompt,
          identity: contestant.identity,
          model: contestant.model,
          battleType: battle.battle_type,
        });
        await sql`
          UPDATE contestants
          SET task_id = ${taskId}, updated_at = clock_timestamp()
          WHERE id = ${contestant.id}
        `;
        log.info(
          { battleId: battle.id, contestantId: contestant.id, taskId },
          'arena-runner: contestant re-dispatched on resume',
        );
        break;
      }
    }
  }

  // ─── cancelBattle ─────────────────────────────────────────────────────────

  async function cancelBattle(battleId: string): Promise<{ cancelled: boolean; taskIds: string[] }> {
    const updated = await sql`
      UPDATE battles SET status = 'cancelled', updated_at = clock_timestamp()
      WHERE id = ${battleId} AND status = 'running'
    `;
    if (updated.count === 0) return { cancelled: false, taskIds: [] };

    // Mark all non-terminal contestants cancelled and collect in-flight task_ids.
    const contestants = await sql<{ id: string; task_id: string | null; status: string }[]>`
      SELECT id, task_id, status FROM contestants
      WHERE battle_id = ${battleId} AND status NOT IN ('done', 'error')
    `;

    if (contestants.length > 0) {
      await sql`
        UPDATE contestants
        SET status = 'error', error = 'battle cancelled', updated_at = clock_timestamp()
        WHERE battle_id = ${battleId} AND status NOT IN ('done', 'error')
      `;
      for (const c of contestants) {
        publishContestantFrame(battleId, c.id, {
          status: 'error',
          error: 'battle cancelled',
          battle_status: 'cancelled',
        });
      }
    }

    const taskIds = contestants
      .filter(
        (c): c is typeof c & { task_id: string } =>
          c.task_id !== null && c.status === 'running',
      )
      .map((c) => c.task_id);

    log.info({ battleId }, 'arena-runner: battle cancelled by request');
    return { cancelled: true, taskIds };
  }

  // ─── triggerAnalysis (Phase 5 seam) ──────────────────────────────────────

  async function triggerAnalysis(battleId: string): Promise<{ triggered: boolean }> {
    const battle = await loadBattle(battleId);
    if (!battle) return { triggered: false };
    log.info({ battleId }, 'arena-runner: triggerAnalysis requested');
    // Calls the injected onBattleComplete seam — Phase 5 replaces this with the
    // real two-stage digest→judge analyzer (see ADR 0002 + plan Phase 5).
    onBattleComplete(battleId);
    return { triggered: true };
  }

  // ─── startCrossExam (Phase 5 seam) ───────────────────────────────────────

  async function startCrossExam(
    battleId: string,
    opts: { identity: string; model: string },
  ): Promise<{ crossExamId: string }> {
    const [row] = await sql<{ id: string }[]>`
      INSERT INTO cross_examinations (battle_id, identity, model)
      VALUES (${battleId}, ${opts.identity}, ${opts.model})
      RETURNING id
    `;
    const crossExamId = row!.id;
    log.info({ battleId, crossExamId, ...opts }, 'arena-runner: cross-exam inserted, triggering analyzer');
    if (onCrossExamStart) {
      try {
        onCrossExamStart({ battleId, crossExamId, identity: opts.identity, model: opts.model });
      } catch (err) {
        log.error({ err: err instanceof Error ? err.message : String(err), battleId, crossExamId }, 'arena-runner: onCrossExamStart threw');
      }
    }
    return { crossExamId };
  }

  // ─── setWinner (user override) ────────────────────────────────────────────

  async function setWinner(
    battleId: string,
    winnerId: string | null,
  ): Promise<{ ok: boolean; notFound?: boolean; invalidContestant?: boolean }> {
    const [row] = await sql<{ id: string }[]>`SELECT id FROM battles WHERE id = ${battleId}`;
    if (!row) return { ok: false, notFound: true };

    if (winnerId !== null) {
      const [c] = await sql<{ id: string }[]>`
        SELECT id FROM contestants WHERE id = ${winnerId} AND battle_id = ${battleId}
      `;
      if (!c) return { ok: false, invalidContestant: true };
    }

    await sql`
      UPDATE battles SET winner_contestant_id = ${winnerId}, updated_at = clock_timestamp()
      WHERE id = ${battleId}
    `;
    publishUser({ type: 'battle_updated', battle_id: battleId, winner_contestant_id: winnerId });
    return { ok: true };
  }

  return { startBattle, handleTaskTerminal, initResume, cancelBattle, triggerAnalysis, startCrossExam, setWinner };
}

function errMsg(e: unknown): string {
  return e instanceof Error ? e.message : String(e);
}