History/server/stub.ts

// Deterministic LLM stub. Used by unit tests and the Playwright e2e (webServer
// sets LLM_STUB=1) so the whole loop is reproducible with no live model. It
// speaks the OpenAI chat-completions wire shape, so it plugs in as a Transport.
import { createLlmClient, type ChatMessage, type Transport } from './llm';
import { addDays, clampStat, type TurnResult } from '../shared/protocol';

export const ADVISOR_MARKER = '<<ADVISOR>>';

/** The parsed body of the user message the engine sends (see engine.buildPrompt). */
export interface StubPayload {
  state?: {
    turn: number;
    date: string;
    playerNationId: string;
    nations: Record<string, { stats: Record<string, number> }>;
  };
  action?: string | null;
  chat?: { to: string; message: string } | null;
}

function truncate(s: string, n: number): string {
  return s.length <= n ? s : s.slice(0, n - 1) + '…';
}

/**
 * Pure deterministic TurnResult for a parsed payload. Bumps the player nation's
 * military by 5 each turn (clamped to 0–100) and emits one event, so a caller can
 * predict the exact next state for assertions.
 */
export function deterministicTurn(p: StubPayload): TurnResult {
  const state = p.state;
  const pid = state?.playerNationId ?? 'uk';
  const player = state?.nations?.[pid];
  const mil = clampStat((player?.stats?.military ?? 50) + 5);
  const otherId = state
    ? (Object.keys(state.nations).find((id) => id !== pid) ?? '')
    : 'us';

  const events = p.action
    ? [
        {
          nation: pid,
          type: 'military' as const,
          title: `Action: ${truncate(p.action, 60)}`,
          text: 'Your move takes effect across the region.',
          outcome: 'success' as const,
        },
      ]
    : p.chat
      ? [
          {
            nation: pid,
            type: 'diplomatic' as const,
            title: 'Diplomatic outreach',
            text: 'Your message is received by the target nation.',
            outcome: 'partial' as const,
          },
        ]
      : [
          {
            nation: pid,
            type: 'political' as const,
            title: 'The world moves on',
            text: 'Weeks pass; the great powers posture and rearm quietly.',
            outcome: 'neutral' as const,
          },
        ];

  return {
    date: addDays(state?.date ?? '1939-09-01', 2),
    events,
    stateChanges: [{ nation: pid, stat: 'military', op: 'set', value: mil }],
    relations: otherId ? [{ a: pid, b: otherId, set: 'ally' }] : [],
    territoryChanges: [],
  };
}

const CANNED_ADVISOR: string[] = [
  'Rearm the navy and expand the shipyards to secure the sea lanes.',
  'Open a back-channel with a rival to trade for rare resources.',
  'Invest in domestic industry to fund a long war effort.',
  'Dispatch envoys to neutral states to lock in non-aggression pacts.',
  'Build coastal fortifications to blunt any surprise invasion.',
];

/**
 * A Transport that deterministically answers LLM requests. Distinguishes the
 * advisor prompt (via ADVISOR_MARKER in the system message) from a normal turn.
 * Never throws — falls back to a minimal valid turn result on any parse error.
 */
export function makeDeterministicTransport(): Transport {
  return async (_url, init) => {
    let content: string;
    try {
      const req = JSON.parse(String(init?.body ?? '{}')) as {
        messages?: ChatMessage[];
      };
      const sys = req.messages?.find((m) => m.role === 'system')?.content ?? '';
      if (sys.includes(ADVISOR_MARKER)) {
        content = JSON.stringify(CANNED_ADVISOR);
      } else {
        const user = req.messages?.find((m) => m.role === 'user');
        const payload = JSON.parse(user?.content ?? '{}') as StubPayload;
        content = JSON.stringify(deterministicTurn(payload));
      }
    } catch {
      content = JSON.stringify(deterministicTurn({}));
    }
    return {
      ok: true,
      status: 200,
      json: async () => ({ choices: [{ message: { content } }] }),
      text: async () => content,
    } as unknown as Response;
  };
}

/** A ready-made LlmClient backed by the deterministic transport. */
export function makeStubLlmClient() {
  return createLlmClient({ transport: makeDeterministicTransport() });
}