station-master/test/mainline-cards.test.ts

/**
 * Mainline modifiers and Maneuver cards (implications.md §7).
 *
 * The 14 cards left after the Enhancements. These are NOT multiplayer-only: Brakeman, Airbrakes,
 * Helpers and Realignment all change your own crossing times, and Red Flags prevents a rear-ender,
 * which happens in solitaire too.
 *
 * Poling is deliberately absent — the recovered sheet records its effect as "TBD in the source", so
 * there is nothing to implement and nothing to test.
 */

import { describe, it } from 'node:test';
import assert from 'node:assert/strict';

import { advance } from '../src/engine/advance.ts';
import { applyIntent, areaOf, check } from '../src/engine/apply.ts';
import { legalActions } from '../src/engine/legal.ts';
import {
  MAINLINE_MODIFIER_CARDS,
  MAINLINE_PROFILES,
  MANEUVER_CARDS,
  REALIGNMENTS,
  crossingStages,
  mainlineModifierRule,
  mainlineProfile,
} from '../src/engine/content.ts';
import { createGame } from '../src/engine/setup.ts';
import type { GameEvent } from '../src/engine/events.ts';
import type { GameConfig, GameState, GridCoord, TrackCard } from '../src/engine/state.ts';
import { coordKey, turnOf } from '../src/engine/state.ts';
import { snapshot } from '../src/sim/view.ts';

const config: GameConfig = {
  mode: 'solitaire',
  victory: 'highestAfterDays',
  length: 'standard',
  optionalRules: { reducedVisibility: false, sisterTrains: false, employeeRotation: false, emergencyToolbox: false },
};
const game = (seed = 5): GameState => createGame({ id: 'g', seed, config, playerNames: ['p'] });
const at = (row: number, col: number): GridCoord => ({ row, col });

/** Puts a card of `kind`/`key` in hand and returns its id. */
function hand(s: GameState, kind: string, key: string): string {
  for (const [id, card] of s.cards) {
    const k = card.kind as { kind: string; key?: string };
    if (k.kind === kind && k.key === key) {
      s.decks.hands.set(0, [id]);
      return id;
    }
  }
  throw new Error(`no ${kind} card: ${key}`);
}

/**
 * Puts a specific TRACK card in hand and returns its id.
 *
 * Track is an ordinary deck card, so building a district means holding the right piece — which is
 * exactly the constraint these tests need to set up deliberately rather than draw for.
 */
function trackInHand(s: GameState, geometry: string, hand: string): string {
  for (const [id, card] of s.cards) {
    const k = card.kind as { kind: string; geometry?: string; hand?: string };
    if (k.kind === 'track' && k.geometry === geometry && k.hand === hand) {
      s.decks.hands.set(0, [id]);
      return id;
    }
  }
  throw new Error(`no track card: ${geometry}/${hand}`);
}

/** A Mainline node pinned to a known terrain, so deck composition cannot decide an outcome. */
function pinned(s: GameState, index: number, card: string) {
  const node = s.division.nodes[index];
  if (node?.kind !== 'mainline') throw new Error(`node ${index} is not a Mainline card`);
  node.card = card as never;
  node.transits = [];
  return node;
}

/** Lets the player actually play a card this turn. */
function drawTurn(s: GameState): void {
  s.clock.phase = 'localOps';
  s.clock.currentActor = 0;
  turnOf(s, 0).option = 'draw';
  turnOf(s, 0).drawnThisTurn = true;
}

// ---------------------------------------------------------------------------

describe('grade modifiers change crossing time', () => {
  it('a Heavy Grade takes two Stages bare', () => {
    assert.equal(crossingStages('heavyGrade', 'fast', false), 2);
  });

  it('Brakeman speeds the descent but not the climb', () => {
    // Q11 — the card prints "(Up)" and "Player sets orientation", so the last argument is which
    // way is UPHILL. With up = east, a westbound train is descending.
    assert.equal(crossingStages('heavyGrade', 'fast', false, ['brakeman'], 'west', 'east'), 1);
    assert.equal(crossingStages('heavyGrade', 'fast', false, ['brakeman'], 'east', 'east'), 2);
  });

  it('follows the orientation the player chose, not a fixed compass direction', () => {
    // The same train on the same card, with the card turned around: Brakeman helps a westbound
    // train on an east-climbing grade, and an eastbound one when the grade climbs west.
    assert.equal(crossingStages('heavyGrade', 'fast', false, ['brakeman'], 'east', 'west'), 1);
    assert.equal(crossingStages('heavyGrade', 'fast', false, ['brakeman'], 'west', 'west'), 2);
    assert.equal(crossingStages('heavyGrade', 'fast', false, ['helpers'], 'west', 'west'), 1);
    assert.equal(crossingStages('heavyGrade', 'fast', false, ['helpers'], 'east', 'west'), 2);
  });

  it('Helpers speed the climb but not the descent', () => {
    assert.equal(crossingStages('heavyGrade', 'fast', false, ['helpers'], 'east', 'east'), 1);
    assert.equal(crossingStages('heavyGrade', 'fast', false, ['helpers'], 'west', 'east'), 2);
  });

  it('Airbrakes stack with Brakeman on a slow train', () => {
    // A slow train pays 3 on a grade; Brakeman and Airbrakes take one Stage each.
    assert.equal(crossingStages('heavyGrade', 'slow', false, [], 'west', 'east'), 3);
    assert.equal(crossingStages('heavyGrade', 'slow', false, ['brakeman'], 'west', 'east'), 2);
    assert.equal(
      crossingStages('heavyGrade', 'slow', false, ['brakeman', 'airbrakes'], 'west', 'east'),
      1,
    );
  });

  it('never lets a train cross in no time', () => {
    assert.equal(
      crossingStages('heavyGrade', 'fast', false, ['brakeman', 'airbrakes'], 'west', 'east'),
      1,
    );
  });

  it('leaves non-grade cards alone', () => {
    // Brakeman on Plains would be an illegal placement anyway; the maths must not move regardless.
    assert.equal(crossingStages('plains', 'fast', false, ['brakeman'], 'west', 'east'), 1);
    assert.equal(crossingStages('curves', 'fast', false, ['helpers'], 'east', 'east'), 2);
  });
});

describe('placing a Mainline modifier', () => {
  it('accepts Brakeman on a grade', () => {
    const s = game();
    pinned(s, 1, 'heavyGrade');
    drawTurn(s);
    const cardId = hand(s, 'mainlineModifier', 'brakeman');
    assert.equal(check(s, 0, { type: 'mainline.modify', cardId, node: 1 }), null);
  });

  it('rejects Brakeman on anything but a grade', () => {
    const s = game();
    pinned(s, 1, 'plains');
    drawTurn(s);
    const cardId = hand(s, 'mainlineModifier', 'brakeman');
    assert.equal(check(s, 0, { type: 'mainline.modify', cardId, node: 1 }), 'NOT_A_GRADE');
  });

  it('requires Brakeman before Airbrakes', () => {
    const s = game();
    const node = pinned(s, 1, 'heavyGrade');
    drawTurn(s);
    const cardId = hand(s, 'mainlineModifier', 'airbrakes');
    assert.equal(check(s, 0, { type: 'mainline.modify', cardId, node: 1 }), 'NOT_CONNECTED');

    node.modifiers = ['brakeman'];
    assert.equal(check(s, 0, { type: 'mainline.modify', cardId, node: 1 }), null);
  });

  it('will not modify a card with a train on it', () => {
    const s = game();
    const node = pinned(s, 1, 'heavyGrade');
    node.transits.push({ tray: 'someone', stagesRemaining: 1, stagesTotal: 1, direction: 'east' });
    drawTurn(s);
    const cardId = hand(s, 'mainlineModifier', 'brakeman');
    assert.equal(check(s, 0, { type: 'mainline.modify', cardId, node: 1 }), 'TRAIN_ON_CARD');
  });

  it('rejects a Mainline node that is not a Mainline card', () => {
    const s = game();
    drawTurn(s);
    const cardId = hand(s, 'mainlineModifier', 'brakeman');
    // Node 0 is the western Division Point.
    assert.equal(check(s, 0, { type: 'mainline.modify', cardId, node: 0 }), 'NO_PLACEMENT');
  });

  it('lays the modifier and spends the card', () => {
    const s = game();
    const node = pinned(s, 1, 'heavyGrade');
    drawTurn(s);
    const cardId = hand(s, 'mainlineModifier', 'brakeman');
    const r = applyIntent(s, 0, { type: 'mainline.modify', cardId, node: 1 });

    assert.ok(r.ok, 'placement should be accepted');
    assert.deepEqual(node.modifiers, ['brakeman']);
    assert.ok(!(s.decks.hands.get(0) ?? []).includes(cardId), 'card should leave the hand');
    assert.ok(s.decks.salvageYard.includes(cardId), 'card should reach the Salvage Yard');
  });
});

describe('Realignment converts one Mainline type to another', () => {
  it('converts according to the table', () => {
    const s = game();
    const node = pinned(s, 1, 'curves');
    drawTurn(s);
    const cardId = hand(s, 'mainlineModifier', 'realignment');
    const r = applyIntent(s, 0, { type: 'mainline.modify', cardId, node: 1 });

    assert.ok(r.ok);
    assert.equal(node.card, 'plains', 'Curves realigns to Plains');
    // The point of the card: Curves is a 30 (two Stages), Plains a 60 (one).
    assert.equal(crossingStages(node.card, 'fast', false), 1);
  });

  it('refuses a card with no conversion listed', () => {
    const s = game();
    // Tunnel is not a `from` in REALIGNMENTS.
    assert.ok(!REALIGNMENTS.some((r) => r.from === 'tunnel'));
    pinned(s, 1, 'tunnel');
    drawTurn(s);
    const cardId = hand(s, 'mainlineModifier', 'realignment');
    assert.equal(check(s, 0, { type: 'mainline.modify', cardId, node: 1 }), 'NO_PLACEMENT');
  });

  it('applies to any Mainline card, not only grades', () => {
    assert.equal(mainlineModifierRule('realignment')?.gradeOnly, false);
  });
});

describe('Red Flags protect a stopped train', () => {
  /** A slow train `behind` closing on a stopped train `ahead`, both eastbound on node 1. */
  function rearEnder(s: GameState) {
    const node = pinned(s, 1, 'plains');
    node.transits.push({ tray: 'ahead', stagesRemaining: 2, stagesTotal: 2, direction: 'east' });
    s.trays.set('ahead', {
      id: 'ahead', trainNumber: 4, trainIsExtra: false, engineAt: 0,
      consist: [], direction: 'east', position: { at: 'mainline', index: 1 }, movesUsed: 0,
    });
    s.trays.set('behind', {
      id: 'behind', trainNumber: 2, trainIsExtra: false, engineAt: 0,
      consist: [], direction: 'east', position: { at: 'divisionPoint', side: 'west' }, movesUsed: 0,
    });
    const dp = s.division.nodes[0];
    if (dp?.kind === 'divisionPoint') dp.holding.push('behind');
    s.clock.phase = 'mainline';
    s.movedThisPhase = new Set();
    return node;
  }

  it('holds the approaching train instead of letting it close', () => {
    const s = game();
    const node = rearEnder(s);
    node.redFlagged = ['ahead'];

    advance(s);
    assert.deepEqual(
      s.trays.get('behind')!.position,
      { at: 'divisionPoint', side: 'west' },
      'the flagged train must not be approached',
    );
  });

  it('comes in when the protected train rolls', () => {
    const s = game();
    const node = rearEnder(s);
    node.redFlagged = ['ahead'];
    // Bring the protected train to the end of its crossing so it leaves the card.
    node.transits[0]!.stagesRemaining = 1;

    for (let i = 0; i < 12 && (node.redFlagged?.length ?? 0) > 0; i++) advance(s);
    assert.deepEqual(node.redFlagged, [], 'flags come in once the train moves off');
  });

  it('only protects a train out on the Mainline', () => {
    const s = game();
    rearEnder(s);
    s.clock.phase = 'localOps';
    s.clock.currentActor = 0;
    const cardId = hand(s, 'maneuver', 'redFlags');

    assert.equal(
      check(s, 0, { type: 'maneuver.redFlags', cardId, trayId: 'behind' }),
      'NO_PLACEMENT',
      'a train sitting at a Division Point cannot be rear-ended',
    );
    assert.equal(check(s, 0, { type: 'maneuver.redFlags', cardId, trayId: 'ahead' }), null);
  });

  it('will not double-flag the same train', () => {
    const s = game();
    const node = rearEnder(s);
    s.clock.phase = 'localOps';
    s.clock.currentActor = 0;
    const cardId = hand(s, 'maneuver', 'redFlags');
    node.redFlagged = ['ahead'];

    assert.equal(
      check(s, 0, { type: 'maneuver.redFlags', cardId, trayId: 'ahead' }),
      'OPTION_ALREADY_CHOSEN',
    );
  });
});

describe('Flying Switch rolls a cut into an industry', () => {
  /** A crew on a siding below the main, with a facility next door along the same track. */
  function setup(s: GameState) {
    const area = areaOf(s, 0);
    // A flying switch rolls the cut ALONG THE TRACK, so the crew and the industry must be joined by
    // rail. Every run is east-west now, and the siding gets down off the main the only way there is:
    //
    //   row  0:  [office] [turnout, leg south]                      <- Running Track
    //   row -1:           [curve ne] [straight, crew] [industry]    <- the siding
    //
    const plain = (): TrackCard => ({
      geometry: { kind: 'track', geometry: 'straight' },
      baseOperationalRail: true,
      standing: [],
      facility: null,
      modifiers: [],
      enhancements: [],
    });
    area.grid.set(coordKey(at(0, 1)), {
      ...plain(),
      geometry: { kind: 'track', geometry: 'turnout', turnout: { stem: 'w', through: 'e', diverge: 's' }, hand: 'left' },
      baseOperationalRail: false,
    });
    area.grid.set(coordKey(at(-1, 1)), { ...plain(), geometry: { kind: 'track', geometry: 'curved', arc: 'ne', hand: 'left' } });
    area.grid.set(coordKey(at(-1, 2)), plain());
    const industry: TrackCard = {
      ...plain(),
      facility: {
        kind: 'freight', subtype: 'mineTipple',
        allows: { outbound: true, inbound: false },
        outboundBox: [], inboundBox: [],
        capacity: { outbound: 1, inbound: 0 },
        menAtWork: [null, null, null],
        industryTrack: { cars: [] },
        laborers: 1, porters: 0,
        usedThisStage: { laborers: 0, porters: 0 },
      },
    };
    area.grid.set(coordKey(at(-1, 3)), industry);

    s.trays.set('crew', {
      id: 'crew', trainNumber: 1, trainIsExtra: false, engineAt: 0,
      consist: [{ type: 'hopper', loaded: false }, { type: 'boxcar', loaded: false }],
      direction: 'east', facing: 'e',
      position: { at: 'grid', seat: 0, coord: at(-1, 2) }, movesUsed: 0,
    });
    s.clock.phase = 'localOps';
    s.clock.currentActor = 0;
    turnOf(s, 0).option = 'switch';
    turnOf(s, 0).movesRemaining = 6;
    return industry;
  }

  it('drops the cut onto the industry track without moving the engine', () => {
    const s = game();
    const industry = setup(s);
    const cardId = hand(s, 'maneuver', 'flyingSwitch');

    const r = applyIntent(s, 0, {
      type: 'maneuver.flyingSwitch', cardId, trayId: 'crew', count: 1, to: at(-1, 3),
    });
    assert.ok(r.ok, 'the flying switch should be accepted');

    assert.equal(industry.facility!.industryTrack.cars.length, 1, 'car reaches the industry track');
    assert.equal(industry.facility!.industryTrack.cars[0]!.type, 'boxcar', '§A.3 — the back car');
    assert.equal(s.trays.get('crew')!.consist.length, 1, 'the cut leaves the consist');
    assert.deepEqual(
      s.trays.get('crew')!.position,
      { at: 'grid', seat: 0, coord: at(-1, 2) },
      'the engine never enters the industry',
    );
    assert.equal(turnOf(s, 0).movesRemaining, 5, 'the manoeuvre costs a Move');
  });

  it('only reaches an adjacent card', () => {
    const s = game();
    setup(s);
    const cardId = hand(s, 'maneuver', 'flyingSwitch');
    assert.equal(
      check(s, 0, { type: 'maneuver.flyingSwitch', cardId, trayId: 'crew', count: 1, to: at(-4, 0) }),
      'NOT_CONNECTED',
    );
  });

  it('rolls into an industry, not onto plain track', () => {
    const s = game();
    setup(s);
    const cardId = hand(s, 'maneuver', 'flyingSwitch');
    // (0,0) is the Office — track-connected to the crew back up through the turnout, but not a
    // freight industry.
    assert.equal(
      check(s, 0, { type: 'maneuver.flyingSwitch', cardId, trayId: 'crew', count: 1, to: at(0, 0) }),
      'CANNOT_DROP_HERE',
    );
  });

  it('will not roll onto a card with no track connection', () => {
    const s = game();
    setup(s);
    const area = areaOf(s, 0);
    // An industry sitting directly beneath the crew. Both cards are plain east-west track, so
    // nothing crosses the edge between them: adjacency is not a rail.
    const stranded = { ...area.grid.get(coordKey(at(-1, 3)))! };
    area.grid.set(coordKey(at(-2, 2)), stranded);
    const cardId = hand(s, 'maneuver', 'flyingSwitch');
    assert.equal(
      check(s, 0, { type: 'maneuver.flyingSwitch', cardId, trayId: 'crew', count: 1, to: at(-2, 2) }),
      'NOT_CONNECTED',
    );
  });

  it('cannot cut more cars than it is hauling', () => {
    const s = game();
    setup(s);
    const cardId = hand(s, 'maneuver', 'flyingSwitch');
    assert.equal(
      check(s, 0, { type: 'maneuver.flyingSwitch', cardId, trayId: 'crew', count: 5, to: at(-1, 3) }),
      'CONSIST_EMPTY',
    );
  });
});

describe('card coverage', () => {
  it('every Mainline modifier is either implemented or a known duplicate', () => {
    for (const card of MAINLINE_MODIFIER_CARDS) {
      const known =
        mainlineModifierRule(card.key) !== null || card.key === 'facingPointLocksMainline';
      assert.ok(known, `${card.key} has no rule and is not the Facing Point Locks duplicate`);
    }
  });

  it('Poling stays unimplemented AND out of the deck, deliberately', () => {
    // Guard against someone "fixing" this by inventing an effect. The sheet says TBD; until the
    // source says otherwise, a silent no-op would be worse than a rejection.
    //
    // It is dealt zero copies rather than deleted: a card that cannot be played is worse in a
    // player's hand than absent from the deck, but the entry stays so the gap remains visible and
    // the card returns the moment its rule is known.
    const poling = MANEUVER_CARDS.find((c) => c.key === 'poling');
    assert.ok(poling, 'the Poling entry should stay, so the gap does not vanish');
    assert.match(poling!.effect, /TBD/i);
    assert.equal(poling!.copies, 0, 'Poling is dealt into the deck but cannot be played');
  });
});

// ---------------------------------------------------------------------------

describe('the Limits sign moves with the Running Track (§2.1, Gap 4a)', () => {
  it('refuses to build beyond the Limits sign', () => {
    // §2.1 — the Running Track runs BETWEEN the Limits, so the sign is the only growth point.
    // Offering the square PAST it produced, from one straight laid at (0,2):
    //   limits · Whistle Post · limits · straight · limits
    // — track on the far side of the sign, and a second sign planted beyond that.
    const s = game();
    const area = areaOf(s, 0);
    turnOf(s, 0).option = 'draw';

    const straight = trackInHand(s, 'straight', 'none');
    const beyondEast = { row: area.runningRow, col: area.limitsEast.col + 1 };
    const beyondWest = { row: area.runningRow, col: area.limitsWest.col - 1 };
    for (const placement of [beyondEast, beyondWest]) {
      assert.notEqual(
        check(s, 0, { type: 'card.play', cardId: straight, placement, variant: 0 }),
        null,
        `(${placement.row},${placement.col}) is outside the Limits and must not be placeable`,
      );
    }

    // The sign itself IS legal — that is how the Running Track grows.
    assert.equal(
      check(s, 0, {
        type: 'card.play', cardId: straight,
        placement: { row: area.runningRow, col: area.limitsEast.col }, variant: 0,
      }),
      null,
      'laying on the Limits sign must be how the track extends',
    );

    // A district opens with a TURNOUT on the Running Track — no office or industry card carries a
    // stub of its own — so extend onto the east sign with one.
    const turnoutCol = area.limitsEast.col;
    assert.ok(
      applyIntent(s, 0, {
        type: 'card.play', cardId: trackInHand(s, 'turnout', 'left'),
        placement: { row: area.runningRow, col: turnoutCol }, variant: 0,
      }).ok,
      'a turnout must be layable on the Limits sign',
    );

    // The district hangs off that turnout and is not bounded by the Limits at all. Variant 1 of a
    // left-hand curve is the `nw` arc — the one on the same diagonal as the turnout's leg.
    assert.equal(
      check(s, 0, {
        type: 'card.play', cardId: trackInHand(s, 'curved', 'left'),
        placement: { row: area.runningRow - 1, col: turnoutCol }, variant: 1,
      }),
      null,
      'Secondary Track below the turnout must stay legal',
    );
  });

  it('keeps the Limits at the ends, never stranded mid-track', () => {
    // Found by watching a replay: a district grew to
    //   (0,-5) (0,-4) (0,-3) (0,-2)=Power Plant [LIMITS] (0,0)=Office [LIMITS] (0,2) …
    // with everything beyond (0,-2) built OUTSIDE a sign that never moved. Not cosmetic — §8.1 and
    // §10 both reason about "the track between the train and the Limits", and running past a
    // player's Limits is what makes a collision his fault.
    const s = game();
    const area = areaOf(s, 0);
    turnOf(s, 0).option = 'draw';

    for (let n = 0; n < 4; n++) {
      const target = { row: area.runningRow, col: area.limitsWest.col };
      const r = applyIntent(s, 0, {
        type: 'card.play', cardId: trackInHand(s, 'straight', 'none'), placement: target, variant: 0,
      });
      assert.ok(r.ok, `extending onto the Limits should be legal (attempt ${n + 1})`);
    }

    const onRunning = [...area.grid.entries()]
      .map(([k, c]) => ({ col: Number(k.split(',')[1]), kind: c.geometry.kind }))
      .filter((x) => !Number.isNaN(x.col));
    const limits = onRunning.filter((x) => x.kind === 'limits').map((x) => x.col).sort((a, b) => a - b);
    const cols = onRunning.map((x) => x.col);

    assert.equal(limits.length, 2, 'there must be exactly two Limits signs');
    assert.equal(limits[0], Math.min(...cols), 'the west sign must be the westernmost card');
    assert.equal(limits[1], Math.max(...cols), 'the east sign must be the easternmost card');
  });
});

// ---------------------------------------------------------------------------

describe('a turnout may be laid on top of a card already down', () => {
  /** Lays a straight inside the Limits and returns where it went. */
  const layStraight = (s: GameState): GridCoord => {
    const area = areaOf(s, 0);
    const target = { row: area.runningRow, col: area.limitsEast.col };
    const r = applyIntent(s, 0, {
      type: 'card.play', cardId: trackInHand(s, 'straight', 'none'), placement: target, variant: 0,
    });
    assert.ok(r.ok, 'the straight should lay on the Limits sign');
    return target;
  };

  it('upgrades a straight, which is how a district opens off track already built', () => {
    // REPORTED from playtesting: a district can only hang off a turnout, so a player who laid a
    // straight along the main and then wanted to branch there had no move at all — the piece had to
    // have been a turnout when it went down.
    const s = game();
    turnOf(s, 0).option = 'draw';
    const at = layStraight(s);

    for (const hand of ['left', 'right'] as const) {
      for (let variant = 0; variant < 2; variant++) {
        assert.equal(
          check(s, 0, { type: 'card.play', cardId: trackInHand(s, 'turnout', hand), placement: at, variant }),
          null,
          `a ${hand}-hand turnout at rotation ${variant} must be able to upgrade a straight`,
        );
      }
    }

    const r = applyIntent(s, 0, {
      type: 'card.play', cardId: trackInHand(s, 'turnout', 'right'), placement: at, variant: 0,
    });
    assert.ok(r.ok);
    const now = areaOf(s, 0).grid.get(`${at.row},${at.col}`)!;
    assert.equal(now.geometry.kind === 'track' && now.geometry.geometry, 'turnout', 'the square is still a straight');
  });

  it('upgrades a curve only when the turnout diverges onto the same arc', () => {
    // The ARC, not merely the diagonal. A `sw` curve and an `ne` one share a slope but leave by
    // opposite edges, so swapping one for the other would move the leg off whatever it joined.
    const s = game();
    const area = areaOf(s, 0);
    turnOf(s, 0).option = 'draw';

    // A turnout on the main, then its matching curve on the row below — the start of every siding.
    const turnoutAt = { row: area.runningRow, col: area.limitsEast.col };
    assert.ok(applyIntent(s, 0, {
      type: 'card.play', cardId: trackInHand(s, 'turnout', 'right'), placement: turnoutAt, variant: 0,
    }).ok, 'the turnout should lay on the Limits sign');

    const curveAt = { row: area.runningRow - 1, col: turnoutAt.col };
    assert.ok(applyIntent(s, 0, {
      type: 'card.play', cardId: trackInHand(s, 'curved', 'right'), placement: curveAt, variant: 1,
    }).ok, 'the matching curve should hang under the turnout');

    const laid = area.grid.get(`${curveAt.row},${curveAt.col}`)!;
    const arc = laid.geometry.kind === 'track' ? laid.geometry.arc : undefined;
    assert.equal(arc, 'ne', 'the fixture is not the curve this test means to upgrade');

    // A turnout whose leg is that same `ne` arc may replace it; every other rotation may not.
    const matching: string[] = [];
    for (const hand of ['left', 'right'] as const) {
      for (let variant = 0; variant < 2; variant++) {
        const code = check(s, 0, {
          type: 'card.play', cardId: trackInHand(s, 'turnout', hand), placement: curveAt, variant,
        });
        if (code === null) matching.push(`${hand}/${variant}`);
        else assert.equal(code, 'NOT_UPGRADEABLE_TRACK', `${hand}/${variant} should be refused as a mismatch`);
      }
    }
    assert.deepEqual(matching, ['right/1'], 'exactly the turnout diverging onto `ne` should be accepted');
  });

  it('refuses to swap the track out from under a car, or out from under an Interlocking', () => {
    const s = game();
    const area = areaOf(s, 0);
    turnOf(s, 0).option = 'draw';
    const at = layStraight(s);
    const square = area.grid.get(`${at.row},${at.col}`)!;

    square.standing = [{ type: 'boxcar', loaded: false }];
    assert.equal(
      check(s, 0, { type: 'card.play', cardId: trackInHand(s, 'turnout', 'right'), placement: at, variant: 0 }),
      'UPGRADE_OCCUPIED',
      'track was swapped out from under a standing car',
    );

    square.standing = [];
    square.enhancements = ['interlocking'];
    assert.equal(
      check(s, 0, { type: 'card.play', cardId: trackInHand(s, 'turnout', 'right'), placement: at, variant: 0 }),
      'UPGRADE_ENHANCED',
      'the upgrade lifted a built Interlocking with the card',
    );
  });

  it('refuses to upgrade anything that is not a straight or a matching curve', () => {
    // The Office is east-west track like a straight, but it is not track to build over.
    const s = game();
    const area = areaOf(s, 0);
    turnOf(s, 0).option = 'draw';
    assert.equal(
      check(s, 0, {
        type: 'card.play', cardId: trackInHand(s, 'turnout', 'right'),
        placement: area.officeCoord, variant: 0,
      }),
      'NOT_UPGRADEABLE_TRACK',
      'a turnout was laid over the Office',
    );
  });

  it('does not let a straight or a curve upgrade anything — only a turnout may', () => {
    const s = game();
    turnOf(s, 0).option = 'draw';
    const at = layStraight(s);

    assert.equal(
      check(s, 0, { type: 'card.play', cardId: trackInHand(s, 'straight', 'none'), placement: at, variant: 0 }),
      'NOT_UPGRADEABLE_TRACK',
      'a straight was laid on top of a straight',
    );
    assert.equal(
      check(s, 0, { type: 'card.play', cardId: trackInHand(s, 'curved', 'right'), placement: at, variant: 0 }),
      'NOT_UPGRADEABLE_TRACK',
      'a curve was laid on top of a straight',
    );
  });

  it('offers the upgrade in the legal-move list, or it may as well not exist', () => {
    // The 18 Enhancement cards were permanently dead for weeks for exactly this reason: the engine
    // allowed the play and `legalIntents` never once offered a square it could go on.
    const s = game();
    turnOf(s, 0).option = 'draw';
    const at = layStraight(s);
    const cardId = trackInHand(s, 'turnout', 'right');

    const offered = legalActions(s, 0).some(
      (i) => i.type === 'card.play' && i.cardId === cardId &&
        i.placement?.row === at.row && i.placement?.col === at.col,
    );
    assert.ok(offered, 'the occupied square is never offered, so the upgrade cannot be played');
  });
});

describe("a train is made up to its card's consist (§8.2)", () => {
  it('takes a caboose when the card calls for one, and refuses a fourth freight car', () => {
    // Train 9 "Heavy Freight" is freight 3 + caboose 1. It was being made up with FOUR hoppers and
    // no caboose, because placeCar checked only MAX_CONSIST and yard availability.
    const s = game();
    s.clock.phase = 'newTrain';
    s.trays.set('t', {
      id: 't', trainNumber: 9, trainIsExtra: false, engineAt: 0,
      consist: [], direction: 'west', position: { at: 'divisionPoint', side: 'east' }, movesUsed: 0,
    });

    for (let n = 0; n < 3; n++) {
      const r = applyIntent(s, 0, { type: 'newTrain.placeCar', trayId: 't', carType: 'hopper', loaded: true });
      assert.ok(r.ok, `hopper ${n + 1} of 3 should be accepted`);
    }
    assert.equal(
      check(s, 0, { type: 'newTrain.placeCar', trayId: 't', carType: 'hopper', loaded: true }),
      'NO_SUITABLE_CAR',
      'a fourth freight car exceeds the card',
    );
    assert.equal(
      check(s, 0, { type: 'newTrain.placeCar', trayId: 't', carType: 'coach', loaded: true }),
      'NO_SUITABLE_CAR',
      'this card carries no coaches',
    );
    assert.equal(
      check(s, 0, { type: 'newTrain.placeCar', trayId: 't', carType: 'caboose', loaded: true }),
      null,
      'the card calls for a caboose',
    );
  });

  it('couples nothing behind the caboose', () => {
    // §A.3 — the caboose rides last.
    const s = game();
    s.clock.phase = 'newTrain';
    s.trays.set('t', {
      id: 't', trainNumber: 9, trainIsExtra: false, engineAt: 0,
      consist: [{ type: 'caboose', loaded: true }],
      direction: 'west', position: { at: 'divisionPoint', side: 'east' }, movesUsed: 0,
    });
    assert.equal(
      check(s, 0, { type: 'newTrain.placeCar', trayId: 't', carType: 'hopper', loaded: true }),
      'NO_SUITABLE_CAR',
    );
  });
});

describe('regions on a Mainline card (§2.1, §8.2)', () => {
  /** Put one train mid-crossing and ask the view where the map should draw it. */
  const regionFor = (stagesTotal: number, stagesRemaining: number): { region: number; regions: number } => {
    const s = createGame({
      id: 'reg',
      seed: 4,
      config: {
        mode: 'solitaire', victory: 'highestAfterDays', length: 'standard',
        optionalRules: { reducedVisibility: false, sisterTrains: false, employeeRotation: false, emergencyToolbox: false },
      },
      playerNames: ['Solitaire'],
    });
    const node = s.division.nodes.find((n) => n.kind === 'mainline');
    assert.ok(node && node.kind === 'mainline');
    const tray = [...s.trays.keys()][0]!;
    node.transits.push({ tray, stagesRemaining, stagesTotal, direction: 'east' });
    const ml = snapshot(s, [], null).division.find((n) => n.kind === 'ml');
    assert.ok(ml, 'no Mainline node in the view');
    const t = ml!.trains.flat()[0]!;
    return { region: t.region as number, regions: ml!.regions as number };
  };

  it('divides every Mainline card into two regions', () => {
    // §2.1, and open-questions.md settled on two deliberately: at three, a Day of 12 Stages leaves
    // trains still on the road when the next Day begins, locking Crew Trays permanently.
    assert.equal(regionFor(2, 2).regions, 2);
  });

  it('moves one region per Stage for an ordinary train on a 30 card', () => {
    // §8.2 exactly. This is the case the printed rule was written for.
    assert.equal(regionFor(2, 2).region, 0);
    assert.equal(regionFor(2, 1).region, 1);
  });

  it('starts a fast train further along the card rather than moving it twice', () => {
    // The printed cards carry Start positions — a 60 card is crossed in one Stage, which the map
    // shows as entering at the second region. That is the same fact as "Brakemen moves your entry
    // point further along", which is how the modifier cards are drawn.
    assert.equal(regionFor(1, 1).region, 1);
  });

  it('holds a slow train at the START, not at the exit', () => {
    // Three Stages will not fit three steps into two regions. The card's distance is fixed, so the
    // extra Stage is spent getting going — clamping the entry instead would park it at the exit and
    // read as a train that raced across and then waited.
    assert.equal(regionFor(3, 3).region, 0);
    assert.equal(regionFor(3, 2).region, 0);
    assert.equal(regionFor(3, 1).region, 1);
  });

  it('never leaves the card it is on', () => {
    for (let total = 1; total <= 4; total++) {
      for (let left = total; left >= 1; left--) {
        const r = regionFor(total, left).region;
        assert.ok(r >= 0 && r <= 1, `total ${total}, ${left} left put the train in region ${r}`);
      }
    }
  });
});

describe('Q13 — a train that catches the one ahead runs into it', () => {
  /**
   * Two trains on one card, same direction, the follower closing. `total 3 / remaining 2` puts the
   * leader in region 1 after it moves; `total 2 / remaining 2` puts the follower in region 0 and
   * then region 1 — so it catches up whichever order the phase happens to process them in.
   */
  const twoTrains = (opts: { absSignals?: boolean } = {}): { s: GameState; events: GameEvent[] } => {
    const s = createGame({
      id: 'rear', seed: 3,
      config: {
        mode: 'solitaire', victory: 'highestAfterDays', length: 'standard',
        optionalRules: { reducedVisibility: false, sisterTrains: false, employeeRotation: false, emergencyToolbox: false },
      },
      playerNames: ['bot'],
    });
    const index = s.division.nodes.findIndex(
      (n) => n.kind === 'mainline' && !mainlineProfile(n.card).trainsMayPass,
    );
    assert.ok(index >= 0, 'no single-track Mainline card in this Division');
    const node = s.division.nodes[index]!;
    assert.ok(node.kind === 'mainline');
    if (node.kind !== 'mainline') throw new Error('unreachable');
    if (opts.absSignals) node.absSignals = true;

    const make = (n: number, total: number, left: number): string => {
      const id = s.freeTrays.pop()!;
      s.trays.set(id, {
        id, trainNumber: n, trainIsExtra: false, engineAt: 0,
        consist: [{ type: 'boxcar', loaded: false }],
        direction: 'east',
        position: { at: 'mainline', index },
      } as never);
      node.transits.push({ tray: id, stagesRemaining: left, stagesTotal: total, direction: 'east' });
      return id;
    };
    make(9, 3, 2);   // the leader
    make(11, 2, 2);  // the follower, closing

    s.clock.phase = 'mainline';
    const events: GameEvent[] = [];
    for (let i = 0; i < 4; i++) events.push(...advance(s).events);
    return { s, events };
  };

  it('destroys both trains, and it is the Superintendent\'s fault (§10)', () => {
    // REGRESSION against a rule that was never implemented. §10 makes a Mainline collision the
    // Superintendent's fault and removes both trains, and ABS Signals exists to prevent rear-enders
    // — but §8.3's trigger list never named one, so granting clearance was FREE: both trains
    // survived, no penalty, and ABS Signals protected against nothing.
    const { events } = twoTrains();
    const smash = events.find((e) => e.type === 'trainsDestroyed');
    assert.ok(smash, 'the follower ran into the leader and nothing happened');
    if (smash.type !== 'trainsDestroyed') throw new Error('unreachable');
    assert.match(smash.reason, /ran into the train ahead/);
    assert.equal(smash.trains.length, 2, 'a rear-ender removes BOTH trains');
    assert.ok(
      events.some((e) => e.type === 'revenueChanged' && e.delta < 0),
      'the collision cost nothing',
    );
  });

  it('ABS Signals holds the follower short instead', () => {
    // "Trains on this card will not rear-end each other; they stop short of a collision."
    const { events } = twoTrains({ absSignals: true });
    assert.ok(
      !events.some((e) => e.type === 'trainsDestroyed'),
      'ABS Signals did not prevent the collision',
    );
    assert.ok(
      events.some((e) => e.type === 'trainHeld' && /ABS Signals/.test(e.reason)),
      'nothing was held short of the train ahead',
    );
  });

  it('leaves trains alone on a card that prints "trains may pass"', () => {
    // Double Track and Uncontrolled Siding hold two trains because they HAVE two roads. Catching up
    // there means going past, which is what the card is for. Without this the mechanic fired 0.41
    // times a game while the bot never once granted clearance — the tell that they were all
    // passing cards.
    const passing = MAINLINE_PROFILES.filter((m) => m.trainsMayPass).map((m) => m.kind);
    assert.deepEqual(passing, ['doubleTrack', 'uncontrolledSiding']);
  });
});