/**
 * Build order step 2 — "Move legality provable against Appendix A's worked switching examples."
 * See architecture/components.md §4.
 */

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

import type {
  GameState,
  GridCoord,
  OfficeArea,
  RollingStock,
  TrackArc,
  TrackCard,
  TurnoutOrientation,
} from '../src/engine/state.ts';
import { coordKey } from '../src/engine/state.ts';
import { createGame } from '../src/engine/setup.ts';
import { applyIntent, areaOf } from '../src/engine/apply.ts';
import type { MoveContext, Occupancy, Port } from '../src/engine/track.ts';
import {
  allReachable,
  canDropCarsAt,
  canPlaceAt,
  exitsFrom,
  hasPort,
  neighbour,
  opposite,
  reachableDestinations,
  connectionsFor,
  joins,
  slopeAt,
  slopeOfPair,
  variantsFor,
} from '../src/engine/track.ts';
import { TRACK_SUPPLY } from '../src/engine/content.ts';

// ---------------------------------------------------------------------------
// Fixture helpers
// ---------------------------------------------------------------------------

const straight = (standing: RollingStock[] = []): TrackCard => ({
  geometry: { kind: 'track', geometry: 'straight' },
  baseOperationalRail: true,
  standing,
  facility: null,
  modifiers: [],
  enhancements: [],
});

/**
 * A curve: along the centre line from the east or west edge to a frog, then out at 45° through the
 * middle of the north or south edge. `ne`/`sw` lie on one diagonal, `nw`/`se` on the other, and only
 * cards on the SAME diagonal meet across a horizontal edge.
 */
const curve = (arc: TrackArc, standing: RollingStock[] = []): TrackCard => ({
  geometry: { kind: 'track', geometry: 'curved', arc },
  baseOperationalRail: true,
  standing,
  facility: null,
  modifiers: [],
  enhancements: [],
});

const turnout = (o?: TurnoutOrientation): TrackCard => ({
  geometry: o
    ? { kind: 'track', geometry: 'turnout', turnout: o }
    : { kind: 'track', geometry: 'turnout' },
  baseOperationalRail: false, // no wheel icon — a train may pass through but not stop (§A.1)
  standing: [],
  facility: null,
  modifiers: [],
  enhancements: [],
});

const officeCard = (): TrackCard => ({
  geometry: { kind: 'office' },
  baseOperationalRail: true,
  standing: [],
  facility: null,
  modifiers: [],
  enhancements: [],
});

const lockedFacility = (): TrackCard => ({
  geometry: { kind: 'facility', facility: 'mineTipple' },
  baseOperationalRail: true,
  standing: [],
  // A load sitting on MEN|AT|WORK locks the industry track down (§9.3).
  facility: {
    kind: 'freight',
    subtype: 'mineTipple',
    allows: { outbound: true, inbound: false },
    outboundBox: [],
    inboundBox: [],
    capacity: { outbound: 3, inbound: 0 },
    menAtWork: [{ type: 'hopper', dir: 'out' }, null, null],
    industryTrack: { length: 4, cars: [] },
    laborers: 3,
    porters: 0,
    usedThisStage: { laborers: 0, porters: 0 },
  },
  modifiers: [],
  enhancements: [],
});

const car = (): RollingStock => ({ type: 'boxcar', loaded: false });

function straightCard(): TrackCard {
  return {
    geometry: { kind: 'track', geometry: 'straight' },
    baseOperationalRail: true, standing: [], facility: null, modifiers: [], enhancements: [],
  };
}

/** A minimal game wrapped around a prepared Office Area, for engine-level switching tests. */
function gameWith(area: OfficeArea): GameState {
  const s = createGame({
    id: 'g', seed: 5,
    config: {
      mode: 'solitaire', victory: 'highestAfterDays', length: 'standard',
      optionalRules: { reducedVisibility: false, sisterTrains: false, employeeRotation: false, emergencyToolbox: false },
    },
    playerNames: ['p'],
  });
  s.officeAreas.set(0, area);
  return s;
}

function areaFrom(cards: Record<string, TrackCard>, officeCoord: GridCoord): OfficeArea {
  const grid = new Map<string, TrackCard>();
  for (const [k, v] of Object.entries(cards)) grid.set(k, v);
  return {
    owner: 0,
    tier: 'whistlePost',
    grid,
    officeCoord,
    runningRow: officeCoord.row,
    limitsWest: { row: officeCoord.row, col: officeCoord.col - 2 },
    limitsEast: { row: officeCoord.row, col: officeCoord.col + 2 },
    adOccupancy: [],
    heldAtLimits: [],
    dispatchUsedToday: [],
    trackSupply: new Map(),
  };
}

const emptyOccupancy: Occupancy = { trayAt: () => null, freeAdTracks: () => 1 };

function ctxFor(area: OfficeArea, over: Partial<MoveContext> = {}): MoveContext {
  return { area, occupancy: emptyOccupancy, consistSize: 0, self: 'tray0', ...over };
}

const at = (r: number, c: number): GridCoord => ({ row: r, col: c });
const has = (dests: { coord: GridCoord }[], r: number, c: number): boolean =>
  dests.some((d) => d.coord.row === r && d.coord.col === c);

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

describe('ports and geometry', () => {
  it('pairs opposite ports and grid neighbours consistently', () => {
    for (const p of ['n', 's', 'e', 'w'] as Port[]) {
      assert.equal(opposite(opposite(p)), p);
      const there = neighbour(at(0, 0), p);
      const back = neighbour(there, opposite(p));
      assert.deepEqual(back, at(0, 0));
    }
  });

  it('joins a turnout stem to both legs but never the legs to each other', () => {
    // §A.1 — the whole point. Entering from the stem reaches either leg; entering from a leg
    // reaches only the stem. This is an ABSENT edge, not a one-way edge.
    const t = turnout({ stem: 'e', through: 'w', diverge: 's' });
    assert.deepEqual(exitsFrom(t, 'e').sort(), ['s', 'w']);
    assert.deepEqual(exitsFrom(t, 'w'), ['e']);
    assert.deepEqual(exitsFrom(t, 's'), ['e']);
  });

  it('lets a train traverse a turnout in both directions', () => {
    const t = turnout({ stem: 'e', through: 'w', diverge: 's' });
    assert.ok(exitsFrom(t, 'e').includes('s'), 'stem to diverging leg');
    assert.ok(exitsFrom(t, 's').includes('e'), 'diverging leg back to stem');
  });

  it('gives the Office card a plain east-west track and no stub at all', () => {
    // Every office and industry card on the printed sheet — Depot, Station, Terminal, Refinery,
    // Freighthouse, Coal Tipple, Manufacturing, Town — is a straight east-west rail with no
    // diverging leg. A district hangs off a TURNOUT laid on the Running Track instead.
    const o = officeCard();
    for (const p of ['e', 'w'] as Port[]) assert.ok(hasPort(o, p), `office port ${p}`);
    for (const p of ['n', 's'] as Port[]) assert.ok(!hasPort(o, p), `office must not have a ${p} stub`);
    assert.deepEqual(exitsFrom(o, 'e'), ['w']);
  });

  it('runs every straight east-west, with no north-south rotation to choose', () => {
    // There is no north-south track anywhere on the printed sheet, and there cannot be: a vertical
    // rail meets a card edge at 90°, and everything crossing a horizontal edge does so at 45°.
    assert.deepEqual(connectionsFor(straight()), [['e', 'w']]);
    assert.equal(variantsFor('straight', 'none').length, 1, 'a straight has ONE orientation');
  });

  it('joins a curve from one end of the through track to the middle of an edge', () => {
    // A curve runs along the centre line to a frog and leaves at 45°. Handedness decides which
    // diagonal; turning the card 180° decides whether the leg goes north or south.
    const r: TrackCard = {
      geometry: { kind: 'track', geometry: 'curved', hand: 'left' },
      baseOperationalRail: true,
      standing: [],
      facility: null,
      modifiers: [],
      enhancements: [],
    };
    assert.deepEqual(exitsFrom(r, 's'), ['w']);
  });
});

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

describe('the 45° matching rule', () => {
  const arcCard = (arc: TrackArc): TrackCard => curve(arc);
  const turnoutOf = (o: TurnoutOrientation): TrackCard => turnout(o);

  it('puts a pair of arcs on each diagonal, named after the pair', () => {
    // The name IS the rule: `ne` and `sw` lie on one line, `nw` and `se` on the other.
    assert.equal(slopeOfPair('n', 'e'), 'ne_sw');
    assert.equal(slopeOfPair('s', 'w'), 'ne_sw');
    assert.equal(slopeOfPair('n', 'w'), 'nw_se');
    assert.equal(slopeOfPair('s', 'e'), 'nw_se');
    assert.equal(slopeOfPair('e', 'w'), null, 'the through track has no slope');
  });

  it('joins two cards stacked vertically only when their legs share a diagonal', () => {
    // `sw` above `ne` is one unbroken 45° rail across the card edge. `sw` above `nw` is a V: both
    // cards have the port, the rails meet at the same point, and they still do not connect.
    assert.ok(joins(arcCard('sw'), 's', arcCard('ne')), 'sw over ne is continuous rail');
    assert.ok(joins(arcCard('se'), 's', arcCard('nw')), 'se over nw is continuous rail');
    assert.ok(!joins(arcCard('sw'), 's', arcCard('nw')), 'sw over nw is a V');
    assert.ok(!joins(arcCard('se'), 's', arcCard('ne')), 'se over ne is a V');
  });

  it('matches a turnout to the curve of the same hand, and to no other', () => {
    // A siding needs one card of each hand: a left turnout to drop off the main, a right curve to
    // climb back. This is the pairing that makes that true.
    const leftTurnout = turnoutOf({ stem: 'w', through: 'e', diverge: 's' });
    const rightTurnout = turnoutOf({ stem: 'e', through: 'w', diverge: 's' });
    assert.ok(joins(leftTurnout, 's', arcCard('ne')));
    assert.ok(!joins(leftTurnout, 's', arcCard('nw')));
    assert.ok(joins(rightTurnout, 's', arcCard('nw')));
    assert.ok(!joins(rightTurnout, 's', arcCard('ne')));
  });

  it('ignores slope on an east-west edge, where every card meets at the same height', () => {
    assert.ok(joins(straight(), 'e', straight()));
    assert.ok(joins(arcCard('ne'), 'e', straight()), 'a curve reaching east meets the through line');
  });

  it('never connects north to south on any card the supply can produce', () => {
    // Descending a row therefore costs at least two cards — leg down, then a curve turning the run
    // back east-west. A district can never be a vertical column, which is what the sheet shows.
    for (const { geometry, hand } of TRACK_SUPPLY) {
      for (const v of variantsFor(geometry, hand)) {
        const card = { ...straight(), geometry: { kind: 'track' as const, geometry, ...v, ...(hand !== 'none' ? { hand } : {}) } };
        for (const [a, b] of connectionsFor(card)) {
          const pair = new Set([a, b]);
          assert.ok(!(pair.has('n') && pair.has('s')), `${geometry}/${hand} joins north to south`);
        }
      }
    }
  });

  it('gives every north or south port exactly one slope', () => {
    // `slopeAt` returns the first matching pair, so a card carrying two differently-sloped legs at
    // one edge would silently answer with whichever came first. Nothing may.
    for (const { geometry, hand } of TRACK_SUPPLY) {
      for (const v of variantsFor(geometry, hand)) {
        const card = { ...straight(), geometry: { kind: 'track' as const, geometry, ...v, ...(hand !== 'none' ? { hand } : {}) } };
        for (const p of ['n', 's'] as Port[]) {
          const slopes = new Set(
            connectionsFor(card).filter(([a, b]) => a === p || b === p).map(([a, b]) => slopeOfPair(a, b)),
          );
          assert.ok(slopes.size <= 1, `${geometry}/${hand} has two slopes at its ${p} edge`);
        }
      }
    }
  });

  it('ties the slope to the printed hand, so turning a card never changes diagonal', () => {
    // A printed card has a back: it turns 180° but never flips. That is the whole reason handedness
    // is not decoration.
    for (const geometry of ['curved', 'sharpCurved', 'turnout'] as const) {
      for (const [hand, expected] of [['left', 'ne_sw'], ['right', 'nw_se']] as const) {
        const variants = variantsFor(geometry, hand);
        assert.equal(variants.length, 2, `${geometry}/${hand} should offer 0° and 180°, and nothing else`);
        const legs = variants.map((v) => {
          const card = { ...straight(), geometry: { kind: 'track' as const, geometry, ...v, hand } };
          return (slopeAt(card, 'n') ?? slopeAt(card, 's'));
        });
        assert.deepEqual(legs, [expected, expected], `${geometry}/${hand} changes diagonal when turned`);
      }
    }
  });
});

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

describe('Move legality', () => {
  //  row  0: [straight] [office] [turnout, leg south] [straight]   <- Running Track
  //  row -1:                     [curve ne         ]               <- Secondary Track
  //
  // The turnout's leg is on the `ne_sw` diagonal ({w,s}), so the card beneath it must be the arc on
  // the SAME diagonal that reaches north — `ne`. `nw` would meet the same point and still not join.
  const basicArea = (): OfficeArea =>
    areaFrom(
      {
        [coordKey(at(0, -1))]: straight(),
        [coordKey(at(0, 0))]: officeCard(),
        [coordKey(at(0, 1))]: turnout({ stem: 'w', through: 'e', diverge: 's' }),
        [coordKey(at(0, 2))]: straight(),
        [coordKey(at(-1, 1))]: curve('ne'),
      },
      at(0, 0),
    );

  it('travels any distance in a straight line', () => {
    // §2.4 — "regardless of distance".
    const area = basicArea();
    const dests = reachableDestinations(ctxFor(area), at(0, 2), 'w');
    assert.ok(has(dests, 0, 0), 'reaches the office');
    assert.ok(has(dests, 0, -1), 'reaches past it');
  });

  it('branches onto Secondary Track through a turnout on the Running Track', () => {
    // The crew must enter the turnout through its STEM (west) to take the diverging leg (§A.1).
    const area = basicArea();
    const dests = reachableDestinations(ctxFor(area), at(0, 0), 'e');
    assert.ok(has(dests, -1, 1), 'reaches the curve below the turnout');
  });

  it('will not take a diverging leg entered from the through end', () => {
    // §A.1 — through and diverge are not joined to each other. Approaching from the east reaches
    // the stem and nothing else, so the leg below is unreachable from that side.
    const area = basicArea();
    const dests = reachableDestinations(ctxFor(area), at(0, 2), 'w');
    assert.ok(!has(dests, -1, 1), 'no route from the through end onto the diverging leg');
  });

  it('never reverses within a single Move', () => {
    // Leaving east from the west end cannot double back to the west end.
    const area = basicArea();
    const dests = reachableDestinations(ctxFor(area), at(0, -1), 'e');
    assert.ok(!has(dests, 0, -1), 'a Move may not return to its own start');
  });

  it('separates forward and reverse into different Moves', () => {
    const area = basicArea();
    const both = allReachable(ctxFor(area), at(0, 0), 'e');
    // Col 1 is the turnout, which carries no wheel icon, so east means running through it to col 2.
    assert.ok(has(both.forward, 0, 2));
    assert.ok(has(both.reverse, 0, -1));
    assert.ok(!has(both.forward, 0, -1), 'reverse destinations must cost their own Move');
  });

  it('will not stop on a turnout', () => {
    //  row  0: [start] [turnout, leg south]
    //  row -1:         [curve ne          ]
    const area = areaFrom(
      {
        [coordKey(at(0, 0))]: straight(),
        [coordKey(at(0, 1))]: turnout({ stem: 'w', through: 'e', diverge: 's' }),
        [coordKey(at(-1, 1))]: curve('ne'),
      },
      at(0, 0),
    );
    const dests = reachableDestinations(ctxFor(area), at(0, 0), 'e');
    assert.ok(!has(dests, 0, 1), 'a turnout carries no wheel icon (§A.1)');
    assert.ok(has(dests, -1, 1), 'but a train may pass through it');
  });

  it('will not stop on a facility whose track is locked by a load', () => {
    // §9.3 — while any load sits on MEN|AT|WORK the industry track is not Operational Rail.
    const area = areaFrom(
      {
        [coordKey(at(0, 0))]: straight(),
        [coordKey(at(0, 1))]: lockedFacility(),
      },
      at(0, 0),
    );
    const dests = reachableDestinations(ctxFor(area), at(0, 0), 'e');
    assert.ok(!has(dests, 0, 1), 'locked industry track must not accept a train');
  });
});

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

describe('coupling', () => {
  const withCars = (n: number): OfficeArea =>
    areaFrom(
      {
        [coordKey(at(0, 0))]: straight(),
        [coordKey(at(0, 1))]: straight(Array.from({ length: n }, car)),
        [coordKey(at(0, 2))]: straight(),
      },
      at(0, 0),
    );

  it('couples cars met along the way, mandatorily', () => {
    // §A.4 — "you MUST pick them up. You may not go around them."
    const dests = reachableDestinations(ctxFor(withCars(2)), at(0, 0), 'e');
    const beyond = dests.find((d) => d.coord.col === 2);
    assert.ok(beyond, 'should be able to pass the occupied card');
    assert.equal(beyond.couples.length, 2, 'cars must be collected, not stepped over');
  });

  it('forbids a Move that would overfill the tray', () => {
    // §A.4 — max four Rolling Stock, cabooses included. Overfilling makes the move illegal
    // rather than picking up fewer cars.
    const ctx = ctxFor(withCars(3), { consistSize: 3 });
    const dests = reachableDestinations(ctx, at(0, 0), 'e');
    assert.equal(dests.length, 0, '3 in tray + 3 standing exceeds 4');
  });

  it('permits a Move that exactly fills the tray', () => {
    const ctx = ctxFor(withCars(2), { consistSize: 2 });
    const dests = reachableDestinations(ctx, at(0, 0), 'e');
    assert.ok(dests.length > 0, '2 + 2 = 4 is legal');
  });
});

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

describe('occupancy', () => {
  const twoTrainArea = (): OfficeArea =>
    areaFrom(
      {
        [coordKey(at(0, 0))]: straight(),
        [coordKey(at(0, 1))]: straight(),
        [coordKey(at(0, 2))]: straight(),
      },
      at(0, 5), // office elsewhere, so col 1 is ordinary track
    );

  it('blocks a card another train occupies', () => {
    // §A.4 — two trains may not share a card or move through each other.
    const occupancy: Occupancy = {
      trayAt: (c) => (c.row === 0 && c.col === 1 ? 'other' : null),
      freeAdTracks: () => 1,
    };
    const dests = reachableDestinations(ctxFor(twoTrainArea(), { occupancy }), at(0, 0), 'e');
    assert.ok(!has(dests, 0, 1), 'occupied card');
    assert.ok(!has(dests, 0, 2), 'and no passing through it');
  });

  it('allows passing through the Office while A/D tracks remain free', () => {
    const area = areaFrom(
      {
        [coordKey(at(0, 0))]: straight(),
        [coordKey(at(0, 1))]: officeCard(),
        [coordKey(at(0, 2))]: straight(),
      },
      at(0, 1),
    );
    const occupancy: Occupancy = {
      trayAt: (c) => (c.row === 0 && c.col === 1 ? 'other' : null),
      freeAdTracks: () => 1,
    };
    const dests = reachableDestinations(ctxFor(area, { occupancy }), at(0, 0), 'e');
    assert.ok(has(dests, 0, 2), 'the Office is the exception to the blocking rule (§A.4)');
  });

  it('blocks the Office once every A/D track is taken', () => {
    const area = areaFrom(
      {
        [coordKey(at(0, 0))]: straight(),
        [coordKey(at(0, 1))]: officeCard(),
        [coordKey(at(0, 2))]: straight(),
      },
      at(0, 1),
    );
    const occupancy: Occupancy = {
      trayAt: (c) => (c.row === 0 && c.col === 1 ? 'other' : null),
      freeAdTracks: () => 0,
    };
    const dests = reachableDestinations(ctxFor(area, { occupancy }), at(0, 0), 'e');
    assert.ok(!has(dests, 0, 2));
  });
});

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

describe('placement and drop-off', () => {
  it('grows a district on BOTH sides of the Running Track', () => {
    // A turnout turned 180° sends its 45° leg north instead of south, so there is nothing special
    // about the rows below. Q7 used to reject everything above the Running Track outright.
    const area = areaFrom(
      {
        [coordKey(at(0, -1))]: straight(),
        [coordKey(at(0, 0))]: officeCard(),
        [coordKey(at(0, 1))]: turnout({ stem: 'w', through: 'e', diverge: 's' }),
        [coordKey(at(0, 2))]: turnout({ stem: 'e', through: 'w', diverge: 'n' }),
      },
      at(0, 0),
    );
    assert.ok(canPlaceAt(area, at(-1, 1), curve('ne')), 'below a south-diverging turnout');
    assert.ok(canPlaceAt(area, at(1, 2), curve('sw')), 'above a north-diverging turnout');
  });

  it('refuses a card whose 45° leg lies on the wrong diagonal', () => {
    // Both cards have the port and both legs meet the same point on the shared edge. They still do
    // not join: one descends to the right and the other to the left, so the rails form a V.
    const area = areaFrom(
      {
        [coordKey(at(0, 0))]: officeCard(),
        [coordKey(at(0, 1))]: turnout({ stem: 'w', through: 'e', diverge: 's' }),
      },
      at(0, 0),
    );
    assert.ok(canPlaceAt(area, at(-1, 1), curve('ne')), 'the matching diagonal joins');
    assert.ok(!canPlaceAt(area, at(-1, 1), curve('nw')), 'the opposite diagonal must not');
  });

  it('refuses a card whose ports do not meet the neighbour it touches', () => {
    // An east-west straight below a turnout has no north port at all, so nothing can reach it.
    const area = areaFrom(
      {
        [coordKey(at(0, 0))]: officeCard(),
        [coordKey(at(0, 1))]: turnout({ stem: 'w', through: 'e', diverge: 's' }),
      },
      at(0, 0),
    );
    assert.ok(!canPlaceAt(area, at(-1, 1), straight()), 'an east-west straight cannot meet a 45° leg');
  });

  it('refuses a card that connects to nothing', () => {
    const area = areaFrom({ [coordKey(at(0, 0))]: officeCard() }, at(0, 0));
    assert.ok(!canPlaceAt(area, at(3, 3), straight()), 'orphaned track is never legal');
  });

  it('refuses to place on an occupied cell', () => {
    const area = areaFrom({ [coordKey(at(0, 0))]: officeCard() }, at(0, 0));
    assert.ok(!canPlaceAt(area, at(0, 0), straight()));
  });

  it('forbids dropping cars at the Office', () => {
    // §A.4 — a passenger platform is no place to switch Rolling Stock.
    const area = areaFrom(
      {
        [coordKey(at(0, 0))]: officeCard(),
        [coordKey(at(0, 1))]: straight(),
      },
      at(0, 0),
    );
    assert.ok(!canDropCarsAt(area, at(0, 0)), 'Office track');
    assert.ok(canDropCarsAt(area, at(0, 1)), 'ordinary Operational Rail');
  });

  it('forbids dropping cars on a locked industry track', () => {
    const area = areaFrom(
      {
        [coordKey(at(0, 0))]: officeCard(),
        [coordKey(at(0, 1))]: lockedFacility(),
      },
      at(0, 0),
    );
    assert.ok(!canDropCarsAt(area, at(0, 1)));
  });
});

describe('curves are arcs, and arcs make sidings possible', () => {
  const arcCard = (arc: 'ne' | 'nw' | 'se' | 'sw', geometry: 'curved' | 'sharpCurved' = 'curved'): TrackCard => ({
    geometry: { kind: 'track', geometry, arc },
    baseOperationalRail: true, standing: [], facility: null, modifiers: [], enhancements: [],
  });

  it('joins exactly two adjacent edges, with no through track', () => {
    // The printed cards (docs/tracks.png, rows 3-4) are a single sweep from edge to edge. Modelling
    // a curve as through-track PLUS a diverging leg made it a turnout, and an exact duplicate of one.
    for (const arc of ['ne', 'nw', 'se', 'sw'] as const) {
      const links = connectionsFor(arcCard(arc));
      assert.equal(links.length, 1, `${arc} should be ONE connection, not a through track plus a leg`);
      assert.deepEqual([...links[0]!].sort(), [...arc].sort(), `${arc} joins the wrong edges`);
    }
  });

  it('is no longer a duplicate of a turnout', () => {
    const norm = (c: readonly (readonly string[])[]): string =>
      c.map((p) => [...p].sort().join('')).sort().join(' ');
    const turnout: TrackCard = {
      geometry: { kind: 'track', geometry: 'turnout', turnout: { stem: 'w', through: 'e', diverge: 's' } },
      baseOperationalRail: true, standing: [], facility: null, modifiers: [], enhancements: [],
    };
    for (const arc of ['ne', 'nw', 'se', 'sw'] as const) {
      assert.notEqual(norm(connectionsFor(arcCard(arc))), norm(connectionsFor(turnout)),
        `a ${arc} curve still has the same connections as a turnout`);
    }
  });

  it('can reach NORTH, which is the only way back up to the Running Track', () => {
    // Without it a district could only ever hang below the main — no siding, no run-around.
    const reachesNorth = (['ne', 'nw'] as const).every((arc) =>
      connectionsFor(arcCard(arc)).some((p) => p.includes('n')),
    );
    assert.ok(reachesNorth, 'a curve still cannot reach north');
  });

  it('offers two rotations, and which two depends on the printed hand', () => {
    // A card turns 180° but never flips, so a left-hand curve reaches `sw` and `ne` and can never
    // become `se` or `nw` — those are the right-hand card.
    for (const g of ['curved', 'sharpCurved'] as const) {
      assert.deepEqual(variantsFor(g, 'left').map((v) => v.arc).sort(), ['ne', 'sw'], `${g} left`);
      assert.deepEqual(variantsFor(g, 'right').map((v) => v.arc).sort(), ['nw', 'se'], `${g} right`);
    }
  });

  it('a sharp curve differs from a curve only in the Moves it costs', () => {
    for (const arc of ['ne', 'nw', 'se', 'sw'] as const) {
      assert.deepEqual(
        connectionsFor(arcCard(arc, 'sharpCurved')),
        connectionsFor(arcCard(arc, 'curved')),
        'a sharp curve should be geometrically identical',
      );
    }
  });

  it('lets a crew run a siding and rejoin the main', () => {
    // THE CANONICAL SIDING, and the reason handedness matters: leave the Running Track on a LEFT
    // turnout, drop onto its matching left curve (`ne`), run parallel, and climb back on a RIGHT
    // curve (`nw`) into a RIGHT turnout. One card of each hand, and the slopes agree at both ends.
    // A crew must ENTER a turnout through its stem to take the diverging leg (§A.1).
    const ew = (): TrackCard => straightCard();
    const grid: Record<string, TrackCard> = {
      '0,-2': ew(),
      '0,-1': { geometry: { kind: 'track', geometry: 'turnout', turnout: { stem: 'w', through: 'e', diverge: 's' }, hand: 'left' }, baseOperationalRail: true, standing: [], facility: null, modifiers: [], enhancements: [] },
      '-1,-1': arcCard('ne'),
      '-1,0': ew(),
      '-1,1': arcCard('nw'),
      '0,1': { geometry: { kind: 'track', geometry: 'turnout', turnout: { stem: 'e', through: 'w', diverge: 's' }, hand: 'right' }, baseOperationalRail: true, standing: [], facility: null, modifiers: [], enhancements: [] },
      '0,0': ew(),
    };
    const area = areaFrom(grid, { row: 0, col: 0 });
    const ctx = { area, occupancy: { trayAt: () => null, freeAdTracks: () => 2 }, consistSize: 0, self: 'crew' as const };

    const seen = new Set<string>(['0,-2']);
    const queue = [{ row: 0, col: -2 }];
    for (let i = 0; i < 40 && queue.length > 0; i++) {
      const at = queue.shift()!;
      for (const facing of ['e', 'w', 'n', 's'] as const) {
        for (const d of reachableDestinations(ctx, at, facing)) {
          const key = `${d.coord.row},${d.coord.col}`;
          if (!seen.has(key)) {
            seen.add(key);
            queue.push(d.coord);
          }
        }
      }
    }
    for (const cell of ['-1,-1', '-1,0', '-1,1']) {
      assert.ok(seen.has(cell), `the siding cell ${cell} is unreachable — no run-around is possible`);
    }
    assert.ok(seen.has('0,1'), 'the siding does not rejoin the Running Track');
  });
});

describe('coupling lifts only the cars the crew ran over', () => {
  it('leaves cars standing elsewhere in the district alone', () => {
    // The reducer cleared EVERY card in the Office Area on any coupling, so picking up one boxcar
    // deleted every car standing anywhere — including loads worked over several Stages onto an
    // industry track the crew never went near.
    const grid: Record<string, TrackCard> = {
      '0,-1': straightCard(),
      '0,0': straightCard(),
      '0,1': straightCard(),
      '0,2': straightCard(),
    };
    grid['0,-1']!.standing.push({ type: 'boxcar', loaded: false });   // on the path
    grid['0,2']!.standing.push({ type: 'hopper', loaded: true });      // nowhere near it

    const area = areaFrom(grid, { row: 0, col: 0 });
    const s = gameWith(area);
    s.trays.set('crew', {
      id: 'crew', trainNumber: 1, trainIsExtra: false, engineAt: 0, consist: [],
      direction: 'west', facing: 'w', position: { at: 'grid', owner: 0, coord: { row: 0, col: 0 } }, movesUsed: 0,
    });
    s.clock.phase = 'localOps';
    s.clock.currentActor = 0;
    s.turn.option = 'switch';
    s.turn.movesRemaining = 6;

    const r = applyIntent(s, 0, { type: 'switch.move', trayId: 'crew', to: { row: 0, col: -1 }, reverse: false });
    assert.ok(r.ok, 'the move should be legal');
    assert.deepEqual(
      s.trays.get('crew')!.consist.map((c: RollingStock) => c.type),
      ['boxcar'],
      'the crew should have picked up the car it ran over',
    );
    assert.equal(
      areaOf(s, 0).grid.get('0,2')!.standing.length,
      1,
      'a car standing off the path was deleted by an unrelated coupling',
    );
  });
});