Keep lower-numbered lobbies during autoscaling

tests/unit/autoscaling-planner.test.ts

@@ -25,7 +25,7 @@ describe('autoscaling policy planning', () => {
         expect(planScaling(policy, { ...snapshot(), keyPool: { total: 2, available: 0 } }, now).reason).toContain('keys are in use');
     });
     it('still removes eligible surplus lobbies when the game pool is exhausted', () => {
-        expect(planScaling(policy, { ...snapshot(member(1), member(2)), keyPool: { total: 2, available: 0 } }, now)).toMatchObject({ action: 'destroy', server: name(1) });
+        expect(planScaling(policy, { ...snapshot(member(1), member(2)), keyPool: { total: 2, available: 0 } }, now)).toMatchObject({ action: 'destroy', server: name(2) });
     });
     it('holds for pending creation, unknown, stopped, or protected members', () => {
         for (const state of ['creating', 'removing', 'unknown', 'stopped', 'protected'] as const) expect(planScaling(policy, snapshot(member(1, state)), now).action).toBe('wait');
@@ -40,10 +40,49 @@ describe('autoscaling policy planning', () => {
         expect(planScaling({ ...policy, maximum: 1 }, snapshot(member(1, 'occupied'), member(2)), now).action).toBe('wait');
         expect(planScaling({ ...policy, minimum: 3 }, snapshot(member(1), member(2), member(3)), now).action).toBe('wait');
     });
-    it('trims one longest-empty lobby and requires a full observed interval', () => {
-        expect(planScaling(policy, snapshot(member(1, 'occupied'), member(2), member(3), member(4)), now)).toMatchObject({ action: 'destroy', server: name(2), desired: 2 });
+    it('trims one highest-numbered surplus lobby and requires a full observed interval', () => {
+        expect(planScaling(policy, snapshot(member(1, 'occupied'), member(2), member(3), member(4)), now)).toMatchObject({ action: 'destroy', server: name(4), desired: 2 });
         expect(planScaling(policy, snapshot({ ...member(1), emptySince: now }, { ...member(2), emptySince: now }), now).action).toBe('wait');
     });
+    it('keeps Lobby 1 when it has been empty longer than a newly empty Lobby 2', () => {
+        const members = [{ ...member(2), emptySince: now }, member(1)];
+        expect(planScaling(policy, snapshot(...members), now)).toMatchObject({ action: 'wait', server: null, desired: 1 });
+        const later = now + policy.emptyMinutes * 60_000;
+        expect(planScaling(policy, snapshot(...members.map(m => ({ ...m, observedAt: later }))), later))
+            .toMatchObject({ action: 'destroy', server: name(2), desired: 1 });
+    });
+    it.each([1, 2])('retains the lowest empty spare while Lobby %i is occupied', occupied => {
+        const members = [member(1, occupied === 1 ? 'occupied' : 'empty'), member(2, occupied === 2 ? 'occupied' : 'empty')];
+        expect(planScaling(policy, snapshot(...members), now)).toMatchObject({ action: 'wait', desired: 2 });
+        expect(planScaling(policy, snapshot(...members, { ...member(3), emptySince: now }), now)).toMatchObject({ action: 'wait', desired: 2 });
+        expect(planScaling(policy, snapshot(...members, member(3)), now)).toMatchObject({ action: 'destroy', server: name(3), desired: 2 });
+    });
+    it('preserves occupied higher-numbered lobbies while trimming empty surplus', () => {
+        expect(planScaling(policy, snapshot(member(1), member(2), member(3), member(4, 'occupied')), now))
+            .toMatchObject({ action: 'destroy', server: name(3), desired: 2 });
+        expect(planScaling(policy, snapshot(member(1), member(4, 'occupied')), now)).toMatchObject({ action: 'wait', desired: 2 });
+    });
+    it.each([
+        { minimum: 3, spares: 1, occupied: null, surplus: 4 },
+        { minimum: 3, spares: 1, occupied: 4, surplus: 3 },
+        { minimum: 1, spares: 2, occupied: 4, surplus: 3 }
+    ])('retains low-numbered empties for minimum $minimum and spares $spares with occupied Lobby $occupied', ({ minimum, spares, occupied, surplus }) => {
+        const settings = { ...policy, minimum, spares };
+        const members = [4, 3, 2, 1].map(n => member(n, n === occupied ? 'occupied' : 'empty'));
+        expect(planScaling(settings, snapshot(...members.map(m => m.name === name(surplus) ? { ...m, emptySince: now } : m)), now))
+            .toMatchObject({ action: 'wait', desired: 3 });
+        expect(planScaling(settings, snapshot(...members), now)).toMatchObject({ action: 'destroy', server: name(surplus), desired: 3 });
+        expect(planScaling(settings, snapshot(...members.filter(m => m.name !== name(surplus))), now)).toMatchObject({ action: 'wait', desired: 3 });
+    });
+    it('orders lobby numbers numerically when choosing survivors and surplus', () => {
+        expect(planScaling(policy, snapshot(member(10), member(2)), now)).toMatchObject({ action: 'destroy', server: name(10) });
+        expect(planScaling(policy, snapshot(member(2), member(10), member(1)), now)).toMatchObject({ action: 'destroy', server: name(10) });
+    });
+    it('keeps an existing solo Lobby 2 and fills Lobby 1 when occupancy requires growth', () => {
+        expect(planScaling(policy, snapshot(member(2)), now)).toMatchObject({ action: 'wait', server: null, desired: 1 });
+        expect(planScaling(policy, snapshot(member(2, 'occupied')), now)).toMatchObject({ action: 'create', server: name(1), desired: 2 });
+        expect(planScaling(policy, snapshot(member(2), member(1)), now)).toMatchObject({ action: 'destroy', server: name(2), desired: 1 });
+    });
     it('preserves map variants and refuses ambiguous engine maps', () => {
         expect(mapIdentity({ name: 'legacy', game: 't6', map: 'zm_transit' })).toBeNull();
         expect(mapIdentity({ name: name(1), map: 'zm_transit' })?.mapSlug).toBe('town');