feat(sim): move items from shelf back to backroom

src/sim/inventory.ts

@@ -64,6 +64,18 @@ export function stockSlot(inv: Inventory, slotId: string, item: ItemRef, qty: nu
   return moved;
 }
 
+/** Moves up to `qty` items from the slot back to the backroom. Returns the quantity moved. */
+export function unstockSlot(inv: Inventory, slotId: string, qty: number): number {
+  const slot = getSlot(inv, slotId);
+  if (!slot.item || slot.qty === 0) return 0;
+  const moved = Math.min(qty, slot.qty);
+  if (moved <= 0) return 0;
+  addToBackroom(inv, slot.item, moved);
+  slot.qty -= moved;
+  if (slot.qty === 0) slot.item = null;
+  return moved;
+}
+
 export function takeFromSlot(inv: Inventory, slotId: string): ItemRef {
   const slot = getSlot(inv, slotId);
   if (!slot.item || slot.qty === 0) throw new Error(`Slot ${slotId} is empty`);

tests/sim/inventory.test.ts

@@ -1,7 +1,7 @@
 import { describe, it, expect } from 'vitest';
 import {
   addToBackroom, backroomQty, createInventory, getSlot, itemKey, parseItemKey,
-  shelfItems, SLOT_CAPACITY, stockSlot, takeFromSlot, totalQty,
+  shelfItems, SLOT_CAPACITY, stockSlot, takeFromSlot, totalQty, unstockSlot,
 } from '../../src/sim/inventory';
 
 const tetrisNew = { gameId: 'tetris', condition: 'new' as const };
@@ -60,6 +60,29 @@ describe('inventory', () => {
     expect(() => takeFromSlot(inv, 'slot-0')).toThrow('Slot slot-0 is empty');
   });
 
+  it('moves items from a slot back to the backroom', () => {
+    const inv = createInventory(1);
+    addToBackroom(inv, tetrisNew, 4);
+    stockSlot(inv, 'slot-0', tetrisNew, 4);
+    expect(unstockSlot(inv, 'slot-0', 3)).toBe(3);
+    expect(getSlot(inv, 'slot-0').qty).toBe(1);
+    expect(backroomQty(inv, tetrisNew)).toBe(3);
+  });
+
+  it('caps unstocking to the slot quantity and clears the slot at 0', () => {
+    const inv = createInventory(1);
+    addToBackroom(inv, tetrisNew, 2);
+    stockSlot(inv, 'slot-0', tetrisNew, 2);
+    expect(unstockSlot(inv, 'slot-0', 10)).toBe(2);
+    expect(getSlot(inv, 'slot-0')).toEqual({ id: 'slot-0', item: null, qty: 0 });
+    expect(backroomQty(inv, tetrisNew)).toBe(2);
+  });
+
+  it('returns 0 when unstocking an empty slot', () => {
+    const inv = createInventory(1);
+    expect(unstockSlot(inv, 'slot-0', 3)).toBe(0);
+  });
+
   it('throws on unknown slot', () => {
     expect(() => getSlot(createInventory(1), 'slot-9')).toThrow('Unknown slot: slot-9');
   });