feat: Make sure the cash balance does not fall below zero

finance/ext/warehouse.py

@@ -361,7 +361,14 @@ def apply_plan(
     ):
         def apply(t, q):
             self.inventory.setdefault(t, 0)
+            while self.inventory["_USD"] - self.current_prices[t] * q < 0:
+                if q > 0:
+                    q -= 1
+                else:
+                    q += 1
             self.inventory["_USD"] -= self.current_prices[t] * q
+            if self.inventory["_USD"] < 0:
+                raise ValueError(f"USD balance cannot be negative: {t}, {q}")
             return self.inventory[t] + q
 
         # 'close' is actually 'adj close', which already includes dividends/stock split/capital gains
@@ -382,5 +389,4 @@ def calc_dividends_sum(
                         if q < 0:
                             raise ValueError(f"Quantity cannot be negative: {t}, {q}")
                         div_sum += div_amount * q
-                        # print(f"div[{t}] = {div_amount} * {q} = {div_amount * q}, {start_dt}, {end_dt}")
-        return div_sum
+        return div_sum