fix calculation of middle between old and new stock in buying price formula

Author: Trolldemorted

src/towns/ware-prices/buying-price.md

@@ -23,7 +23,7 @@ where \\(w\_i\\) is the amount being bought from \\(i\\) and \\(f\\) is defined
 \\[
 \begin{aligned}
     f\_4 &= 0.6\\\\
-    f_{i} &=  m_i - v_i \underbrace{\frac{w\_{relative\\_stock} - w\_{relative\\_remain}}{2 * \text{interval_width}}}\_{\in [0; 1]}
+    f_{i} &=  m_i - v_i \underbrace{\frac{w\_{relative\\_stock} + w\_{relative\\_remain}}{2 * \text{interval_width}}}\_{\in [0; 1]}
 \end{aligned}
 \\]
 

src/towns/ware-prices/selling-price.py

@@ -156,11 +156,21 @@ def tests():
     price = get_price(stock, sell_amount, SAMPLE_THRESHOLDS, PIG_IRON_BASE_PRICE, 2)
     assert price == pytest.approx(1566.400097608566)
 
+    stock = 0
+    sell_amount = 50 * 2000
+    price = get_price(stock, sell_amount, SAMPLE_THRESHOLDS, PIG_IRON_BASE_PRICE, 0)
+    assert price == pytest.approx(47740.00297486782)
+
     stock = 0
     sell_amount = 50 * 2000
     price = get_price(stock, sell_amount, SAMPLE_THRESHOLDS, PIG_IRON_BASE_PRICE, 1)
     assert price == pytest.approx(46860.00292003155)
 
+    stock = 0
+    sell_amount = 50 * 2000
+    price = get_price(stock, sell_amount, SAMPLE_THRESHOLDS, PIG_IRON_BASE_PRICE, 2)
+    assert price == pytest.approx(45980.00286519527)
+
     stock = 17000
     sell_amount = 1 * 2000
     price = get_price(stock, sell_amount, SAMPLE_THRESHOLDS, PIG_IRON_BASE_PRICE, 1)