fix calc_dip

tests/cidr4_merge/test_cidr4_merger.py

@@ -88,6 +88,13 @@ def test_find_parent__with_exception():
 
 
 def test_calc_dip():
+    assert calc_dip(26, 26) == 0
+
     assert calc_dip(26, 27) == 67108864
+    assert calc_dip(27, 26) == 67108864
+
     assert calc_dip(26, 28) == 201326592
+    assert calc_dip(28, 26) == 201326592
+
     assert calc_dip(26, 29) == 469762048
+    assert calc_dip(29, 26) == 469762048

vpn_manager/cidr4_merge/cidr4_merger.py

@@ -44,9 +44,10 @@ def find_parent(a: Node, b: Node) -> Node:
 
 def calc_dip(mask_len_a: int, mask_len_b: int) -> int:
     dip = 0
-    while mask_len_a < mask_len_b:
-        dip += 1 << mask_len_a
-        mask_len_a += 1
+    len_min, len_max = sorted((mask_len_a, mask_len_b))
+    while len_min < len_max:
+        dip += 1 << len_min
+        len_min += 1
     return dip