add calc_dip function

tests/cidr4_merge/test_cidr4_merger.py

@@ -2,6 +2,7 @@ import pytest
 
 from vpn_manager.cidr4_merge.cidr4_merger import (
     Cidr4MergerError,
+    calc_dip,
     cidr4_to_node,
     find_parent,
     make_cidr4,
@@ -84,3 +85,9 @@ def test_find_parent__with_exception():
         == "Error! Trying to find common parent of network and subnet! parent_node=(0, 0), a=(0, 0), b=(3221225472, 2)."
     )
     assert exc_info.type is Cidr4MergerError
+
+
+def test_calc_dip():
+    assert calc_dip(26, 27) == 67108864
+    assert calc_dip(26, 28) == 201326592
+    assert calc_dip(26, 29) == 469762048

vpn_manager/cidr4_merge/cidr4_merger.py

@@ -42,6 +42,14 @@ def find_parent(a: Node, b: Node) -> Node:
     return parent_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
+    return dip
+
+
 def make_cidr4(ip, mask_len) -> str:
     lst = [str(ip >> (i << 3) & 0xFF) for i in reversed(range(4))]
     ip_address = ".".join(lst)