Solve 2020 day 13 CRT jojo

lib.py

@@ -280,3 +280,18 @@ def get_data(filename):
         assert os.path.isfile(txt_file), "Could not download AoC file"
         with open(txt_file) as f:
             return f.read()
+
+
+def mod_inverse(a, m):
+    def egcd(a, b):
+        if a == 0:
+            return b, 0, 1
+        else:
+            g, y, x = egcd(b % a, a)
+            return g, x - (b // a) * y, y
+
+    g, x, _ = egcd(a, m)
+    if g != 1:
+        raise Exception('Modular inverse does not exist')
+    else:
+        return x % m