add two new functions

- `encoding_overhead` for a given source length
- `max_encoded_length` for a given source length
- Unittests for both functions

Author: robamu

python3/cobs/cobs/__init__.py

@@ -27,3 +27,14 @@
 
 from .._version import *
 
+
+def encoding_overhead(source_len: int) -> int:
+    """Calculates the maximum overhead when encoding a message with the given length.
+    The overhead is a maximum of [n/254] bytes (one in 254 bytes) rounded up."""
+    return (source_len + 254 - 1) // 254
+
+
+def max_encoded_length(source_len: int) -> int:
+    """Calculates how maximum possible size of an encoded message given the length of the
+    source message."""
+    return source_len + encoding_overhead(source_len)

python3/cobs/cobs/test.py

@@ -13,7 +13,6 @@
 from .. import cobs as cobs
 #from ..cobs import _cobs_py as cobs
 
-
 def infinite_non_zero_generator():
     while True:
         for i in range(1,50):
@@ -191,6 +190,25 @@ def test_array_of_half_words(self):
                 cobs.decode(array_encoded_string)
 
 
+class UtilTests(unittest.TestCase):
+
+    def test_encoded_len_calc(self):
+        self.assertEqual(cobs.encoding_overhead(5), 1)
+        self.assertEqual(cobs.max_encoded_length(5), 6)
+
+    def test_encoded_len_calc_empty_packet(self):
+        self.assertEqual(cobs.encoding_overhead(0), 0)
+        self.assertEqual(cobs.max_encoded_length(0), 0)
+
+    def test_encoded_len_calc_still_one_byte_overhead(self):
+        self.assertEqual(cobs.encoding_overhead(254), 1)
+        self.assertEqual(cobs.max_encoded_length(254), 255)
+
+    def test_encoded_len_calc_two_byte_overhead(self):
+        self.assertEqual(cobs.encoding_overhead(255), 2)
+        self.assertEqual(cobs.max_encoded_length(255), 257)
+
+
 def runtests():
     unittest.main()