dexter has uploaded this change for review.
construct: move __bytes_required to utils and finish the implementation
The method __bytes_required in the GreedyInteger class computes how many
byte a given integer number requires, when it is encoded. This method
currently only works for unsigned integers. Let's move it to utils and
add support for signed integers.
Related: SYS#7094
Change-Id: I9ae4acc30dbd5fb6a6b24b10254ffb205bfde52d
---
M src/osmocom/construct.py
M src/osmocom/utils.py
M tests/test_utils.py
3 files changed, 77 insertions(+), 21 deletions(-)
git pull ssh://gerrit.osmocom.org:29418/python/pyosmocom refs/changes/89/38289/1
diff --git a/src/osmocom/construct.py b/src/osmocom/construct.py
index def5d85..7c0f034 100644
--- a/src/osmocom/construct.py
+++ b/src/osmocom/construct.py
@@ -16,7 +16,7 @@
from construct.core import evaluate
from construct.lib import integertypes
-from osmocom.utils import b2h, h2b, swap_nibbles
+from osmocom.utils import b2h, h2b, swap_nibbles, int_bytes_required
# (C) 2021-2022 by Harald Welte <laforge@osmocom.org>
#
@@ -587,29 +587,10 @@
except ValueError as e:
raise IntegerError(str(e), path=path)
- def __bytes_required(self, i, minlen=0):
- if self.signed:
- raise NotImplementedError("FIXME: Implement support for encoding signed integer")
-
- # compute how many bytes we need
- nbytes = 1
- while True:
- i = i >> 8
- if i == 0:
- break
- else:
- nbytes = nbytes + 1
-
- # round up to the minimum number
- # of bytes we anticipate
- nbytes = max(nbytes, minlen)
-
- return nbytes
-
def _build(self, obj, stream, context, path):
if not isinstance(obj, integertypes):
raise IntegerError(f"value {obj} is not an integer", path=path)
- length = self.__bytes_required(obj, self.minlen)
+ length = int_bytes_required(obj, self.minlen, self.signed)
try:
data = obj.to_bytes(length, byteorder='big', signed=self.signed)
except ValueError as e:
diff --git a/src/osmocom/utils.py b/src/osmocom/utils.py
index 009eb04..5947453 100644
--- a/src/osmocom/utils.py
+++ b/src/osmocom/utils.py
@@ -137,6 +137,44 @@
return (n + 1)//2
+def int_bytes_required(number: int, minlen:int = 0, signed:bool = False):
+ """compute how many bytes an integer requires when it is encoded into bytes
+ Args:
+ number : integer number
+ minlen : minimum length
+ signed : compute the number of bytes for a signed integer (two's complement)
+ Returns:
+ Integer 'nbytes', which is the number of bytes required to encode 'number'
+ """
+
+ if signed == False and number < 0:
+ raise ValueError("expecting a positive number")
+
+ # Compute how many bytes we need for the absolute (positive) value of the given number
+ nbytes = 1
+ i = abs(number)
+ while True:
+ i = i >> 8
+ if i == 0:
+ break
+ else:
+ nbytes = nbytes + 1
+
+ # When we deal with signed numbers, then the two's complement applies. This means that we must check if the given
+ # number would still fit in the value range of the number of bytes we have calculated above. If not, one more
+ # byte is required.
+ if signed:
+ value_range_limit = pow(2,nbytes*8) // 2
+ if number < -value_range_limit:
+ nbytes = nbytes + 1
+ elif number >= value_range_limit:
+ nbytes = nbytes + 1
+
+ # round up to the minimum number of bytes we anticipate
+ nbytes = max(nbytes, minlen)
+ return nbytes
+
+
def str_sanitize(s: str) -> str:
"""replace all non printable chars, line breaks and whitespaces, with ' ', make sure that
there are no whitespaces at the end and at the beginning of the string.
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 8e4732f..0f22e35 100755
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -54,5 +54,42 @@
self.assertEqual(str(hexstr('ABCD')), 'abcd')
+class Test_int_bytes_required(unittest.TestCase):
+
+ def test_int_bytes_required(self):
+
+ tests = [
+ # unsigned positive numbers
+ ( 1, 0, False ),
+ ( 1, 1, False ),
+ ( 1, 255, False ),
+ ( 2, 256, False ),
+ ( 2, 65535, False ),
+ ( 3, 65536, False ),
+ ( 2, 65535, False ),
+ ( 3, 16777215, False ),
+ ( 4, 16777216, False ),
+
+ # signed positive numbers
+ ( 1, 0, True ),
+ ( 1, 1, True ),
+ ( 1, 127, True ),
+ ( 2, 128, True ),
+ ( 2, 32767, True ),
+ ( 3, 32768, True ),
+
+ # signed negative numbers
+ ( 1, -0, True ),
+ ( 1, -1, True ),
+ ( 1, -128, True ),
+ ( 2, -129, True ),
+ ( 2, -32768, True ),
+ ( 3, -32769, True ),
+ ]
+
+ for t in tests:
+ self.assertEqual(t[0], int_bytes_required(t[1], signed = t[2]))
+
+
if __name__ == "__main__":
unittest.main()
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