construct: Add Construct for variable-length int 'GreedyInteger'

We have a number of integers with variable-length encoding, so
add a Construct for this.  Naming inspired by GreedyBytes.

Related to https://github.com/construct/construct/issues/962

Change-Id: Ic6049b74ea3705fda24855f34b4a1d5f2c9327f7

pySim/construct.py

@@ -2,12 +2,14 @@ from construct.lib.containers import Container, ListContainer
 from construct.core import EnumIntegerString
 import typing
 from construct import *
+from construct.core import evaluate, bytes2integer, integer2bytes
+from construct.lib import integertypes
 from pySim.utils import b2h, h2b, swap_nibbles
 import gsm0338
 
 """Utility code related to the integration of the 'construct' declarative parser."""
 
-# (C) 2021 by Harald Welte <laforge@osmocom.org>
+# (C) 2021-2022 by Harald Welte <laforge@osmocom.org>
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -184,3 +186,45 @@ def GsmString(n):
         n (Integer): Fixed length of the encoded byte string
     '''
     return GsmStringAdapter(Rpad(Bytes(n), pattern=b'\xff'), codec='gsm03.38')
+
+class GreedyInteger(Construct):
+    """A variable-length integer implementation, think of combining GrredyBytes with BytesInteger."""
+    def __init__(self, signed=False, swapped=False):
+        super().__init__()
+        self.signed = signed
+        self.swapped = swapped
+
+    def _parse(self, stream, context, path):
+        data = stream_read_entire(stream, path)
+        if evaluate(self.swapped, context):
+            data = swapbytes(data)
+        try:
+            return bytes2integer(data, self.signed)
+        except ValueError as e:
+            raise IntegerError(str(e), path=path)
+
+    def __bytes_required(self, i):
+        if self.signed:
+            raise NotImplementedError("FIXME: Implement support for encoding signed integer")
+        nbytes = 1
+        while True:
+            i = i >> 8
+            if i == 0:
+                return nbytes
+            else:
+                nbytes = nbytes + 1
+        # this should never happen, above loop must return eventually...
+        raise IntegerError(f"value {i} is out of range")
+
+    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)
+        try:
+            data = integer2bytes(obj, length, self.signed)
+        except ValueError as e:
+            raise IntegerError(str(e), path=path)
+        if evaluate(self.swapped, context):
+            data = swapbytes(data)
+        stream_write(stream, data, length, path)
+        return obj

tests/test_construct.py

@@ -0,0 +1,26 @@
+#!/usr/bin/env python3
+
+import unittest
+from pySim.construct import GreedyInteger
+
+tests = [
+        ( b'\x80', 0x80 ),
+        ( b'\x80\x01', 0x8001 ),
+        ( b'\x80\x00\x01', 0x800001 ),
+        ( b'\x80\x23\x42\x01', 0x80234201 ),
+        ]
+
+class TestGreedyInt(unittest.TestCase):
+    def test_GreedyInt_decoder(self):
+        gi = GreedyInteger()
+        for t in tests:
+            self.assertEqual(gi.parse(t[0]), t[1])
+    def test_GreedyInt_encoder(self):
+        gi = GreedyInteger()
+        for t in tests:
+            self.assertEqual(t[0], gi.build(t[1]))
+        pass
+
+
+if __name__ == "__main__":
+	unittest.main()