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osmocom.construct.Asn1DerInteger

This is a 'construct' type which can be used for encoding/decoding
integer values according to ASN.1 DER encoding rules.

Change-Id: I0cfe97daf957919de86453d6d44f9c99ab3075ac
---
M src/osmocom/construct.py
1 file changed, 15 insertions(+), 0 deletions(-)

git pull ssh://gerrit.osmocom.org:29418/python/pyosmocom refs/changes/72/38272/1

diff --git a/src/osmocom/construct.py b/src/osmocom/construct.py
index def5d85..cd1fbe2 100644
--- a/src/osmocom/construct.py
+++ b/src/osmocom/construct.py
@@ -16,6 +16,7 @@
 from construct.core import evaluate
 from construct.lib import integertypes
 
+from osmocom import tlv
 from osmocom.utils import b2h, h2b, swap_nibbles
 
 # (C) 2021-2022 by Harald Welte <laforge@osmocom.org>
@@ -619,6 +620,20 @@
         stream_write(stream, data, length, path)
         return obj
 
+class Asn1DerInteger(Construct):
+    """An integer value using ASN.1 DER encoding rules."""
+    def _parse(self, stream, context, path):
+        data = stream_read_entire(stream, path)
+        val, remainder = tlv.bertlv_parse_len(data)
+        return val
+
+    def _build(self, obj, stream, context, path):
+        if not isinstance(obj, integertypes):
+            raise IntegerError(f"value {obj} is not an integer", path=path)
+        val = tlv.bertlv_encode_len(obj)
+        stream_write(stream, val, len(val), path)
+        return obj
+
 # merged definitions of 24.008 + 23.040
 TypeOfNumber = Enum(BitsInteger(3), unknown=0, international=1, national=2, network_specific=3,
                     short_code=4, alphanumeric=5, abbreviated=6, reserved_for_extension=7)

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