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The encoder wrongly assumed that a known multiplier string (such as
IA5String) encoded as exactly 16 bits did not need to be aligned to an
octet boundary. X.691 (07/2002) 27.5.7 says that it does. Since an
OCTET STRING encoded to 16 bits (two octets) should not be aligned to
an octet boundary, that means that asnct_imm:dec_string() needs an
additional parameter to determine whether a string of a given length
needs to be aligned.
Furthermore, there is another subtle rule difference: An OCTET STRING
which does not have fixed length is always aligned (in PER), but
a known multiplier string is aligned if its upper bound is greater
than or equal to 16.
In encoding, make sure that short known multiplier strings and
OCTET STRINGs with extensible sizes are not aligned when they are
below the appropriate limit.
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Optimize the decoding of CHOICE. Most important is to inline decoding
of the extension bit (if present) and decoding of the choice index
to give the BEAM compiler more opportunities for optimization.
We will also change the structure of the generated code. The current
code uses a flattened case for both the root and extension alternatives:
case Choice + NumRootChoices * Ext of
%% Root alternatives.
0 - ...;
:
LastRootAlternative -> ...;
%% Extension alternatives.
LastRootAlternative+1 -> ...;
:
%% Unknown extension.
_ -> ...;
end
We will instead generate nested cases:
case Ext of
0 ->
case Choice of
%% Root alternatives.
0 - ...;
:
LastRootAlternative -> ...
end;
1 ->
%% Extension alternatives.
<Decode the open type here>
case Choice of
0 -> ...;
:
LastExtensionAlternative -> ...;
%% Unknown extension.
_ -> ...;
end
end
Nested cases should be slightly faster. For decoding of the extensions,
it also makes it possible to hoist the decoding of the open type up
from each case to before the case switching on the extension index,
thus reducing the size of the generated code.
We will also do another change to the structure. Currently, the
big flat clase is wrapped in code that repackages the return values:
{Alt,{Value,RemainingEncodedData}} =
case Choice + NumRootChoices * Ext of
:
end,
{{Value,Alt},RemainingEncodedData}.
We still need to do the repackaging, but we can push it down to
the case arm for decoding each alternative. In many cases, that
will give the BEAM compiler the opportunity to avoid building the
temporary tuples.
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An ENUMERATED with as single value is not encoded. The decoder
incorrectly assumed that it was encoded in one bit.
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Dialyzer will warn for default clauses that cannot possible match.
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Refactor decoding of BIT STRINGs so that the run-time code does
not spend testing conditions that are known already at compile-time
(which wastes time and produces unnecessary Dialyzer warnings).
There are three ways to decode BIT STRINGs:
1) To a list of bit names (and {bit,Position} for unnamed positions)
if the BIT STRING type has any bit names.
2a) To a list of ones and zeros if there are no named bits, and the
compact_bit_string option was NOT given.
2b) To a {Unused,Bin} tuple if there are no named bits compact_bit_string
option WAS given.
Structure the decoding functions in the same way.
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The template modules (asn1rtt_*.erl) are based on the existing
run-time modules, but with some simplifications and improvements,
for example:
The run-time functions for BER encoding took a Constraint argument which
was not used. It has been eliminated, along with the unused StringType
argument for the encode_restricted_string function.
The Range argument for decode_enumerated() has been dropped since it
was not used.
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Decoding of fragmented OCTET STRINGs was only implemented when the size
was constrained to a single value. While at it, support decoding
fragmented OCTET STRINGS in all circumstances.
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