fix i256 key type conversion by initialize buffer before usage

[learnforpractice]

fix i256 key type conversion by initialize buffer before usage

export_bits doesn't fill every byte in the buffer, need to ensure that 'buffer' is initialized before usage:

https://github.com/boostorg/multiprecision/blob/5f81a78cdb38ea35f98ef7a655d2535773c1261b/include/boost/multiprecision/cpp_int/import_export.hpp#L236

[greg7mdp]

Thanks for the fix, good catch! It would definitely work fine.
A thought, since export_bits returns the pointer to the byte just after the last one written, we can do a small optimization by zeroing only the remaining bytes.

Suggested change

	memset(buffer, 0, 32);
	boost::multiprecision::export_bits(v, buffer, 8, false);
	auto out = boost::multiprecision::export_bits(v, buffer, 8, false);
	memset(out, 0, 32 - (out - buffer)); // zero out remaining bytes

[spoonincode]

My hunch is the original is faster (although personally I'd prefer the uint8_t buffer[32] = {} style) because the call to memset() is completely removed (and there isn't a read dependency on the register).

https://godbolt.org/z/axGa6e4bK

[greg7mdp]

I have no issue with @spoonincode 's suggestion. Hard to know which version would be fastest without testing, but it probably doesn't matter in this instance.

[heifner]

I'll add my vote for the more c++ style of uint8_t buffer[32] = {}

[learnforpractice]

Updated. Thanks for the suggestion!

[greg7mdp]

@learnforpractice you should be able to just push the merge button to merge your PR.

[spoonincode]

thanks @learnforpractice