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https://issues.apache.org/jira/browse/ARROW-786?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16101887#comment-16101887
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Phillip Cloud edited comment on ARROW-786 at 7/26/17 4:33 PM:
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[~jnadeau] {{__int128_t}} doesn't work on Windows (with visual cpp) and when I
originally wrote the decimal code for arrow-cpp, it was buggy with clang. The
symbols required to link in libc++ code necessary to use that type were not
exported by clang. See here: https://bugs.llvm.org//show_bug.cgi?id=26156.
We ultimately went with the boost multiprecision representation (which is sign
magnitude) because of desire to reuse existing libraries and cross platform
capabilities out of the box.
One possible alternative (depending on whether clang issues have been resolved)
is to write our own pared down of version of something like boost
multiprecision that uses {{__int128_t}} on UNIXes and two {{int64_t}} s on
Windows. It wouldn't need to have any operations at the moment, just the
ability to print itself like a decimal number and convert decimal strings to
the underlying type. Even those may be able to be functions and not methods on
the class.
was (Author: cpcloud):
[~jnadeau] {{__int128_t}} doesn't work on Windows (with visual cpp) and when I
originally wrote the decimal code for arrow-cpp, it was buggy with clang. The
symbols required to link in libc++ code necessary to use that type were not
exported by clang. See here: https://bugs.llvm.org//show_bug.cgi?id=26156.
We ultimately went with the boost multiprecision representation (which is sign
magnitude) because of desire to reuse existing libraries and cross platform
capabilities out of the box.
One possible alternative (depending on whether clang issues have been resolved)
is to write our own pared down of version of something like boost
multiprecision that uses {{__int128_t}} on UNIXes and two {{int64_t}}s on
Windows. It wouldn't need to have any operations at the moment, just the
ability to print itself like a decimal number and convert decimal strings to
the underlying type. Even those may be able to be functions and not methods on
the class.
> [Format] In-memory format for 128-bit Decimals, handling of sign bit
> --------------------------------------------------------------------
>
> Key: ARROW-786
> URL: https://issues.apache.org/jira/browse/ARROW-786
> Project: Apache Arrow
> Issue Type: Improvement
> Components: Format
> Reporter: Wes McKinney
> Fix For: 0.6.0
>
>
> cc [~cpcloud]
> We found in ARROW-655 that we needed to add an extra bit for signedness for
> decimals stored as 128-bit values to be able to use the Boost multiprecision
> libraries. This makes Decimal128 not fit completely neatly as a 16-byte fixed
> size binary value, and more of a {{struct<sign_bitmap: boolean, data:
> fixed_size_binary(16)>}}. What is the current formata in the Java
> implementation? We will need to document the memory layout for decimals that
> maximizes compatibility across languages and eventually implement integration
> tests for IPC.
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