[jira] [Commented] (ARROW-1749) [C++] Handle range of Decimal128 values that require 39 digits to be displayed

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-1749?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16239234#comment-16239234 ] 

ASF GitHub Bot commented on ARROW-1749:
---------------------------------------

cpcloud commented on a change in pull request #1282: ARROW-1749: [C++] Handle range of Decimal128 values that require 39 digits to be displayed
URL: https://github.com/apache/arrow/pull/1282#discussion_r148941301
 
 

 ##########
 File path: cpp/src/arrow/util/bit-util.h
 ##########
 @@ -296,6 +297,25 @@ static inline int Log2(uint64_t x) {
   return result;
 }
 
+/// \brief Count the number of leading zeros in a 32 bit integer.
+static inline int64_t CountLeadingZeros(uint32_t value) {
+  DCHECK_NE(value, 0);
+#if defined(__clang__) || defined(__GNUC__)
+  return static_cast<int64_t>(__builtin_clz(value));
 
 Review comment:
   Interesting tidbit here is that `__builtin_clz` compiles to `31 - bit scan reverse` on gcc and clang.

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> [C++] Handle range of Decimal128 values that require 39 digits to be displayed
> ------------------------------------------------------------------------------
>
>                 Key: ARROW-1749
>                 URL: https://issues.apache.org/jira/browse/ARROW-1749
>             Project: Apache Arrow
>          Issue Type: Bug
>          Components: C++
>    Affects Versions: 0.7.1
>            Reporter: Phillip Cloud
>            Assignee: Phillip Cloud
>            Priority: Major
>              Labels: pull-request-available
>             Fix For: 0.8.0
>
>
> {{2 ** 127 - 1}} and {{-2 ** 127}} both require a 39th digit to be displayed as a decimal string.
> This means we do not have the ability to _display_ decimal values above {{10 ** 38 - 1}} even though we have the ability to _store_ them.
> For reference, Impala returns {{NULL}} when trying to cast {{2 ** 127 - 1}} to {{DECIMAL(38, 0)}}.
> A few next steps are in order:
> # Explicitly test this behavior
> # Avoid crashing or displaying garbage when we cannot display a number that can be stored
> # Make a decision about whether we want to eventually support range of values from {{10 ** 38}} to {{2 ** 127 - 1}} positive and {{-2 ** 127}} to {{-10 ** 38}} negative. This would require a larger integer type to hold the value just before being converted to a string.



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