[jira] [Commented] (AVRO-2620) Max precision on decimal/fixed LogicalType is wrong

["Ryan Skraba (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/AVRO-2620?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16968231#comment-16968231 ] 

Ryan Skraba commented on AVRO-2620:
-----------------------------------

All credit to [~kojiromike] who noticed this in the python implementation!

> Max precision on decimal/fixed LogicalType is wrong
> ---------------------------------------------------
>
>                 Key: AVRO-2620
>                 URL: https://issues.apache.org/jira/browse/AVRO-2620
>             Project: Apache Avro
>          Issue Type: Bug
>          Components: java
>            Reporter: Ryan Skraba
>            Priority: Minor
>
> A decimal logical type with an underlying fixed bytes type is invalid if the precision is greater than can be contained in the fixed bytes.  The calculation for 
>  [max precision|https://github.com/apache/avro/blob/a2098bd88361b5ff9298c462a2dccafbb0c2c508/lang/java/avro/src/main/java/org/apache/avro/LogicalTypes.java#L262] can return a wrong value when the fixed bytes size is greater than 128.
> For a fixed size of *129*, the {{Math.pow(2, 8 * size - 1)}} part of this formula will return {{Double.INFINITY}}, and the max precision will be considered {{Long.MAX_VALUE}}.  The expected value is 310 digits precision.
> We can apply the log power rule to avoid overflow, or some bit-twiddling approximation (like [java.math.BigDecimal|https://github.com/openjdk/jdk/blob/jdk8-b120/jdk/src/share/classes/java/math/BigDecimal.java#L3779]).



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