[jira] [Resolved] (ARROW-17223) [C#] DecimalArray incorrectly appends values greater than MaxValue / 2 and less than MinValue / 2

["Eric Erhardt (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-17223?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Eric Erhardt resolved ARROW-17223.
----------------------------------
    Fix Version/s: 10.0.0
       Resolution: Fixed

Issue resolved by pull request 13732
[https://github.com/apache/arrow/pull/13732]

> [C#] DecimalArray incorrectly appends values greater than MaxValue / 2 and less than MinValue / 2
> -------------------------------------------------------------------------------------------------
>
>                 Key: ARROW-17223
>                 URL: https://issues.apache.org/jira/browse/ARROW-17223
>             Project: Apache Arrow
>          Issue Type: Bug
>          Components: C#
>    Affects Versions: 4.0.0, 4.0.1, 5.0.0, 6.0.0, 6.0.1, 7.0.0, 8.0.0, 6.0.2, 7.0.1
>            Reporter: Alexey Smirnov
>            Priority: Critical
>              Labels: pull-request-available
>             Fix For: 10.0.0
>
>          Time Spent: 2h
>  Remaining Estimate: 0h
>
> I try to append values into Decimal arrays (Decimal128Array or Decimal256Array), but for values greater than Decimal.MaxValue / 2 and less than Decimal.MinValue / 2, it fails to do it correctly.
> For example for DecimalArray128 I created simple unit test:
> {code:c#}
> [Fact]
> public void AppendMaxMinDecimal()                
> {
>   // Assert
>   var builder = new Decimal128Array.Builder(new Decimal128Type(29, 0));
>   var max = Decimal.MaxValue;
>   var min = Decimal.MinValue; 
>  
>   // Act
>   builder.Append(max);
>   builder.Append(min); 
>   // Assert
>   var array = builder.Build();
>   Assert.Equal(max, array.GetValue(0));
>   Assert.Equal(min, array.GetValue(1));
> }
> {code}
> I assume it to work correctly, but get:
> {color:#de350b}_Assert.Equal() Failure Expected: 79228162514264337593543950335 Actual:   -1_{color}
> Looks like the root cause is in _GetBytes_ method of _DecimalUtility_ class:
> {code:c#}
> //...
> Span<byte> bigIntBytes = stackalloc byte[12];
> for (int i = 0; i < 3; i++)
> {
>        int bit = decimalBits[i];
>        Span<byte> intBytes = stackalloc byte[4];
>        if (!BitConverter.TryWriteBytes(intBytes, bit))
>            throw new OverflowException($"Could not extract bytes from int {bit}");
>        for (int j = 0; j < 4; j++)
>        {
>              bigIntBytes[4 * i + j] = intBytes[j];
>        }
>  }
>  bigInt = new BigInteger(bigIntBytes);
> //...
> {code}
> according to MSDN: "The binary representation of a {{Decimal}} value is 128-bits consisting of a 96-bit integer number, and a 32-bit set of flags representing things such as the sign and scaling factor used to specify what portion of it is a decimal fraction".
> In 12 bytes BigInteger only 95 bits are used to numbers and 1 bit for a sign.
> So code:
> {code:c#}
> var newBigInt = new BigInteger(Decimal.MaxValue);
> var arr = newBigInt.ToByteArray();
> {code}
> will produce array of 13 bytes long, not 12. 
> I tried to change 
> _Span<byte> bigIntBytes = stackalloc byte[12];_
> to
> _Span<byte> bigIntBytes = stackalloc byte[13];_
> and this solved the issue.
> PR: https://github.com/apache/arrow/pull/13732



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