[I] Casting decimal -> decimal with large scale is buggy [arrow-rs]

["neilconway (via GitHub)" <gi...@apache.org>]

neilconway opened a new issue, #10946:
URL: https://github.com/apache/arrow-rs/issues/10946

   ### Describe the bug
   
   Casting decimal -> decimal with a much larger scale can produce incorrect values, including panics in debug mode.
   
   ### To Reproduce
   
   ```rust
   use arrow_array::{Array, Decimal256Array};
   use arrow_buffer::i256;
   use arrow_cast::{cast_with_options, CastOptions};
   use arrow_schema::DataType;
   
   let array = Decimal256Array::from(vec![
       i256::from_i128(1),
       i256::from_i128(10_i128.pow(16)),
       i256::from_i128(10_i128.pow(17)),
   ])
   .with_precision_and_scale(76, 0)
   .unwrap();
   
   // Strict mode; safe mode returns the same values (no nulls, no error)
   let options = CastOptions { safe: false, ..Default::default() };
   let result = cast_with_options(&array, &DataType::Decimal256(76, 60), &options).unwrap();
   ```
   
   Results:
   
   ```
   1                  -> 1000000000000000000000000000000000000000000000000000000000000
   10000000000000000  -> 10000000000000000000000000000000000000000000000000000000000000000000000000000
   100000000000000000 -> -15792089237316195423570985008687907853269984665640564039457584007913129639936
   ```
   
   The first result is correct.
   
   The second result exceeds the maximum value representable at precision 76 but is returned without error.
   
   The third result is just wrong.
   
   ### Expected behavior
   
   _No response_
   
   ### Additional context
   
   _No response_


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Re: [I] Casting decimal -> decimal with large scale is buggy [arrow-rs]

["neilconway (via GitHub)" <gi...@apache.org>]

neilconway commented on issue #10946:
URL: https://github.com/apache/arrow-rs/issues/10946#issuecomment-5498966357

   take


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