pitrou opened a new pull request, #35997:
URL: https://github.com/apache/arrow/pull/35997
The original algorithm for real-to-decimal conversion did its computations
in the floating-point domain, accumulating rounding errors especially for large
scale or precision values, such as:
```
>>> pa.array([1234567890.]).cast(pa.decimal128(38, 11))
<pyarrow.lib.Decimal128Array object at 0x7f05f4a3f1c0>
[
1234567889.99999995904
]
>>> pa.array([1234567890.]).cast(pa.decimal128(38, 12))
<pyarrow.lib.Decimal128Array object at 0x7f05f494f9a0>
[
1234567890.000000057344
]
```
The new algorithm strives to avoid precision loss by doing all its
computations in the decimal domain. However, negative scales, which are
presumably infrequent, fall back on the old algorithm.
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