scovich commented on code in PR #9689:
URL: https://github.com/apache/arrow-rs/pull/9689#discussion_r3081924941
##########
arrow-cast/src/cast/mod.rs:
##########
@@ -89,7 +89,7 @@ where
D: DecimalType,
F: Fn(D::Native) -> f64,
{
- f(x) / 10_f64.powi(scale)
+ f(x) * 10_f64.powi(-scale)
Review Comment:
Pure luck that `1f` didn't trigger anything. A more complete
[example](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=dea495dbe79b072e3e6fb45f8d5c4806)
finds plenty of values just in `-10..=10`:
```
a = -10, b = -30
(a as f64) / 10_f64.powi(b) = -1.00000000000000007618e31
(a as f64) * 10_f64.powi(-b) = -9.99999999999999963590e30
a = -10, b = -26
(a as f64) / 10_f64.powi(b) = -1.00000000000000015073e27
(a as f64) * 10_f64.powi(-b) = -1.00000000000000001329e27
a = -10, b = -23
(a as f64) / 10_f64.powi(b) = -9.99999999999999849005e23
(a as f64) * 10_f64.powi(-b) = -9.99999999999999983223e23
a = -10, b = -17
(a as f64) / 10_f64.powi(b) = -9.99999999999999872000e17
(a as f64) * 10_f64.powi(-b) = -1.00000000000000000000e18
a = -10, b = -5
(a as f64) / 10_f64.powi(b) = -9.99999999999999883585e5
(a as f64) * 10_f64.powi(-b) = -1.00000000000000000000e6
a = -10, b = 6
(a as f64) / 10_f64.powi(b) = -1.00000000000000008180e-5
(a as f64) * 10_f64.powi(-b) = -9.99999999999999912396e-6
a = -10, b = 11
(a as f64) / 10_f64.powi(b) = -1.00000000000000003643e-10
(a as f64) * 10_f64.powi(-b) = -9.99999999999999907185e-11
a = -10, b = 15
(a as f64) / 10_f64.powi(b) = -9.99999999999999998819e-15
(a as f64) * 10_f64.powi(-b) = -1.00000000000000015659e-14
a = -10, b = 17
(a as f64) / 10_f64.powi(b) = -9.99999999999999979098e-17
(a as f64) * 10_f64.powi(-b) = -1.00000000000000010236e-16
Found 246 examples in all
```
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