jaideeppyne commented on code in PR #10567:
URL: https://github.com/apache/arrow-rs/pull/10567#discussion_r3730302050
##########
arrow-cast/src/cast/mod.rs:
##########
@@ -77,6 +77,105 @@ pub use decimal::{
};
pub use string::cast_single_string_to_boolean_default;
+// IEEE-754 bit patterns of `10_f64.powi(0..=Decimal256Type::MAX_SCALE)`.
+// Values are taken from `10_f64.powi` so lookups are bit-identical to the
previous
+// calculation for every valid non-negative decimal scale.
+const DECIMAL_F64_POWERS: [u64; Decimal256Type::MAX_SCALE as usize + 1] = [
+ 0x3ff0000000000000,
+ 0x4024000000000000,
+ 0x4059000000000000,
+ 0x408f400000000000,
+ 0x40c3880000000000,
+ 0x40f86a0000000000,
+ 0x412e848000000000,
+ 0x416312d000000000,
+ 0x4197d78400000000,
+ 0x41cdcd6500000000,
+ 0x4202a05f20000000,
+ 0x42374876e8000000,
+ 0x426d1a94a2000000,
+ 0x42a2309ce5400000,
+ 0x42d6bcc41e900000,
+ 0x430c6bf526340000,
+ 0x4341c37937e08000,
+ 0x4376345785d8a000,
+ 0x43abc16d674ec800,
+ 0x43e158e460913d00,
+ 0x4415af1d78b58c40,
+ 0x444b1ae4d6e2ef50,
+ 0x4480f0cf064dd592,
+ 0x44b52d02c7e14af6,
+ 0x44ea784379d99db4,
+ 0x45208b2a2c280291,
+ 0x4554adf4b7320335,
+ 0x4589d971e4fe8402,
+ 0x45c027e72f1f1281,
+ 0x45f431e0fae6d721,
+ 0x46293e5939a08cea,
+ 0x465f8def8808b024,
+ 0x4693b8b5b5056e17,
+ 0x46c8a6e32246c99d,
+ 0x46fed09bead87c04,
+ 0x4733426172c74d82,
+ 0x476812f9cf7920e3,
+ 0x479e17b84357691c,
+ 0x47d2ced32a16a1b1,
+ 0x48078287f49c4a1e,
+ 0x483d6329f1c35ca5,
+ 0x48725dfa371a19e7,
+ 0x48a6f578c4e0a061,
+ 0x48dcb2d6f618c879,
+ 0x4911efc659cf7d4c,
+ 0x49466bb7f0435c9f,
+ 0x497c06a5ec5433c6,
+ 0x49b18427b3b4a05c,
+ 0x49e5e531a0a1c873,
+ 0x4a1b5e7e08ca3a90,
+ 0x4a511b0ec57e649a,
+ 0x4a8561d276ddfdc0,
+ 0x4ababa4714957d30,
+ 0x4af0b46c6cdd6e3e,
+ 0x4b24e1878814c9ce,
+ 0x4b5a19e96a19fc41,
+ 0x4b905031e2503da9,
+ 0x4bc4643e5ae44d14,
+ 0x4bf97d4df19d6058,
+ 0x4c2fdca16e04b86e,
+ 0x4c63e9e4e4c2f344,
+ 0x4c98e45e1df3b015,
+ 0x4ccf1d75a5709c1b,
+ 0x4d03726987666191,
+ 0x4d384f03e93ff9f6,
+ 0x4d6e62c4e38ff874,
+ 0x4da2fdbb0e39fb48,
+ 0x4dd7bd29d1c87a1a,
+ 0x4e0dac74463a98a1,
+ 0x4e428bc8abe49f64,
+ 0x4e772ebad6ddc73e,
+ 0x4eacfa698c95390d,
+ 0x4ee21c81f7dd43a8,
+ 0x4f16a3a275d49492,
+ 0x4f4c4c8b1349b9b7,
+ 0x4f81afd6ec0e1412,
+ 0x4fb61bcca7119917,
+];
+
+/// Return `10^scale` as `f64` for decimal cast scaling.
+///
+/// For `0 <= scale <= Decimal256Type::MAX_SCALE`, returns a precomputed value
that is
+/// bit-identical to `10_f64.powi(scale)`. Negative or out-of-range scales
fall back to
+/// `powi` to preserve prior behavior.
+#[inline]
+pub fn decimal_f64_power(scale: i32) -> f64 {
Review Comment:
Yes — intentional: **negative scales do not use the LUT**.
### Behavior (now documented on `decimal_f64_power`)
| `scale` | Path | Why |
|--------|------|-----|
| `0..=Decimal256Type::MAX_SCALE` (0..=76) | `DECIMAL_F64_POWERS[scale]` |
Valid non-negative Arrow decimal scales; bit-identical to `10_f64.powi` |
| `scale < 0` | `10_f64.powi(scale)` | Negative powers are `1/10^\|scale\|`;
not stored in the table. `usize::try_from(scale)` fails → fallback |
| `scale > MAX_SCALE` | `10_f64.powi(scale)` | Outside table length |
Evidence in code: the first branch is only taken when
`usize::try_from(scale)` succeeds (non-negative).
Pushed a follow-up that:
1. Documents this in the rustdoc on `decimal_f64_power`
2. Splits tests into LUT-only vs fallback-only so negative-scale coverage is
not confused with table coverage
##########
arrow-cast/src/cast/mod.rs:
##########
@@ -2851,6 +2950,17 @@ where
#[cfg(test)]
mod tests {
use super::*;
+
+ #[test]
+ fn test_decimal_f64_power_matches_powi() {
+ for scale in -(Decimal256Type::MAX_SCALE as
i32)..=Decimal256Type::MAX_SCALE as i32 {
+ assert_eq!(
+ decimal_f64_power(scale).to_bits(),
+ 10_f64.powi(scale).to_bits(),
+ "scale {scale}"
Review Comment:
Agreed — the old loop mixed LUT and fallback paths.
Fixed in the latest commit by splitting tests:
1. `test_decimal_f64_power_lut_matches_powi_for_non_negative_scales` —
**only** `0..=MAX_SCALE` (table path)
2.
`test_decimal_f64_power_fallback_matches_powi_for_negative_and_out_of_range` —
**only** negative / `> MAX_SCALE` (explicitly documents it is fallback parity,
not table coverage)
Local: both tests pass under `cargo test -p arrow-cast
test_decimal_f64_power --lib`.
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