ethantang93 commented on code in PR #10567:
URL: https://github.com/apache/arrow-rs/pull/10567#discussion_r3730111896
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arrow-cast/src/cast/mod.rs:
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@@ -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:
curious how do we handle negative scale values? or is the intention here to
just let it do the calculation without LUT? seems like the current
implementation only use the LUT for values ranging from 0 to 76
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