davidlghellin commented on code in PR #10509:
URL: https://github.com/apache/arrow-rs/pull/10509#discussion_r4176970657
##########
arrow-cast/src/cast/mod.rs:
##########
@@ -92,7 +100,124 @@ where
D: DecimalType,
F: Fn(D::Native) -> f64,
{
- f(x) / 10_f64.powi(scale)
+ let unscaled = f(x);
+ // Both operands are exact in this range, so this rounds once. A negative
scale
+ // multiplies, since `10^-scale` is the exact power of ten there.
+ if (-22..=22).contains(&scale) && unscaled.abs() < F64_EXACT_INT_LIMIT {
+ return if scale >= 0 {
+ unscaled / 10_f64.powi(scale)
+ } else {
+ unscaled * 10_f64.powi(-scale)
+ };
+ }
+ match i8::try_from(scale) {
+ Ok(scale) => decimal_to_f64_rounded_once::<D>(x, scale, unscaled),
+ Err(_) => unscaled / 10_f64.powi(scale),
+ }
+}
+
+/// Rounds once for the values the arithmetic cannot convert exactly, by
parsing
+/// the decimal's own text.
+#[cold]
+#[inline(never)]
+fn decimal_to_f64_rounded_once<D: DecimalType>(x: D::Native, scale: i8,
unscaled: f64) -> f64 {
+ D::format_decimal(x, u8::MAX, scale)
+ .parse::<f64>()
+ .unwrap_or_else(|_| unscaled / 10_f64.powi(scale.into()))
+}
+
+/// As [`decimal_to_f64_rounded_once`], but narrowing to `f32` in one step.
+/// Rounding to `f64` first and then to `f32` rounds twice: a decimal just
above an
+/// `f32` midpoint can collapse onto that midpoint in `f64`, and
round-half-even
+/// then sends it the wrong way.
+#[cold]
+#[inline(never)]
+fn decimal_to_f32_rounded_once<D: DecimalType>(x: D::Native, scale: i8,
unscaled: f64) -> f32 {
+ D::format_decimal(x, u8::MAX, scale)
+ .parse::<f32>()
+ .unwrap_or_else(|_| unscaled as f32 / 10_f32.powi(scale.into()))
Review Comment:
Same here — the note points at the `f64` one rather than repeating it.
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