scovich commented on code in PR #8689:
URL: https://github.com/apache/arrow-rs/pull/8689#discussion_r2456646090
##########
arrow-cast/src/cast/decimal.rs:
##########
@@ -166,50 +166,86 @@ where
}
}
-pub(crate) fn convert_to_smaller_scale_decimal<I, O>(
- array: &PrimitiveArray<I>,
+/// Construct closures to upscale decimals from `(input_precision,
input_scale)` to
+/// `(output_precision, output_scale)`.
+///
+/// Returns `None` if the required scale increase `delta_scale = output_scale
- input_scale`
+/// exceeds the supported precomputed precision table
`O::MAX_FOR_EACH_PRECISION`.
+/// In that case, the caller should treat this as an overflow for the output
scale
+/// and handle it accordingly (e.g., return a cast error).
+#[allow(clippy::type_complexity)]
+pub fn make_upscaler<I: DecimalType, O: DecimalType>(
input_precision: u8,
input_scale: i8,
output_precision: u8,
output_scale: i8,
- cast_options: &CastOptions,
-) -> Result<PrimitiveArray<O>, ArrowError>
+) -> Option<(
+ impl Fn(I::Native) -> Option<O::Native>,
+ Option<impl Fn(I::Native) -> O::Native>,
+)>
where
- I: DecimalType,
- O: DecimalType,
I::Native: DecimalCast + ArrowNativeTypeOp,
O::Native: DecimalCast + ArrowNativeTypeOp,
{
- let error = cast_decimal_to_decimal_error::<I, O>(output_precision,
output_scale);
- let delta_scale = input_scale - output_scale;
- // if the reduction of the input number through scaling (dividing) is
greater
- // than a possible precision loss (plus potential increase via rounding)
- // every input number will fit into the output type
+ let delta_scale = output_scale - input_scale;
+
+ // O::MAX_FOR_EACH_PRECISION[k] stores 10^k - 1 (e.g., 9, 99, 999, ...).
+ // Adding 1 yields exactly 10^k without computing a power at runtime.
+ // Using the precomputed table avoids pow(10, k) and its checked/overflow
+ // handling, which is faster and simpler for scaling by 10^delta_scale.
+ let max = O::MAX_FOR_EACH_PRECISION.get(delta_scale as usize)?;
+ let mul = max.add_wrapping(O::Native::ONE);
+ let f = move |x| O::Native::from_decimal(x).and_then(|x|
x.mul_checked(mul).ok());
+
+ // if the gain in precision (digits) is greater than the multiplication
due to scaling
+ // every number will fit into the output type
// Example: If we are starting with any number of precision 5 [xxxxx],
- // then and decrease the scale by 3 will have the following effect on the
representation:
- // [xxxxx] -> [xx] (+ 1 possibly, due to rounding).
- // The rounding may add an additional digit, so the cast to be infallible,
- // the output type needs to have at least 3 digits of precision.
- // e.g. Decimal(5, 3) 99.999 to Decimal(3, 0) will result in 100:
- // [99999] -> [99] + 1 = [100], a cast to Decimal(2, 0) would not be
possible
- let is_infallible_cast = (input_precision as i8) - delta_scale <
(output_precision as i8);
+ // then an increase of scale by 3 will have the following effect on the
representation:
+ // [xxxxx] -> [xxxxx000], so for the cast to be infallible, the output type
+ // needs to provide at least 8 digits precision
+ let is_infallible_cast = (input_precision as i8) + delta_scale <=
(output_precision as i8);
+ let f_infallible = is_infallible_cast
+ .then_some(move |x|
O::Native::from_decimal(x).unwrap().mul_wrapping(mul));
+ Some((f, f_infallible))
+}
+
+/// Construct closures to downscale decimals from `(input_precision,
input_scale)` to
+/// `(output_precision, output_scale)`.
+///
+/// Returns `None` if the required scale reduction `delta_scale = input_scale
- output_scale`
+/// exceeds the supported precomputed precision table
`I::MAX_FOR_EACH_PRECISION`.
+/// In this scenario, any value would round to zero (e.g., dividing by 10^k
where k exceeds the
+/// available precision). Callers should therefore produce zero values
(preserving nulls) rather
+/// than returning an error.
+#[allow(clippy::type_complexity)]
+pub fn make_downscaler<I: DecimalType, O: DecimalType>(
+ input_precision: u8,
+ input_scale: i8,
+ output_precision: u8,
+ output_scale: i8,
+) -> Option<(
+ impl Fn(I::Native) -> Option<O::Native>,
+ Option<impl Fn(I::Native) -> O::Native>,
+)>
+where
+ I::Native: DecimalCast + ArrowNativeTypeOp,
+ O::Native: DecimalCast + ArrowNativeTypeOp,
+{
+ let delta_scale = input_scale - output_scale;
Review Comment:
It can absolutely be negative. It corresponds to removing significant digits
from the answer. The code is specifically designed to handle that (including
cases where the final scale is negative, e.g. `123` with scale -3 is
interpreted as `123000`).
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