[GitHub] [arrow-rs] alamb commented on a diff in pull request #4088: optimize cast for same decimal type and same scale

["alamb (via GitHub)" <gi...@apache.org>]

alamb commented on code in PR #4088:
URL: https://github.com/apache/arrow-rs/pull/4088#discussion_r1167159839


##########
arrow-cast/src/cast.rs:
##########
@@ -2081,29 +2081,102 @@ impl DecimalCast for i256 {
     }
 }
 
-fn cast_decimal_to_decimal<I, O>(
-    array: &PrimitiveArray<I>,
-    input_scale: i8,
+fn cast_decimal_to_decimal_error<O>(
     output_precision: u8,
     output_scale: i8,
-    cast_options: &CastOptions,
-) -> Result<ArrayRef, ArrowError>
+) -> impl Fn(<O as ArrowPrimitiveType>::Native) -> ArrowError
 where
-    I: DecimalType,
     O: DecimalType,
-    I::Native: DecimalCast + ArrowNativeTypeOp,
     O::Native: DecimalCast + ArrowNativeTypeOp,
 {
-    let error = |x| {
+    move |x: O::Native| {
         ArrowError::CastError(format!(
             "Cannot cast to {}({}, {}). Overflowing on {:?}",
             O::PREFIX,
             output_precision,
             output_scale,
             x
         ))
+    }
+}
+
+fn cast_decimal_to_decimal_same_type<T>(

Review Comment:
   Could you please add some comments here about what the difference between `cast_decimal_to_decimal_same_type` and `cast_decimal_to_decimal` are, especially given their arguments appear to be the the same. 
   



##########
arrow-cast/src/cast.rs:
##########
@@ -2081,29 +2081,102 @@ impl DecimalCast for i256 {
     }
 }
 
-fn cast_decimal_to_decimal<I, O>(
-    array: &PrimitiveArray<I>,
-    input_scale: i8,
+fn cast_decimal_to_decimal_error<O>(
     output_precision: u8,
     output_scale: i8,
-    cast_options: &CastOptions,
-) -> Result<ArrayRef, ArrowError>
+) -> impl Fn(<O as ArrowPrimitiveType>::Native) -> ArrowError
 where
-    I: DecimalType,
     O: DecimalType,
-    I::Native: DecimalCast + ArrowNativeTypeOp,
     O::Native: DecimalCast + ArrowNativeTypeOp,
 {
-    let error = |x| {
+    move |x: O::Native| {
         ArrowError::CastError(format!(
             "Cannot cast to {}({}, {}). Overflowing on {:?}",
             O::PREFIX,
             output_precision,
             output_scale,
             x
         ))
+    }
+}
+
+fn cast_decimal_to_decimal_same_type<T>(
+    array: &PrimitiveArray<T>,
+    input_scale: i8,
+    output_precision: u8,
+    output_scale: i8,
+    cast_options: &CastOptions,
+) -> Result<ArrayRef, ArrowError>
+where
+    T: DecimalType,
+    T::Native: DecimalCast + ArrowNativeTypeOp,
+{
+    let error = cast_decimal_to_decimal_error::<T>(output_precision, output_scale);
+
+    let array: PrimitiveArray<T> = if input_scale == output_scale {
+        // the scale doesn't change, the native value don't need to be changed
+        array.clone()
+    } else if input_scale > output_scale {

Review Comment:
   this code and below looks very similar to what is in `cast_decimal_to_decimal`
   
   Is there any way they can be combined to reduce the maintenance burden longer term?



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