etseidl commented on code in PR #6252:
URL: https://github.com/apache/arrow-rs/pull/6252#discussion_r1723674743
##########
arrow-array/src/array/primitive_array.rs:
##########
@@ -1016,6 +1016,32 @@ impl<T: ArrowPrimitiveType> PrimitiveArray<T> {
PrimitiveArray::new(values, Some(nulls))
}
+ /// Applies a unary infallible function to each value in an array,
producing a
+ /// new primitive array.
+ ///
+ /// # Null Handling
+ ///
+ /// Applies the function for all values, including those on null slots.
This
+ /// will often allow the compiler to generate faster vectorized code, but
+ /// requires that the operation must be infallible (not error/panic) for
any
+ /// value of the corresponding type or this function may panic.
+ pub fn from_unary<U: ArrayAccessor, F>(left: U, mut op: F) -> Self
+ where
+ F: FnMut(U::Item) -> T::Native,
+ {
+ let nulls = left.logical_nulls();
+ let mut values: Vec<T::Native> = vec![T::Native::default();
left.len()];
+
+ for (i, val) in values.iter_mut().enumerate().take(left.len()) {
Review Comment:
On my workstation that caused a 10-20% regression for fixed_len_byte_arrays.
:(
How about the following (which avoids the vector initialization):
```rust
let mut values: Vec<T::Native> = Vec::with_capacity(left.len());
unsafe {
// SAFETY: setting len to capacity, and then iterating over
range 0..len
values.set_len(left.len());
for (i, val) in values.iter_mut().enumerate().take(left.len()) {
*val = op(left.value_unchecked(i));
}
}
```
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