[GitHub] [arrow-rs] viirya commented on a diff in pull request #4393: Add Scalar/Datum abstraction (#1047)

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

viirya commented on code in PR #4393:
URL: https://github.com/apache/arrow-rs/pull/4393#discussion_r1247230350


##########
arrow-array/src/scalar.rs:
##########
@@ -0,0 +1,116 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use crate::Array;
+
+/// A possibly [`Scalar`] [`Array`]
+///
+/// This allows optimised binary kernels where one or more arguments are constant
+///
+/// ```
+/// # use arrow_array::*;
+/// # use arrow_buffer::{BooleanBuffer, MutableBuffer, NullBuffer};
+/// # use arrow_schema::ArrowError;
+/// #
+/// fn eq_impl<T: ArrowPrimitiveType>(
+///     a: &PrimitiveArray<T>,
+///     a_scalar: bool,
+///     b: &PrimitiveArray<T>,
+///     b_scalar: bool,
+/// ) -> BooleanArray {
+///     let (array, scalar) = match (a_scalar, b_scalar) {
+///         (true, true) | (false, false) => {
+///             let len = a.len().min(b.len());
+///             let nulls = NullBuffer::union(a.nulls(), b.nulls());
+///             let buffer = BooleanBuffer::collect_bool(len, |idx| a.value(idx) == b.value(idx));
+///             return BooleanArray::new(buffer, nulls);
+///         }
+///         (true, false) => (b, (a.null_count() == 0).then(|| a.value(0))),
+///         (false, true) => (a, (b.null_count() == 0).then(|| b.value(0))),
+///     };
+///     match scalar {
+///         Some(v) => {
+///             let len = array.len();
+///             let nulls = array.nulls().cloned();
+///             let buffer = BooleanBuffer::collect_bool(len, |idx| array.value(idx) == v);
+///             BooleanArray::new(buffer, nulls)
+///         }
+///         None => BooleanArray::new_null(array.len()),
+///     }
+/// }
+///
+/// pub fn eq(l: &dyn Datum, r: &dyn Datum) -> Result<BooleanArray, ArrowError> {
+///     let (l_array, l_scalar) = l.get();
+///     let (r_array, r_scalar) = r.get();
+///     downcast_primitive_array!(
+///         (l_array, r_array) => Ok(eq_impl(l_array, l_scalar, r_array, r_scalar)),
+///         (a, b) => Err(ArrowError::NotYetImplemented(format!("{a} == {b}"))),
+///     )
+/// }
+///
+/// // Comparison of two arrays
+/// let a = Int32Array::from(vec![1, 2, 3, 4, 5]);
+/// let b = Int32Array::from(vec![1, 2, 4, 7, 3]);
+/// let r = eq(&a, &b).unwrap();
+/// let values: Vec<_> = r.values().iter().collect();
+/// assert_eq!(values, &[true, true, false, false, false]);
+///
+/// // Comparison of an array and a scalar
+/// let a = Int32Array::from(vec![1, 2, 3, 4, 5]);
+/// let b = Int32Array::from(vec![1]);
+/// let r = eq(&a, &Scalar::new(&b)).unwrap();
+/// let values: Vec<_> = r.values().iter().collect();
+/// assert_eq!(values, &[true, false, false, false, false]);
+pub trait Datum {
+    /// Returns the value for this [`Datum`] and a boolean indicating if the value is scalar
+    fn get(&self) -> (&dyn Array, bool);
+}
+
+impl<T: Array> Datum for T {
+    fn get(&self) -> (&dyn Array, bool) {
+        (self, false)
+    }
+}

Review Comment:
   Hmm, so an Array with length 1 will also return false indicating it is not a scalar? Only if we explicitly wrap it with a `Scalar` so we can get correct output?



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