chore: use NullBuffer::union for Spark concat (apache#18087)

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- Closes #.

Followup on
apache#18063 (review)

## Rationale for this change

Use cheaper `NullBuffer::union` to apply null mask instead of iterator
approach

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(cherry picked from commit 337378a)

Author: comphead

datafusion/spark/src/function/string/concat.rs

@@ -15,7 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
-use arrow::array::{Array, ArrayBuilder};
+use arrow::array::Array;
+use arrow::buffer::NullBuffer;
 use arrow::datatypes::DataType;
 use datafusion_common::{Result, ScalarValue};
 use datafusion_expr::{
@@ -31,6 +32,10 @@ use std::sync::Arc;
 ///
 /// Concatenates multiple input strings into a single string.
 /// Returns NULL if any input is NULL.
+///
+/// Differences with DataFusion concat:
+/// - Support 0 arguments
+/// - Return NULL if any input is NULL
 #[derive(Debug, PartialEq, Eq, Hash)]
 pub struct SparkConcat {
     signature: Signature,
@@ -80,6 +85,16 @@ impl ScalarUDFImpl for SparkConcat {
     }
 }
 
+/// Represents the null state for Spark concat
+enum NullMaskResolution {
+    /// Return NULL as the result (e.g., scalar inputs with at least one NULL)
+    ReturnNull,
+    /// No null mask needed (e.g., all scalar inputs are non-NULL)
+    NoMask,
+    /// Null mask to apply for arrays
+    Apply(NullBuffer),
+}
+
 /// Concatenates strings, returning NULL if any input is NULL
 /// This is a Spark-specific wrapper around DataFusion's concat that returns NULL
 /// if any argument is NULL (Spark behavior), whereas DataFusion's concat ignores NULLs.
@@ -103,7 +118,7 @@ fn spark_concat(args: ScalarFunctionArgs) -> Result<ColumnarValue> {
     let null_mask = compute_null_mask(&arg_values, number_rows)?;
 
     // If all scalars and any is NULL, return NULL immediately
-    if null_mask.is_none() {
+    if matches!(null_mask, NullMaskResolution::ReturnNull) {
         return Ok(ColumnarValue::Scalar(ScalarValue::Utf8(None)));
     }
 
@@ -122,13 +137,11 @@ fn spark_concat(args: ScalarFunctionArgs) -> Result<ColumnarValue> {
     apply_null_mask(result, null_mask)
 }
 
-/// Compute NULL mask for the arguments
-/// Returns None if all scalars and any is NULL, or a Vector of
-/// boolean representing the null mask for incoming arrays
+/// Compute NULL mask for the arguments using NullBuffer::union
 fn compute_null_mask(
     args: &[ColumnarValue],
     number_rows: usize,
-) -> Result<Option<Vec<bool>>> {
+) -> Result<NullMaskResolution> {
     // Check if all arguments are scalars
     let all_scalars = args
         .iter()
@@ -139,15 +152,14 @@ fn compute_null_mask(
         for arg in args {
             if let ColumnarValue::Scalar(scalar) = arg {
                 if scalar.is_null() {
-                    // Return None to indicate all values should be NULL
-                    return Ok(None);
+                    return Ok(NullMaskResolution::ReturnNull);
                 }
             }
         }
         // No NULLs in scalars
-        Ok(Some(vec![]))
+        Ok(NullMaskResolution::NoMask)
     } else {
-        // For arrays, compute NULL mask for each row
+        // For arrays, compute NULL mask for each row using NullBuffer::union
         let array_len = args
             .iter()
             .find_map(|arg| match arg {
@@ -166,99 +178,50 @@ fn compute_null_mask(
             .collect();
         let arrays = arrays?;
 
-        // Compute NULL mask
-        let mut null_mask = vec![false; array_len];
-        for array in &arrays {
-            for (i, null_flag) in null_mask.iter_mut().enumerate().take(array_len) {
-                if array.is_null(i) {
-                    *null_flag = true;
-                }
-            }
-        }
+        // Use NullBuffer::union to combine all null buffers
+        let combined_nulls = arrays
+            .iter()
+            .map(|arr| arr.nulls())
+            .fold(None, |acc, nulls| NullBuffer::union(acc.as_ref(), nulls));
 
-        Ok(Some(null_mask))
+        match combined_nulls {
+            Some(nulls) => Ok(NullMaskResolution::Apply(nulls)),
+            None => Ok(NullMaskResolution::NoMask),
+        }
     }
 }
 
-/// Apply NULL mask to the result
+/// Apply NULL mask to the result using NullBuffer::union
 fn apply_null_mask(
     result: ColumnarValue,
-    null_mask: Option<Vec<bool>>,
+    null_mask: NullMaskResolution,
 ) -> Result<ColumnarValue> {
     match (result, null_mask) {
-        // Scalar with NULL mask means return NULL
-        (ColumnarValue::Scalar(_), None) => {
+        // Scalar with ReturnNull mask means return NULL
+        (ColumnarValue::Scalar(_), NullMaskResolution::ReturnNull) => {
             Ok(ColumnarValue::Scalar(ScalarValue::Utf8(None)))
         }
-        // Scalar without NULL mask, return as-is
-        (scalar @ ColumnarValue::Scalar(_), Some(mask)) if mask.is_empty() => Ok(scalar),
-        // Array with NULL mask
-        (ColumnarValue::Array(array), Some(null_mask)) if !null_mask.is_empty() => {
-            let array_len = array.len();
-            let return_type = array.data_type();
+        // Scalar without mask, return as-is
+        (scalar @ ColumnarValue::Scalar(_), NullMaskResolution::NoMask) => Ok(scalar),
+        // Array with NULL mask - use NullBuffer::union to combine nulls
+        (ColumnarValue::Array(array), NullMaskResolution::Apply(null_mask)) => {
+            // Combine the result's existing nulls with our computed null mask
+            let combined_nulls = NullBuffer::union(array.nulls(), Some(&null_mask));
 
-            let mut builder: Box<dyn ArrayBuilder> = match return_type {
-                DataType::Utf8 => {
-                    let string_array = array
-                        .as_any()
-                        .downcast_ref::<arrow::array::StringArray>()
-                        .unwrap();
-                    let mut builder =
-                        arrow::array::StringBuilder::with_capacity(array_len, 0);
-                    for (i, &is_null) in null_mask.iter().enumerate().take(array_len) {
-                        if is_null || string_array.is_null(i) {
-                            builder.append_null();
-                        } else {
-                            builder.append_value(string_array.value(i));
-                        }
-                    }
-                    Box::new(builder)
-                }
-                DataType::LargeUtf8 => {
-                    let string_array = array
-                        .as_any()
-                        .downcast_ref::<arrow::array::LargeStringArray>()
-                        .unwrap();
-                    let mut builder =
-                        arrow::array::LargeStringBuilder::with_capacity(array_len, 0);
-                    for (i, &is_null) in null_mask.iter().enumerate().take(array_len) {
-                        if is_null || string_array.is_null(i) {
-                            builder.append_null();
-                        } else {
-                            builder.append_value(string_array.value(i));
-                        }
-                    }
-                    Box::new(builder)
-                }
-                DataType::Utf8View => {
-                    let string_array = array
-                        .as_any()
-                        .downcast_ref::<arrow::array::StringViewArray>()
-                        .unwrap();
-                    let mut builder =
-                        arrow::array::StringViewBuilder::with_capacity(array_len);
-                    for (i, &is_null) in null_mask.iter().enumerate().take(array_len) {
-                        if is_null || string_array.is_null(i) {
-                            builder.append_null();
-                        } else {
-                            builder.append_value(string_array.value(i));
-                        }
-                    }
-                    Box::new(builder)
-                }
-                _ => {
-                    return datafusion_common::exec_err!(
-                        "Unsupported return type for concat: {:?}",
-                        return_type
-                    );
-                }
-            };
+            // Create new array with combined nulls
+            let new_array = array
+                .into_data()
+                .into_builder()
+                .nulls(combined_nulls)
+                .build()?;
 
-            Ok(ColumnarValue::Array(builder.finish()))
+            Ok(ColumnarValue::Array(Arc::new(arrow::array::make_array(
+                new_array,
+            ))))
         }
         // Array without NULL mask, return as-is
-        (array @ ColumnarValue::Array(_), _) => Ok(array),
-        // Shouldn't happen
+        (array @ ColumnarValue::Array(_), NullMaskResolution::NoMask) => Ok(array),
+        // Edge cases that shouldn't happen in practice
         (scalar, _) => Ok(scalar),
     }
 }