andygrove commented on code in PR #15168:
URL: https://github.com/apache/datafusion/pull/15168#discussion_r2051099886


##########
datafusion/spark/src/function/math/expm1.rs:
##########
@@ -0,0 +1,169 @@
+// 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::function::error_utils::{
+    invalid_arg_count_exec_err, unsupported_data_type_exec_err,
+};
+use arrow::array::{ArrayRef, AsArray};
+use arrow::datatypes::{DataType, Float64Type};
+use datafusion_common::{Result, ScalarValue};
+use datafusion_expr::{
+    ColumnarValue, ScalarFunctionArgs, ScalarUDFImpl, Signature, Volatility,
+};
+use datafusion_macros::user_doc;
+use std::any::Any;
+use std::sync::Arc;
+
+#[user_doc(
+    doc_section(label = "Spark Math Functions"),
+    description = "Returns exp(expr) - 1 as a Float64.",
+    syntax_example = "expm1(expr)",
+    sql_example = r#"```sql
+> select expm1(0);
++--------------------+
+| expm1(0)           |
++--------------------+
+| 0.0                |
++--------------------+
+> select expm1(1);
++----------------+
+| expm1(1)       |
++----------------+
+| 50             |
++----------------+
+```"#,
+    argument(
+        name = "expr",
+        description = "An expression that evaluates to a numeric."
+    )
+)]
+#[derive(Debug)]
+pub struct SparkExpm1 {
+    signature: Signature,
+    aliases: Vec<String>,
+}
+
+impl Default for SparkExpm1 {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl SparkExpm1 {
+    pub fn new() -> Self {
+        Self {
+            signature: Signature::user_defined(Volatility::Immutable),
+            aliases: vec!["spark_expm1".to_string()],
+        }
+    }
+}
+
+impl ScalarUDFImpl for SparkExpm1 {
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+
+    fn name(&self) -> &str {
+        "expm1"
+    }
+
+    fn signature(&self) -> &Signature {
+        &self.signature
+    }
+
+    fn return_type(&self, _arg_types: &[DataType]) -> Result<DataType> {
+        Ok(DataType::Float64)
+    }
+
+    fn invoke_with_args(&self, args: ScalarFunctionArgs) -> 
Result<ColumnarValue> {
+        if args.args.len() != 1 {
+            return Err(invalid_arg_count_exec_err("expm1", (1, 1), 
args.args.len()));
+        }
+        match &args.args[0] {
+            ColumnarValue::Scalar(ScalarValue::Float64(value)) => Ok(
+                ColumnarValue::Scalar(ScalarValue::Float64(value.map(|x| 
x.exp_m1()))),
+            ),
+            ColumnarValue::Array(array) => match array.data_type() {
+                DataType::Float64 => Ok(ColumnarValue::Array(Arc::new(
+                    array
+                        .as_primitive::<Float64Type>()
+                        .unary::<_, Float64Type>(|x| x.exp_m1()),
+                )
+                    as ArrayRef)),
+                other => Err(unsupported_data_type_exec_err(
+                    "expm1",
+                    format!("{}", DataType::Float64).as_str(),
+                    other,
+                )),
+            },
+            other => Err(unsupported_data_type_exec_err(
+                "expm1",
+                format!("{}", DataType::Float64).as_str(),
+                &other.data_type(),
+            )),
+        }
+    }
+
+    fn aliases(&self) -> &[String] {
+        &self.aliases
+    }
+
+    fn coerce_types(&self, arg_types: &[DataType]) -> Result<Vec<DataType>> {
+        if arg_types.len() != 1 {
+            return Err(invalid_arg_count_exec_err("expm1", (1, 1), 
arg_types.len()));
+        }
+        if arg_types[0].is_numeric() {
+            Ok(vec![DataType::Float64])
+        } else {
+            Err(unsupported_data_type_exec_err(
+                "expm1",
+                "Numeric Type",
+                &arg_types[0],
+            ))
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::function::math::expm1::SparkExpm1;
+    use crate::function::utils::test::test_scalar_function;
+    use arrow::array::{Array, Float64Array};
+    use arrow::datatypes::DataType::Float64;
+    use datafusion_common::{Result, ScalarValue};
+    use datafusion_expr::{ColumnarValue, ScalarUDFImpl};
+
+    macro_rules! test_expm1_float64_invoke {
+        ($INPUT:expr, $EXPECTED:expr) => {
+            test_scalar_function!(
+                SparkExpm1::new(),
+                vec![ColumnarValue::Scalar(ScalarValue::Float64($INPUT))],
+                $EXPECTED,
+                f64,
+                Float64,
+                Float64Array
+            );
+        };
+    }
+
+    #[test]

Review Comment:
   Sorry for the late response here. I had a vacation and have been busy with 
Comet priorities since getting back. I would like to help with the review here. 
I do wonder if we could start with a smaller scope PR to get the initial crate 
in place.
   
   I would also like to contribute some Spark-compatible shuffle implementation 
from Comet so that we can re-use it in Ballista.
   



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