[datafusion-spark] Add Spark-compatible char expression (#15994)

* Add Spark-compatible char expression

* Add slt test

Author: andygrove

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

@@ -0,0 +1,130 @@
+// 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 std::{any::Any, sync::Arc};
+
+use arrow::{
+    array::{ArrayRef, StringArray},
+    datatypes::{
+        DataType,
+        DataType::{Int64, Utf8},
+    },
+};
+
+use datafusion_common::{cast::as_int64_array, exec_err, Result, ScalarValue};
+use datafusion_expr::{
+    ColumnarValue, ScalarFunctionArgs, ScalarUDFImpl, Signature, Volatility,
+};
+
+/// Spark-compatible `char` expression
+/// <https://spark.apache.org/docs/latest/api/sql/index.html#char>
+#[derive(Debug)]
+pub struct SparkChar {
+    signature: Signature,
+}
+
+impl Default for SparkChar {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl SparkChar {
+    pub fn new() -> Self {
+        Self {
+            signature: Signature::uniform(1, vec![Int64], Volatility::Immutable),
+        }
+    }
+}
+
+impl ScalarUDFImpl for SparkChar {
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+
+    fn name(&self) -> &str {
+        "char"
+    }
+
+    fn signature(&self) -> &Signature {
+        &self.signature
+    }
+
+    fn return_type(&self, _arg_types: &[DataType]) -> Result<DataType> {
+        Ok(Utf8)
+    }
+
+    fn invoke_with_args(&self, args: ScalarFunctionArgs) -> Result<ColumnarValue> {
+        spark_chr(&args.args)
+    }
+}
+
+/// Returns the ASCII character having the binary equivalent to the input expression.
+/// E.g., chr(65) = 'A'.
+/// Compatible with Apache Spark's Chr function
+fn spark_chr(args: &[ColumnarValue]) -> Result<ColumnarValue> {
+    let array = args[0].clone();
+    match array {
+        ColumnarValue::Array(array) => {
+            let array = chr(&[array])?;
+            Ok(ColumnarValue::Array(array))
+        }
+        ColumnarValue::Scalar(ScalarValue::Int64(Some(value))) => {
+            if value < 0 {
+                Ok(ColumnarValue::Scalar(ScalarValue::Utf8(Some(
+                    "".to_string(),
+                ))))
+            } else {
+                match core::char::from_u32((value % 256) as u32) {
+                    Some(ch) => Ok(ColumnarValue::Scalar(ScalarValue::Utf8(Some(
+                        ch.to_string(),
+                    )))),
+                    None => {
+                        exec_err!("requested character was incompatible for encoding.")
+                    }
+                }
+            }
+        }
+        _ => exec_err!("The argument must be an Int64 array or scalar."),
+    }
+}
+
+fn chr(args: &[ArrayRef]) -> Result<ArrayRef> {
+    let integer_array = as_int64_array(&args[0])?;
+
+    // first map is the iterator, second is for the `Option<_>`
+    let result = integer_array
+        .iter()
+        .map(|integer: Option<i64>| {
+            integer
+                .map(|integer| {
+                    if integer < 0 {
+                        return Ok("".to_string()); // Return empty string for negative integers
+                    }
+                    match core::char::from_u32((integer % 256) as u32) {
+                        Some(ch) => Ok(ch.to_string()),
+                        None => {
+                            exec_err!("requested character not compatible for encoding.")
+                        }
+                    }
+                })
+                .transpose()
+        })
+        .collect::<Result<StringArray>>()?;
+
+    Ok(Arc::new(result) as ArrayRef)
+}

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

@@ -16,12 +16,14 @@
 // under the License.
 
 pub mod ascii;
+pub mod char;
 
 use datafusion_expr::ScalarUDF;
 use datafusion_functions::make_udf_function;
 use std::sync::Arc;
 
 make_udf_function!(ascii::SparkAscii, ascii);
+make_udf_function!(char::SparkChar, char);
 
 pub mod expr_fn {
     use datafusion_functions::export_functions;
@@ -31,8 +33,13 @@ pub mod expr_fn {
         "Returns the ASCII code point of the first character of string.",
         arg1
     ));
+    export_functions!((
+        char,
+        "Returns the ASCII character having the binary equivalent to col. If col is larger than 256 the result is equivalent to char(col % 256).",
+        arg1
+    ));
 }
 
 pub fn functions() -> Vec<Arc<ScalarUDF>> {
-    vec![ascii()]
+    vec![ascii(), char()]
 }