perf: Optimize scalar path for ascii function (#19951)

## Which issue does this PR close?

<!--
We generally require a GitHub issue to be filed for all bug fixes and
enhancements and this helps us generate change logs for our releases.
You can link an issue to this PR using the GitHub syntax. For example
`Closes #123` indicates that this PR will close issue #123.
-->

- Part of apache/datafusion-comet#2986.

## Rationale for this change


The `ascii` function currently converts scalar inputs to arrays before
processing via `make_scalar_function`. Adding a scalar fast path avoids
this overhead and improves performance.

<!--
Why are you proposing this change? If this is already explained clearly
in the issue then this section is not needed.
Explaining clearly why changes are proposed helps reviewers understand
your changes and offer better suggestions for fixes.
-->

## What changes are included in this PR?

1. Refactored `invoke_with_args` to use `match` statement for handling
both scalar and array inputs
2. Added scalar fast path for `Utf8`, `LargeUtf8`, and `Utf8View` scalar
inputs

| Type | Before | After | Speedup |
|------|--------|-------|---------|
| **ascii/scalar_utf8** | 234 ns | 56 ns | **4.2x** |
| **ascii/scalar_utf8view** | 206 ns | 57 ns | **3.6x** |

<!--
There is no need to duplicate the description in the issue here but it
is sometimes worth providing a summary of the individual changes in this
PR.
-->

## Are these changes tested?

Yes

<!--
We typically require tests for all PRs in order to:
1. Prevent the code from being accidentally broken by subsequent changes
2. Serve as another way to document the expected behavior of the code

If tests are not included in your PR, please explain why (for example,
are they covered by existing tests)?
-->

## Are there any user-facing changes?

No

<!--
If there are user-facing changes then we may require documentation to be
updated before approving the PR.
-->

<!--
If there are any breaking changes to public APIs, please add the `api
change` label.
-->

Author: kumarUjjawal

datafusion/functions/benches/ascii.rs

@@ -20,14 +20,43 @@ mod helper;
 
 use arrow::datatypes::{DataType, Field};
 use criterion::{Criterion, criterion_group, criterion_main};
+use datafusion_common::ScalarValue;
 use datafusion_common::config::ConfigOptions;
-use datafusion_expr::ScalarFunctionArgs;
+use datafusion_expr::{ColumnarValue, ScalarFunctionArgs};
 use helper::gen_string_array;
 use std::hint::black_box;
 use std::sync::Arc;
 
 fn criterion_benchmark(c: &mut Criterion) {
     let ascii = datafusion_functions::string::ascii();
+    let config_options = Arc::new(ConfigOptions::default());
+
+    // Scalar benchmarks (outside loop)
+    c.bench_function("ascii/scalar_utf8", |b| {
+        let args = ScalarFunctionArgs {
+            args: vec![ColumnarValue::Scalar(ScalarValue::Utf8(Some(
+                "hello".to_string(),
+            )))],
+            arg_fields: vec![Field::new("a", DataType::Utf8, false).into()],
+            number_rows: 1,
+            return_field: Field::new("f", DataType::Int32, true).into(),
+            config_options: Arc::clone(&config_options),
+        };
+        b.iter(|| black_box(ascii.invoke_with_args(args.clone()).unwrap()))
+    });
+
+    c.bench_function("ascii/scalar_utf8view", |b| {
+        let args = ScalarFunctionArgs {
+            args: vec![ColumnarValue::Scalar(ScalarValue::Utf8View(Some(
+                "hello".to_string(),
+            )))],
+            arg_fields: vec![Field::new("a", DataType::Utf8View, false).into()],
+            number_rows: 1,
+            return_field: Field::new("f", DataType::Int32, true).into(),
+            config_options: Arc::clone(&config_options),
+        };
+        b.iter(|| black_box(ascii.invoke_with_args(args.clone()).unwrap()))
+    });
 
     // All benches are single batch run with 8192 rows
     const N_ROWS: usize = 8192;

datafusion/functions/src/string/ascii.rs

@@ -15,12 +15,12 @@
 // specific language governing permissions and limitations
 // under the License.
 
-use crate::utils::make_scalar_function;
 use arrow::array::{ArrayRef, AsArray, Int32Array, StringArrayType};
 use arrow::datatypes::DataType;
 use arrow::error::ArrowError;
 use datafusion_common::types::logical_string;
-use datafusion_common::{Result, internal_err};
+use datafusion_common::utils::take_function_args;
+use datafusion_common::{Result, ScalarValue, internal_err};
 use datafusion_expr::{ColumnarValue, Documentation, TypeSignatureClass};
 use datafusion_expr::{ScalarFunctionArgs, ScalarUDFImpl, Signature, Volatility};
 use datafusion_expr_common::signature::Coercion;
@@ -91,7 +91,31 @@ impl ScalarUDFImpl for AsciiFunc {
     }
 
     fn invoke_with_args(&self, args: ScalarFunctionArgs) -> Result<ColumnarValue> {
-        make_scalar_function(ascii, vec![])(&args.args)
+        let [arg] = take_function_args(self.name(), args.args)?;
+
+        match arg {
+            ColumnarValue::Scalar(scalar) => {
+                if scalar.is_null() {
+                    return Ok(ColumnarValue::Scalar(ScalarValue::Int32(None)));
+                }
+
+                match scalar {
+                    ScalarValue::Utf8(Some(s))
+                    | ScalarValue::LargeUtf8(Some(s))
+                    | ScalarValue::Utf8View(Some(s)) => {
+                        let result = s.chars().next().map_or(0, |c| c as i32);
+                        Ok(ColumnarValue::Scalar(ScalarValue::Int32(Some(result))))
+                    }
+                    _ => {
+                        internal_err!(
+                            "Unexpected data type {:?} for function ascii",
+                            scalar.data_type()
+                        )
+                    }
+                }
+            }
+            ColumnarValue::Array(array) => Ok(ColumnarValue::Array(ascii(&[array])?)),
+        }
     }
 
     fn documentation(&self) -> Option<&Documentation> {