perf: Optimize repeat function for scalar and array fast (#19976)

## 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 `repeat` function currently converts scalar inputs to arrays before
processing via `make_scalar_function`. Adding a scalar fast path avoids
this overhead and improves performance for constant folding and scalar
expression evaluation.

<!--
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 handle scalar inputs directly
without array conversion
2. Added scalar fast path for `Utf8`, `LargeUtf8`, and `Utf8View` scalar
inputs
3. Added array fast path that skips per-element null checks when
`null_count() == 0`

| Type | Before | After | Speedup |
|------|--------|-------|---------|
| **repeat/scalar_utf8** | 519 ns | 91 ns | **5.7x** |
| **repeat/scalar_utf8view** | 437 ns | 91 ns | **4.8x** |

<!--
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/repeat.rs

@@ -24,6 +24,7 @@ use arrow::util::bench_util::{
 };
 use criterion::{Criterion, SamplingMode, criterion_group, criterion_main};
 use datafusion_common::DataFusionError;
+use datafusion_common::ScalarValue;
 use datafusion_common::config::ConfigOptions;
 use datafusion_expr::{ColumnarValue, ScalarFunctionArgs};
 use datafusion_functions::string;
@@ -80,6 +81,44 @@ fn invoke_repeat_with_args(
 }
 
 fn criterion_benchmark(c: &mut Criterion) {
+    let repeat_fn = string::repeat();
+    let config_options = Arc::new(ConfigOptions::default());
+
+    // Scalar benchmarks (outside loop)
+    c.bench_function("repeat/scalar_utf8", |b| {
+        let args = ScalarFunctionArgs {
+            args: vec![
+                ColumnarValue::Scalar(ScalarValue::Utf8(Some("hello".to_string()))),
+                ColumnarValue::Scalar(ScalarValue::Int64(Some(3))),
+            ],
+            arg_fields: vec![
+                Field::new("a", DataType::Utf8, false).into(),
+                Field::new("b", DataType::Int64, false).into(),
+            ],
+            number_rows: 1,
+            return_field: Field::new("f", DataType::Utf8, true).into(),
+            config_options: Arc::clone(&config_options),
+        };
+        b.iter(|| black_box(repeat_fn.invoke_with_args(args.clone()).unwrap()))
+    });
+
+    c.bench_function("repeat/scalar_utf8view", |b| {
+        let args = ScalarFunctionArgs {
+            args: vec![
+                ColumnarValue::Scalar(ScalarValue::Utf8View(Some("hello".to_string()))),
+                ColumnarValue::Scalar(ScalarValue::Int64(Some(3))),
+            ],
+            arg_fields: vec![
+                Field::new("a", DataType::Utf8View, false).into(),
+                Field::new("b", DataType::Int64, false).into(),
+            ],
+            number_rows: 1,
+            return_field: Field::new("f", DataType::Utf8, true).into(),
+            config_options: Arc::clone(&config_options),
+        };
+        b.iter(|| black_box(repeat_fn.invoke_with_args(args.clone()).unwrap()))
+    });
+
     for size in [1024, 4096] {
         // REPEAT 3 TIMES
         let repeat_times = 3;

datafusion/functions/src/string/repeat.rs

@@ -18,16 +18,17 @@
 use std::any::Any;
 use std::sync::Arc;
 
-use crate::utils::{make_scalar_function, utf8_to_str_type};
+use crate::utils::utf8_to_str_type;
 use arrow::array::{
-    ArrayRef, AsArray, GenericStringArray, GenericStringBuilder, Int64Array,
+    Array, ArrayRef, AsArray, GenericStringArray, GenericStringBuilder, Int64Array,
     OffsetSizeTrait, StringArrayType, StringViewArray,
 };
 use arrow::datatypes::DataType;
 use arrow::datatypes::DataType::{LargeUtf8, Utf8, Utf8View};
 use datafusion_common::cast::as_int64_array;
 use datafusion_common::types::{NativeType, logical_int64, logical_string};
-use datafusion_common::{DataFusionError, Result, exec_err};
+use datafusion_common::utils::take_function_args;
+use datafusion_common::{DataFusionError, Result, ScalarValue, exec_err, internal_err};
 use datafusion_expr::{ColumnarValue, Documentation, Volatility};
 use datafusion_expr::{ScalarFunctionArgs, ScalarUDFImpl, Signature};
 use datafusion_expr_common::signature::{Coercion, TypeSignatureClass};
@@ -99,39 +100,112 @@ impl ScalarUDFImpl for RepeatFunc {
     }
 
     fn invoke_with_args(&self, args: ScalarFunctionArgs) -> Result<ColumnarValue> {
-        make_scalar_function(repeat, vec![])(&args.args)
+        let return_type = args.return_field.data_type().clone();
+        let [string_arg, count_arg] = take_function_args(self.name(), args.args)?;
+
+        // Early return if either argument is a scalar null
+        if let ColumnarValue::Scalar(s) = &string_arg
+            && s.is_null()
+        {
+            return Ok(ColumnarValue::Scalar(ScalarValue::try_from(&return_type)?));
+        }
+        if let ColumnarValue::Scalar(c) = &count_arg
+            && c.is_null()
+        {
+            return Ok(ColumnarValue::Scalar(ScalarValue::try_from(&return_type)?));
+        }
+
+        match (&string_arg, &count_arg) {
+            (
+                ColumnarValue::Scalar(string_scalar),
+                ColumnarValue::Scalar(count_scalar),
+            ) => {
+                let count = match count_scalar {
+                    ScalarValue::Int64(Some(n)) => *n,
+                    _ => {
+                        return internal_err!(
+                            "Unexpected data type {:?} for repeat count",
+                            count_scalar.data_type()
+                        );
+                    }
+                };
+
+                let result = match string_scalar {
+                    ScalarValue::Utf8(Some(s)) | ScalarValue::Utf8View(Some(s)) => {
+                        ScalarValue::Utf8(Some(compute_repeat(
+                            s,
+                            count,
+                            i32::MAX as usize,
+                        )?))
+                    }
+                    ScalarValue::LargeUtf8(Some(s)) => ScalarValue::LargeUtf8(Some(
+                        compute_repeat(s, count, i64::MAX as usize)?,
+                    )),
+                    _ => {
+                        return internal_err!(
+                            "Unexpected data type {:?} for function repeat",
+                            string_scalar.data_type()
+                        );
+                    }
+                };
+
+                Ok(ColumnarValue::Scalar(result))
+            }
+            _ => {
+                let string_array = string_arg.to_array(args.number_rows)?;
+                let count_array = count_arg.to_array(args.number_rows)?;
+                Ok(ColumnarValue::Array(repeat(&string_array, &count_array)?))
+            }
+        }
     }
 
     fn documentation(&self) -> Option<&Documentation> {
         self.doc()
     }
 }
 
+/// Computes repeat for a single string value with max size check
+#[inline]
+fn compute_repeat(s: &str, count: i64, max_size: usize) -> Result<String> {
+    if count <= 0 {
+        return Ok(String::new());
+    }
+    let result_len = s.len().saturating_mul(count as usize);
+    if result_len > max_size {
+        return exec_err!(
+            "string size overflow on repeat, max size is {}, but got {}",
+            max_size,
+            result_len
+        );
+    }
+    Ok(s.repeat(count as usize))
+}
+
 /// Repeats string the specified number of times.
 /// repeat('Pg', 4) = 'PgPgPgPg'
-fn repeat(args: &[ArrayRef]) -> Result<ArrayRef> {
-    let number_array = as_int64_array(&args[1])?;
-    match args[0].data_type() {
+fn repeat(string_array: &ArrayRef, count_array: &ArrayRef) -> Result<ArrayRef> {
+    let number_array = as_int64_array(count_array)?;
+    match string_array.data_type() {
         Utf8View => {
-            let string_view_array = args[0].as_string_view();
+            let string_view_array = string_array.as_string_view();
             repeat_impl::<i32, &StringViewArray>(
                 &string_view_array,
                 number_array,
                 i32::MAX as usize,
             )
         }
         Utf8 => {
-            let string_array = args[0].as_string::<i32>();
+            let string_arr = string_array.as_string::<i32>();
             repeat_impl::<i32, &GenericStringArray<i32>>(
-                &string_array,
+                &string_arr,
                 number_array,
                 i32::MAX as usize,
             )
         }
         LargeUtf8 => {
-            let string_array = args[0].as_string::<i64>();
+            let string_arr = string_array.as_string::<i64>();
             repeat_impl::<i64, &GenericStringArray<i64>>(
-                &string_array,
+                &string_arr,
                 number_array,
                 i64::MAX as usize,
             )
@@ -150,7 +224,7 @@ fn repeat_impl<'a, T, S>(
 ) -> Result<ArrayRef>
 where
     T: OffsetSizeTrait,
-    S: StringArrayType<'a>,
+    S: StringArrayType<'a> + 'a,
 {
     let mut total_capacity = 0;
     let mut max_item_capacity = 0;
@@ -181,37 +255,55 @@ where
     // Reusable buffer to avoid allocations in string.repeat()
     let mut buffer = Vec::<u8>::with_capacity(max_item_capacity);
 
-    string_array
-        .iter()
-        .zip(number_array.iter())
-        .for_each(|(string, number)| {
+    // Helper function to repeat a string into a buffer using doubling strategy
+    // count must be > 0
+    #[inline]
+    fn repeat_to_buffer(buffer: &mut Vec<u8>, string: &str, count: usize) {
+        buffer.clear();
+        if !string.is_empty() {
+            let src = string.as_bytes();
+            // Initial copy
+            buffer.extend_from_slice(src);
+            // Doubling strategy: copy what we have so far until we reach the target
+            while buffer.len() < src.len() * count {
+                let copy_len = buffer.len().min(src.len() * count - buffer.len());
+                // SAFETY: we're copying valid UTF-8 bytes that we already verified
+                buffer.extend_from_within(..copy_len);
+            }
+        }
+    }
+
+    // Fast path: no nulls in either array
+    if string_array.null_count() == 0 && number_array.null_count() == 0 {
+        for i in 0..string_array.len() {
+            // SAFETY: i is within bounds (0..len) and null_count() == 0 guarantees valid value
+            let string = unsafe { string_array.value_unchecked(i) };
+            let count = number_array.value(i);
+            if count > 0 {
+                repeat_to_buffer(&mut buffer, string, count as usize);
+                // SAFETY: buffer contains valid UTF-8 since we only copy from a valid &str
+                builder.append_value(unsafe { std::str::from_utf8_unchecked(&buffer) });
+            } else {
+                builder.append_value("");
+            }
+        }
+    } else {
+        // Slow path: handle nulls
+        for (string, number) in string_array.iter().zip(number_array.iter()) {
             match (string, number) {
-                (Some(string), Some(number)) if number >= 0 => {
-                    buffer.clear();
-                    let count = number as usize;
-                    if count > 0 && !string.is_empty() {
-                        let src = string.as_bytes();
-                        // Initial copy
-                        buffer.extend_from_slice(src);
-                        // Doubling strategy: copy what we have so far until we reach the target
-                        while buffer.len() < src.len() * count {
-                            let copy_len =
-                                buffer.len().min(src.len() * count - buffer.len());
-                            // SAFETY: we're copying valid UTF-8 bytes that we already verified
-                            buffer.extend_from_within(..copy_len);
-                        }
-                    }
-                    // SAFETY: buffer contains valid UTF-8 since we only ever copy from a valid &str
+                (Some(string), Some(count)) if count > 0 => {
+                    repeat_to_buffer(&mut buffer, string, count as usize);
+                    // SAFETY: buffer contains valid UTF-8 since we only copy from a valid &str
                     builder
                         .append_value(unsafe { std::str::from_utf8_unchecked(&buffer) });
                 }
                 (Some(_), Some(_)) => builder.append_value(""),
                 _ => builder.append_null(),
             }
-        });
-    let array = builder.finish();
+        }
+    }
 
-    Ok(Arc::new(array) as ArrayRef)
+    Ok(Arc::new(builder.finish()) as ArrayRef)
 }
 
 #[cfg(test)]