datafusion_functions/unicode/
reverse.rs

// 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;
use std::sync::{Arc, OnceLock};

use crate::utils::{make_scalar_function, utf8_to_str_type};
use arrow::array::{
    Array, ArrayAccessor, ArrayIter, ArrayRef, AsArray, GenericStringArray,
    OffsetSizeTrait,
};
use arrow::datatypes::DataType;
use datafusion_common::{exec_err, Result};
use datafusion_expr::scalar_doc_sections::DOC_SECTION_STRING;
use datafusion_expr::{
    ColumnarValue, Documentation, ScalarUDFImpl, Signature, Volatility,
};
use DataType::{LargeUtf8, Utf8, Utf8View};

#[derive(Debug)]
pub struct ReverseFunc {
    signature: Signature,
}

impl Default for ReverseFunc {
    fn default() -> Self {
        Self::new()
    }
}

impl ReverseFunc {
    pub fn new() -> Self {
        use DataType::*;
        Self {
            signature: Signature::uniform(
                1,
                vec![Utf8View, Utf8, LargeUtf8],
                Volatility::Immutable,
            ),
        }
    }
}

impl ScalarUDFImpl for ReverseFunc {
    fn as_any(&self) -> &dyn Any {
        self
    }

    fn name(&self) -> &str {
        "reverse"
    }

    fn signature(&self) -> &Signature {
        &self.signature
    }

    fn return_type(&self, arg_types: &[DataType]) -> Result<DataType> {
        utf8_to_str_type(&arg_types[0], "reverse")
    }

    fn invoke(&self, args: &[ColumnarValue]) -> Result<ColumnarValue> {
        match args[0].data_type() {
            Utf8 | Utf8View => make_scalar_function(reverse::<i32>, vec![])(args),
            LargeUtf8 => make_scalar_function(reverse::<i64>, vec![])(args),
            other => {
                exec_err!("Unsupported data type {other:?} for function reverse")
            }
        }
    }

    fn documentation(&self) -> Option<&Documentation> {
        Some(get_reverse_doc())
    }
}

static DOCUMENTATION: OnceLock<Documentation> = OnceLock::new();

fn get_reverse_doc() -> &'static Documentation {
    DOCUMENTATION.get_or_init(|| {
        Documentation::builder()
            .with_doc_section(DOC_SECTION_STRING)
            .with_description("Reverses the character order of a string.")
            .with_syntax_example("reverse(str)")
            .with_sql_example(
                r#"```sql
> select reverse('datafusion');
+-----------------------------+
| reverse(Utf8("datafusion")) |
+-----------------------------+
| noisufatad                  |
+-----------------------------+
```"#,
            )
            .with_standard_argument("str", Some("String"))
            .build()
            .unwrap()
    })
}

/// Reverses the order of the characters in the string.
/// reverse('abcde') = 'edcba'
/// The implementation uses UTF-8 code points as characters
pub fn reverse<T: OffsetSizeTrait>(args: &[ArrayRef]) -> Result<ArrayRef> {
    if args[0].data_type() == &Utf8View {
        reverse_impl::<T, _>(args[0].as_string_view())
    } else {
        reverse_impl::<T, _>(args[0].as_string::<T>())
    }
}

fn reverse_impl<'a, T: OffsetSizeTrait, V: ArrayAccessor<Item = &'a str>>(
    string_array: V,
) -> Result<ArrayRef> {
    let result = ArrayIter::new(string_array)
        .map(|string| string.map(|string: &str| string.chars().rev().collect::<String>()))
        .collect::<GenericStringArray<T>>();

    Ok(Arc::new(result) as ArrayRef)
}

#[cfg(test)]
mod tests {
    use arrow::array::{Array, LargeStringArray, StringArray};
    use arrow::datatypes::DataType::{LargeUtf8, Utf8};

    use datafusion_common::{Result, ScalarValue};
    use datafusion_expr::{ColumnarValue, ScalarUDFImpl};

    use crate::unicode::reverse::ReverseFunc;
    use crate::utils::test::test_function;

    macro_rules! test_reverse {
        ($INPUT:expr, $EXPECTED:expr) => {
            test_function!(
                ReverseFunc::new(),
                &[ColumnarValue::Scalar(ScalarValue::Utf8($INPUT))],
                $EXPECTED,
                &str,
                Utf8,
                StringArray
            );

            test_function!(
                ReverseFunc::new(),
                &[ColumnarValue::Scalar(ScalarValue::LargeUtf8($INPUT))],
                $EXPECTED,
                &str,
                LargeUtf8,
                LargeStringArray
            );

            test_function!(
                ReverseFunc::new(),
                &[ColumnarValue::Scalar(ScalarValue::Utf8View($INPUT))],
                $EXPECTED,
                &str,
                Utf8,
                StringArray
            );
        };
    }

    #[test]
    fn test_functions() -> Result<()> {
        test_reverse!(Some("abcde".into()), Ok(Some("edcba")));
        test_reverse!(Some("loẅks".into()), Ok(Some("sk̈wol")));
        test_reverse!(Some("loẅks".into()), Ok(Some("sk̈wol")));
        test_reverse!(None, Ok(None));
        #[cfg(not(feature = "unicode_expressions"))]
        test_reverse!(
            Some("abcde".into()),
            internal_err!(
                "function reverse requires compilation with feature flag: unicode_expressions."
            ),
        );

        Ok(())
    }
}
datafusion_functions/unicode/reverse.rs

datafusion_functions/unicode/
reverse.rs
