Added lower

Cargo.toml

@@ -171,6 +171,7 @@ compute_substring = []
 compute_take = []
 compute_temporal = []
 compute_window = ["compute_concatenate"]
+compute_lower = []
 compute = [
     "compute_aggregate",
     "compute_arithmetics",
@@ -196,6 +197,7 @@ compute = [
     "compute_take",
     "compute_temporal",
     "compute_window",
+    "compute_lower",
 ]
 # base64 + io_ipc because arrow schemas are stored as base64-encoded ipc format.
 io_parquet = ["parquet2", "io_ipc", "base64", "futures"]
@@ -298,4 +300,4 @@ harness = false
 
 [[bench]]
 name = "bitwise"
-harness = false
+harness = false

src/compute/lower.rs

@@ -0,0 +1,67 @@
+// 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.
+
+//! Defines kernel to extract a lower case of a \[Large\]StringArray
+
+use super::utils::utf8_apply;
+use crate::array::*;
+use crate::{
+    datatypes::DataType,
+    error::{ArrowError, Result},
+};
+
+/// Returns a new `Array` where each of each of the elements is lower-cased.
+/// this function errors when the passed array is not a \[Large\]String array.
+pub fn lower(array: &dyn Array) -> Result<Box<dyn Array>> {
+    match array.data_type() {
+        DataType::LargeUtf8 => Ok(Box::new(utf8_apply(
+            str::to_lowercase,
+            array
+                .as_any()
+                .downcast_ref::<Utf8Array<i64>>()
+                .expect("A large string is expected"),
+        ))),
+        DataType::Utf8 => Ok(Box::new(utf8_apply(
+            str::to_lowercase,
+            array
+                .as_any()
+                .downcast_ref::<Utf8Array<i32>>()
+                .expect("A string is expected"),
+        ))),
+        _ => Err(ArrowError::InvalidArgumentError(format!(
+            "lower does not support type {:?}",
+            array.data_type()
+        ))),
+    }
+}
+
+/// Checks if an array of type `datatype` can perform lower operation
+///
+/// # Examples
+/// ```
+/// use arrow2::compute::lower::can_lower;
+/// use arrow2::datatypes::{DataType};
+///
+/// let data_type = DataType::Utf8;
+/// assert_eq!(can_lower(&data_type), true);
+///
+/// let data_type = DataType::Null;
+/// assert_eq!(can_lower(&data_type), false);
+/// ```
+pub fn can_lower(data_type: &DataType) -> bool {
+    matches!(data_type, DataType::LargeUtf8 | DataType::Utf8)
+}

src/compute/mod.rs

@@ -58,6 +58,9 @@ pub mod like;
 #[cfg(feature = "compute_limit")]
 #[cfg_attr(docsrs, doc(cfg(feature = "compute_limit")))]
 pub mod limit;
+#[cfg(feature = "compute_lower")]
+#[cfg_attr(docsrs, doc(cfg(feature = "compute_lower")))]
+pub mod lower;
 #[cfg(feature = "compute_merge_sort")]
 #[cfg_attr(docsrs, doc(cfg(feature = "compute_merge_sort")))]
 pub mod merge_sort;

src/compute/utils.rs

@@ -30,6 +30,14 @@ pub fn unary_utf8_boolean<O: Offset, F: Fn(&str) -> bool>(
     BooleanArray::from_data(DataType::Boolean, values, validity)
 }
 
+/// utf8_apply will apply `Fn(&str) -> String` to every value in Utf8Array.
+pub fn utf8_apply<O: Offset, F: Fn(&str) -> String>(f: F, array: &Utf8Array<O>) -> Utf8Array<O> {
+    let iter = array.values_iter().map(f);
+
+    let new = Utf8Array::<O>::from_trusted_len_values_iter(iter);
+    new.with_validity(array.validity().cloned())
+}
+
 // Errors iff the two arrays have a different length.
 #[inline]
 pub fn check_same_len(lhs: &dyn Array, rhs: &dyn Array) -> Result<()> {

tests/it/compute/lower.rs

@@ -0,0 +1,186 @@
+use arrow2::{array::*, compute::lower::*, error::Result};
+
+fn with_nulls_utf8<O: Offset>() -> Result<()> {
+    let cases = vec![
+        // identity
+        (
+            vec![Some("hello"), None, Some("world")],
+            vec![Some("hello"), None, Some("world")],
+        ),
+        // part of input
+        (
+            vec![Some("Hello"), None, Some("wOrld")],
+            vec![Some("hello"), None, Some("world")],
+        ),
+        // all input
+        (
+            vec![Some("HELLO"), None, Some("WORLD")],
+            vec![Some("hello"), None, Some("world")],
+        ),
+        // UTF8 characters
+        (
+            vec![
+                None,
+                Some("السلام عليكم"),
+                Some("Dobrý den"),
+                Some("שָׁלוֹם"),
+                Some("नमस्ते"),
+                Some("こんにちは"),
+                Some("안녕하세요"),
+                Some("你好"),
+                Some("Olá"),
+                Some("Здравствуйте"),
+                Some("Hola"),
+            ],
+            vec![
+                None,
+                Some("السلام عليكم"),
+                Some("dobrý den"),
+                Some("שָׁלוֹם"),
+                Some("नमस्ते"),
+                Some("こんにちは"),
+                Some("안녕하세요"),
+                Some("你好"),
+                Some("olá"),
+                Some("здравствуйте"),
+                Some("hola"),
+            ],
+        ),
+    ];
+
+    cases
+        .into_iter()
+        .try_for_each::<_, Result<()>>(|(array, expected)| {
+            let array = Utf8Array::<O>::from(&array);
+            let result = lower(&array)?;
+            assert_eq!(array.len(), result.len());
+
+            let result = result.as_any().downcast_ref::<Utf8Array<O>>().unwrap();
+            let expected = Utf8Array::<O>::from(&expected);
+
+            assert_eq!(&expected, result);
+            Ok(())
+        })?;
+
+    Ok(())
+}
+
+#[test]
+fn with_nulls_string() -> Result<()> {
+    with_nulls_utf8::<i32>()
+}
+
+#[test]
+fn with_nulls_large_string() -> Result<()> {
+    with_nulls_utf8::<i64>()
+}
+
+fn without_nulls_utf8<O: Offset>() -> Result<()> {
+    let cases = vec![
+        // identity
+        (vec!["hello", "world"], vec!["hello", "world"]),
+        // part of input
+        (vec!["Hello", "wOrld"], vec!["hello", "world"]),
+        // all input
+        (vec!["HELLO", "WORLD"], vec!["hello", "world"]),
+        // UTF8 characters
+        (
+            vec![
+                "السلام عليكم",
+                "Dobrý den",
+                "שָׁלוֹם",
+                "नमस्ते",
+                "こんにちは",
+                "안녕하세요",
+                "你好",
+                "Olá",
+                "Здравствуйте",
+                "Hola",
+            ],
+            vec![
+                "السلام عليكم",
+                "dobrý den",
+                "שָׁלוֹם",
+                "नमस्ते",
+                "こんにちは",
+                "안녕하세요",
+                "你好",
+                "olá",
+                "здравствуйте",
+                "hola",
+            ],
+        ),
+    ];
+
+    cases
+        .into_iter()
+        .try_for_each::<_, Result<()>>(|(array, expected)| {
+            let array = Utf8Array::<O>::from_slice(&array);
+            let result = lower(&array)?;
+            assert_eq!(array.len(), result.len());
+
+            let result = result.as_any().downcast_ref::<Utf8Array<O>>().unwrap();
+            let expected = Utf8Array::<O>::from_slice(&expected);
+            assert_eq!(&expected, result);
+            Ok(())
+        })?;
+
+    Ok(())
+}
+
+#[test]
+fn without_nulls_string() -> Result<()> {
+    without_nulls_utf8::<i32>()
+}
+
+#[test]
+fn without_nulls_large_string() -> Result<()> {
+    without_nulls_utf8::<i64>()
+}
+
+#[test]
+fn consistency() {
+    use arrow2::datatypes::DataType::*;
+    use arrow2::datatypes::TimeUnit;
+    let datatypes = vec![
+        Null,
+        Boolean,
+        UInt8,
+        UInt16,
+        UInt32,
+        UInt64,
+        Int8,
+        Int16,
+        Int32,
+        Int64,
+        Float32,
+        Float64,
+        Timestamp(TimeUnit::Second, None),
+        Timestamp(TimeUnit::Millisecond, None),
+        Timestamp(TimeUnit::Microsecond, None),
+        Timestamp(TimeUnit::Nanosecond, None),
+        Time64(TimeUnit::Microsecond),
+        Time64(TimeUnit::Nanosecond),
+        Date32,
+        Time32(TimeUnit::Second),
+        Time32(TimeUnit::Millisecond),
+        Date64,
+        Utf8,
+        LargeUtf8,
+        Binary,
+        LargeBinary,
+        Duration(TimeUnit::Second),
+        Duration(TimeUnit::Millisecond),
+        Duration(TimeUnit::Microsecond),
+        Duration(TimeUnit::Nanosecond),
+    ];
+
+    datatypes.into_iter().for_each(|d1| {
+        let array = new_null_array(d1.clone(), 10);
+        if can_lower(&d1) {
+            assert!(lower(array.as_ref()).is_ok());
+        } else {
+            assert!(lower(array.as_ref()).is_err());
+        }
+    });
+}

tests/it/compute/mod.rs

@@ -28,6 +28,8 @@ mod length;
 mod like;
 #[cfg(feature = "compute_limit")]
 mod limit;
+#[cfg(feature = "compute_lower")]
+mod lower;
 #[cfg(feature = "compute_merge_sort")]
 mod merge_sort;
 #[cfg(feature = "compute_partition")]