[GitHub] [arrow] nevi-me commented on a change in pull request #8199: ARROW-10019: [Rust] Add substring kernel

[GitBox <gi...@apache.org>]

nevi-me commented on a change in pull request #8199:
URL: https://github.com/apache/arrow/pull/8199#discussion_r491702713



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File path: rust/arrow/src/compute/kernels/substring.rs
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@@ -0,0 +1,226 @@
+// 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 substring of a StringArray
+
+use crate::{array::*, buffer::Buffer, datatypes::ToByteSlice};
+use crate::{
+    datatypes::DataType,
+    error::{ArrowError, Result},
+};
+use std::sync::Arc;
+
+/// Returns an ArrayRef with a substring starting from `start` and with optional length `length` of each of the elements in `array`.
+/// `start` can be negative, in which case the start counts from the end of the string.
+pub fn substring(array: &Array, start: i32, length: &Option<u32>) -> Result<ArrayRef> {
+    match array.data_type() {
+        DataType::Utf8 => {
+            // compute current offsets
+            let offsets = array.data_ref().clone().buffers()[0].clone();
+            let offsets: &[u32] = unsafe { offsets.typed_data::<u32>() };
+
+            // compute null bitmap (copy)
+            let null_bit_buffer = array.data_ref().null_buffer().map(|e| e.clone());
+
+            // compute values
+            let values = &array.data_ref().buffers()[1];
+            let data = values.data();
+
+            let mut new_values = Vec::new(); // we have no way to estimate how much this will be.
+            let mut new_offsets = Vec::with_capacity(array.len() + 1);
+
+            let mut length_so_far = 0i32;
+            new_offsets.push(length_so_far);
+            (0..array.len()).for_each(|i| {
+                // the length of this entry
+                let lenght_i = offsets[i + 1] - offsets[i];
+                // compute where we should start slicing this entry
+                let start = offsets[i] as i32
+                    + if start >= 0 {
+                        start
+                    } else {
+                        lenght_i as i32 + start
+                    };
+
+                let start =
+                    start.max(offsets[i] as i32).min(offsets[i + 1] as i32) as usize;
+                // compute the lenght of the slice
+                let length = length
+                    .unwrap_or(lenght_i)
+                    // .max(0) is not needed as it is guaranteed
+                    .min(offsets[i + 1] - start as u32) // so we do not go beyond this entry
+                    as i32;
+
+                length_so_far += length;
+
+                new_offsets.push(length_so_far);
+                new_values.extend_from_slice(&data[start..start + length as usize]);
+            });
+
+            let data = ArrayData::new(
+                DataType::Utf8,
+                array.len(),
+                None,
+                null_bit_buffer,
+                0,
+                vec![
+                    Buffer::from(new_offsets.to_byte_slice()),
+                    Buffer::from(&new_values[..]),
+                ],
+                vec![],
+            );
+            Ok(Arc::new(StringArray::from(Arc::new(data))))
+        }
+        _ => Err(ArrowError::ComputeError(format!(

Review comment:
       We should also support `DataType::LargeUtf8`. We're likely to get a scenario where Arrow users who deal with large data would want to use the `LargeList` and `LargeString` arrays by default (something I would also do), so it's ideal to support both string arrays from the onset going forward.




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