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

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

jhorstmann commented on a change in pull request #8199:
URL: https://github.com/apache/arrow/pull/8199#discussion_r489406369



##########
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.

Review comment:
       I think we can estimate the capacity as `length * array.len()` if the length is specified and `values.len() - (start*array.len())` otherwise.
   
   Hm, actually depends on the semantics of the `length` parameter, is it supposed to be a maximum length or should it be an error if `start+length` goes out of bounds of one entry?




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