[GitHub] [arrow] nevi-me commented on a change in pull request #7878: ARROW-9617: [Rust] [DataFusion] Add length of string array

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

nevi-me commented on a change in pull request #7878:
URL: https://github.com/apache/arrow/pull/7878#discussion_r464059368



##########
File path: rust/arrow/src/compute/kernels/length.rs
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@@ -0,0 +1,197 @@
+// 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 for length of a string array
+
+use crate::array::*;
+use crate::{
+    datatypes::DataType,
+    datatypes::UInt32Type,
+    error::{ArrowError, Result},
+};
+use std::sync::Arc;
+
+fn as_u32_le(array: &[u8]) -> u32 {
+    // le: little endian
+    ((array[0] as u32) << 0)
+        + ((array[1] as u32) << 8)
+        + ((array[2] as u32) << 16)
+        + ((array[3] as u32) << 24)
+}
+
+/// Returns an array of UInt32 denoting the number of characters in each string in the array.
+///
+/// * this only accepts StringArray
+/// * lenght of null is null.
+/// * length of utf8 with more than one code point is the number of code points
+pub fn length(array: &Array) -> Result<UInt32Array> {
+    match array.data_type() {
+        DataType::Utf8 => {
+            // note: offsets are stored as u8, but they can be interpreted as u32
+            let offsets = array.data_ref().clone().buffers()[0].clone();
+            let offsets = offsets.data();
+
+            let mut builder = UInt32BufferBuilder::new(array.len());
+
+            let mut previous_offset = 0;
+            // the first u32 offset is always 0 and is not needed to compute lengths
+            for i in (4..offsets.len()).step_by(4) {
+                // interpret 4 u8 as 1 u32:
+                let offset = as_u32_le(&offsets[i..(i + 4)]);

Review comment:
       You can use `typed_data` to do the byte conversion for you (https://github.com/apache/arrow/blob/e771b94e63e1287bf61ee905f39799aa433f4326/rust/arrow/src/buffer.rs#L247). Then you could maybe iterate through a window (https://github.com/apache/arrow/blob/e771b94e63e1287bf61ee905f39799aa433f4326/rust/arrow/src/array/builder.rs#L756)




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