[GitHub] [arrow] alamb commented on a change in pull request #8598: ARROW-10500: [Rust] Refactor bit slice, bit view iterator for array buffers

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

alamb commented on a change in pull request #8598:
URL: https://github.com/apache/arrow/pull/8598#discussion_r518730329



##########
File path: rust/arrow/src/util/bit_slice_iterator.rs
##########
@@ -0,0 +1,210 @@
+// 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 crate::buffer::Buffer;
+
+use bitvec::prelude::*;
+use bitvec::slice::ChunksExact;
+
+use std::fmt::Debug;
+
+///
+/// Bit slice representation of buffer data

Review comment:
       I wasn't familiar with the term "bit slice" before reading this PR (It is mentioned in the docs of https://docs.rs/bitvec/0.19.4/bitvec/, where perhaps the term came from.
   
   ```suggestion
   /// Bit slice representation of buffer data. A bit slice is a
   /// view on top of a buffer of bytes which can be used to
   /// access each bit.
   ```

##########
File path: rust/arrow/src/buffer.rs
##########
@@ -259,16 +260,20 @@ impl Buffer {
     /// Returns a slice of this buffer starting at a certain bit offset.
     /// If the offset is byte-aligned the returned buffer is a shallow clone,
     /// otherwise a new buffer is allocated and filled with a copy of the bits in the range.
-    pub fn bit_slice(&self, offset: usize, len: usize) -> Self {
-        if offset % 8 == 0 && len % 8 == 0 {
-            return self.slice(offset / 8);
+    pub fn bit_view(&self, offset_in_bits: usize, len_in_bits: usize) -> Self {
+        if offset_in_bits % 8 == 0 && len_in_bits % 8 == 0 {
+            self.slice(offset_in_bits / 8)

Review comment:
       I don't fully understand why this doesn't need to refer to `len_in_bits` -- how do we know that `len_in_bits` covers the entire buffer? Maybe this should be `self.slice(len_in_bits/8)`?

##########
File path: rust/arrow/src/util/bit_slice_iterator.rs
##########
@@ -0,0 +1,210 @@
+// 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 crate::buffer::Buffer;
+
+use bitvec::prelude::*;
+use bitvec::slice::ChunksExact;
+
+use std::fmt::Debug;
+
+///
+/// Bit slice representation of buffer data
+#[derive(Debug)]
+pub struct BufferBitSlice<'a> {
+    buffer_data: &'a [u8],
+    bit_slice: &'a BitSlice<LocalBits, u8>,
+}
+
+impl<'a> BufferBitSlice<'a> {
+    ///
+    /// Creates a bit slice over the given data
+    #[inline]
+    pub fn new(buffer_data: &'a [u8]) -> Self {
+        let bit_slice = BitSlice::<LocalBits, _>::from_slice(buffer_data).unwrap();
+
+        BufferBitSlice {
+            buffer_data,
+            bit_slice,
+        }
+    }
+
+    ///
+    /// Returns immutable view with the given offset in bits and length in bits.

Review comment:
       ```suggestion
       /// Returns a new bit slice relative to self, with the given offset in bits and length in bits.
   ```




----------------------------------------------------------------
This is an automated message from the Apache Git Service.
To respond to the message, please log on to GitHub and use the
URL above to go to the specific comment.

For queries about this service, please contact Infrastructure at:
users@infra.apache.org