[GitHub] [arrow] jhorstmann commented on a change in pull request #8262: ARROW-10040: [Rust] Iterate over and combine boolean buffers with arbitrary offsets

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

jhorstmann commented on a change in pull request #8262:
URL: https://github.com/apache/arrow/pull/8262#discussion_r501270867



##########
File path: rust/arrow/src/util/bit_chunk_iterator.rs
##########
@@ -0,0 +1,223 @@
+// 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 crate::util::bit_util::ceil;
+use std::fmt::Debug;
+
+#[derive(Debug)]
+pub struct BitChunks<'a> {
+    buffer: &'a Buffer,
+    raw_data: *const u8,
+    offset: usize,
+    chunk_len: usize,
+    remainder_len: usize,
+}
+
+impl<'a> BitChunks<'a> {
+    pub fn new(buffer: &'a Buffer, offset: usize, len: usize) -> Self {
+        assert!(ceil(offset + len, 8) <= buffer.len() * 8);
+
+        let byte_offset = offset / 8;
+        let offset = offset % 8;
+
+        let raw_data = unsafe { buffer.raw_data().add(byte_offset) };
+
+        let chunk_bits = 64;
+
+        let chunk_len = len / chunk_bits;
+        let remainder_len = len & (chunk_bits - 1);
+
+        BitChunks::<'a> {
+            buffer: &buffer,
+            raw_data,
+            offset,
+            chunk_len,
+            remainder_len,
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct BitChunkIterator<'a> {
+    buffer: &'a Buffer,
+    raw_data: *const u8,
+    offset: usize,
+    chunk_len: usize,
+    index: usize,
+}
+
+impl<'a> BitChunks<'a> {
+    #[inline]
+    pub fn remainder_len(&self) -> usize {
+        self.remainder_len
+    }
+
+    #[inline]
+    pub fn remainder_bits(&self) -> u64 {
+        let bit_len = self.remainder_len;
+        if bit_len == 0 {
+            0
+        } else {
+            let byte_len = ceil(bit_len, 8);
+
+            let mut bits = 0;
+            for i in 0..byte_len {
+                let byte = unsafe {
+                    std::ptr::read(
+                        self.raw_data
+                            .add(self.chunk_len * std::mem::size_of::<u64>() + i),
+                    )
+                };
+                bits |= (byte as u64) << (i * 8);
+            }
+
+            let offset = self.offset as u64;
+
+            (bits >> offset) & ((1 << bit_len) - 1)
+        }
+    }
+
+    #[inline]
+    pub fn iter(&self) -> BitChunkIterator<'a> {
+        BitChunkIterator::<'a> {
+            buffer: self.buffer,
+            raw_data: self.raw_data,
+            offset: self.offset,
+            chunk_len: self.chunk_len,
+            index: 0,
+        }
+    }
+}
+
+impl<'a> IntoIterator for BitChunks<'a> {
+    type Item = u64;
+    type IntoIter = BitChunkIterator<'a>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
+impl Iterator for BitChunkIterator<'_> {
+    type Item = u64;
+
+    #[inline]
+    fn next(&mut self) -> Option<u64> {
+        if self.index >= self.chunk_len {
+            return None;
+        }
+
+        // cast to *const u64 should be fine since we are using read_unaligned
+        #[allow(clippy::cast_ptr_alignment)]
+        let current = unsafe {
+            std::ptr::read_unaligned((self.raw_data as *const u64).add(self.index))
+        };
+
+        let combined = if self.offset == 0 {
+            current
+        } else {
+            // cast to *const u64 should be fine since we are using read_unaligned
+            #[allow(clippy::cast_ptr_alignment)]
+            let next = unsafe {
+                std::ptr::read_unaligned(
+                    (self.raw_data as *const u64).add(self.index + 1),
+                )
+            };
+            current >> self.offset
+                | (next & ((1 << self.offset) - 1)) << (64 - self.offset)
+        };
+
+        self.index += 1;
+
+        Some(combined)
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (
+            self.chunk_len - self.index,
+            Some(self.chunk_len - self.index),
+        )
+    }
+}
+
+impl ExactSizeIterator for BitChunkIterator<'_> {
+    #[inline]
+    fn len(&self) -> usize {
+        self.chunk_len - self.index
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::buffer::Buffer;
+
+    #[test]
+    fn test_iter_aligned() {

Review comment:
       Endianness also confuses me, but I think this test is correct. It might have been clearer if I had also written it using binary literals.
   
   We are counting bits starting from the least significant, since the data is using bytes it should not matter whether it's big- or little-endian. So bit 0 is the least significant bit in the first byte, bit 8 is the least significant in the second byte. The order in the u64 should be the same, so bit 8, starting from the least significant on the right is the 1 one from the second input byte. I hope this makes sense.




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