Dandandan commented on code in PR #8854:
URL: https://github.com/apache/arrow-rs/pull/8854#discussion_r2542028664
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arrow-buffer/src/buffer/immutable.rs:
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@@ -115,6 +114,150 @@ impl Buffer {
Self::from(bytes)
}
+ /// Create a new [`Buffer`] by applying the bitwise operation `op` to two
input buffers.
+ ///
+ /// This function is highly optimized for bitwise operations on large
+ /// bitmaps by processing input buffers in chunks of 64 bits (8 bytes) at a
+ /// time, and thus is much faster than applying the operation bit by bit.
+ ///
+ /// # Notes:
+ /// * `op` takes two `u64` inputs and produces one `u64` output,
+ /// operating on 64 bits at a time. **It must only apply bitwise
operations
+ /// on the relevant bits, as the input `u64` may contain irrelevant bits
+ /// and may be processed differently on different endian architectures.**
+ /// * The inputs are treated as bitmaps, meaning that offsets and length
+ /// are specified in number of bits.
+ /// * The output always has zero offset
+ ///
+ /// # See Also
+ /// - [`Buffer::from_bitwise_unary_op`] for unary operations on a single
input buffer.
+ /// - [`apply_bitwise_binary_op`](bit_util::apply_bitwise_binary_op) for
in-place binary bitwise operations
+ ///
+ /// # Example: Create new [`Buffer`] from bitwise `AND` of two [`Buffer`]s
+ /// ```
+ /// # use arrow_buffer::Buffer;
+ /// let left = Buffer::from(&[0b11001100u8, 0b10111010u8]); // 2 bytes =
16 bits
+ /// let right = Buffer::from(&[0b10101010u8, 0b11011100u8, 0b11110000u8]);
// 3 bytes = 24 bits
+ /// // AND of the first 12 bits
+ /// let result = Buffer::from_bitwise_binary_op(
+ /// &left, 0, &right, 0, 12, |a, b| a & b
+ /// );
+ /// assert_eq!(result.as_slice(), &[0b10001000u8, 0b00001000u8]);
+ /// ```
+ ///
+ /// # Example: Create new [`Buffer`] from bitwise `OR` of two byte slices
+ /// ```
+ /// # use arrow_buffer::Buffer;
+ /// let left = [0b11001100u8, 0b10111010u8];
+ /// let right = [0b10101010u8, 0b11011100u8];
+ /// // OR of bits 4..16 from left and bits 0..12 from right
+ /// let result = Buffer::from_bitwise_binary_op(
+ /// &left, 4, &right, 0, 12, |a, b| a | b
+ /// );
+ /// assert_eq!(result.as_slice(), &[0b10101110u8, 0b00001111u8]);
+ /// ```
+ pub fn from_bitwise_binary_op<F>(
+ left: impl AsRef<[u8]>,
+ left_offset_in_bits: usize,
+ right: impl AsRef<[u8]>,
+ right_offset_in_bits: usize,
+ len_in_bits: usize,
+ mut op: F,
+ ) -> Buffer
+ where
+ F: FnMut(u64, u64) -> u64,
+ {
+ let left_chunks = BitChunks::new(left.as_ref(), left_offset_in_bits,
len_in_bits);
+ let right_chunks = BitChunks::new(right.as_ref(),
right_offset_in_bits, len_in_bits);
+
+ let chunks = left_chunks
+ .iter()
+ .zip(right_chunks.iter())
+ .map(|(left, right)| op(left, right));
+ // Soundness: `BitChunks` is a `BitChunks` iterator which
+ // correctly reports its upper bound
+ let mut buffer = unsafe { MutableBuffer::from_trusted_len_iter(chunks)
};
+
+ let remainder_bytes = ceil(left_chunks.remainder_len(), 8);
+ let rem = op(left_chunks.remainder_bits(),
right_chunks.remainder_bits());
+ // we are counting its starting from the least significant bit, to
to_le_bytes should be correct
+ let rem = &rem.to_le_bytes()[0..remainder_bytes];
+ buffer.extend_from_slice(rem);
Review Comment:
There is also a extend from trusted iter in Mutable buffer? Other option is
to use `Vec::extend` here as well.
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