yordan-pavlov commented on a change in pull request #9759:
URL: https://github.com/apache/arrow/pull/9759#discussion_r598276202
##########
File path: rust/arrow/src/buffer/mutable.rs
##########
@@ -415,6 +415,61 @@ impl MutableBuffer {
buffer
}
+ /// Creates a [`MutableBuffer`] from a boolean [`Iterator`] with a trusted
(upper) length.
+ /// # use arrow::buffer::MutableBuffer;
+ /// # Example
+ /// ```
+ /// # use arrow::buffer::MutableBuffer;
+ /// let v = vec![false, true, false];
+ /// let iter = v.iter().map(|x| *x || true);
+ /// let buffer = unsafe { MutableBuffer::from_trusted_len_iter_bool(iter)
};
+ /// assert_eq!(buffer.len(), 1) // 3 booleans have 1 byte
+ /// ```
+ /// # Safety
+ /// This method assumes that the iterator's size is correct and is
undefined behavior
+ /// to use it on an iterator that reports an incorrect length.
+ // This implementation is required for two reasons:
+ // 1. there is no trait `TrustedLen` in stable rust and therefore
+ // we can't specialize `extend` for `TrustedLen` like `Vec` does.
+ // 2. `from_trusted_len_iter_bool` is faster.
+ pub unsafe fn from_trusted_len_iter_bool<I: Iterator<Item = bool>>(
+ mut iterator: I,
+ ) -> Self {
+ let (_, upper) = iterator.size_hint();
+ let upper = upper.expect("from_trusted_len_iter requires an upper
limit");
+
+ let mut result = {
+ let byte_capacity: usize = upper.saturating_add(7) / 8;
+ MutableBuffer::new(byte_capacity)
+ };
+
+ 'a: loop {
+ let mut byte_accum: u8 = 0;
+ let mut mask: u8 = 1;
+
+ //collect (up to) 8 bits into a byte
+ while mask != 0 {
+ if let Some(value) = iterator.next() {
+ byte_accum |= match value {
Review comment:
@Dandandan @jorgecarleitao it looks like this implementation merges in
the same loop (1) the equality check / comparison operation and (2) setting the
corresponding bit in the bitmap. I wonder what difference would it make if the
comparison operation and the bitmap setting operation are done separately:
first the comparison operation generates an iterator of bool, then that bool
iterator is translated into a bitmap (similar to how it's done in the
implementation above).
Another interesting thing to try is to fully separate the comparison
operation (so that the CPU can focus on one type of operation at a time), by
materializing the comparison iterator into a `Vec` using `collect()` before
using the result to generate a bitmap.
Or may be you've already tried this. Any thoughts?
--
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:
[email protected]
