Dandandan commented on a change in pull request #9759:
URL: https://github.com/apache/arrow/pull/9759#discussion_r598282062
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File path: rust/arrow/src/buffer/mutable.rs
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@@ -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:
@yordan-pavlov
I am not sure where the win would be in that case.
I would expect the first idea to be compiled to roughly the same code (if
all compiler optimizations work out)?
For the `Vec` one - I would expect that would be slower as it introduces an
extra loop / allocation and barrier for optimization?
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