yordan-pavlov commented on a change in pull request #9759:
URL: https://github.com/apache/arrow/pull/9759#discussion_r598154125
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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:
I wonder if the bool iterator could be split into chunks (for example,
using https://docs.rs/itertools/0.4.2/itertools/struct.Chunks.html or
alternatively using
https://doc.rust-lang.org/std/primitive.slice.html#method.chunks) of 8 bool
values, then each chunk is mapped into a byte by converting each bool value
into a byte (for example using std::mem::transmute::<bool, u8>), then shifting
according to the position in the chunk, and applying in the output byte, and
finally the resulting byte iterator would be used to build the buffer directly.
This is the fastest implementation I can imagine because it eliminates as many
conditions / checks as possible (and conditions are the enemy of fast).
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