arrow_buffer/util/bit_iterator.rs
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// 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.
//! Types for iterating over packed bitmasks
use crate::bit_chunk_iterator::{UnalignedBitChunk, UnalignedBitChunkIterator};
use crate::bit_util::{ceil, get_bit_raw};
/// Iterator over the bits within a packed bitmask
///
/// To efficiently iterate over just the set bits see [`BitIndexIterator`] and [`BitSliceIterator`]
pub struct BitIterator<'a> {
buffer: &'a [u8],
current_offset: usize,
end_offset: usize,
}
impl<'a> BitIterator<'a> {
/// Create a new [`BitIterator`] from the provided `buffer`,
/// and `offset` and `len` in bits
///
/// # Panic
///
/// Panics if `buffer` is too short for the provided offset and length
pub fn new(buffer: &'a [u8], offset: usize, len: usize) -> Self {
let end_offset = offset.checked_add(len).unwrap();
let required_len = ceil(end_offset, 8);
assert!(
buffer.len() >= required_len,
"BitIterator buffer too small, expected {required_len} got {}",
buffer.len()
);
Self {
buffer,
current_offset: offset,
end_offset,
}
}
}
impl<'a> Iterator for BitIterator<'a> {
type Item = bool;
fn next(&mut self) -> Option<Self::Item> {
if self.current_offset == self.end_offset {
return None;
}
// Safety:
// offsets in bounds
let v = unsafe { get_bit_raw(self.buffer.as_ptr(), self.current_offset) };
self.current_offset += 1;
Some(v)
}
}
impl<'a> ExactSizeIterator for BitIterator<'a> {}
impl<'a> DoubleEndedIterator for BitIterator<'a> {
fn next_back(&mut self) -> Option<Self::Item> {
if self.current_offset == self.end_offset {
return None;
}
self.end_offset -= 1;
// Safety:
// offsets in bounds
let v = unsafe { get_bit_raw(self.buffer.as_ptr(), self.end_offset) };
Some(v)
}
}
/// Iterator of contiguous ranges of set bits within a provided packed bitmask
///
/// Returns `(usize, usize)` each representing an interval where the corresponding
/// bits in the provides mask are set
///
#[derive(Debug)]
pub struct BitSliceIterator<'a> {
iter: UnalignedBitChunkIterator<'a>,
len: usize,
current_offset: i64,
current_chunk: u64,
}
impl<'a> BitSliceIterator<'a> {
/// Create a new [`BitSliceIterator`] from the provided `buffer`,
/// and `offset` and `len` in bits
pub fn new(buffer: &'a [u8], offset: usize, len: usize) -> Self {
let chunk = UnalignedBitChunk::new(buffer, offset, len);
let mut iter = chunk.iter();
let current_offset = -(chunk.lead_padding() as i64);
let current_chunk = iter.next().unwrap_or(0);
Self {
iter,
len,
current_offset,
current_chunk,
}
}
/// Returns `Some((chunk_offset, bit_offset))` for the next chunk that has at
/// least one bit set, or None if there is no such chunk.
///
/// Where `chunk_offset` is the bit offset to the current `u64` chunk
/// and `bit_offset` is the offset of the first `1` bit in that chunk
fn advance_to_set_bit(&mut self) -> Option<(i64, u32)> {
loop {
if self.current_chunk != 0 {
// Find the index of the first 1
let bit_pos = self.current_chunk.trailing_zeros();
return Some((self.current_offset, bit_pos));
}
self.current_chunk = self.iter.next()?;
self.current_offset += 64;
}
}
}
impl<'a> Iterator for BitSliceIterator<'a> {
type Item = (usize, usize);
fn next(&mut self) -> Option<Self::Item> {
// Used as termination condition
if self.len == 0 {
return None;
}
let (start_chunk, start_bit) = self.advance_to_set_bit()?;
// Set bits up to start
self.current_chunk |= (1 << start_bit) - 1;
loop {
if self.current_chunk != u64::MAX {
// Find the index of the first 0
let end_bit = self.current_chunk.trailing_ones();
// Zero out up to end_bit
self.current_chunk &= !((1 << end_bit) - 1);
return Some((
(start_chunk + start_bit as i64) as usize,
(self.current_offset + end_bit as i64) as usize,
));
}
match self.iter.next() {
Some(next) => {
self.current_chunk = next;
self.current_offset += 64;
}
None => {
return Some((
(start_chunk + start_bit as i64) as usize,
std::mem::replace(&mut self.len, 0),
));
}
}
}
}
}
/// An iterator of `usize` whose index in a provided bitmask is true
///
/// This provides the best performance on most masks, apart from those which contain
/// large runs and therefore favour [`BitSliceIterator`]
#[derive(Debug)]
pub struct BitIndexIterator<'a> {
current_chunk: u64,
chunk_offset: i64,
iter: UnalignedBitChunkIterator<'a>,
}
impl<'a> BitIndexIterator<'a> {
/// Create a new [`BitIndexIterator`] from the provide `buffer`,
/// and `offset` and `len` in bits
pub fn new(buffer: &'a [u8], offset: usize, len: usize) -> Self {
let chunks = UnalignedBitChunk::new(buffer, offset, len);
let mut iter = chunks.iter();
let current_chunk = iter.next().unwrap_or(0);
let chunk_offset = -(chunks.lead_padding() as i64);
Self {
current_chunk,
chunk_offset,
iter,
}
}
}
impl<'a> Iterator for BitIndexIterator<'a> {
type Item = usize;
fn next(&mut self) -> Option<Self::Item> {
loop {
if self.current_chunk != 0 {
let bit_pos = self.current_chunk.trailing_zeros();
self.current_chunk ^= 1 << bit_pos;
return Some((self.chunk_offset + bit_pos as i64) as usize);
}
self.current_chunk = self.iter.next()?;
self.chunk_offset += 64;
}
}
}
/// Calls the provided closure for each index in the provided null mask that is set,
/// using an adaptive strategy based on the null count
///
/// Ideally this would be encapsulated in an [`Iterator`] that would determine the optimal
/// strategy up front, and then yield indexes based on this.
///
/// Unfortunately, external iteration based on the resulting [`Iterator`] would match the strategy
/// variant on each call to [`Iterator::next`], and LLVM generally cannot eliminate this.
///
/// One solution to this might be internal iteration, e.g. [`Iterator::try_fold`], however,
/// it is currently [not possible] to override this for custom iterators in stable Rust.
///
/// As such this is the next best option
///
/// [not possible]: https://github.com/rust-lang/rust/issues/69595
#[inline]
pub fn try_for_each_valid_idx<E, F: FnMut(usize) -> Result<(), E>>(
len: usize,
offset: usize,
null_count: usize,
nulls: Option<&[u8]>,
f: F,
) -> Result<(), E> {
let valid_count = len - null_count;
if valid_count == len {
(0..len).try_for_each(f)
} else if null_count != len {
BitIndexIterator::new(nulls.unwrap(), offset, len).try_for_each(f)
} else {
Ok(())
}
}
// Note: further tests located in arrow_select::filter module
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_bit_iterator() {
let mask = &[0b00010010, 0b00100011, 0b00000101, 0b00010001, 0b10010011];
let actual: Vec<_> = BitIterator::new(mask, 0, 5).collect();
assert_eq!(actual, &[false, true, false, false, true]);
let actual: Vec<_> = BitIterator::new(mask, 4, 5).collect();
assert_eq!(actual, &[true, false, false, false, true]);
let actual: Vec<_> = BitIterator::new(mask, 12, 14).collect();
assert_eq!(
actual,
&[
false, true, false, false, true, false, true, false, false, false, false, false,
true, false
]
);
assert_eq!(BitIterator::new(mask, 0, 0).count(), 0);
assert_eq!(BitIterator::new(mask, 40, 0).count(), 0);
}
#[test]
#[should_panic(expected = "BitIterator buffer too small, expected 3 got 2")]
fn test_bit_iterator_bounds() {
let mask = &[223, 23];
BitIterator::new(mask, 17, 0);
}
}