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use super::imp::{
BitMaskWord, NonZeroBitMaskWord, BITMASK_ITER_MASK, BITMASK_MASK, BITMASK_STRIDE,
};
/// A bit mask which contains the result of a `Match` operation on a `Group` and
/// allows iterating through them.
///
/// The bit mask is arranged so that low-order bits represent lower memory
/// addresses for group match results.
///
/// For implementation reasons, the bits in the set may be sparsely packed with
/// groups of 8 bits representing one element. If any of these bits are non-zero
/// then this element is considered to true in the mask. If this is the
/// case, `BITMASK_STRIDE` will be 8 to indicate a divide-by-8 should be
/// performed on counts/indices to normalize this difference. `BITMASK_MASK` is
/// similarly a mask of all the actually-used bits.
///
/// To iterate over a bit mask, it must be converted to a form where only 1 bit
/// is set per element. This is done by applying `BITMASK_ITER_MASK` on the
/// mask bits.
#[derive(Copy, Clone)]
pub(crate) struct BitMask(pub(crate) BitMaskWord);
#[allow(clippy::use_self)]
impl BitMask {
/// Returns a new `BitMask` with all bits inverted.
#[inline]
#[must_use]
#[allow(dead_code)]
pub(crate) fn invert(self) -> Self {
BitMask(self.0 ^ BITMASK_MASK)
}
/// Returns a new `BitMask` with the lowest bit removed.
#[inline]
#[must_use]
fn remove_lowest_bit(self) -> Self {
BitMask(self.0 & (self.0 - 1))
}
/// Returns whether the `BitMask` has at least one set bit.
#[inline]
pub(crate) fn any_bit_set(self) -> bool {
self.0 != 0
}
/// Returns the first set bit in the `BitMask`, if there is one.
#[inline]
pub(crate) fn lowest_set_bit(self) -> Option<usize> {
if let Some(nonzero) = NonZeroBitMaskWord::new(self.0) {
Some(Self::nonzero_trailing_zeros(nonzero))
} else {
None
}
}
/// Returns the number of trailing zeroes in the `BitMask`.
#[inline]
pub(crate) fn trailing_zeros(self) -> usize {
// ARM doesn't have a trailing_zeroes instruction, and instead uses
// reverse_bits (RBIT) + leading_zeroes (CLZ). However older ARM
// versions (pre-ARMv7) don't have RBIT and need to emulate it
// instead. Since we only have 1 bit set in each byte on ARM, we can
// use swap_bytes (REV) + leading_zeroes instead.
if cfg!(target_arch = "arm") && BITMASK_STRIDE % 8 == 0 {
self.0.swap_bytes().leading_zeros() as usize / BITMASK_STRIDE
} else {
self.0.trailing_zeros() as usize / BITMASK_STRIDE
}
}
/// Same as above but takes a `NonZeroBitMaskWord`.
#[inline]
fn nonzero_trailing_zeros(nonzero: NonZeroBitMaskWord) -> usize {
if cfg!(target_arch = "arm") && BITMASK_STRIDE % 8 == 0 {
// SAFETY: A byte-swapped non-zero value is still non-zero.
let swapped = unsafe { NonZeroBitMaskWord::new_unchecked(nonzero.get().swap_bytes()) };
swapped.leading_zeros() as usize / BITMASK_STRIDE
} else {
nonzero.trailing_zeros() as usize / BITMASK_STRIDE
}
}
/// Returns the number of leading zeroes in the `BitMask`.
#[inline]
pub(crate) fn leading_zeros(self) -> usize {
self.0.leading_zeros() as usize / BITMASK_STRIDE
}
}
impl IntoIterator for BitMask {
type Item = usize;
type IntoIter = BitMaskIter;
#[inline]
fn into_iter(self) -> BitMaskIter {
// A BitMask only requires each element (group of bits) to be non-zero.
// However for iteration we need each element to only contain 1 bit.
BitMaskIter(BitMask(self.0 & BITMASK_ITER_MASK))
}
}
/// Iterator over the contents of a `BitMask`, returning the indices of set
/// bits.
#[derive(Copy, Clone)]
pub(crate) struct BitMaskIter(pub(crate) BitMask);
impl BitMaskIter {
/// Flip the bit in the mask for the entry at the given index.
///
/// Returns the bit's previous state.
#[inline]
#[allow(clippy::cast_ptr_alignment)]
#[cfg(feature = "raw")]
pub(crate) unsafe fn flip(&mut self, index: usize) -> bool {
// NOTE: The + BITMASK_STRIDE - 1 is to set the high bit.
let mask = 1 << (index * BITMASK_STRIDE + BITMASK_STRIDE - 1);
self.0 .0 ^= mask;
// The bit was set if the bit is now 0.
self.0 .0 & mask == 0
}
}
impl Iterator for BitMaskIter {
type Item = usize;
#[inline]
fn next(&mut self) -> Option<usize> {
let bit = self.0.lowest_set_bit()?;
self.0 = self.0.remove_lowest_bit();
Some(bit)
}
}