small-map 0.1.5

use super::imp::{BitMaskWord, NonZeroBitMaskWord, BITMASK_ITER_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 {
    #[inline]
    pub(crate) fn and(self, n: BitMaskWord) -> Self {
        Self(self.0 & n)
    }

    /// 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 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
        }
    }

    /// Same as above but takes a `NonZeroBitMaskWord`.
    #[inline]
    fn nonzero_trailing_zeros(nonzero: NonZeroBitMaskWord) -> usize {
        if cfg!(target_arch = "arm") && BITMASK_STRIDE.is_multiple_of(8) {
            // 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
        }
    }
}

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(BitMask);

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)
    }
}