1use super::group::{
2 BitMaskWord, NonZeroBitMaskWord, BITMASK_ITER_MASK, BITMASK_MASK, BITMASK_STRIDE,
3};
4
5/// A bit mask which contains the result of a `Match` operation on a `Group` and
6/// allows iterating through them.
7///
8/// The bit mask is arranged so that low-order bits represent lower memory
9/// addresses for group match results.
10///
11/// For implementation reasons, the bits in the set may be sparsely packed with
12/// groups of 8 bits representing one element. If any of these bits are non-zero
13/// then this element is considered to true in the mask. If this is the
14/// case, `BITMASK_STRIDE` will be 8 to indicate a divide-by-8 should be
15/// performed on counts/indices to normalize this difference. `BITMASK_MASK` is
16/// similarly a mask of all the actually-used bits.
17///
18/// To iterate over a bit mask, it must be converted to a form where only 1 bit
19/// is set per element. This is done by applying `BITMASK_ITER_MASK` on the
20/// mask bits.
21#[derive(Copy, Clone)]
22pub(crate) struct BitMask(pub(crate) BitMaskWord);
23
24#[allow(clippy::use_self)]
25impl BitMask {
26 /// Returns a new `BitMask` with all bits inverted.
27 #[inline]
28 #[must_use]
29 #[allow(dead_code)]
30 pub(crate) fn invert(self) -> Self {
31 BitMask(self.0 ^ BITMASK_MASK)
32 }
33
34 /// Returns a new `BitMask` with the lowest bit removed.
35 #[inline]
36 #[must_use]
37 fn remove_lowest_bit(self) -> Self {
38 BitMask(self.0 & (self.0 - 1))
39 }
40
41 /// Returns whether the `BitMask` has at least one set bit.
42 #[inline]
43 pub(crate) fn any_bit_set(self) -> bool {
44 self.0 != 0
45 }
46
47 /// Returns the first set bit in the `BitMask`, if there is one.
48 #[inline]
49 pub(crate) fn lowest_set_bit(self) -> Option<usize> {
50 if let Some(nonzero) = NonZeroBitMaskWord::new(self.0) {
51 Some(Self::nonzero_trailing_zeros(nonzero))
52 } else {
53 None
54 }
55 }
56
57 /// Returns the number of trailing zeroes in the `BitMask`.
58 #[inline]
59 pub(crate) fn trailing_zeros(self) -> usize {
60 // ARM doesn't have a trailing_zeroes instruction, and instead uses
61 // reverse_bits (RBIT) + leading_zeroes (CLZ). However older ARM
62 // versions (pre-ARMv7) don't have RBIT and need to emulate it
63 // instead. Since we only have 1 bit set in each byte on ARM, we can
64 // use swap_bytes (REV) + leading_zeroes instead.
65 if cfg!(target_arch = "arm") && BITMASK_STRIDE % 8 == 0 {
66 self.0.swap_bytes().leading_zeros() as usize / BITMASK_STRIDE
67 } else {
68 self.0.trailing_zeros() as usize / BITMASK_STRIDE
69 }
70 }
71
72 /// Same as above but takes a `NonZeroBitMaskWord`.
73 #[inline]
74 fn nonzero_trailing_zeros(nonzero: NonZeroBitMaskWord) -> usize {
75 if cfg!(target_arch = "arm") && BITMASK_STRIDE % 8 == 0 {
76 // SAFETY: A byte-swapped non-zero value is still non-zero.
77 let swapped = unsafe { NonZeroBitMaskWord::new_unchecked(nonzero.get().swap_bytes()) };
78 swapped.leading_zeros() as usize / BITMASK_STRIDE
79 } else {
80 nonzero.trailing_zeros() as usize / BITMASK_STRIDE
81 }
82 }
83
84 /// Returns the number of leading zeroes in the `BitMask`.
85 #[inline]
86 pub(crate) fn leading_zeros(self) -> usize {
87 self.0.leading_zeros() as usize / BITMASK_STRIDE
88 }
89}
90
91impl IntoIterator for BitMask {
92 type Item = usize;
93 type IntoIter = BitMaskIter;
94
95 #[inline]
96 fn into_iter(self) -> BitMaskIter {
97 // A BitMask only requires each element (group of bits) to be non-zero.
98 // However for iteration we need each element to only contain 1 bit.
99 BitMaskIter(BitMask(self.0 & BITMASK_ITER_MASK))
100 }
101}
102
103/// Iterator over the contents of a `BitMask`, returning the indices of set
104/// bits.
105#[derive(Clone)]
106pub(crate) struct BitMaskIter(pub(crate) BitMask);
107
108impl Iterator for BitMaskIter {
109 type Item = usize;
110
111 #[inline]
112 fn next(&mut self) -> Option<usize> {
113 let bit: usize = self.0.lowest_set_bit()?;
114 self.0 = self.0.remove_lowest_bit();
115 Some(bit)
116 }
117}
118