1 | use std::fmt; |
2 | |
3 | #[derive(Clone, Copy, PartialEq)] |
4 | pub(crate) struct Pack { |
5 | mask: usize, |
6 | shift: u32, |
7 | } |
8 | |
9 | impl Pack { |
10 | /// Value is packed in the `width` least-significant bits. |
11 | pub(crate) const fn least_significant(width: u32) -> Pack { |
12 | let mask = mask_for(width); |
13 | |
14 | Pack { mask, shift: 0 } |
15 | } |
16 | |
17 | /// Value is packed in the `width` more-significant bits. |
18 | pub(crate) const fn then(&self, width: u32) -> Pack { |
19 | let shift = pointer_width() - self.mask.leading_zeros(); |
20 | let mask = mask_for(width) << shift; |
21 | |
22 | Pack { mask, shift } |
23 | } |
24 | |
25 | /// Width, in bits, dedicated to storing the value. |
26 | pub(crate) const fn width(&self) -> u32 { |
27 | pointer_width() - (self.mask >> self.shift).leading_zeros() |
28 | } |
29 | |
30 | /// Max representable value. |
31 | pub(crate) const fn max_value(&self) -> usize { |
32 | (1 << self.width()) - 1 |
33 | } |
34 | |
35 | pub(crate) fn pack(&self, value: usize, base: usize) -> usize { |
36 | assert!(value <= self.max_value()); |
37 | (base & !self.mask) | (value << self.shift) |
38 | } |
39 | |
40 | pub(crate) fn unpack(&self, src: usize) -> usize { |
41 | unpack(src, self.mask, self.shift) |
42 | } |
43 | } |
44 | |
45 | impl fmt::Debug for Pack { |
46 | fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { |
47 | write!( |
48 | fmt, |
49 | "Pack {{ mask: {:b}, shift: {} }}" , |
50 | self.mask, self.shift |
51 | ) |
52 | } |
53 | } |
54 | |
55 | /// Returns the width of a pointer in bits. |
56 | pub(crate) const fn pointer_width() -> u32 { |
57 | std::mem::size_of::<usize>() as u32 * 8 |
58 | } |
59 | |
60 | /// Returns a `usize` with the right-most `n` bits set. |
61 | pub(crate) const fn mask_for(n: u32) -> usize { |
62 | let shift = 1usize.wrapping_shl(n - 1); |
63 | shift | (shift - 1) |
64 | } |
65 | |
66 | /// Unpacks a value using a mask & shift. |
67 | pub(crate) const fn unpack(src: usize, mask: usize, shift: u32) -> usize { |
68 | (src & mask) >> shift |
69 | } |
70 | |