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    use std::fmt;

    #[derive(Clone, Copy, PartialEq)]
    pub(crate) struct Pack {
        mask: usize,
        shift: u32,
    }

    impl Pack {
        /// Value is packed in the `width` least-significant bits.
        pub(crate) const fn least_significant(width: u32) -> Pack {
            let mask = mask_for(width);

            Pack { mask, shift: 0 }
        }

        /// Value is packed in the `width` more-significant bits.
        pub(crate) const fn then(&self, width: u32) -> Pack {
            let shift = pointer_width() - self.mask.leading_zeros();
            let mask = mask_for(width) << shift;

            Pack { mask, shift }
        }

        /// Width, in bits, dedicated to storing the value.
        pub(crate) const fn width(&self) -> u32 {
            pointer_width() - (self.mask >> self.shift).leading_zeros()
        }

        /// Max representable value
        pub(crate) const fn max_value(&self) -> usize {
            (1 << self.width()) - 1
        }

        pub(crate) fn pack(&self, value: usize, base: usize) -> usize {
            assert!(value <= self.max_value());
            (base & !self.mask) | (value << self.shift)
        }

        /// Packs the value with `base`, losing any bits of `value` that fit.
        ///
        /// If `value` is larger than the max value that can be represented by the
        /// allotted width, the most significant bits are truncated.
        pub(crate) fn pack_lossy(&self, value: usize, base: usize) -> usize {
            self.pack(value & self.max_value(), base)
        }

        pub(crate) fn unpack(&self, src: usize) -> usize {
            unpack(src, self.mask, self.shift)
        }
    }

    impl fmt::Debug for Pack {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(
                fmt,
                "Pack {{ mask: {:b}, shift: {} }}",
                self.mask, self.shift
            )
        }
    }

    /// Returns the width of a pointer in bits
    pub(crate) const fn pointer_width() -> u32 {
        std::mem::size_of::<usize>() as u32 * 8
    }

    /// Returns a `usize` with the right-most `n` bits set.
    pub(crate) const fn mask_for(n: u32) -> usize {
        let shift = 1usize.wrapping_shl(n - 1);
        shift | (shift - 1)
    }

    /// Unpack a value using a mask & shift
    pub(crate) const fn unpack(src: usize, mask: usize, shift: u32) -> usize {
        (src & mask) >> shift
    }