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 = usize::BITS - 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 {

        usize::BITS - (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)

    }

    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 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)

}

/// Unpacks a value using a mask & shift.

pub(crate) const fn unpack(src: usize, mask: usize, shift: u32) -> usize {

    (src & mask) >> shift

}