iris/core/src/ui/holds.rs

use crate::{Len, Px, REL_SHIFT, fixed::div_toward, fixed::narrow};
use std::ops::RangeInclusive;

/// The lengths of a box, in pixels, that one drawing of a widget holds for:
/// give the widget any box in this range and it draws the same thing and
/// reports the same size. A widget that never reads its box in pixels holds
/// for every length; one that does holds for the one it read unless it says
/// otherwise, and a parent holds for whatever keeps every child it asked
/// about or drew inside its own range.
///
/// The ends are lengths on the grid rather than floats with a tolerance
/// around them: a box offered back at the length a widget reported comes back
/// as the same number, so a range means what it says. The one place a range
/// is wider than the length it came from is [`Self::through`], and what it is
/// wider by is the floor that inverting a fraction undoes.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Holds {
    pub lo: Px,
    pub hi: Px,
}

impl Holds {
    pub const ANY: Self = Self {
        lo: Px::MIN,
        hi: Px::MAX,
    };

    pub const fn at(len: Px) -> Self {
        Self { lo: len, hi: len }
    }

    pub const fn contains(&self, len: Px) -> bool {
        len.raw() >= self.lo.raw() && len.raw() <= self.hi.raw()
    }

    pub const fn and(self, other: Self) -> Self {
        Self {
            lo: self.lo.max(other.lo),
            hi: self.hi.min(other.hi),
        }
    }

    /// What a box has to be for a part of it, `len` of the box long, to stay
    /// in this range: the exact preimage of `px + floor(rel * box)`, which is
    /// the one way a box in pixels is reached. A part with no relative extent
    /// is a fixed length -- it was drawn at that length and any box keeps it
    /// there.
    ///
    /// The answer is an interval even where this range is a single length,
    /// because the multiply on the way in drops to the step below and many
    /// boxes therefore give one length. That is a floor rather than an
    /// allowance: inverting it is two divisions and nothing else, and the
    /// whole of a box maps back to itself.
    pub const fn through(self, len: Len) -> Self {
        let rel = len.rel.raw() as i64;
        if rel == 0 {
            return Self::ANY;
        }
        let px = len.px.raw() as i64;
        // `floor(rel * box) >= lo - px` is `rel * box >= (lo - px) << REL`, and
        // `floor(rel * box) <= hi - px` is `rel * box < (hi - px + 1) << REL`.
        let lo = (self.lo.raw() as i64 - px) << REL_SHIFT;
        let hi = (((self.hi.raw() as i64 - px) + 1) << REL_SHIFT) - 1;
        // Dividing by a negative fraction turns the ends around, so which
        // bound each comes from is decided before dividing rather than by
        // taking the min and max of four divisions.
        match rel > 0 {
            true => Self::raws(div_toward(lo, rel, true), div_toward(hi, rel, false)),
            false => Self::raws(div_toward(hi, rel, true), div_toward(lo, rel, false)),
        }
    }

    const fn raws(lo: i64, hi: i64) -> Self {
        Self {
            lo: Px::from_raw(narrow(lo)),
            hi: Px::from_raw(narrow(hi)),
        }
    }
}

impl From<RangeInclusive<Px>> for Holds {
    fn from(range: RangeInclusive<Px>) -> Self {
        Self {
            lo: *range.start(),
            hi: *range.end(),
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::Rel;

    #[test]
    fn through_reverses_a_range_for_a_negative_fraction() {
        // `10 - box / 2` is between 20 and 40 for boxes from -60 to -20.
        let part = Len::from_parts(Rel::from_f32(-0.5), Px::from_int(10));
        let holds = Holds::from(Px::from_int(20)..=Px::from_int(40)).through(part);
        assert!(holds.contains(Px::from_int(-60)) && holds.contains(Px::from_int(-20)));
        assert!(!holds.contains(Px::from_int(-61)) && !holds.contains(Px::from_int(-19)));
    }

    /// The case the widening is for: a part that holds only for the length it
    /// was drawn at has to hold for the box it was drawn in, and a third of a
    /// box is not a whole number of steps.
    #[test]
    fn a_part_maps_back_onto_the_box_it_was_measured_in() {
        let part = Len::from_parts(Rel::from_f32(1.0 / 3.0), Px::from_int(-146));
        for box_len in (440..460).map(Px::from_int) {
            let holds = Holds::at(part.to_px(box_len)).through(part);
            assert!(holds.contains(box_len), "{box_len:?} left out by {holds:?}");
        }
    }

    /// A widget handed the whole of its parent's box, with or without pixels
    /// taken off it, has no fraction to invert: multiplying by one is exact
    /// and taking the pixels off again is too, so the box maps back to
    /// itself. Allowing for anything here compounded a step a level down a
    /// chain of widgets each taking the whole of its parent.
    #[test]
    fn the_whole_of_a_box_maps_back_to_itself() {
        let at = Px::from_int(956);
        assert_eq!(Holds::at(at).through(Len::FULL), Holds::at(at));
        let less_eight = Len::from_parts(Rel::ONE, Px::from_int(-8));
        assert_eq!(
            Holds::at(at).through(less_eight),
            Holds::at(at + Px::from_int(8))
        );
    }

    /// The range is the exact preimage at both ends, so a box one step
    /// outside it really does give a length outside this range. What a wider
    /// range costs is a drawing reused where it does not hold.
    #[test]
    fn a_box_one_step_outside_the_range_is_outside_it() {
        let part = Len::from_parts(Rel::from_f32(1.0 / 3.0), Px::from_int(-146));
        let at = Px::from_int(300);
        let holds = Holds::at(at).through(part);
        for inside in [holds.lo, holds.hi] {
            assert_eq!(part.to_px(inside), at, "{inside:?} left out of {holds:?}");
        }
        for outside in [holds.lo.next_down(), holds.hi.next_up()] {
            assert_ne!(part.to_px(outside), at, "{outside:?} admitted by {holds:?}");
        }
    }

    /// A truncating multiply only ever drops, so the step it needs allowing
    /// for on the way in belongs at the top of the range and not the bottom.
    #[test]
    fn a_fraction_widens_further_up_than_down() {
        let half = Len::from_parts(Rel::from_f32(0.5), Px::ZERO);
        let holds = Holds::at(Px::from_int(100)).through(half);
        let box_len = Px::from_int(200);
        assert!(holds.hi - box_len > box_len - holds.lo, "{holds:?}");
    }

    #[test]
    fn a_boundary_the_next_step_along_does_not_admit_it() {
        let boundary = Px::from_int(10);
        let above = Holds::from(boundary.next_up()..=Px::MAX);
        assert!(!above.contains(boundary));
        assert!(above.contains(boundary.next_up()));
    }
}