iris/src/widget/position/span.rs

use crate::prelude::*;
use std::marker::PhantomData;

pub struct Span {
    pub children: Vec<StrongWidget>,
    pub dir: Dir,
    pub gap: Px,
}

impl Widget for Span {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        let axis = self.dir.axis;
        // The row: this span's own box, as a length of the frame its children
        // are laid out against. Its start is nothing's business -- a slot is
        // a length from it -- so what this reads is the length alone.
        let far = painter.extent_len(axis);
        let along = |from: Len, to: Len| match self.dir.sign {
            Sign::Pos => UiSpan::new(from, to),
            Sign::Neg => UiSpan::new(far - to, far - from),
        };
        // Across itself the child sits where its own alignment says, in the
        // whole of the row: a span is what contains its children there, and
        // nothing divides that axis.
        let across = Place::Within(Part::All);
        // A length for every child before their final slots are chosen. The
        // frame passes through unchanged, so `rel(0.5)` is half the area this
        // span was given whatever else is in it and wherever this child sits
        // among them; what it is drawn in is the room left from the cursor,
        // because a text has to wrap at the width actually there.
        let mut cursor = Len::rel_min();
        let mut lens = Vec::with_capacity(self.children.len());
        for child in &self.children {
            let room = Place::Within(Part::From(along(cursor, far)));
            let len = painter
                .widget_at(child, UiRegion::FULL, axis.pair(room, across))
                .len(axis);
            cursor.px += len.px + self.gap;
            cursor.rel += len.rel;
            lens.push(len);
        }

        let gaps = self
            .gap
            .mul_int(self.children.len().saturating_sub(1) as i32);
        let total = lens.iter().fold(
            LayoutLen {
                px: gaps,
                ..LayoutLen::ZERO
            },
            |sum, len| sum + *len,
        );

        // What is left for the shares to divide: the row less everything
        // fixed, as a length of the frame rather than a number of pixels.
        let room = far - Len::from_parts(total.rel, total.px);
        // Whether anything is left over is a question in pixels: `rel(0.5)`
        // beside 300 px is full at 600 and overfull at 400. Asked of `room`
        // itself, and answered back through the same expression, so the
        // boundary is the drawing's own and not a second way of finding it:
        // the three cases a rounded division needed -- the fixed parts
        // growing slower than the box, faster, or exactly with it -- are the
        // sign of `room.rel`, which `through` already reads. What the
        // generated oracle checks is the consequence, since which children
        // exist at all turns on this.
        let mut shares = false;
        if total.leftover > Weight::ZERO {
            shares = room.to_px(painter.frame_px_len(axis)) > Px::ZERO;
            let holds = match shares {
                true => Holds::from(Px::STEP..=Px::MAX),
                false => Holds::from(Px::MIN..=Px::ZERO),
            };
            painter.frame_holds(axis, holds.through(room));
        }

        // Across itself a span is as long as its longest child -- unless a
        // rule beside it gives that length outright, and then reading them
        // answers nothing and makes its size depend on theirs for it. A rule
        // that only bounds the length does not count: the answer is still
        // this span's to give.
        let shrinks = !painter.has_exact_size(!axis);
        // What the fixed parts and the gaps before here take, which is a sum
        // of lengths and exact, and how much of the leftover weight is
        // spoken for. A position is one from the other rather than a step
        // from the last child: the share of the room is rounded, and taking
        // each from the one before it would carry every rounding along the
        // row.
        let mut fixed = Len::rel_min();
        let mut taken = Weight::ZERO;
        let mut start = Len::rel_min();
        let mut ortho = LayoutLen::ZERO;
        for (child, len) in self.children.iter().zip(&lens) {
            // A child asking for nothing but a part of what is left over,
            // when nothing is, is not drawn at all. One that also asked for
            // pixels or a fraction keeps those and overflows.
            if len.leftover > Weight::ZERO && len.px == Px::ZERO && len.rel == Rel::ZERO && !shares
            {
                painter.undraw(child);
                fixed.px += self.gap;
                continue;
            }
            let from = start;
            if len.leftover > Weight::ZERO && shares {
                taken += len.leftover;
            }
            fixed.px += len.px;
            fixed.rel += len.rel;
            start = shared(fixed, taken, total.leftover, room);
            // Along the row the span says where the child goes, and that slot
            // is the drawing's box outright rather than something to place an
            // answer inside again.
            let slot = Place::Fill(Part::From(along(from, start)));
            let placed = painter.widget_at(child, UiRegion::FULL, axis.pair(slot, across));
            if shrinks {
                let used = placed.len(!axis);
                // Choosing between a fixed and a relative length from the
                // span's own eventual width admits multiple fixed points.
                // A scalable child therefore makes Children scalable too;
                // only fixed children are compared with one another.
                if used.rel != Rel::ZERO || used.leftover != Weight::ZERO {
                    ortho = LayoutLen::LEFTOVER;
                } else if ortho.leftover == Weight::ZERO {
                    ortho.px = ortho.px.max(used.px);
                }
            }
            fixed.px += self.gap;
            start = shared(fixed, taken, total.leftover, room);
        }

        // Carried whole rather than collapsed to one share: a span that sizes
        // from its children does not resolve `leftover`, it passes the weight up,
        // so nesting spans divides the same space rather than re-dividing a
        // share of it. Four `leftover(1)` children under two spans under one span
        // get a quarter each, which collapsing to `leftover(1)` per level does
        // not give. Resolution happens at the nearest ancestor with a length,
        // and the root always has one.
        let along = total;
        let ortho = match shrinks {
            true => ortho,
            false => LayoutLen::rel(1.0),
        };
        Size::from_axis(axis, along, ortho)
    }
}

/// Where a row has reached: everything fixed before this point, which is a
/// sum and exact, plus the share of the room the weights so far are worth,
/// which is one rounding wherever it is asked for.
fn shared(fixed: Len, taken: Weight, weight: Weight, room: Len) -> Len {
    if taken == Weight::ZERO {
        return fixed;
    }
    fixed + room.scale(Rel::ratio(taken, weight))
}

impl Span {
    pub fn empty(dir: Dir) -> Self {
        Self {
            children: Vec::new(),
            dir,
            gap: Px::ZERO,
        }
    }

    pub fn gap(mut self, gap: impl UiNum) -> Self {
        self.gap = Px::from_num(gap);
        self
    }

    pub fn push(&mut self, w: StrongWidget) {
        self.children.push(w);
    }

    pub fn pop(&mut self) -> Option<StrongWidget> {
        self.children.pop()
    }
}

pub struct SpanBuilder<State, const LEN: usize, Wa: WidgetArrLike<State, LEN, Tag>, Tag> {
    pub children: Wa,
    pub dir: Dir,
    pub gap: Px,
    _pd: PhantomData<(State, Tag)>,
}

impl<Rsc, const LEN: usize, Wa: WidgetArrLike<Rsc, LEN, Tag>, Tag> WidgetFnTrait<Rsc>
    for SpanBuilder<Rsc, LEN, Wa, Tag>
{
    type Widget = Span;

    #[track_caller]
    fn run(self, rsc: &mut Rsc) -> Self::Widget {
        Span {
            children: self.children.add(rsc).arr.into_iter().collect(),
            dir: self.dir,
            gap: self.gap,
        }
    }
}

impl<State, const LEN: usize, Wa: WidgetArrLike<State, LEN, Tag>, Tag>
    SpanBuilder<State, LEN, Wa, Tag>
{
    pub fn new(children: Wa, dir: Dir) -> Self {
        Self {
            children,
            dir,
            gap: Px::ZERO,
            _pd: PhantomData,
        }
    }

    pub fn gap(mut self, gap: impl UiNum) -> Self {
        self.gap = Px::from_num(gap);
        self
    }
}

impl std::ops::Deref for Span {
    type Target = Vec<StrongWidget>;

    fn deref(&self) -> &Self::Target {
        &self.children
    }
}

impl std::ops::DerefMut for Span {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.children
    }
}