#[cfg(feature = "layout-diagnostics")]
use crate::layout_diagnostics::{self as diag, Counter};
use crate::{
    Axis, DrawRegion, Holds, LayoutHolds, LayoutLen, Len, Px, PxVec2, RegionAlign, RenderedText,
    RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData, TextureHandle,
    UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
    render::{
        GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveInst, PrimitiveKind,
        TexturePrimitive,
    },
    ui::render_state::DrawInfo,
};
const AXES: [Axis; 2] = [Axis::X, Axis::Y];

/// makes your surfaces look pretty
pub struct Painter<'a> {
    pub(super) state: &'a mut UiRenderState,
    pub(super) rsc: &'a mut dyn UiRsc,

    /// The box its parent gave it, in the coordinates of `move_idx`: what a
    /// fraction of this widget's area is a fraction of, and what every region
    /// it writes composes within. The same box on the ask that measures and
    /// the ask that places, which is what keeps a fraction under it from
    /// being resolved twice.
    pub(super) region: UiRegion,
    /// Where this widget's drawing sits inside that box, in the box's own
    /// coordinates: `FULL` while its answer is not yet known, and the box
    /// its answer or its parent chose once one of them has.
    pub(super) placement: UiRegion,
    /// Whether this draw read its placement, which makes the drawing one
    /// that holds for that placement alone -- the way reading a length in
    /// pixels makes it hold for that length.
    pub(super) reads_placement: bool,
    /// That box in pixels, which its children's are a length of: threaded
    /// down from the box this widget was given rather than composed back up
    /// the chain, so every length in layout is one multiply from its
    /// parent's and [`Holds::through`] inverts exactly that.
    pub(super) px: PxVec2,
    pub(super) mask: MaskIdx,
    pub(super) textures: Vec<TextureHandle>,
    pub(super) primitives: Vec<RetainedPrimitive>,
    pub(super) mask_region: Option<DrawRegion>,
    pub(super) inherited_children: Vec<WidgetId>,
    pub(super) extent_own: [Holds; 2],
    /// Only children whose answers were read constrain this widget's answer.
    pub(super) answer_under: LayoutHolds,
    pub(super) children: Vec<WidgetId>,
    /// The children asked about so far, so the first box each was asked in
    /// is the one recorded as its offer.
    pub(super) offered: Vec<WidgetId>,
    /// The lengths of the box this widget was first asked about in, in
    /// pixels. Its children's offers are a fraction of it.
    pub(super) offered_px: PxVec2,
    /// Whether this draw is in a box of those lengths, which makes the
    /// questions it asks the ones a cold layout asks and their answers the
    /// ones to keep.
    pub(super) at_offer: bool,
    /// The children whose size this widget read while drawing.
    pub(super) size_deps: Vec<WidgetId>,
    /// What this draw itself read of its box in pixels, per axis: every
    /// length until it reads one, then that one, unless it says otherwise.
    pub(super) own: [Holds; 2],
    /// Dependencies of every child drawing, including unmeasured overlays.
    pub(super) under: LayoutHolds,
    /// The movable region this widget's primitives are positioned through:
    /// its own when opted in, otherwise the nearest ancestor's.
    pub(super) move_idx: MoveIdx,
    pub layer: usize,
    /// The layer this widget was entered on, which its children's layers are
    /// counted from however far `layer` has walked.
    pub(super) own_layer: usize,
    pub(super) depth: usize,
    pub(super) id: WidgetId,
}

impl<'a> Painter<'a> {
    fn primitive_at<P: Primitive>(&mut self, primitive: P, region: DrawRegion) {
        let kind = self.rsc.ui_mut().primitives.kind::<P>();
        self.write(kind, primitive, region);
    }

    /// Takes the kind, for a caller writing many of one primitive.
    fn write<P: Primitive>(&mut self, kind: PrimitiveKind<P>, primitive: P, region: DrawRegion) {
        self.write_resolved(
            kind,
            primitive,
            region,
            region.resolve(self.region, self.placement),
        );
    }

    fn write_resolved<P: Primitive>(
        &mut self,
        kind: PrimitiveKind<P>,
        primitive: P,
        region: DrawRegion,
        resolved: UiRegion,
    ) {
        #[cfg(feature = "layout-diagnostics")]
        diag::bump(Counter::PrimitiveWrites);
        let h = self.state.layers.write(
            self.layer,
            PrimitiveInst {
                kind,
                id: self.id,
                primitive,
                region: resolved,
                mask_idx: self.mask,
                move_idx: self.move_idx,
            },
        );
        self.push_primitive(RetainedPrimitive { handle: h, region });
    }

    fn push_primitive(&mut self, h: RetainedPrimitive) {
        if self.mask != MaskIdx::NONE {
            // TODO: I have no clue if this works at all :joy:
            self.rsc.ui_mut().masks.push_ref(self.mask);
        }
        self.primitives.push(h);
    }

    /// Writes a primitive over the whole of this widget's own box.
    pub fn primitive(&mut self, primitive: impl PrimitiveLike) {
        let at = DrawRegion::Extent(UiRegion::FULL);
        let primitive = primitive.into_primitive(self);
        self.primitive_at(primitive, at)
    }

    /// Writes in the frame by default. `DrawRegion::Extent` keeps the local
    /// geometry attached to this widget's box without reading its placement.
    pub fn primitive_within(
        &mut self,
        primitive: impl PrimitiveLike,
        region: impl Into<DrawRegion>,
    ) {
        let primitive = primitive.into_primitive(self);
        self.primitive_at(primitive, region.into());
    }

    /// Sets a mask in the selected frame or extent coordinates.
    pub fn set_mask(&mut self, region: impl Into<DrawRegion>) {
        let region = region.into();
        self.mask_region = Some(region);
        assert!(self.mask == MaskIdx::NONE);
        self.mask = self.rsc.ui_mut().masks.push(Mask {
            region: region.resolve(self.region, self.placement),
            move_idx: self.move_idx,
        });
    }

    /// Draws a widget in the whole of this widget's own box: it gets the
    /// same region -- the same area for its fractions to be of -- and is put
    /// where this widget was put. What a container that is only a wrapper
    /// around one child wants, since its box is the child's.
    pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
        let own = self.placement;
        self.widget_at_inner(id, UiRegion::FULL, [Some(own.x), Some(own.y)], true, false)
    }

    /// What a widget's rules declare its lengths to be, which whoever draws
    /// it resolves into its box. Reading them depends on nothing -- the box
    /// that comes of them is kept on the child, and `redraw` compares it
    /// there.
    fn declared_lens<W: ?Sized>(&self, id: &StrongWidget<W>) -> [Option<LayoutLen>; 2] {
        declared_lens(self.rsc.widgets(), id.id())
    }

    /// Takes back a child that was drawn only to find out how long it is.
    /// Its drawing is dropped and it is not one of this widget's children
    /// this frame; what it answered is still something this widget asked.
    pub fn undraw<W: ?Sized>(&mut self, id: &StrongWidget<W>) {
        self.children.retain(|child| *child != id.id());
        self.inherited_children.retain(|child| *child != id.id());
        self.state.undraw_rec(id.id(), self.rsc);
    }

    /// Draws a child in `region`, relative to this widget's frame. The child
    /// resolves declared lengths and reports against that frame, then places
    /// its drawing by its own alignment.
    pub fn widget_within<'s, W: ?Sized>(
        &'s mut self,
        id: &'s StrongWidget<W>,
        region: UiRegion,
    ) -> DrawResult<'s, 'a, W> {
        self.widget_at(id, region, [None; 2])
    }

    /// Draws a widget in `region`, saying where in it the drawing goes.
    ///
    /// `region` is the child's own area: what a fraction it declares or
    /// reports is a fraction of, and the coordinates the regions it writes
    /// compose within. It is the same box on the ask that measures and the
    /// ask that places, which is what stops a fraction under it being
    /// resolved twice.
    ///
    /// `placement` is what of that region the child's drawing takes, per
    /// axis, wherever this widget is choosing. `None` leaves the axis to the
    /// child's own answer and alignment, which is what
    /// [`Self::widget_within`] passes. A span passes the whole row as the
    /// region, so `rel(0.5)` is half the row wherever the child sits in it,
    /// and places the child by passing the slot along its axis.
    pub fn widget_at<'s, W: ?Sized>(
        &'s mut self,
        id: &'s StrongWidget<W>,
        region: UiRegion,
        placement: [Option<UiSpan>; 2],
    ) -> DrawResult<'s, 'a, W> {
        self.widget_at_inner(id, region, placement, false, false)
    }

    fn widget_at_inner<'s, W: ?Sized>(
        &'s mut self,
        id: &'s StrongWidget<W>,
        region: UiRegion,
        placement: [Option<UiSpan>; 2],
        inherited: bool,
        measuring: bool,
    ) -> DrawResult<'s, 'a, W> {
        if inherited {
            if !self.inherited_children.contains(&id.id()) {
                self.inherited_children.push(id.id());
            }
        } else {
            self.inherited_children.retain(|child| *child != id.id());
        }
        let region_node = self.rsc.widgets().is_region_node(id.id());
        let declared = self.declared_lens(id);
        let align = self.rsc.widgets().alignment(id.id());
        let (local, placement) = ask_box(region, declared, align, placement);
        let within = match local == UiRegion::FULL {
            true => self.region,
            false => local.within(&self.region),
        };
        #[cfg(feature = "layout-diagnostics")]
        if region_node {
            diag::bump(Counter::RegionNodeDraws);
            diag::region_node(id.id(), self.id, within);
        }
        // A child listed twice would be moved twice.
        if !self.children.contains(&id.id()) {
            self.children.push(id.id());
        }
        let first_ask = self.offer(id.id());
        let given_len = local.size();
        let offer_len = match first_ask {
            true => given_len,
            false => self
                .state
                .active
                .get(&id.id())
                .map_or(given_len, |a| a.offer_len),
        };
        let offer_placement = if first_ask {
            placement
        } else {
            self.state
                .active
                .get(&id.id())
                .map_or(placement, |a| a.offer_placement)
        };
        let px = given_len.to_px(self.px);
        let offered_px = offer_len.to_px(self.offered_px);
        // The answer and what it holds for, both about the box asked in. The
        // child's record may say something else once its drawing has been
        // placed: a drawing made again in its placed box holds for that box.
        let (size, answer_holds, holds) = self.state.draw_inner(
            id.id(),
            within,
            DrawInfo {
                layer: self.layer,
                parent: Some(self.id),
                depth: self.depth + 1,
                parent_move: self.move_idx,
                region_node,
                mask: self.mask,
                given_region: local,
                offer_len,
                offer_placement,
                px,
                offered_px,
                placement,
            },
            None,
            measuring,
            self.rsc,
        );
        let in_parent = |holds: LayoutHolds| {
            let mut result = LayoutHolds::ANY;
            for axis in AXES {
                let n = axis as usize;
                result.frame[n] = holds.frame[n].through(local.axis(axis).len());
                if inherited && declared[n].is_none() {
                    result.extent[n] = holds.extent[n];
                    if holds.placement.is_some() {
                        result.placement = Some(self.placement);
                    }
                } else {
                    let chosen = placement[n].unwrap_or(UiSpan::FULL).len();
                    result.frame[n] = result.frame[n].and(
                        holds.extent[n]
                            .through(chosen)
                            .through(local.axis(axis).len()),
                    );
                }
            }
            result
        };
        self.under = self.under.and(in_parent(holds));
        let answer_holds = in_parent(answer_holds);
        DrawResult {
            child: id,
            painter: self,
            size: in_parent_frame(size, local.size(), declared),
            answer_holds,
        }
    }

    /// What a child says its length is without being drawn, if it can say.
    /// Asking counts as reading its size.
    pub fn size_hint<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<LayoutLen> {
        let widgets = self.rsc.widgets();
        // A rule is the answer where there is one: it wins over whatever the
        // widget would draw, so it has to win over what the widget says too.
        let hint = widgets.size_rules(id.id()).axis(axis).exact().or_else(|| {
            widgets
                .get_dyn(id.id())
                .and_then(|widget| widget.size_hint(axis))
        });
        #[cfg(feature = "layout-diagnostics")]
        diag::hint_read(id.id(), self.id, axis, hint);
        match hint {
            Some(hint) => {
                #[cfg(feature = "layout-diagnostics")]
                diag::bump(Counter::HintHits);
                self.depend_on(id);
                Some(hint)
            }
            None => {
                #[cfg(feature = "layout-diagnostics")]
                diag::bump(Counter::HintMisses);
                None
            }
        }
    }

    /// Measures a child's length from its hint, a retained answer, or `draw`.
    /// A fresh draw evaluates the offer without placing its answer. The caller
    /// must later place or undraw the child.
    pub fn measure_len<W: ?Sized>(
        &mut self,
        child: &StrongWidget<W>,
        axis: Axis,
        region: UiRegion,
        placement: [Option<UiSpan>; 2],
    ) -> LayoutLen {
        let offered = placement;
        let declared = self.declared_lens(child);
        let align = self.rsc.widgets().alignment(child.id());
        let (local, placement) = ask_box(region, declared, align, placement);
        let first_ask = self.at_offer && !self.offered.contains(&child.id());

        if let Some(hint) = self.size_hint(child, axis) {
            return hint;
        }
        let px = local.size().to_px(self.px);
        let retained =
            self.state
                .retained_size(child.id(), px, placement, self.move_idx, self.rsc.widgets());
        let Some((size, holds)) = retained else {
            return self
                .widget_at_inner(child, region, offered, false, true)
                .len(axis);
        };
        #[cfg(feature = "layout-diagnostics")]
        diag::bump(Counter::RetainedSizeHits);
        self.depend_on(child);
        if first_ask {
            self.offered.push(child.id());
            let active = self.state.active.get_mut(&child.id()).unwrap();
            active.offer_len = local.size();
            active.offer_placement = placement;
        }
        let placement = UiRegion {
            x: placement[0].unwrap_or(UiSpan::FULL),
            y: placement[1].unwrap_or(UiSpan::FULL),
        };
        let holds = holds.in_frame(placement);
        for (axis, under) in AXES.into_iter().zip(self.answer_under.frame.iter_mut()) {
            *under = under.and(holds[axis as usize].through(local.axis(axis).len()));
        }
        in_parent_frame(size, local.size(), declared).axis(axis)
    }

    /// Whether this is the first box a child is asked about in during a draw
    /// that is itself in the box it was asked in -- the question a cold
    /// layout asks, whose answer is the one to keep.
    fn offer(&mut self, child: WidgetId) -> bool {
        if !self.at_offer || self.offered.contains(&child) {
            return false;
        }
        self.offered.push(child);
        true
    }

    fn depend_on<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
        if !self.size_deps.contains(&child.id()) {
            self.size_deps.push(child.id());
        }
    }

    pub fn render_text<'b>(
        &mut self,
        buffer: &'b mut TextBuffer,
        attrs: &TextAttrs,
        width: Option<f32>,
    ) -> &'b RenderedText {
        #[cfg(feature = "layout-diagnostics")]
        diag::render_text(self.id, self.rsc.widgets().label(self.id), width);
        let ui = self.rsc.ui_mut();
        ui.text.render(buffer, attrs, width)
    }

    /// Writes glyphs in the selected frame or extent coordinates.
    // TODO: merge the text methods into the primitive ones.
    pub fn glyphs(&mut self, text: &RenderedText, origin: impl Into<DrawRegion>) {
        let origin = origin.into();
        // Glyph offsets and sizes are pixels, which compose additively.
        // Only the shared origin needs the frame/extent composition.
        let resolved = origin.resolve(self.region, self.placement);
        let kind = self.rsc.ui_mut().primitives.kind::<GlyphPrimitive>();
        for glyph in text.glyphs.iter() {
            let place = |mut region: UiRegion| {
                region.x.end = region.x.start;
                region.y.end = region.y.start;
                let mut region = region.offset(UiVec2::from_px(glyph.offset));
                let size = PxVec2::new(
                    Px::from_int(glyph.entry.width as i32),
                    Px::from_int(glyph.entry.height as i32),
                );
                region.x.end = region.x.start.offset(size.x);
                region.y.end = region.y.start.offset(size.y);
                region
            };
            self.write_resolved(
                kind,
                GlyphPrimitive {
                    uv_min: glyph.entry.uv_min,
                    uv_max: glyph.entry.uv_max,
                    layer: glyph.entry.layer,
                    color: text.color,
                    flags: glyph.entry.flags(),
                },
                origin.map(place),
                place(resolved),
            );
        }
    }

    /// The box this widget's parent gave it, in the coordinates its own
    /// primitives are written in -- so a region composed `within` it may be
    /// drawn directly. Its own box is [`Self::placement`] of this one.
    pub fn region(&self) -> UiRegion {
        self.region
    }

    /// Where this widget's drawing goes inside the box it was given, in that
    /// box's coordinates: what its own answer took of it, or what its parent
    /// chose for it. `FULL` on the ask that measures, since nothing has been
    /// placed yet.
    ///
    /// Reading it is what says the drawing depends on it, so a widget that
    /// positions its own content reads it and is drawn again once its box is
    /// known, and one that fills whatever it is given never is.
    pub fn placement(&mut self) -> UiRegion {
        self.reads_placement = true;
        self.placement
    }

    /// Where this widget sits in a box longer than the length it takes. A
    /// widget that positions its own content reads it to place that content
    /// the way the box around it would have placed the widget.
    pub fn alignment(&self) -> RegionAlign {
        self.rsc.widgets().alignment(self.id)
    }

    /// Whether a rule beside this widget gives its length on `axis` outright,
    /// which makes whatever it reports for that axis moot. A rule that only
    /// bounds the length is not one of these: the answer is still the
    /// widget's to give, and something still has to work it out.
    ///
    /// The widget under a rule does not otherwise learn of it -- this is for
    /// a container deciding whether reading its children across an axis is
    /// worth anything, since reading one is also what makes its own size
    /// depend on it.
    pub fn has_exact_size(&self, axis: Axis) -> bool {
        self.rsc
            .widgets()
            .size_rules(self.id)
            .axis(axis)
            .exact()
            .is_some()
    }

    /// The part of this widget's box that something of `size` takes, at the
    /// near edge. A container that reports one child's size gives every child
    /// this, so what it draws is inside what it says it occupies.
    pub fn box_of(&self, size: Size) -> UiRegion {
        let lens = placed_lens(size, [None; 2], [false; 2]);
        placed_box(UiRegion::FULL, lens, RegionAlign::NEAR)
    }

    /// This widget's own box in pixels. Reading it makes the drawing one
    /// that holds for this box only, until `holds` says how far it goes.
    pub fn px_size(&mut self) -> PxVec2 {
        PxVec2::new(self.px_len(Axis::X), self.px_len(Axis::Y))
    }

    /// One axis of this widget's own box in pixels. Prefer this to
    /// [`Self::px_size`] when the other axis cannot affect the drawing.
    pub fn px_len(&mut self, axis: Axis) -> Px {
        let part = self.placement.axis(axis).len();
        let len = part.to_px(self.px.axis(axis));
        let own = &mut self.extent_own[axis as usize];
        if *own == Holds::ANY {
            *own = Holds::at(len);
        }
        len
    }

    /// The lengths of this widget's own box on `axis` that what it is drawing
    /// holds for -- the same primitives, in the same fractions and offsets
    /// of the box, and the same reported size. A widget that read its length
    /// in pixels holds for that one alone until it says otherwise.
    pub fn holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
        let part = self.placement.axis(axis).len();
        let holds = holds.into();
        debug_assert!(
            holds.contains(part.to_px(self.px.axis(axis))),
            "'{}' ({:?}) says its drawing holds for lengths that leave out its own box",
            self.label(),
            self.id
        );
        self.extent_own[axis as usize] = holds;
    }

    /// One axis of the box this widget's parent gave it, in pixels -- what a
    /// fraction of its area resolves against, and so what a container divides
    /// among its children. Its own box is a part of this one.
    pub fn region_px_len(&mut self, axis: Axis) -> Px {
        let len = self.px.axis(axis);
        let own = &mut self.own[axis as usize];
        if *own == Holds::ANY {
            *own = Holds::at(len);
        }
        len
    }

    /// [`Self::holds`] stated about the region rather than about this
    /// widget's own box, for a container whose drawing turns on the box it
    /// was given rather than on the part of it it took.
    pub fn region_holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
        let holds = holds.into();
        debug_assert!(
            holds.contains(self.px.axis(axis)),
            "'{}' ({:?}) says its drawing holds for lengths that leave out its region",
            self.label(),
            self.id
        );
        self.own[axis as usize] = holds;
    }

    pub fn text_data(&mut self) -> &mut TextData {
        &mut self.rsc.ui_mut().text
    }

    pub fn child_layer(&mut self) {
        self.layer = self.state.layers.child(self.layer);
    }

    /// The layer this widget's `n`th child draws on, addressed rather than
    /// walked to. A container that measures one child by drawing it can ask
    /// on the layer that child will end up on, and then the second ask is a
    /// reuse rather than a second drawing on another layer.
    pub fn child_layer_at(&mut self, n: usize) {
        let mut at = self.state.layers.child(self.own_layer);
        for _ in 0..n {
            at = self.state.layers.next(at);
        }
        self.layer = at;
    }

    pub fn next_layer(&mut self) {
        self.layer = self.state.layers.next(self.layer);
    }

    pub fn label(&self) -> &str {
        &self.rsc.widgets().data(self.id).unwrap().label
    }

    pub fn id(&self) -> &WidgetId {
        &self.id
    }
}

/// A child that has just been drawn. Reading its size records that this
/// widget's own size depends on it; dropping it without reading draws the
/// child and leaves the parent independent of what it came to.
pub struct DrawResult<'p, 'a, W: ?Sized> {
    painter: &'p mut Painter<'a>,
    child: &'p StrongWidget<W>,
    size: Size,
    answer_holds: LayoutHolds,
}

impl<W: ?Sized> DrawResult<'_, '_, W> {
    pub fn size(self) -> Size {
        #[cfg(feature = "layout-diagnostics")]
        {
            diag::bump(Counter::SizeReads);
            diag::size_read(self.child.id(), self.painter.id, self.size);
        }
        self.painter.depend_on(self.child);
        self.painter.answer_under = self.painter.answer_under.and(self.answer_holds);
        self.size
    }

    pub fn len(self, axis: Axis) -> LayoutLen {
        self.size().axis(axis)
    }
}

/// What `Painter::primitive` takes: a primitive, or something that yields one
/// and does whatever else drawing it needs.
pub trait PrimitiveLike {
    type Primitive: Primitive;
    fn into_primitive(self, painter: &mut Painter) -> Self::Primitive;
}

impl<P: Primitive> PrimitiveLike for P {
    type Primitive = P;
    fn into_primitive(self, _: &mut Painter) -> P {
        self
    }
}

impl PrimitiveLike for &TextureHandle {
    type Primitive = TexturePrimitive;

    /// Retains a share of the handle, so the slot the primitive names cannot
    /// be freed and reused while it is still drawn.
    fn into_primitive(self, painter: &mut Painter) -> TexturePrimitive {
        painter.textures.push(self.clone());
        self.into()
    }
}

/// A child's answer as lengths of the parent's own region. A widget reports
/// a fraction of its own region, and `of` is that region as a length of this
/// one. Pixels come through untouched, being that many pixels wherever they
/// end up. A declared axis is already the parent's: it resolved the rule in
/// its own region, and the rule is what the report says.
fn in_parent_frame(size: Size, of: UiVec2, declared: [Option<LayoutLen>; 2]) -> Size {
    let mut size = size;
    for (axis, declared) in AXES.into_iter().zip(declared) {
        if declared.is_none() {
            *size.axis_mut(axis) = size.axis(axis).within_len(of.axis(axis));
        }
    }
    size
}

/// What a widget declares a length of its box to be. `leftover` is not one: a
/// share of what is left over is only a length to the widget dividing one,
/// so it passes up in the size instead.
pub(crate) fn declared_lens(widgets: &Widgets, id: WidgetId) -> [Option<LayoutLen>; 2] {
    let rules = widgets.size_rules(id);
    let widget = widgets.get_dyn(id);
    AXES.map(|axis| {
        rules.axis(axis).declared().or_else(|| {
            // A hint still narrows the box where no rule does, which is how a
            // widget with a natural pixel size -- an image, a gap -- gets that
            // size rather than the whole offer. That is the offer's business
            // rather than a declaration's, and this falls away once a widget
            // occupies its reported size inside the box it was offered.
            widget
                .and_then(|widget| widget.size_hint(axis))
                .filter(|len| len.leftover == Weight::ZERO)
        })
    })
}

/// Whether what a widget reported along an axis is the whole of the box it
/// is in rather than a part to be placed inside it. A share fills, because a
/// share is a length only to whoever divides one, and whoever did is the one
/// that handed down this box. A declared axis does too: the rule already gave
/// the region its length, and the rule's length is what the widget reports
/// there. And an axis the parent decided from the answer is
/// the answer already.
pub(crate) fn fills(reported: LayoutLen, declared: Option<LayoutLen>, decided: bool) -> bool {
    reported.leftover != Weight::ZERO || declared.is_some() || decided
}

/// What of the box it was given a widget's drawing occupies, as lengths of
/// that box: the size it reported wherever that is a part to be placed, and
/// the whole of the box wherever the answer fills it.
///
/// A reported fraction is a fraction of the box the widget drew in, where a
/// declared one is a fraction of the box its parent handed down -- a span
/// reporting `rel(1.0)` means all of what it was given, whatever that was a
/// fraction of. So this is a length of the box rather than a length composed
/// into it, and a box in pixels is this step from the given box's pixels.
pub(crate) fn placed_lens(
    size: Size,
    declared: [Option<LayoutLen>; 2],
    decided: [bool; 2],
) -> UiVec2 {
    let mut lens = UiVec2::FULL_SIZE;
    for (axis, (declared, decided)) in AXES.into_iter().zip(declared.into_iter().zip(decided)) {
        let reported = size.axis(axis);
        if !fills(reported, declared, decided) {
            *lens.axis_mut(axis) = Len::from_parts(reported.rel, reported.px);
        }
    }
    lens
}

/// Where that drawing sits: those lengths taken of the box the widget was
/// asked in, on the side of it that the widget's alignment says.
pub(crate) fn placed_box(region: UiRegion, lens: UiVec2, align: RegionAlign) -> UiRegion {
    let mut placed = region;
    for axis in AXES {
        // The whole of the box is already where it sits, and the arithmetic
        // below is the identity for it.
        if lens.axis(axis) == Len::FULL {
            continue;
        }
        let span = placed.axis_mut(axis);
        let len = lens.axis(axis).within_len(span.len());
        span.start += (span.len() - len).scale(align.axis(axis).rel());
        span.end = span.start + len;
    }
    placed
}

/// A declared axis gets a frame of that length, aligned within the parent's
/// slot (or the offer). Undeclared axes keep the offered frame and chosen
/// placement, so their reported fractions retain that reference.
pub(crate) fn ask_box(
    mut region: UiRegion,
    declared: [Option<LayoutLen>; 2],
    align: RegionAlign,
    placement: [Option<UiSpan>; 2],
) -> (UiRegion, [Option<UiSpan>; 2]) {
    let mut placed = [None; 2];
    for (axis, (len, chosen)) in AXES.into_iter().zip(declared.into_iter().zip(placement)) {
        let Some(len) = len else {
            placed[axis as usize] = chosen;
            continue;
        };
        let span = region.axis_mut(axis);
        let len = Len::from_parts(len.rel, len.px);
        let slot = chosen.unwrap_or(*span);
        span.start = slot.start + (slot.len() - len).scale(align.axis(axis).rel());
        span.end = span.start + len;
    }
    (region, placed)
}