#[cfg(feature = "layout-diagnostics")]
use crate::layout_diagnostics::{self as diag, Counter};
use crate::{
    Axis, Bound, Bounds, Declared, DrawScratch, Holds, LayoutHolds, LayoutLen, Len, PlaceDesc,
    PlaceFit, Px, PxVec2, RegionAlign, Rel, RenderedText, RequestArena, RequestedLen,
    RetainedPrimitive, Size, SizeRequests, StrongWidget, TextAttrs, TextBuffer, TextureHandle,
    UiRegion, UiRenderState, UiRsc, UiVec2, Weight, WidgetId, Widgets,
    render::{
        GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveInst, PrimitiveKind,
        TexturePrimitive,
    },
    ui::{
        place::{PlaceSpan, RelBase},
        render_state::{DrawInfo, Placing},
    },
};

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

    /// This widget's rel base, per axis: a length of the window, and what a
    /// fraction it or anything under it declares or reports is a fraction
    /// of. A length rather than a box, so padding can take from both the
    /// rel base and the box without either becoming the other.
    pub(super) rel_base: UiVec2,
    /// The box this widget was asked in, in its region node's coordinates:
    /// what it draws in, and what its children's places are parts of.
    pub(super) region: UiRegion,
    /// The window in pixels. Frames and boxes become pixels against this one
    /// unit, regardless of region-node boundaries.
    pub(super) window: PxVec2,
    pub(super) mask: MaskIdx,
    pub(super) textures: Vec<TextureHandle>,
    pub(super) primitives: Vec<RetainedPrimitive>,
    pub(super) mask_region: Option<UiRegion>,
    /// The previous drawing's owned mask, available for this draw to reclaim.
    pub(super) mask_slot: Option<MaskIdx>,
    /// Only children whose answers were read constrain this widget's answer.
    pub(super) answer_under: LayoutHolds,
    pub(super) children: Vec<WidgetId>,
    /// The children whose size this widget read while drawing.
    pub(super) size_deps: Vec<WidgetId>,
    pub(super) request_deps: Vec<WidgetId>,
    pub(super) scratch: DrawScratch,
    /// What this draw itself reads, as against what its children's drawings
    /// hold for: every window and every length of its own region until it
    /// reads one, then that one unless it says otherwise, and the rel base or
    /// region length it read symbolically, each of which makes the drawing
    /// hold for that length alone.
    pub(super) own: LayoutHolds,
    /// What each child's drawing depends on. Asking a child again replaces
    /// its drawing, so it replaces this too rather than narrowing it.
    pub(super) under: Vec<(WidgetId, 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> {
    /// Reuses this widget's allocation buffers across draws. A child drawn
    /// part way through gets a painter of its own, with buffers of its own,
    /// so nothing it does while this one is mid-row can reach these.
    pub fn with_requests<T>(
        &mut self,
        f: impl FnOnce(&mut Self, &mut Vec<RequestedLen>, &mut Vec<Px>) -> T,
    ) -> T {
        let mut requests = std::mem::take(&mut self.scratch.requests);
        let mut lengths = std::mem::take(&mut self.scratch.lengths);
        requests.clear();
        lengths.clear();
        let result = f(self, &mut requests, &mut lengths);
        self.scratch.requests = requests;
        self.scratch.lengths = lengths;
        result
    }

    /// Discovers a composable request without painting a provisional box.
    pub fn size_request<W: ?Sized>(
        &mut self,
        child: &StrongWidget<W>,
        axis: Axis,
    ) -> Option<RequestedLen> {
        if let Some(len) = self.size_hint(child, axis) {
            self.request_deps.push(child.id());
            return Some(len.into());
        }
        let start = self.request_deps.len();
        let mut requests = SizeRequests {
            arena: &mut self.state.requests,
            measured: None,
            widgets: self.rsc.widgets(),
            dependencies: &mut self.request_deps,
            rel_base: self.rel_base[axis],
        };
        // Intrinsic fixed content must keep its offered box for wrapping;
        // only a declaration or a share chooses the box it is drawn in.
        let request = requests.widget(child, axis).filter(|request| {
            request.has_leftover()
                || self.rsc.widgets().size_rules(child.id())[axis]
                    .request
                    .is_some()
        });
        if request.is_some() {
            self.rel_base(axis);
        } else {
            // A discarded request contributes no dependency: the measured
            // draw below records the size and box it actually used instead.
            self.request_deps.truncate(start);
        }
        request
    }

    /// Completes discovery after a child was measured. Only this call may use
    /// drawn answers: before the ask they could belong to an obsolete box.
    pub fn measured_request<W: ?Sized>(
        &mut self,
        child: &StrongWidget<W>,
        axis: Axis,
        len: LayoutLen,
    ) -> RequestedLen {
        let start = self.request_deps.len();
        let bound = self.rsc.widgets().size_rules(child.id())[axis].bound;
        let mut requests = SizeRequests {
            arena: &mut self.state.requests,
            measured: Some(&self.state.active),
            widgets: self.rsc.widgets(),
            dependencies: &mut self.request_deps,
            rel_base: self.rel_base[axis],
        };
        if let Some(request) = requests.widget(child, axis)
            && request.linear().is_none()
            && request.has_leftover()
        {
            self.rel_base(axis);
            return request;
        }
        let shares = len.leftover > Weight::ZERO;
        let request = match shares {
            true => requests.bounded(len.into(), bound),
            false => len.into(),
        };
        self.request_deps.truncate(start);
        // Only a bound that is a fraction was read against the rel base; one
        // in pixels binds at the same length under any of them.
        if shares && bound.has_fraction() {
            self.rel_base(axis);
        }
        request
    }

    /// Divides `room` between `requests`, one length per request in
    /// `output`. A deferred comparison is decided here, against this window:
    /// which side of a crossing the solution falls is a question in pixels,
    /// so the drawing holds only for the window that answered it.
    pub fn allocate(
        &mut self,
        requests: &[RequestedLen],
        room: Len,
        axis: Axis,
        output: &mut Vec<Px>,
    ) {
        let window = self.window[axis];
        self.own[axis].window = self.own[axis].window.and(Holds::at(window));
        output.clear();
        output.extend(
            self.state
                .requests
                .allocate(requests, room.to_px(window), window),
        )
    }

    /// The least a request can come to, which is what it takes of the row
    /// before anything is divided. A comparison is read at no share at all.
    pub fn minimum_request(&mut self, request: &RequestedLen, axis: Axis) -> Len {
        match request.linear() {
            Some(len) => len.without_leftover(),
            None => {
                let window = self.window[axis];
                self.own[axis].window = self.own[axis].window.and(Holds::at(window));
                Len::from_parts(Rel::ZERO, self.state.requests.minimum(*request, window))
            }
        }
    }

    fn primitive_at<P: Primitive>(&mut self, primitive: P, region: UiRegion) {
        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: UiRegion) {
        self.write_resolved(kind, primitive, region, self.resolve(region));
    }

    /// A box in this widget's region, composed into its region node's
    /// coordinates.
    fn resolve(&self, region: UiRegion) -> UiRegion {
        region.within(&self.region)
    }

    fn write_resolved<P: Primitive>(
        &mut self,
        kind: PrimitiveKind<P>,
        primitive: P,
        region: UiRegion,
        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 {
            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 primitive = primitive.into_primitive(self);
        self.primitive_at(primitive, UiRegion::FULL)
    }

    /// Writes a primitive in a part of this widget's own box, in that box's
    /// coordinates.
    pub fn primitive_within(&mut self, primitive: impl PrimitiveLike, region: UiRegion) {
        let primitive = primitive.into_primitive(self);
        self.primitive_at(primitive, region);
    }

    /// Sets a mask, in this widget's own box's coordinates.
    pub fn set_mask(&mut self, region: UiRegion) {
        self.mask_region = Some(region);
        assert!(self.mask == MaskIdx::NONE);
        let resolved = self.resolve(region);
        let move_idx = self.move_idx;
        let mask = Mask {
            region: resolved,
            move_idx,
        };
        let masks = &mut self.rsc.ui_mut().masks;
        self.mask = match self.mask_slot.take() {
            Some(idx) => {
                *masks.get_mut(idx) = mask;
                idx
            }
            None => {
                let idx = masks.push(mask);
                // The owner keeps the slot alive even with no primitives.
                masks.push_ref(idx);
                idx
            }
        };
    }

    /// Draws a widget in the whole of this widget's own box, with the rel
    /// base forwarded unchanged: what a container that is only a wrapper
    /// around one child wants.
    pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
        self.widget_at(id, UiRegion::FULL)
    }

    /// Resolves what the place says about the child's rel base into a length,
    /// where that is this widget's own narrowed the way the region is. An
    /// axis the region leaves whole is not read at all, so a wrapper that
    /// only moves its child does not pin its drawing to a rel base.
    fn resolve_rel_base(&mut self, mut place: PlaceDesc) -> PlaceDesc {
        for axis in Axis::BOTH {
            let at = &mut place[axis];
            let (RelBase::WithRegion, PlaceSpan::Within(span)) = (at.rel_base, at.span) else {
                continue;
            };
            let len = span.len();
            at.rel_base = match len == Len::FULL {
                true => RelBase::Inherit,
                false => RelBase::Len(len.within_len(self.rel_base(axis))),
            };
        }
        place
    }

    /// Asks a child, saying what its fractions are of and where it is asked.
    ///
    /// `place` says where the child goes and what its fractions are of:
    /// see [`PlaceDesc`]. A `UiRegion` converts into the common case, which
    /// is a box of this widget's own with the answer placed inside it.
    ///
    /// The child draws once, in the region that comes of it, and its answer
    /// is placed inside that region by re-expressing the drawing. Nothing is
    /// drawn again in a box an answer chose; a container that puts the
    /// answer somewhere else says so with [`Self::place_at`].
    pub fn widget_at<'s, W: ?Sized>(
        &'s mut self,
        id: &'s StrongWidget<W>,
        place: impl Into<PlaceDesc>,
    ) -> DrawResult<'s, 'a, W> {
        let offer = self.resolve_rel_base(place.into());
        let Ask {
            rel_base,
            region,
            place,
            declared,
            bounds,
            holds: ask_holds,
            inputs,
        } = self.placing().ask(
            self.rsc.widgets(),
            &mut self.state.requests,
            self.window,
            id.id(),
            offer,
        );
        self.own = self.own.and(inputs);
        let region_node = self.rsc.widgets().is_region_node(id.id());
        #[cfg(feature = "layout-diagnostics")]
        if region_node {
            diag::bump(Counter::RegionNodeDraws);
            diag::region_node(id.id(), self.id, region);
        }
        // A child listed twice would be moved twice.
        let re_asked = self.children.contains(&id.id());
        if !re_asked {
            self.children.push(id.id());
        }
        let drawn = self.state.draw_inner(
            id.id(),
            DrawInfo {
                layer: self.layer,
                parent: Some(self.id),
                depth: self.depth + 1,
                parent_move: self.move_idx,
                region_node,
                mask: self.mask,
                rel_base,
                region,
                placed: place,
                asked: offer,
                declared,
                bounds,
                ask_holds,
                re_asked,
            },
            None,
            self.rsc,
        );
        let holds = self.in_parent(drawn.drawing_holds, region, place, declared);
        let answer_holds = self.in_parent(drawn.answer.holds, region, place, declared);
        match self.under.iter_mut().find(|(child, _)| *child == id.id()) {
            Some((_, kept)) => *kept = holds,
            None => self.under.push((id.id(), holds)),
        }
        DrawResult {
            child: id,
            painter: self,
            size: drawn.answer.size,
            answer_holds,
        }
    }

    /// 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.under.retain(|(child, _)| *child != id.id());
        self.state.undraw_rec(id.id(), self.rsc);
    }

    /// Puts a child in `place` of this widget's box, where that box is the
    /// answer the child already gave: the drawing is re-expressed there
    /// rather than made again -- what a row does once it knows every slot,
    /// having measured each child from its cursor.
    ///
    /// A child this draw has not asked about, and one whose rel base this
    /// narrows, is asked here instead: there is no answer to re-express, or
    /// the question has changed. So a container that places every child the
    /// same way says it once, and which of the two happens is this widget's
    /// business rather than the caller's.
    pub fn place_at<'s, W: ?Sized>(
        &'s mut self,
        id: &'s StrongWidget<W>,
        place: impl Into<PlaceDesc>,
    ) -> DrawResult<'s, 'a, W> {
        let place = self.resolve_rel_base(place.into());
        let states_rel_base = Axis::BOTH
            .iter()
            .any(|&axis| matches!(place[axis].rel_base, RelBase::Len(_)));
        if states_rel_base || !self.children.contains(&id.id()) {
            return self.widget_at(id, place);
        }
        let at = self.placing();
        self.state.place_in(id.id(), &at, place, self.rsc);
        let active = &self.state.active[&id.id()];
        let size = active.measured().unwrap_or(active.size);
        DrawResult {
            child: id,
            painter: self,
            size,
            // Read where it was asked; moving it is not a second answer.
            answer_holds: LayoutHolds::ANY,
        }
    }

    /// This widget as the thing its children are placed within.
    fn placing(&self) -> Placing {
        Placing {
            id: Some(self.id),
            region: self.region,
            rel_base: self.rel_base,
            depth: self.depth,
            move_idx: self.move_idx,
            mask: self.mask,
        }
    }

    /// What a child says its length is without being drawn, if it can say,
    /// as the length its draw would report: a fraction in it is resolved
    /// against this widget's rel base, which is the rel base a child asked with
    /// nothing narrowed gets. Asking counts as reading its size.
    pub fn size_hint<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<LayoutLen> {
        // A bound is composed into a request rather than applied to a hint,
        // so a bounded widget cannot say its length without being asked: what
        // it comes to is a comparison only the ask or the allocator makes.
        // A miss rather than no read at all, so the counters see it.
        let bounded = self.rsc.widgets().size_rules(id.id())[axis].bound != Bound::ANY;
        let hint = (!bounded)
            .then(|| self.rsc.widgets().exact_len(id.id(), axis))
            .flatten();
        let rel_base = self.rel_base[axis];
        let resolved = hint.map(|hint| hint.within_len(rel_base));
        #[cfg(feature = "layout-diagnostics")]
        {
            diag::hint_read(id.id(), self.id, axis, resolved);
            diag::bump(match resolved {
                Some(_) => Counter::HintHits,
                None => Counter::HintMisses,
            });
        }
        if let Some(hint) = hint {
            self.depend_on(id);
            // Resolving a fraction against this rel base makes this draw a
            // function of the rel base's length. The fraction to ask about is
            // the child's own: resolved against a rel base of pixels, none is
            // left to see it by.
            if hint.rel != Rel::ZERO {
                self.own[axis].rel_base = Some(rel_base);
            }
        }
        resolved
    }

    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 rel base or region coordinates.
    // TODO: merge the text methods into the primitive ones.
    pub fn glyphs(&mut self, text: &RenderedText, origin: UiRegion) {
        // Glyph offsets and sizes are pixels, which compose additively.
        // Only the shared origin needs composing through the region.
        let resolved = self.resolve(origin);
        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(),
                },
                place(origin),
                place(resolved),
            );
        }
    }

    /// The symbolic length of this widget's own box along one axis, in the
    /// lengths of its rel base that it places its children in. Reading it pins
    /// the drawing to that length -- and to nothing about where the box
    /// starts, which is what lets a container move without being drawn
    /// again. One axis at a time, because a container that divides one axis
    /// holds for any length of the other.
    pub fn region_len(&mut self, axis: Axis) -> Len {
        let len = self.region[axis].len();
        self.own[axis].region_len = Some(len);
        len
    }

    /// This widget's rel base along one axis: what a fraction it or anything
    /// under it declares or reports is a fraction of. A container reads it
    /// to hand a length of it down -- padding, which takes its pixels off.
    /// Reading it pins the drawing to that rel base, the way
    /// [`Self::region_len`] pins it to the box.
    pub fn rel_base(&mut self, axis: Axis) -> Len {
        let len = self.rel_base[axis];
        self.own[axis].rel_base = Some(len);
        len
    }

    /// 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]
            .request
            .is_some()
    }

    /// 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 len = self.region[axis].len();
        let px = len.to_px(self.window[axis]);
        let own = &mut self.own[axis].region;
        if *own == Holds::ANY {
            *own = Holds::at(px);
        }
        px
    }

    /// 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 len = self.region[axis].len();
        let holds = holds.into();
        debug_assert!(
            holds.contains(len.to_px(self.window[axis])),
            "'{}' ({:?}) says its drawing holds for lengths that leave out its own box",
            self.label(),
            self.id
        );
        self.own[axis].region = holds;
    }

    /// A window length in pixels, which is what every length in layout is
    /// measured in. Reading one pins the drawing to this window wherever the
    /// length is a fraction of it; one that is only pixels is that many
    /// pixels in any window and pins nothing.
    pub fn to_px(&mut self, len: Len, axis: Axis) -> Px {
        let window = self.window[axis];
        if len.rel != Rel::ZERO {
            let own = &mut self.own[axis].window;
            if *own == Holds::ANY {
                *own = Holds::at(window);
            }
        }
        len.to_px(window)
    }

    /// [`Len::longer_than`], asked on this widget's behalf: the windows the
    /// comparison comes out the same way on are windows its drawing holds
    /// for, and nowhere else does it. What a container has left for the
    /// shares it divides is the one thing that asks.
    ///
    /// Narrowed rather than stated, because whatever else this widget read
    /// about the window is a reason its drawing holds where it does too.
    pub fn longer_than(&mut self, len: Len, than: Len, axis: Axis) -> bool {
        let window = self.window[axis];
        let (longer, holds) = len.longer_than(than, window);
        debug_assert!(
            holds.contains(window),
            "'{}' ({:?}) compared two lengths and kept a range without this window",
            self.label(),
            self.id
        );
        self.own[axis].window = self.own[axis].window.and(holds);
        longer
    }

    /// The windows this drawing holds for, stated rather than taken: a
    /// container that branched on a length in pixels says which side of the
    /// boundary it was on, which is wider than the one window reading that
    /// length pins, and replaces it.
    pub fn window_holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
        let holds = holds.into();
        debug_assert!(
            holds.contains(self.window[axis]),
            "'{}' ({:?}) says its drawing holds for windows that leave out this one",
            self.label(),
            self.id
        );
        self.own[axis].window = holds;
    }

    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]
    }
}

/// 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()
    }
}

impl Painter<'_> {
    /// Moves what a child depends on into this widget's own terms, taking
    /// only what the child was asked with.
    ///
    /// Window ranges are already about the one unit and combine directly.
    /// A rel base pin becomes this widget's own rel base wherever a length of it
    /// is what reached the child; where only pixels did, no length of this
    /// rel base can change the child's and the pin stops here.
    ///
    /// A child's validity maps back through the part of this widget's box,
    /// where the box the child was asked in is that part; a declared length
    /// places the box inside the part instead, and then only that length
    /// reaches the child. A narrowed rel base is not one of these: it decides
    /// what fractions under the child mean and leaves the box the part it
    /// was given.
    fn in_parent(
        &self,
        holds: LayoutHolds,
        region: UiRegion,
        place: PlaceDesc,
        declared: Declared,
    ) -> LayoutHolds {
        let mut result = LayoutHolds::ANY;
        for axis in Axis::BOTH {
            let declared = declared[axis];
            let holds = holds[axis];
            let at = place[axis];
            let result = &mut result[axis];
            // Every read became pixels against the window, so a range on
            // it is already in this widget's terms.
            result.window = holds.window;
            // A length this widget named -- a resolved share, a box a sibling
            // decided, a box it sized outright, which is its own base -- is
            // not a length of this widget's rel base, so a pin on it stops
            // here. So does a declaration in pixels: no length of either base
            // is in it to see.
            let reaches = !matches!(at.rel_base, RelBase::Len(_))
                && declared.is_none_or(|len| len.rel != Rel::ZERO);
            result.rel_base = holds.rel_base.and(reaches.then(|| self.rel_base[axis]));
            match (at.span, declared) {
                // Its box is a part of this widget's own box, in that box's
                // own lengths, so what it holds for maps back through that
                // part into a range on this widget's box. A length it pinned
                // is this widget's length less the part's pixels where the
                // part is the whole of the box less pixels, which is the one
                // shape that inverts exactly; any other part pins this
                // widget's own length.
                (PlaceSpan::Within(span), None) => {
                    let part_len = span.len();
                    result.region = holds.region.through(part_len);
                    result.region_len = holds.region_len.map(|pinned| match part_len.rel {
                        Rel::ONE => pinned - Len::from_parts(Rel::ZERO, part_len.px),
                        _ => self.region[axis].len(),
                    });
                }
                // Its box is a length this widget decided, from its own
                // rel base or from a sibling's answer: no length of this
                // widget's box reaches it, so what it holds for is a range
                // on the window and none of it on that box.
                _ => {
                    result.window = result.window.and(holds.region.through(region[axis].len()));
                }
            }
        }
        result
    }
}

impl Widgets {
    /// What says a widget's length on one axis without drawing it, if anything
    /// does. 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 -- a
    /// share included, since a share is a length only to whoever divides one,
    /// and that is the parent rather than this widget.
    fn exact_len(&self, id: WidgetId, axis: Axis) -> Option<LayoutLen> {
        // A request is a length the rule gives, and the hint below must not
        // narrow the box in its place: what the request comes to is not known
        // until the parent allocates, and it is the parent's answer, not this
        // widget's.
        let rule = &self.size_rules(id)[axis];
        if rule.deferred().is_some() {
            return None;
        }
        rule.exact().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.
            self.get_dyn(id)?.size_hint(axis)
        })
    }
}

/// One ask of a widget: the box it draws in, what its fractions are of, and
/// what deciding those read.
pub(super) struct Ask {
    pub rel_base: UiVec2,
    pub region: UiRegion,
    /// The place the ask came to, which a rule of the widget's own can take
    /// past the box its parent offered.
    pub place: PlaceDesc,
    /// What the widget's box is on each axis where something says so
    /// outright: its rule, its hint, or a bound the offer fell outside.
    /// An intrinsic answer can still occupy less than a capped box.
    pub declared: Declared,
    /// Its bounds, resolved against the rel base its rules were resolved
    /// against, for the answer to be held to where the box was not.
    pub bounds: Bounds,
    /// What the ask itself holds for, kept on the widget asked about: a rule
    /// compared against the offer in pixels holds only for the windows on its
    /// side of the crossing, and that range reaches whoever asked through the
    /// drawing it is part of. Kept on the widget asked about rather than on
    /// the asker because the root has no asker.
    pub holds: LayoutHolds,
    /// Inputs read against the parent before declarations choose a new base.
    /// These belong to the asker; the widget's own holds describe its output box.
    pub inputs: LayoutHolds,
}

impl Placing {
    /// Asks about a widget at `place` of this box, with the widget's own
    /// rules applied to what the place offers it. `place` is resolved: what
    /// a rel base of the asker's is a fraction of, the asker worked out.
    ///
    /// Every ask is this one, the root's included -- there the box is the
    /// window and nothing above narrowed it, which is what [`Self::WINDOW`]
    /// says.
    pub(super) fn ask(
        &self,
        widgets: &Widgets,
        requests: &mut RequestArena,
        window: PxVec2,
        id: WidgetId,
        mut place: PlaceDesc,
    ) -> Ask {
        let align = widgets.alignment(id);
        let rules = widgets.size_rules(id);
        let mut holds = LayoutHolds::ANY;
        let mut inputs = LayoutHolds::ANY;
        let mut declared = Declared::NONE;
        let mut bounds = Bounds::ANY;
        for axis in Axis::BOTH {
            let base = place.base(axis, self.rel_base);
            let stated = widgets.exact_len(id, axis);
            if let Some(len) = stated.and_then(|len| len.declared()) {
                if len.rel != Rel::ZERO {
                    inputs[axis].rel_base = Some(self.rel_base[axis]);
                }
                declared[axis] = Some(len.within_len(base));
            }
            if let Some(request) = rules[axis].deferred() {
                inputs[axis].rel_base = Some(self.rel_base[axis]);
                inputs[axis].region_len = Some(self.region[axis].len());
                inputs[axis].window = Holds::at(window[axis]);
                let offer = place.of(self.region, align)[axis].len();
                let px = if place[axis].fit == PlaceFit::Allocated {
                    offer.to_px(window[axis])
                } else {
                    let request = requests.import(request, base);
                    let request = requests.bounded(request, rules[axis].bound.within_len(base));
                    holds[axis].window = Holds::at(window[axis]);
                    requests
                        .allocate(&[request], offer.to_px(window[axis]), window[axis])
                        .next()
                        .unwrap()
                };
                let len = Len::from_parts(Rel::ZERO, px);
                place[axis].rel_base = RelBase::Len(len);
                declared[axis] = Some(len);
                holds[axis].rel_base = Some(len);
                continue;
            }
            // A share fills what the pixels and fraction beside it leave of
            // the box and overflows where they are longer, which is the rule
            // a span follows with one child. Only the overflow is a box of
            // the widget's own: a share that fits is the box it was given,
            // which is what this place already says.
            let (share, kept) = self.share_past_the_offer(stated, window[axis], place, align, axis);
            holds[axis].window = holds[axis].window.and(kept);
            if let Some(len) = share {
                place[axis] = len.as_desc().fills();
            }
            let bound = rules[axis].bound;
            if bound != Bound::ANY {
                if bound.has_fraction() {
                    inputs[axis].rel_base = Some(self.rel_base[axis]);
                }
                // A solved slot may narrow the widget's rel base, but the
                // allocator evaluated its bounds against this parent's base.
                let bound_base = if place[axis].fit == PlaceFit::Allocated {
                    self.rel_base[axis]
                } else {
                    base
                };
                bounds[axis] = bound.within_len(bound_base);
                let offer =
                    declared[axis].unwrap_or_else(|| place.of(self.region, align)[axis].len());
                let (held, kept) = bounds[axis].outside(offer, window[axis]);
                holds[axis].window = holds[axis].window.and(kept);
                // The comparison reads the incoming box, before a bound
                // replaces it. Placement keeps that decision; only an ask
                // may compare a new offer.
                if declared[axis].is_none() {
                    inputs[axis].region_len = Some(self.region[axis].len());
                }
                if let Some(len) = held {
                    declared[axis] = Some(len);
                }
            }
        }
        let (rel_base, region) =
            place.rel_base_and_region(self.region, self.rel_base, declared, align);
        Ask {
            rel_base,
            region,
            place,
            declared,
            bounds,
            holds,
            inputs,
        }
    }

    /// The box a widget's own share asks for where that is longer than the
    /// box `place` gives it, and nothing where the share fits -- with the
    /// windows that answer holds for, which is a range either way.
    ///
    /// A share is a length only to whoever divides one, and nothing divides a
    /// box handed to one child: what is left of it after the pixels and the
    /// fraction beside the share is what the share takes, so the length comes
    /// to the whole box until those are longer than it and to them once they
    /// are. Only that second case is a box its parent did not give, and the
    /// crossing between them is a question in pixels.
    fn share_past_the_offer(
        &self,
        stated: Option<LayoutLen>,
        window: Px,
        place: PlaceDesc,
        align: RegionAlign,
        axis: Axis,
    ) -> (Option<Len>, Holds) {
        // A place that is the widget's placement outright is a box its parent
        // decided, and a parent that divides one has already given the share
        // whatever it was owed. Only an offer -- a box with the answer still
        // to be placed inside it -- is a box a share reads.
        if place[axis].fit.fills() {
            return (None, Holds::ANY);
        }
        // A share with nothing beside it is the box whatever the box is, so
        // there is no comparison to make and no range to keep for one.
        let Some(stated) = stated else {
            return (None, Holds::ANY);
        };
        if stated.leftover == Weight::ZERO || stated.is_only_leftover() {
            return (None, Holds::ANY);
        }
        let fixed = stated
            .without_leftover()
            .within_len(place.base(axis, self.rel_base));
        let offer = place.of(self.region, align)[axis].len();
        let (longer, holds) = fixed.longer_than(offer, window);
        (longer.then_some(fixed), holds)
    }
}

impl LayoutLen {
    /// 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. An axis the parent decided
    /// from the answer is the answer already.
    pub(super) fn fills(&self, decided: bool) -> bool {
        self.leftover != Weight::ZERO || decided
    }
}

impl PlaceDesc {
    /// Where a widget's drawing goes inside the part its parent gave it: what
    /// it reported, on the side of the part its alignment says, and the whole
    /// part wherever the answer fills it.
    ///
    /// The length it reported is a length of its rel base, and the part is one
    /// too, so this takes one from the other rather than composing it into the
    /// part. That is what makes a fraction the same fraction wherever the part
    /// it is placed in sits and however long it is -- the fraction is resolved
    /// once, here, against the rel base it was reported of.
    pub(super) fn placement(self, region: UiRegion, size: Size, align: RegionAlign) -> UiRegion {
        let mut placed = region;
        for axis in Axis::BOTH {
            let reported = size[axis];
            if reported.fills(self[axis].fit.fills()) {
                continue;
            }
            placed[axis] = placed[axis].place(reported.without_leftover(), align[axis]);
        }
        placed
    }

    /// The rel base length and the box a child is asked in, in the coordinates the
    /// widget asking draws in.
    ///
    /// `own` is that widget's own box, and `place` what of it the child is
    /// given, including any rel base it states -- a row's slot, or padding's rel
    /// base less its pixels. The ask has already resolved declarations into
    /// window lengths, so placement only aligns them inside the given box.
    pub(super) fn rel_base_and_region(
        self,
        own: UiRegion,
        parent_rel_base: UiVec2,
        declared: Declared,
        align: RegionAlign,
    ) -> (UiVec2, UiRegion) {
        let given = self.of(own, align);
        let mut rel_base = parent_rel_base;
        let mut region = given;
        for axis in Axis::BOTH {
            let base = self.base(axis, parent_rel_base);
            let len = declared[axis].unwrap_or(base);
            rel_base[axis] = len;
            if declared[axis].is_some() {
                region[axis] = given[axis].place(len, align[axis]);
            }
        }
        (rel_base, region)
    }
}