use crate::prelude::*;
use std::{cell::Cell, cell::RefCell, rc::Rc};

pub(crate) struct TestRsc {
    pub(crate) ui: Ui,
}

impl UiRsc for TestRsc {
    fn ui(&self) -> &Ui {
        &self.ui
    }
    fn ui_mut(&mut self) -> &mut Ui {
        &mut self.ui
    }
}

struct FixedRect(f32);

impl Widget for FixedRect {
    fn draw(&mut self, painter: &mut Painter) {
        let size = Size::from_axis(Axis::Y, Len::abs(self.0), Len::REST);
        let paint = painter.paint(&PaintId::WHITE);
        painter.primitive_within(
            RectPrimitive::color(paint),
            size.to_uivec2(painter.density())
                .align(RegionAlign::TOP_LEFT),
        );
        painter.set_size(size);
    }
}

struct ChildOffset {
    child: StrongWidget,
    offset: Vec2,
}

impl Widget for ChildOffset {
    fn draw(&mut self, painter: &mut Painter) {
        painter.set_child_offset(self.offset);
        let size = painter.widget(&self.child).size();
        painter.set_size(size);
    }
}

struct TracedLeaf {
    height: f32,
    trace: Rc<RefCell<Vec<&'static str>>>,
}

impl Widget for TracedLeaf {
    fn draw(&mut self, painter: &mut Painter) {
        self.trace.borrow_mut().push("child");
        painter.set_size(Size::from_axis(Axis::Y, Len::abs(self.height), Len::REST));
    }
}

struct TracedParent {
    child: StrongWidget,
    reads_child_size: bool,
    trace: Rc<RefCell<Vec<&'static str>>>,
}

struct CountedLeaf {
    height: f32,
    draws: Rc<Cell<u32>>,
}

impl Widget for CountedLeaf {
    fn draw(&mut self, painter: &mut Painter) {
        self.draws.set(self.draws.get() + 1);
        painter.set_size(Size::from_axis(Axis::Y, Len::abs(self.height), Len::REST));
    }
}

impl Widget for TracedParent {
    fn draw(&mut self, painter: &mut Painter) {
        self.trace.borrow_mut().push("parent");
        let child = painter.widget(&self.child);
        let size = if self.reads_child_size {
            child.size()
        } else {
            Size::REST
        };
        painter.set_size(size);
    }
}

#[test]
fn a_widget_retains_its_entry_layer_not_its_child_cursor() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let back = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE));
    let front = rsc.ui.widgets.add_strong(Rect::new(PaintId::RED));
    let stack = rsc.ui.widgets.add_strong(Stack {
        children: vec![back.any(), front.any()],
        size: StackSize::Default,
    });
    let stack_id = stack.id();
    let outer = rsc.ui.widgets.add_strong(Sized {
        inner: stack.any(),
        x: None,
        y: None,
    });
    let outer_weak = outer.weak();
    let root = outer.any();
    let mut render = UiRenderState::new();
    render.resize((100.0, 100.0));
    render.update(&root, &mut rsc);

    assert_eq!(
        render.active[&stack_id].layer, 0,
        "Stack entered on layer 0; layers 1 and 2 belong only to its children"
    );

    render.take_counters();
    rsc.ui.widgets.get_mut(&outer_weak).unwrap().x = None;
    render.update(&root, &mut rsc);
    assert_eq!(
        render.take_counters().draws,
        1,
        "redrawing the parent should retain the unchanged Stack subtree"
    );
}

#[test]
fn a_size_dependent_parent_is_invalidated_before_layout_runs_downward() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let trace = Rc::new(RefCell::new(Vec::new()));
    let child = rsc.ui.widgets.add_strong(TracedLeaf {
        height: 20.0,
        trace: trace.clone(),
    });
    let child_weak = child.weak();
    let parent = rsc.ui.widgets.add_strong(TracedParent {
        child: child.any(),
        reads_child_size: true,
        trace: trace.clone(),
    });
    let parent_weak = parent.weak();
    let root = parent.any();
    let mut render = UiRenderState::new();
    render.resize((100.0, 100.0));
    render.update(&root, &mut rsc);
    trace.borrow_mut().clear();

    rsc.ui.widgets.get_mut(&child_weak).unwrap().height = 40.0;
    render.update(&root, &mut rsc);

    assert_eq!(&*trace.borrow(), &["parent", "child"]);
    assert_eq!(render.active[&parent_weak.id()].size.y, Len::abs(40.0));
}

#[test]
fn a_parent_that_ignores_child_size_is_not_invalidated_with_it() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let trace = Rc::new(RefCell::new(Vec::new()));
    let child = rsc.ui.widgets.add_strong(TracedLeaf {
        height: 20.0,
        trace: trace.clone(),
    });
    let child_weak = child.weak();
    let parent = rsc.ui.widgets.add_strong(TracedParent {
        child: child.any(),
        reads_child_size: false,
        trace: trace.clone(),
    });
    let root = parent.any();
    let mut render = UiRenderState::new();
    render.resize((100.0, 100.0));
    render.update(&root, &mut rsc);
    trace.borrow_mut().clear();

    rsc.ui.widgets.get_mut(&child_weak).unwrap().height = 40.0;
    render.update(&root, &mut rsc);

    assert_eq!(&*trace.borrow(), &["child"]);
}

#[test]
fn a_span_reuses_unchanged_sibling_sizes_when_only_its_along_extent_changes() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let changed_draws = Rc::new(Cell::new(0));
    let sibling_draws = Rc::new(Cell::new(0));
    let changed = rsc.ui.widgets.add_strong(CountedLeaf {
        height: 20.0,
        draws: changed_draws.clone(),
    });
    let changed_weak = changed.weak();
    let sibling = rsc.ui.widgets.add_strong(CountedLeaf {
        height: 20.0,
        draws: sibling_draws.clone(),
    });
    let span = rsc.ui.widgets.add_strong(Span {
        children: vec![changed.any(), sibling.any()],
        dir: Dir::DOWN,
        gap: Len::ZERO,
    });
    let root = rsc
        .ui
        .widgets
        .add_strong(Sized {
            inner: span.any(),
            x: None,
            y: None,
        })
        .any();
    let mut render = UiRenderState::new();
    render.resize((100.0, 100.0));
    render.update(&root, &mut rsc);
    changed_draws.set(0);
    sibling_draws.set(0);

    rsc.ui.widgets.get_mut(&changed_weak).unwrap().height = 40.0;
    render.update(&root, &mut rsc);

    assert!(changed_draws.get() > 0, "the changed child was not redrawn");
    assert_eq!(
        sibling_draws.get(),
        0,
        "a vertical size change redrew a sibling whose width stayed fixed"
    );
}

#[test]
fn a_child_coordinate_offset_moves_only_the_child_subtree() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let child = rsc.ui.widgets.add_strong(FixedRect(40.0));
    let child_weak = child.weak();
    let parent = rsc.ui.widgets.add_strong(ChildOffset {
        child: child.any(),
        offset: vec2(0.0, 15.0),
    });
    let parent_weak = parent.weak();
    let outer = rsc.ui.widgets.add_strong(Sized {
        inner: parent.any(),
        x: None,
        y: None,
    });
    let outer_weak = outer.weak();
    let root = outer.any();
    let mut render = UiRenderState::new();
    render.resize((100.0, 100.0));
    render.update(&root, &mut rsc);
    render.take_counters();

    let parent_before = render.window_region(&parent_weak, &rsc).unwrap();
    let child_before = render.window_region(&child_weak, &rsc).unwrap();
    assert!((parent_before.top_left.y - 0.0).abs() < 0.01);
    assert!((child_before.top_left.y - 15.0).abs() < 0.01);

    rsc.ui.widgets.get_mut(&parent_weak).unwrap().offset.y = 35.0;
    rsc.ui.widgets.get_mut(&outer_weak).unwrap().x = None;
    render.update(&root, &mut rsc);
    let RenderCounters {
        draws,
        region_rewrites: rewrites,
        moves,
        ..
    } = render.take_counters();
    let parent_after = render.window_region(&parent_weak, &rsc).unwrap();
    let child_after = render.window_region(&child_weak, &rsc).unwrap();

    assert_eq!((draws, rewrites, moves), (2, 0, 1));
    assert_eq!(
        parent_after, parent_before,
        "the container itself stays put"
    );
    assert!((child_after.top_left.y - 35.0).abs() < 0.01);
}

#[test]
fn a_hinted_rest_draws_once_and_only_moves_the_fixed_child_after_it() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let first = rsc.ui.widgets.add_strong(FixedRect(40.0));
    let fill = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE));
    let fill = rsc.ui.widgets.add_strong(Sized {
        inner: fill.any(),
        x: None,
        y: Some(Len::REST),
    });
    let last = rsc.ui.widgets.add_strong(FixedRect(40.0));
    let last_w = last.weak();
    let mut span = Span::empty(Dir::DOWN);
    span.push(first.any());
    span.push(fill.any());
    span.push(last.any());
    let root = rsc.ui.widgets.add_strong(span).any();

    let mut render = UiRenderState::new();
    render.resize((200.0, 300.0));
    render.update(&root, &mut rsc);
    let RenderCounters { draws, moves, .. } = render.take_counters();

    assert_eq!(draws, 5);
    assert_eq!(moves, 1);
    let last = render.window_region(&last_w, &rsc).unwrap();
    assert!((last.top_left.y - 260.0).abs() < 0.01, "{last:?}");
}

fn scrolled_rects(
    rsc: &mut TestRsc,
    n: usize,
) -> (WeakWidget<ScrollArea>, StrongWidget, Vec<WeakWidget<Rect>>) {
    let mut span = Span::empty(Dir::DOWN);
    let mut rects = Vec::with_capacity(n);
    for _ in 0..n {
        let rect = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE));
        rects.push(rect.weak());
        let row = rsc.ui.widgets.add_strong(Sized {
            inner: rect.any(),
            x: None,
            y: Some(Len::abs(10.0)),
        });
        span.push(row.any());
    }
    let span = rsc.ui.widgets.add_strong(span);
    let scroll = rsc
        .ui
        .widgets
        .add_strong(ScrollArea::new(span.any(), Axis::Y, Pin::Start));
    let weak = scroll.weak();
    (weak, scroll.any(), rects)
}

#[test]
fn an_unchanged_frame_draws_and_rewrites_nothing() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let (_scroll, root, _rects) = scrolled_rects(&mut rsc, 500);
    let mut render = UiRenderState::new();
    render.resize((800.0, 20000.0));

    render.update(&root, &mut rsc);
    render.update(&root, &mut rsc);
    render.take_counters(); // discard the first, real draws

    render.update(&root, &mut rsc);
    let RenderCounters {
        draws,
        region_rewrites: rewrites,
        moves,
        ..
    } = render.take_counters();
    assert_eq!((draws, rewrites, moves), (0, 0, 0));
}

#[test]
fn scrolling_moves_in_o1_without_a_redraw() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let (scroll, root, _rects) = scrolled_rects(&mut rsc, 500);
    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));

    render.update(&root, &mut rsc);
    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(0.0);
    render.update(&root, &mut rsc);
    render.take_counters();

    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(-40.0);
    render.update(&root, &mut rsc);
    let RenderCounters { draws, moves, .. } = render.take_counters();

    assert_eq!(draws, 1, "only Scroll itself should redraw");
    assert_eq!(moves, 1, "the scrolled subtree should move in one write");
}

#[test]
fn fractional_scrolling_keeps_primitive_edges_on_physical_pixels() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let (scroll, root, rects) = scrolled_rects(&mut rsc, 20);
    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));

    render.update(&root, &mut rsc);
    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(0.0);
    render.update(&root, &mut rsc);
    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(-37.25);
    render.update(&root, &mut rsc);

    let slot = render
        .first_primitive(rects[2].id())
        .expect("the scrolled rect has a primitive");
    let corners = render.primitive_corners(slot, &rsc);
    for edge in [
        corners.top_left.x,
        corners.top_left.y,
        corners.bot_right.x,
        corners.bot_right.y,
    ] {
        assert_eq!(edge.fract(), 0.0, "fractional rendered edge: {corners:?}");
    }
}

#[test]
fn hit_testing_follows_a_scrolled_widget() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let (scroll, root, rects) = scrolled_rects(&mut rsc, 500);
    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));
    render.update(&root, &mut rsc);
    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(0.0);
    render.update(&root, &mut rsc);

    let target = &rects[2];
    let before = render.resolved_region(target, &rsc).unwrap();

    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(-37.0);
    render.update(&root, &mut rsc);

    let after = render.resolved_region(target, &rsc).unwrap();
    let before_px = before.to_px((800.0, 600.0).into());
    let after_px = after.to_px((800.0, 600.0).into());

    assert!(
        (after_px.top_left.y - (before_px.top_left.y - 37.0)).abs() < 0.01,
        "before={before_px:?} after={after_px:?}"
    );
}

#[test]
fn redrawing_a_masked_widget_does_not_nest_its_own_mask() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let (_scroll, inner_root, _rects) = scrolled_rects(&mut rsc, 8);
    let masked = rsc.ui.widgets.add_strong(Masked {
        shape: None,
        inner: inner_root,
    });
    let masked_id = masked.id();
    let root = masked.any();
    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));
    render.update(&root, &mut rsc);

    render.redraw(masked_id, &mut rsc);
    render.redraw(masked_id, &mut rsc);

    assert_eq!(
        render.active.get(&masked_id).unwrap().mask,
        MaskIdx::NONE,
        "a `Masked` at the root is drawn under no mask of its own"
    );
}

#[test]
fn a_mask_stays_put_while_its_scrolled_content_moves() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let (scroll, inner_root, _rects) = scrolled_rects(&mut rsc, 500);
    let masked = rsc.ui.widgets.add_strong(Masked {
        shape: None,
        inner: inner_root,
    });
    let masked_id = masked.id();
    let root = masked.any();
    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));

    render.update(&root, &mut rsc);
    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(0.0);
    render.update(&root, &mut rsc);

    let masked_slot_before = render.active.get(&masked_id).unwrap().move_slot;
    let mask_delta_before = rsc.ui.move_offsets[masked_slot_before.idx()].delta;

    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(-40.0);
    render.update(&root, &mut rsc);

    let masked_slot_after = render.active.get(&masked_id).unwrap().move_slot;
    let mask_delta_after = rsc.ui.move_offsets[masked_slot_after.idx()].delta;

    assert_eq!(mask_delta_before, [0.0, 0.0]);
    assert_eq!(mask_delta_after, [0.0, 0.0]);
}

fn composer_like_tree(rsc: &mut TestRsc) -> (WeakWidget<TextEdit>, StrongWidget) {
    let field = wtext("")
        .editable(EditMode::MultiLine)
        .text_align(Align::LEFT)
        .wrap(true)
        .size(18)
        .color(PaintId::WHITE)
        .add(rsc);
    let bar = (field.pad(dp(12)).width(rest(1)),)
        .span(Dir::RIGHT)
        .background(rect(Srgba8::new(40, 40, 46, 255)))
        .add(rsc);
    let list_stand_in = rect(PaintId::BLACK).height(rest(1)).add(rsc);
    let tree = (list_stand_in, bar).span(Dir::DOWN).add_strong(rsc).any();
    (field, tree)
}

#[test]
fn composing_text_after_a_keyboard_resize_lands_in_the_bars_own_region() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let (field, root) = composer_like_tree(&mut rsc);
    let mut render = UiRenderState::new();

    render.resize((1080.0, 2298.0));
    render.update(&root, &mut rsc);
    field(&mut rsc).select(vec2(40.0, 2250.0), vec2(1080.0, 2298.0), false, false);
    field(&mut rsc).insert("a");
    render.update(&root, &mut rsc);

    let before_px = render.window_region(&field, &rsc).unwrap();
    assert!(
        before_px.bot_right.y - before_px.top_left.y < 200.0,
        "before a resize: {before_px:?}"
    );
    assert!(
        before_px.top_left.y > 1800.0,
        "expected the bar near the bottom before a resize: {before_px:?}"
    );

    render.resize((1080.0, 1478.0));
    render.update(&root, &mut rsc);
    field(&mut rsc).insert("b");
    render.update(&root, &mut rsc);

    let after_px = render.window_region(&field, &rsc).unwrap();
    assert!(
        after_px.bot_right.y - after_px.top_left.y < 200.0,
        "after a resize + keystroke: {after_px:?}"
    );
    assert!(
        after_px.top_left.y > 1200.0,
        "expected the bar near the bottom of the shorter window: {after_px:?}"
    );
}

#[test]
fn a_scroll_measures_the_box_it_was_offered_not_the_window() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let rect = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE));
    let tall = rsc.ui.widgets.add_strong(Sized {
        inner: rect.any(),
        x: None,
        y: Some(Len::abs(1000.0)),
    });
    let scroll = rsc
        .ui
        .widgets
        .add_strong(ScrollArea::new(tall.any(), Axis::Y, Pin::Start));
    let scroll_w = scroll.weak();
    let scroll_id = scroll.id();
    let capped = rsc.ui.widgets.add_strong(MaxSize {
        inner: scroll.any(),
        x: None,
        y: Some(Len::abs(100.0)),
    });
    let capped_id = capped.id();
    let root = capped.any();

    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));
    render.update(&root, &mut rsc);
    rsc.ui.widgets.get_mut(&scroll_w).unwrap().scroll(0.0);
    render.update(&root, &mut rsc);

    assert_eq!(
        render.active.get(&scroll_id).unwrap().size.y,
        Len::abs(1000.0)
    );
    assert_eq!(
        render.active.get(&capped_id).unwrap().size.y,
        Len::abs(100.0),
        "the cap, not the content and not the window"
    );

    rsc.ui.widgets.get_mut(&scroll_w).unwrap().scroll(-10_000.0);
    render.update(&root, &mut rsc);
    assert!(
        (rsc.ui.widgets.get_mut(&scroll_w).unwrap().amt() - 900.0).abs() < 0.01,
        "amt={}",
        rsc.ui.widgets.get_mut(&scroll_w).unwrap().amt()
    );
}

#[test]
fn a_panned_widgets_own_hit_box_moves_exactly_once() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let rect = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE));
    let tall = rsc.ui.widgets.add_strong(Sized {
        inner: rect.any(),
        x: None,
        y: Some(Len::abs(1000.0)),
    });
    let tall_w = tall.weak();
    let scroll = rsc
        .ui
        .widgets
        .add_strong(ScrollArea::new(tall.any(), Axis::Y, Pin::Start));
    let scroll_w = scroll.weak();
    let root = scroll.any();

    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));
    render.update(&root, &mut rsc);
    rsc.ui.widgets.get_mut(&scroll_w).unwrap().scroll(0.0);
    render.update(&root, &mut rsc);

    let before = render.window_region(&tall_w, &rsc).unwrap();
    rsc.ui.widgets.get_mut(&scroll_w).unwrap().scroll(-37.0);
    render.update(&root, &mut rsc);
    let after = render.window_region(&tall_w, &rsc).unwrap();

    assert!(
        (after.top_left.y - (before.top_left.y - 37.0)).abs() < 0.01,
        "the pan was applied twice: before={before:?} after={after:?}"
    );
}

#[test]
fn a_masked_widget_keeps_one_mask_slot_that_is_always_its_own_region() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let (_scroll, inner_root, _rects) = scrolled_rects(&mut rsc, 8);
    let masked = rsc.ui.widgets.add_strong(Masked {
        shape: None,
        inner: inner_root,
    });
    let masked_id = masked.id();
    let filler = rsc.ui.widgets.add_strong(Rect::new(PaintId::BLACK));
    let filler = rsc.ui.widgets.add_strong(Sized {
        inner: filler.any(),
        x: None,
        y: Some(rest(1)),
    });
    let capped = rsc.ui.widgets.add_strong(MaxSize {
        inner: masked.any(),
        x: None,
        y: Some(Len::abs(60.0)),
    });
    let mut span = Span::empty(Dir::DOWN);
    span.push(filler.any());
    span.push(capped.any());
    let root = rsc.ui.widgets.add_strong(span).any();

    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));
    for _ in 0..3 {
        render.update(&root, &mut rsc);
        render.redraw(masked_id, &mut rsc);
    }

    assert_eq!(
        rsc.ui.masks.iter().count(),
        1,
        "one `Masked` must own exactly one mask slot, however often it is redrawn"
    );
    let mask = *rsc.ui.masks.iter().next().unwrap();
    assert_eq!(
        render.primitives.instance(mask.primitive).region,
        render.active.get(&masked_id).unwrap().region,
        "the mask a descendant clips against must be this widget's current box"
    );
}

#[test]
fn a_dp_cap_is_reported_in_pixels_so_a_span_can_place_it() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let rect = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE));
    let tall = rsc.ui.widgets.add_strong(Sized {
        inner: rect.any(),
        x: None,
        y: Some(Len::abs(1000.0)),
    });
    let capped = rsc.ui.widgets.add_strong(MaxSize {
        inner: tall.any(),
        x: None,
        y: Some(Len::dp(100.0)),
    });
    let capped_w = capped.weak();
    let filler = rsc.ui.widgets.add_strong(Rect::new(PaintId::BLACK));
    let filler = rsc.ui.widgets.add_strong(Sized {
        inner: filler.any(),
        x: None,
        y: Some(rest(1)),
    });
    let mut span = Span::empty(Dir::DOWN);
    span.push(filler.any());
    span.push(capped.any());
    let root = rsc.ui.widgets.add_strong(span).any();

    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));
    render.set_density(2.5);
    render.update(&root, &mut rsc);
    render.update(&root, &mut rsc);

    let box_px = render.window_region(&capped_w, &rsc).unwrap();
    let height = box_px.bot_right.y - box_px.top_left.y;
    assert!(
        (height - 250.0).abs() < 0.01,
        "expected the 100dp cap at density 2.5 to be a 250px slot, got {height} ({box_px:?})"
    );
}

#[test]
fn a_size_independent_widget_moved_by_its_parent_has_the_hit_box_it_is_drawn_at() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let top = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE));
    let spacer = rsc.ui.widgets.add_strong(Sized {
        inner: top.any(),
        x: None,
        y: Some(Len::abs(100.0)),
    });
    let spacer_w = spacer.weak();
    let below = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE));
    let below_w = below.weak();
    let mut span = Span::empty(Dir::DOWN);
    span.push(spacer.any());
    span.push(below.any());
    let root = rsc.ui.widgets.add_strong(span).any();

    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));
    render.update(&root, &mut rsc);
    let first = render.window_region(&below_w, &rsc).unwrap();
    assert!(
        (first.top_left.y - 100.0).abs() < 0.01,
        "hit box at {:?}, drawn at y=100",
        first.top_left
    );

    rsc.ui.widgets.get_mut(&spacer_w).unwrap().y = Some(Len::abs(250.0));
    render.update(&root, &mut rsc);
    let after = render.window_region(&below_w, &rsc).unwrap();
    assert!(
        (after.top_left.y - 250.0).abs() < 0.01,
        "hit box at {:?}, drawn at y=250",
        after.top_left
    );
}

struct MoveThenPlace {
    inner: StrongWidget,
    offer_top: f32,
    place_top: f32,
}

impl Widget for MoveThenPlace {
    fn draw(&mut self, painter: &mut Painter) {
        let offer = UiRegion::new(
            UiSpan::FULL,
            UiSpan::new(
                UiScalar::abs(self.offer_top),
                UiScalar::abs(self.offer_top + 40.0),
            ),
        );
        painter.widget_within(&self.inner, offer);
        let place = UiRegion::new(
            UiSpan::FULL,
            UiSpan::new(
                UiScalar::abs(self.place_top),
                UiScalar::abs(self.place_top + 40.0),
            ),
        );
        painter.place(&self.inner, place);
        painter.set_size(Size::default());
    }
}

#[test]
fn a_widget_moved_by_its_parent_and_then_placed_inside_it_lands_at_the_placement() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let rect = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE));
    let child = rsc.ui.widgets.add_strong(Sized {
        inner: rect.any(),
        x: None,
        y: Some(Len::abs(40.0)),
    });
    let child_w = child.weak();
    let parent = rsc.ui.widgets.add_strong(MoveThenPlace {
        inner: child.any(),
        offer_top: 0.0,
        place_top: 100.0,
    });
    let parent_w = parent.weak();
    let root = parent.any();

    let mut render = UiRenderState::new();
    render.resize((200.0, 400.0));
    render.update(&root, &mut rsc);
    let before = render.window_region(&child_w, &rsc).unwrap();
    assert!(
        (before.top_left.y - 100.0).abs() < 0.01,
        "the child should be drawn where it was placed, not where it was offered: {before:?}"
    );

    {
        let parent = rsc.ui.widgets.get_mut(&parent_w).unwrap();
        parent.offer_top = 200.0;
    }
    rsc.ui.widgets.needs_redraw.insert(parent_w.id());
    render.update(&root, &mut rsc);
    let after = render.window_region(&child_w, &rsc).unwrap();
    assert!(
        (after.top_left.y - 100.0).abs() < 0.01,
        "the placement did not change, so neither should the child: before={before:?} \
         after={after:?}"
    );
}

const RADIUS: f32 = 20.0;

fn rounded_container(rsc: &mut TestRsc) -> (UiRenderState, MaskIdx, WidgetId, u32) {
    let child = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE));
    let child_id = child.id();
    let shape = rsc
        .ui
        .widgets
        .add_strong(Rect::new(PaintId::BLACK).radius(Len::abs(RADIUS)));
    let shape_id = shape.id();
    let root = rsc
        .ui
        .widgets
        .add_strong(Masked {
            shape: Some(shape.any()),
            inner: child.any(),
        })
        .any();

    let mut render = UiRenderState::new();
    render.resize((200.0, 100.0));
    render.update(&root, rsc);

    let mask = render
        .active
        .get(&child_id)
        .expect("the child is drawn")
        .mask;
    assert_ne!(
        mask,
        MaskIdx::NONE,
        "the child was drawn with no clip at all"
    );
    let slot = render
        .first_primitive(shape_id)
        .expect("the shape widget drew a rect");
    (render, mask, child_id, slot)
}

#[test]
fn a_masked_child_is_clipped_by_its_container_s_own_corner() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let (render, mask, _child, slot) = rounded_container(&mut rsc);
    let corners = render.primitive_corners(slot, &rsc);
    let radius = render
        .primitives
        .primitive_data::<RectPrimitive>(slot)
        .expect("a mask's shape is a rect")
        .radius;

    let arc_center = corners.top_left + Vec2::new(radius, radius);
    let (mut outside, mut inside) = (0, 0);
    for i in 0..=20 {
        let angle = std::f32::consts::FRAC_PI_2 * i as f32 / 20.0;
        let dir = Vec2::new(-angle.cos(), -angle.sin());
        for out in [-1.5f32, 0.0, 1.5] {
            let pos = arc_center + dir * (radius + out);
            let container = rounded_rect_coverage(pos, corners.top_left, corners.bot_right, radius);
            assert_eq!(
                render.mask_coverage(mask, pos, &rsc),
                container,
                "at {pos:?} the child's clip and the container's own edge disagree",
            );
            if container < 0.5 {
                outside += 1;
            } else {
                inside += 1;
            }
        }
    }
    assert!(
        outside > 0 && inside > 0,
        "the sweep stayed on one side of the curve ({outside} out, {inside} in), so it proved \
         nothing about the corner"
    );
}

#[test]
fn a_mask_s_shape_decides_what_can_be_pressed() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let (render, mask, _child, slot) = rounded_container(&mut rsc);
    let corners = render.primitive_corners(slot, &rsc);

    let cut = corners.top_left + Vec2::new(1.0, 1.0);
    assert!(
        !render.mask_admits(mask, cut, &rsc),
        "the corner the container rounded away is still pressable",
    );
    let inside = corners.top_left + Vec2::new(RADIUS, RADIUS);
    assert!(
        render.mask_admits(mask, inside, &rsc),
        "a point well inside the curve is not pressable",
    );
    let edge = Vec2::new(
        (corners.top_left.x + corners.bot_right.x) / 2.0,
        corners.top_left.y + 1.0,
    );
    assert!(
        render.mask_admits(mask, edge, &rsc),
        "a straight edge between two corners is not pressable",
    );
}

#[test]
fn nested_masks_multiply_their_coverage() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let child = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE));
    let child_id = child.id();

    let inner_shape = rsc
        .ui
        .widgets
        .add_strong(Rect::new(PaintId::BLACK).radius(Len::abs(RADIUS)));
    let inner_shape_id = inner_shape.id();
    let inner = rsc.ui.widgets.add_strong(Masked {
        shape: Some(inner_shape.any()),
        inner: child.any(),
    });
    let outer_shape = rsc
        .ui
        .widgets
        .add_strong(Rect::new(PaintId::BLACK).radius(Len::abs(RADIUS)));
    let outer_shape_id = outer_shape.id();
    let root = rsc
        .ui
        .widgets
        .add_strong(Masked {
            shape: Some(outer_shape.any()),
            inner: inner.any(),
        })
        .any();

    let mut render = UiRenderState::new();
    render.resize((200.0, 100.0));
    render.update(&root, &mut rsc);

    let mask = render.active.get(&child_id).expect("drawn").mask;
    let one = |render: &UiRenderState, rsc: &TestRsc, id, pos| {
        let slot = render.first_primitive(id).expect("a shape rect");
        let c = render.primitive_corners(slot, rsc);
        let radius = render
            .primitives
            .primitive_data::<RectPrimitive>(slot)
            .unwrap()
            .radius;
        rounded_rect_coverage(pos, c.top_left, c.bot_right, radius)
    };

    let slot = render.first_primitive(inner_shape_id).unwrap();
    let corners = render.primitive_corners(slot, &rsc);
    let pos = corners.top_left + Vec2::new(RADIUS * 0.3, RADIUS * 0.3);
    let inner_cov = one(&render, &rsc, inner_shape_id, pos);
    let outer_cov = one(&render, &rsc, outer_shape_id, pos);
    assert!(
        inner_cov > 0.0 && inner_cov < 1.0,
        "the sample point is not inside a feather ({inner_cov}), so this proves nothing"
    );
    assert_eq!(
        render.mask_coverage(mask, pos, &rsc),
        inner_cov * outer_cov,
        "two nested masks must multiply, not intersect",
    );
}

#[test]
fn a_plain_mask_still_clips_to_a_square_box() {
    let mut rsc = TestRsc { ui: Ui::default() };
    let (_scroll, inner_root, _rects) = scrolled_rects(&mut rsc, 8);
    let root = rsc
        .ui
        .widgets
        .add_strong(Masked {
            shape: None,
            inner: inner_root,
        })
        .any();
    let mut render = UiRenderState::new();
    render.resize((200.0, 100.0));
    render.update(&root, &mut rsc);

    let mask = *rsc.ui.masks.iter().next().expect("one mask");
    let corners = render.primitive_corners(mask.primitive, &rsc);
    let mask_idx = MaskIdx::preset(0);
    assert!(
        render.mask_admits(mask_idx, corners.top_left + Vec2::new(0.5, 0.5), &rsc),
        "a square clip must admit its own corner pixel",
    );
    assert!(
        !render.mask_admits(mask_idx, corners.top_left - Vec2::new(2.0, 2.0), &rsc),
        "a square clip must reject a point outside it",
    );
}

#[test]
fn a_scroll_area_opens_at_the_start_of_content_it_has_not_measured_yet() {
    for (name, pin, want) in [("read", Pin::Start, 0.0), ("written", Pin::End, 4900.0)] {
        let mut rsc = TestRsc { ui: Ui::default() };
        let fill = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE)).any();
        let tall = rsc.ui.widgets.add_strong(Sized {
            inner: fill,
            x: None,
            y: Some(Len::abs(5000.0)),
        });
        let scroll = rsc
            .ui
            .widgets
            .add_strong(ScrollArea::new(tall.any(), Axis::Y, pin));
        let weak = scroll.weak();
        let root = scroll.any();

        let mut render = UiRenderState::new();
        render.resize((800.0, 100.0));
        render.update(&root, &mut rsc);
        let _ = rsc.ui.widgets.get_mut(&weak);
        render.update(&root, &mut rsc);

        let amt = rsc.ui.widgets.get(&weak).unwrap().amt();
        assert!(
            (amt - want).abs() < 0.01,
            "an area to be {name} should have opened at {want}, got {amt}"
        );
    }
}

#[test]
fn a_span_of_padded_children_inside_a_span_draws_each_where_its_box_is() {
    const PAD: f32 = 4.0;
    const ROW: f32 = 20.0;
    const HEADER: f32 = 30.0;
    let mut rsc = TestRsc { ui: Ui::default() };
    let header_fill = rsc.ui.widgets.add_strong(Rect::new(PaintId::RED)).any();
    let header_id = header_fill.id();
    let header = rsc.ui.widgets.add_strong(Sized {
        inner: header_fill,
        x: None,
        y: Some(Len::abs(HEADER)),
    });
    let mut inner = Span::empty(Dir::DOWN);
    let mut rects = Vec::new();
    for _ in 0..3 {
        let rect = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE));
        rects.push(rect.weak());
        let sized = rsc.ui.widgets.add_strong(Sized {
            inner: rect.any(),
            x: None,
            y: Some(Len::abs(ROW)),
        });
        let padded = rsc.ui.widgets.add_strong(Pad {
            padding: Padding::uniform(PAD),
            inner: sized.any(),
            exact_region: false,
        });
        let fill = rsc.ui.widgets.add_strong(Rect::new(PaintId::BLUE)).any();
        let card = rsc.ui.widgets.add_strong(Stack {
            children: vec![fill, padded.any()],
            size: StackSize::Child(1),
        });
        let wide = rsc.ui.widgets.add_strong(Sized {
            inner: card.any(),
            x: Some(Len::rest(1.0)),
            y: None,
        });
        inner.push(wide.any());
    }
    let inner = rsc.ui.widgets.add_strong(inner);
    let outer = rsc.ui.widgets.add_strong(Span {
        children: vec![header.any(), inner.any()],
        dir: Dir::DOWN,
        gap: Len::ZERO,
    });
    let mut list = LazySpan::new(Dir::DOWN, Pin::End);
    list.push_back(LazyItem::new(0, outer.any()));
    let list = rsc.ui.widgets.add_strong(list);
    let root = rsc
        .ui
        .widgets
        .add_strong(Masked {
            shape: None,
            inner: list.any(),
        })
        .any();

    let mut render = UiRenderState::new();
    render.resize((200.0, 400.0));
    render.update(&root, &mut rsc);
    render.update(&root, &mut rsc);

    let head_slot = render
        .first_primitive(header_id)
        .expect("the header drew a primitive");
    let head_top = render.primitive_corners(head_slot, &rsc).top_left.y;
    for (i, rect) in rects.iter().enumerate() {
        let want = head_top + HEADER + (ROW + 2.0 * PAD) * i as f32 + PAD;
        let slot = render
            .first_primitive(rect.id())
            .expect("each rect drew a primitive");
        let drawn = render.primitive_corners(slot, &rsc);
        assert!(
            (drawn.top_left.y - want).abs() < 0.01,
            "row {i} should be drawn at y={want}, got {drawn:?}"
        );
    }
}

#[test]
fn a_new_child_in_a_growing_lazy_row_uses_its_final_box_immediately() {
    const FIRST: f32 = 30.0;
    const SECOND: f32 = 70.0;
    const GAP: f32 = 8.0;

    let mut rsc = TestRsc { ui: Ui::default() };
    let first = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE));
    let first_id = first.id();
    let first = rsc.ui.widgets.add_strong(Sized {
        inner: first.any(),
        x: None,
        y: Some(Len::abs(FIRST)),
    });
    let mut contents = Span::empty(Dir::DOWN).gap(Len::abs(GAP));
    contents.push(first.any());
    let contents = rsc.ui.widgets.add_strong(contents);
    let contents_w = contents.weak();
    let row = rsc.ui.widgets.add_strong(Sized {
        inner: contents.any(),
        x: Some(Len::rest(1.0)),
        y: None,
    });
    let mut list = LazySpan::new(Dir::DOWN, Pin::End);
    list.push_back(LazyItem::new(0, row.any()));
    let root = rsc.ui.widgets.add_strong(list).any();

    let mut render = UiRenderState::new();
    render.resize((200.0, 200.0));
    render.update(&root, &mut rsc);

    let second = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE));
    let second_id = second.id();
    let second = rsc.ui.widgets.add_strong(Sized {
        inner: second.any(),
        x: None,
        y: Some(Len::abs(SECOND)),
    });
    rsc.ui
        .widgets
        .get_mut(&contents_w)
        .unwrap()
        .push(second.any());
    render.update(&root, &mut rsc);

    let first = render.primitive_corners(render.first_primitive(first_id).unwrap(), &rsc);
    let second = render.primitive_corners(render.first_primitive(second_id).unwrap(), &rsc);
    assert!(
        (second.top_left.y - (first.bot_right.y + GAP)).abs() < 0.01,
        "the new child should start after the old child and its gap: first={first:?} second={second:?}"
    );
    assert!(
        (second.bot_right.y - second.top_left.y - SECOND).abs() < 0.01,
        "the new child retained its provisional box: {second:?}"
    );
}