tawasuyu/taffy

1//! Alignment of tracks and final positioning of items
2use super::types::GridTrack;
3use crate::axis::InBothAbsAxis;
4use crate::compute::common::alignment::compute_alignment_offset;
5use crate::compute::{GenericAlgorithm, LayoutAlgorithm};
6use crate::geometry::{Line, Point, Rect, Size};
7use crate::layout::{Layout, SizingMode};
8use crate::math::MaybeMath;
9use crate::node::Node;
10use crate::resolve::{MaybeResolve, ResolveOrZero};
11use crate::style::{AlignContent, AlignItems, AlignSelf, AvailableSpace, Position};
12use crate::sys::f32_max;
13use crate::tree::LayoutTree;
14
15/// Align the grid tracks within the grid according to the align-content (rows) or
16/// justify-content (columns) property. This only does anything if the size of the
17/// grid is not equal to the size of the grid container in the axis being aligned.
18pub(super) fn align_tracks(
19    grid_container_content_box_size: f32,
20    padding: Line<f32>,
21    border: Line<f32>,
22    tracks: &mut [GridTrack],
23    track_alignment_style: AlignContent,
24) {
25    let used_size: f32 = tracks.iter().map(|track| track.base_size).sum();
26    let free_space = grid_container_content_box_size - used_size;
27    let origin = padding.start + border.start;
28
29    // Count the number of non-collapsed tracks (not counting gutters)
30    let num_tracks = tracks.iter().skip(1).step_by(2).filter(|track| !track.is_collapsed).count();
31
32    // Grid layout treats gaps as full tracks rather than applying them at alignment so we
33    // simply pass zero here. Grid layout is never reversed.
34    let gap = 0.0;
35    let layout_is_reversed = false;
36
37    // Compute offsets
38    let mut total_offset = origin;
39    tracks.iter_mut().enumerate().for_each(|(i, track)| {
40        // Odd tracks are gutters (but slices are zero-indexed, so odd tracks have even indicies)
41        let is_gutter = i % 2 == 0;
42
43        // The first non-gutter track is index 1
44        let is_first = i == 1;
45
46        let offset = if is_gutter {
47            0.0
48        } else {
49            compute_alignment_offset(free_space, num_tracks, gap, track_alignment_style, layout_is_reversed, is_first)
50        };
51
52        track.offset = total_offset + offset;
53        total_offset = total_offset + offset + track.base_size;
54    });
55}
56
57/// Align and size a grid item into it's final position
58pub(super) fn align_and_position_item(
59    tree: &mut impl LayoutTree,
60    node: Node,
61    order: u32,
62    grid_area: Rect<f32>,
63    container_alignment_styles: InBothAbsAxis<Option<AlignItems>>,
64    baseline_shim: f32,
65) {
66    let grid_area_size = Size { width: grid_area.right - grid_area.left, height: grid_area.bottom - grid_area.top };
67
68    let style = tree.style(node);
69    let aspect_ratio = style.aspect_ratio;
70    let justify_self = style.justify_self;
71    let align_self = style.align_self;
72
73    let position = style.position;
74    let inset_horizontal = style.inset.horizontal_components().map(|size| size.resolve_to_option(grid_area_size.width));
75    let inset_vertical = style.inset.vertical_components().map(|size| size.resolve_to_option(grid_area_size.height));
76    let padding = style.padding.map(|p| p.resolve_or_zero(Some(grid_area_size.width)));
77    let border = style.border.map(|p| p.resolve_or_zero(Some(grid_area_size.width)));
78    let padding_border_size = (padding + border).sum_axes();
79    let inherent_size = style.size.maybe_resolve(grid_area_size).maybe_apply_aspect_ratio(aspect_ratio);
80    let min_size = style
81        .min_size
82        .maybe_resolve(grid_area_size)
83        .or(padding_border_size.map(Some))
84        .maybe_max(padding_border_size)
85        .maybe_apply_aspect_ratio(aspect_ratio);
86    let max_size = style.max_size.maybe_resolve(grid_area_size).maybe_apply_aspect_ratio(aspect_ratio);
87
88    // Resolve default alignment styles if they are set on neither the parent or the node itself
89    // Note: if the child has a preferred aspect ratio but neither width or height are set, then the width is stretched
90    // and the then height is calculated from the width according the aspect ratio
91    // See: https://www.w3.org/TR/css-grid-1/#grid-item-sizing
92    let alignment_styles = InBothAbsAxis {
93        horizontal: justify_self.or(container_alignment_styles.horizontal).unwrap_or_else(|| {
94            if inherent_size.width.is_some() {
95                AlignSelf::Start
96            } else {
97                AlignSelf::Stretch
98            }
99        }),
100        vertical: align_self.or(container_alignment_styles.vertical).unwrap_or_else(|| {
101            if inherent_size.height.is_some() || aspect_ratio.is_some() {
102                AlignSelf::Start
103            } else {
104                AlignSelf::Stretch
105            }
106        }),
107    };
108
109    // Note: This is not a bug. It is part of the CSS spec that both horizontal and vertical margins
110    // resolve against the WIDTH of the grid area.
111    let margin = style.margin.map(|margin| margin.resolve_to_option(grid_area_size.width));
112
113    let grid_area_minus_item_margins_size = Size {
114        width: grid_area_size.width.maybe_sub(margin.left).maybe_sub(margin.right),
115        height: grid_area_size.height.maybe_sub(margin.top).maybe_sub(margin.bottom) - baseline_shim,
116    };
117
118    // If node is absolutely positioned and width is not set explicitly, then deduce it
119    // from left, right and container_content_box if both are set.
120    let width = inherent_size.width.or_else(|| {
121        // Apply width derived from both the left and right properties of an absolutely
122        // positioned element being set
123        if position == Position::Absolute {
124            if let (Some(left), Some(right)) = (inset_horizontal.start, inset_horizontal.end) {
125                return Some(f32_max(grid_area_minus_item_margins_size.width - left - right, 0.0));
126            }
127        }
128
129        // Apply width based on stretch alignment if:
130        //  - Alignment style is "stretch"
131        //  - The node is not absolutely positioned
132        //  - The node does not have auto margins in this axis.
133        if margin.left.is_some()
134            && margin.right.is_some()
135            && alignment_styles.horizontal == AlignSelf::Stretch
136            && position != Position::Absolute
137        {
138            return Some(grid_area_minus_item_margins_size.width);
139        }
140
141        None
142    });
143
144    // Reapply aspect ratio after stretch and absolute position width adjustments
145    let Size { width, height } = Size { width, height: inherent_size.height }.maybe_apply_aspect_ratio(aspect_ratio);
146
147    let height = height.or_else(|| {
148        if position == Position::Absolute {
149            if let (Some(top), Some(bottom)) = (inset_vertical.start, inset_vertical.end) {
150                return Some(f32_max(grid_area_minus_item_margins_size.height - top - bottom, 0.0));
151            }
152        }
153
154        // Apply height based on stretch alignment if:
155        //  - Alignment style is "stretch"
156        //  - The node is not absolutely positioned
157        //  - The node does not have auto margins in this axis.
158        if margin.top.is_some()
159            && margin.bottom.is_some()
160            && alignment_styles.vertical == AlignSelf::Stretch
161            && position != Position::Absolute
162        {
163            return Some(grid_area_minus_item_margins_size.height);
164        }
165
166        None
167    });
168    // Reapply aspect ratio after stretch and absolute position height adjustments
169    let Size { width, height } = Size { width, height }.maybe_apply_aspect_ratio(aspect_ratio);
170
171    // Clamp size by min and max width/height
172    let Size { width, height } = Size { width, height }.maybe_clamp(min_size, max_size);
173
174    // Layout node
175    let measured_size_and_baselines = GenericAlgorithm::perform_layout(
176        tree,
177        node,
178        Size { width, height },
179        grid_area_size.map(Option::Some),
180        grid_area_minus_item_margins_size.map(AvailableSpace::Definite),
181        SizingMode::InherentSize,
182    );
183
184    // Resolve final size
185    let Size { width, height } =
186        Size { width, height }.unwrap_or(measured_size_and_baselines.size).maybe_clamp(min_size, max_size);
187
188    let x = align_item_within_area(
189        Line { start: grid_area.left, end: grid_area.right },
190        justify_self.unwrap_or(alignment_styles.horizontal),
191        width,
192        position,
193        inset_horizontal,
194        margin.horizontal_components(),
195        0.0,
196    );
197    let y = align_item_within_area(
198        Line { start: grid_area.top, end: grid_area.bottom },
199        align_self.unwrap_or(alignment_styles.vertical),
200        height,
201        position,
202        inset_vertical,
203        margin.vertical_components(),
204        baseline_shim,
205    );
206
207    *tree.layout_mut(node) = Layout { order, size: Size { width, height }, location: Point { x, y } };
208}
209
210/// Align and size a grid item along a single axis
211pub(super) fn align_item_within_area(
212    grid_area: Line<f32>,
213    alignment_style: AlignSelf,
214    resolved_size: f32,
215    position: Position,
216    inset: Line<Option<f32>>,
217    margin: Line<Option<f32>>,
218    baseline_shim: f32,
219) -> f32 {
220    // Calculate grid area dimension in the axis
221    let non_auto_margin = Line { start: margin.start.unwrap_or(0.0) + baseline_shim, end: margin.end.unwrap_or(0.0) };
222    let grid_area_size = f32_max(grid_area.end - grid_area.start, 0.0);
223    let free_space = f32_max(grid_area_size - resolved_size - non_auto_margin.sum(), 0.0);
224
225    // Expand auto margins to fill available space
226    let auto_margin_count = margin.start.is_none() as u8 + margin.end.is_none() as u8;
227    let auto_margin_size = if auto_margin_count > 0 { free_space / auto_margin_count as f32 } else { 0.0 };
228    let resolved_margin = Line {
229        start: margin.start.unwrap_or(auto_margin_size) + baseline_shim,
230        end: margin.end.unwrap_or(auto_margin_size),
231    };
232
233    // Compute offset in the axis
234    let alignment_based_offset = match alignment_style {
235        AlignSelf::Start | AlignSelf::FlexStart => resolved_margin.start,
236        AlignSelf::End | AlignSelf::FlexEnd => grid_area_size - resolved_size - resolved_margin.end,
237        AlignSelf::Center => (grid_area_size - resolved_size + resolved_margin.start - resolved_margin.end) / 2.0,
238        // TODO: Add support for baseline alignment. For now we treat it as "start".
239        AlignSelf::Baseline => resolved_margin.start,
240        AlignSelf::Stretch => resolved_margin.start,
241    };
242
243    let offset_within_area = if position == Position::Absolute {
244        if let Some(start) = inset.start {
245            start + non_auto_margin.start
246        } else if let Some(end) = inset.end {
247            grid_area_size - end - resolved_size - non_auto_margin.end
248        } else {
249            alignment_based_offset
250        }
251    } else {
252        alignment_based_offset
253    };
254
255    let mut start = grid_area.start + offset_within_area;
256    if position == Position::Relative {
257        start += inset.start.or(inset.end.map(|pos| -pos)).unwrap_or(0.0);
258    }
259
260    start
261}