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;
11use crate::style::{AlignContent, AlignItems, AlignSelf, AvailableSpace, Position};
12use crate::sys::{f32_max, f32_min};
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 size_diff = grid_container_content_box_size - used_size;
27    let free_space = f32_max(size_diff, 0.0);
28    let overflow = f32_min(size_diff, 0.0);
29
30    // If the used_size > grid_container_size then the tracks must overflow their container
31    // The direction in which they do so is determined by the alignment style
32    let origin = padding.start
33        + border.start
34        + match track_alignment_style {
35            AlignContent::Start => 0.0,
36            AlignContent::FlexStart => 0.0,
37            AlignContent::End => overflow,
38            AlignContent::FlexEnd => overflow,
39            AlignContent::Center => overflow / 2.0,
40            AlignContent::Stretch => 0.0,
41            AlignContent::SpaceBetween => 0.0,
42            AlignContent::SpaceEvenly => 0.0,
43            AlignContent::SpaceAround => 0.0,
44        };
45
46    // Count the number of non-collapsed tracks (not counting gutters)
47    let num_tracks = tracks.iter().skip(1).step_by(2).filter(|track| !track.is_collapsed).count();
48
49    // Grid layout treats gaps as full tracks rather than applying them at alignment so we
50    // simply pass zero here. Grid layout is never reversed.
51    let gap = 0.0;
52    let layout_is_reversed = false;
53
54    // Compute offsets
55    let mut total_offset = origin;
56    tracks.iter_mut().enumerate().for_each(|(i, track)| {
57        // Odd tracks are gutters (but slices are zero-indexed, so odd tracks have even indicies)
58        let is_gutter = i % 2 == 0;
59
60        // The first non-gutter track is index 1
61        let is_first = i == 1;
62
63        let offset = if is_gutter {
64            0.0
65        } else {
66            compute_alignment_offset(free_space, num_tracks, gap, track_alignment_style, layout_is_reversed, is_first)
67        };
68
69        track.offset = total_offset + offset;
70        total_offset = total_offset + offset + track.base_size;
71    });
72}
73
74/// Align and size a grid item into it's final position
75pub(super) fn align_and_position_item(
76    tree: &mut impl LayoutTree,
77    node: Node,
78    order: u32,
79    grid_area: Rect<f32>,
80    container_alignment_styles: InBothAbsAxis<Option<AlignItems>>,
81    baseline_shim: f32,
82) {
83    let grid_area_size = Size { width: grid_area.right - grid_area.left, height: grid_area.bottom - grid_area.top };
84
85    let style = tree.style(node);
86    let aspect_ratio = style.aspect_ratio;
87    let justify_self = style.justify_self;
88    let align_self = style.align_self;
89
90    let position = style.position;
91    let inset_horizontal = style.inset.horizontal_components().map(|size| size.resolve_to_option(grid_area_size.width));
92    let inset_vertical = style.inset.vertical_components().map(|size| size.resolve_to_option(grid_area_size.height));
93    let inherent_size = style.size.maybe_resolve(grid_area_size).maybe_apply_aspect_ratio(aspect_ratio);
94    let min_size = style.min_size.maybe_resolve(grid_area_size).maybe_apply_aspect_ratio(aspect_ratio);
95    let max_size = style.max_size.maybe_resolve(grid_area_size).maybe_apply_aspect_ratio(aspect_ratio);
96
97    // Resolve default alignment styles if they are set on neither the parent or the node itself
98    // Note: if the child has a preferred aspect ratio but neither width or height are set, then the width is stretched
99    // and the then height is calculated from the width according the aspect ratio
100    // See: https://www.w3.org/TR/css-grid-1/#grid-item-sizing
101    let alignment_styles = InBothAbsAxis {
102        horizontal: justify_self.or(container_alignment_styles.horizontal).unwrap_or_else(|| {
103            if inherent_size.width.is_some() {
104                AlignSelf::Start
105            } else {
106                AlignSelf::Stretch
107            }
108        }),
109        vertical: align_self.or(container_alignment_styles.vertical).unwrap_or_else(|| {
110            if inherent_size.height.is_some() || aspect_ratio.is_some() {
111                AlignSelf::Start
112            } else {
113                AlignSelf::Stretch
114            }
115        }),
116    };
117
118    // Note: This is not a bug. It is part of the CSS spec that both horizontal and vertical margins
119    // resolve against the WIDTH of the grid area.
120    let margin = style.margin.map(|margin| margin.resolve_to_option(grid_area_size.width));
121
122    let grid_area_minus_item_margins_size = Size {
123        width: grid_area_size.width.maybe_sub(margin.left).maybe_sub(margin.right),
124        height: grid_area_size.height.maybe_sub(margin.top).maybe_sub(margin.bottom) - baseline_shim,
125    };
126
127    // If node is absolutely positioned and width is not set explicitly, then deduce it
128    // from left, right and container_content_box if both are set.
129    let width = inherent_size.width.or_else(|| {
130        // Apply width derived from both the left and right properties of an absolutely
131        // positioned element being set
132        if position == Position::Absolute {
133            if let (Some(left), Some(right)) = (inset_horizontal.start, inset_horizontal.end) {
134                return Some(f32_max(grid_area_minus_item_margins_size.width - left - right, 0.0));
135            }
136        }
137
138        // Apply width based on stretch alignment if:
139        //  - Alignment style is "stretch"
140        //  - The node is not absolutely positioned
141        //  - The node does not have auto margins in this axis.
142        if margin.left.is_some()
143            && margin.right.is_some()
144            && alignment_styles.horizontal == AlignSelf::Stretch
145            && position != Position::Absolute
146        {
147            return Some(grid_area_minus_item_margins_size.width);
148        }
149
150        None
151    });
152    // Reapply aspect ratio after stretch and absolute position width adjustments
153    let Size { width, height } = Size { width, height: inherent_size.height }.maybe_apply_aspect_ratio(aspect_ratio);
154
155    let height = height.or_else(|| {
156        if position == Position::Absolute {
157            if let (Some(top), Some(bottom)) = (inset_vertical.start, inset_vertical.end) {
158                return Some(f32_max(grid_area_minus_item_margins_size.height - top - bottom, 0.0));
159            }
160        }
161
162        // Apply height based on stretch alignment if:
163        //  - Alignment style is "stretch"
164        //  - The node is not absolutely positioned
165        //  - The node does not have auto margins in this axis.
166        if margin.top.is_some()
167            && margin.bottom.is_some()
168            && alignment_styles.vertical == AlignSelf::Stretch
169            && position != Position::Absolute
170        {
171            return Some(grid_area_minus_item_margins_size.height);
172        }
173
174        None
175    });
176    // Reapply aspect ratio after stretch and absolute position height adjustments
177    let Size { width, height } = Size { width, height }.maybe_apply_aspect_ratio(aspect_ratio);
178
179    // Clamp size by min and max width/height
180    let Size { width, height } = Size { width, height }.maybe_clamp(min_size, max_size);
181
182    // Layout node
183    let measured_size_and_baselines = GenericAlgorithm::perform_layout(
184        tree,
185        node,
186        Size { width, height },
187        grid_area_size.map(Option::Some),
188        grid_area_minus_item_margins_size.map(AvailableSpace::Definite),
189        SizingMode::InherentSize,
190    );
191
192    // Resolve final size
193    let Size { width, height } =
194        Size { width, height }.unwrap_or(measured_size_and_baselines.size).maybe_clamp(min_size, max_size);
195
196    let x = align_item_within_area(
197        Line { start: grid_area.left, end: grid_area.right },
198        justify_self.unwrap_or(alignment_styles.horizontal),
199        width,
200        position,
201        inset_horizontal,
202        margin.horizontal_components(),
203        0.0,
204    );
205    let y = align_item_within_area(
206        Line { start: grid_area.top, end: grid_area.bottom },
207        align_self.unwrap_or(alignment_styles.vertical),
208        height,
209        position,
210        inset_vertical,
211        margin.vertical_components(),
212        baseline_shim,
213    );
214
215    *tree.layout_mut(node) = Layout { order, size: Size { width, height }, location: Point { x, y } };
216}
217
218/// Align and size a grid item along a single axis
219pub(super) fn align_item_within_area(
220    grid_area: Line<f32>,
221    alignment_style: AlignSelf,
222    resolved_size: f32,
223    position: Position,
224    inset: Line<Option<f32>>,
225    margin: Line<Option<f32>>,
226    baseline_shim: f32,
227) -> f32 {
228    // Calculate grid area dimension in the axis
229    let non_auto_margin = Line { start: margin.start.unwrap_or(0.0) + baseline_shim, end: margin.end.unwrap_or(0.0) };
230    let grid_area_size = f32_max(grid_area.end - grid_area.start, 0.0);
231    let free_space = f32_max(grid_area_size - resolved_size - non_auto_margin.sum(), 0.0);
232
233    // Expand auto margins to fill available space
234    let auto_margin_count = margin.start.is_none() as u8 + margin.end.is_none() as u8;
235    let auto_margin_size = if auto_margin_count > 0 { free_space / auto_margin_count as f32 } else { 0.0 };
236    let resolved_margin = Line {
237        start: margin.start.unwrap_or(auto_margin_size) + baseline_shim,
238        end: margin.end.unwrap_or(auto_margin_size),
239    };
240
241    // Compute offset in the axis
242    let alignment_based_offset = match alignment_style {
243        AlignSelf::Start | AlignSelf::FlexStart => resolved_margin.start,
244        AlignSelf::End | AlignSelf::FlexEnd => grid_area_size - resolved_size - resolved_margin.end,
245        AlignSelf::Center => (grid_area_size - resolved_size + resolved_margin.start - resolved_margin.end) / 2.0,
246        // TODO: Add support for baseline alignment. For now we treat it as "start".
247        AlignSelf::Baseline => resolved_margin.start,
248        AlignSelf::Stretch => resolved_margin.start,
249    };
250
251    let offset_within_area = if position == Position::Absolute {
252        if let Some(start) = inset.start {
253            start + non_auto_margin.start
254        } else if let Some(end) = inset.end {
255            grid_area_size - end - resolved_size - non_auto_margin.end
256        } else {
257            alignment_based_offset
258        }
259    } else {
260        alignment_based_offset
261    };
262
263    let mut start = grid_area.start + offset_within_area;
264    if position == Position::Relative {
265        start += inset.start.or(inset.end.map(|pos| -pos)).unwrap_or(0.0);
266    }
267
268    start
269}