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cssbox_core/
float.rs

1//! Float layout per CSS 2.1 §9.5.1.
2//!
3//! This module implements CSS float positioning, which removes elements from normal flow
4//! and positions them to the left or right side of their containing block. Subsequent
5//! content flows around the floated elements.
6
7use crate::fragment::Fragment;
8use crate::geometry::{Point, Rect};
9use crate::style::{Clear, Float};
10
11/// Tracks placed floats and provides exclusion zone queries.
12///
13/// FloatContext maintains separate lists of left and right floats and computes
14/// available space for content that flows around them.
15#[derive(Debug, Clone)]
16pub struct FloatContext {
17    /// Width of the containing block.
18    containing_width: f32,
19    /// Left-floated boxes, in order of placement.
20    left_floats: Vec<FloatBox>,
21    /// Right-floated boxes, in order of placement.
22    right_floats: Vec<FloatBox>,
23}
24
25/// A placed float box with its margin box rectangle.
26#[derive(Debug, Clone, Copy, PartialEq)]
27struct FloatBox {
28    /// Position and size of the float's margin box.
29    rect: Rect,
30}
31
32impl FloatContext {
33    /// Create a new float context for a containing block.
34    ///
35    /// # Arguments
36    /// * `containing_width` - The width of the containing block in pixels.
37    pub fn new(containing_width: f32) -> Self {
38        Self {
39            containing_width,
40            left_floats: Vec::new(),
41            right_floats: Vec::new(),
42        }
43    }
44
45    /// Place a float left or right, below any existing floats that would overlap.
46    ///
47    /// # Arguments
48    /// * `fragment` - The fragment to float (will be mutated with position).
49    /// * `float_type` - Whether to float left or right.
50    /// * `cursor_y` - The vertical position of the generating box (float cannot be placed above this).
51    ///
52    /// # Returns
53    /// The fragment with its position field updated to the final placement.
54    ///
55    /// # Algorithm (per CSS 2.1 §9.5.1)
56    /// 1. Float cannot be placed above cursor_y (the top of its generating box).
57    /// 2. Float cannot be placed above any previously placed float.
58    /// 3. Left float is placed as far left as possible, right float as far right as possible.
59    /// 4. If float doesn't fit horizontally at current y, move down until it fits.
60    pub fn place_float(
61        &mut self,
62        mut fragment: Fragment,
63        float_type: Float,
64        cursor_y: f32,
65    ) -> Fragment {
66        let margin_box = fragment.margin_box();
67        let width = margin_box.width;
68        let height = margin_box.height;
69
70        // Start at cursor_y but ensure we're not above any existing floats
71        let min_y = self.compute_min_y_for_float(&float_type, cursor_y);
72        let mut y = min_y;
73
74        // Find a vertical position where the float fits horizontally
75        loop {
76            let (left_offset, available_width) = self.available_width_at(y, height);
77
78            if available_width >= width {
79                // Found a position where it fits
80                let x = match float_type {
81                    Float::Left => left_offset,
82                    Float::Right => left_offset + available_width - width,
83                    Float::None => {
84                        // Should not happen, but place at left as fallback
85                        left_offset
86                    }
87                };
88
89                // Calculate the border box position from the margin box position
90                // margin_box.x is relative to border_box position with negative margin offset
91                let border_x = x + fragment.margin.left;
92                let border_y = y + fragment.margin.top;
93
94                fragment.position = Point::new(border_x, border_y);
95
96                // Record this float
97                let float_box = FloatBox {
98                    rect: Rect::new(x, y, width, height),
99                };
100
101                match float_type {
102                    Float::Left => self.left_floats.push(float_box),
103                    Float::Right => self.right_floats.push(float_box),
104                    Float::None => {}
105                }
106
107                return fragment;
108            }
109
110            // Doesn't fit; move down to the next constraint
111            y = self.next_y_position(y, height);
112
113            // Safety check to prevent infinite loops
114            if y > cursor_y + 100000.0 {
115                // Place at minimum viable position as fallback
116                let x = match float_type {
117                    Float::Left => 0.0,
118                    Float::Right => self.containing_width - width,
119                    Float::None => 0.0,
120                }
121                .max(0.0);
122
123                let border_x = x + fragment.margin.left;
124                let border_y = y + fragment.margin.top;
125                fragment.position = Point::new(border_x, border_y);
126
127                let float_box = FloatBox {
128                    rect: Rect::new(x, y, width, height),
129                };
130
131                match float_type {
132                    Float::Left => self.left_floats.push(float_box),
133                    Float::Right => self.right_floats.push(float_box),
134                    Float::None => {}
135                }
136
137                return fragment;
138            }
139        }
140    }
141
142    /// Get available width at a vertical position accounting for float exclusions.
143    ///
144    /// # Arguments
145    /// * `y` - Vertical position to query.
146    /// * `height` - Height of the content being placed (to check overlap).
147    ///
148    /// # Returns
149    /// A tuple of (left_offset, available_width):
150    /// - `left_offset`: The x-coordinate where content can start.
151    /// - `available_width`: The width available for content.
152    pub fn available_width_at(&self, y: f32, height: f32) -> (f32, f32) {
153        let bottom = y + height;
154
155        // Find the rightmost left float that overlaps [y, bottom)
156        let left_edge = self
157            .left_floats
158            .iter()
159            .filter(|f| f.rect.y < bottom && f.rect.bottom() > y)
160            .map(|f| f.rect.right())
161            .fold(0.0_f32, f32::max);
162
163        // Find the leftmost right float that overlaps [y, bottom)
164        let right_edge = self
165            .right_floats
166            .iter()
167            .filter(|f| f.rect.y < bottom && f.rect.bottom() > y)
168            .map(|f| f.rect.x)
169            .fold(self.containing_width, f32::min);
170
171        let available = (right_edge - left_edge).max(0.0);
172        (left_edge, available)
173    }
174
175    /// Get the y position that clears floats according to the clear property.
176    ///
177    /// # Arguments
178    /// * `clear` - The clear value (Left, Right, Both, or None).
179    ///
180    /// # Returns
181    /// The y position below the relevant floats, or 0.0 for Clear::None.
182    pub fn clear(&self, clear: Clear) -> f32 {
183        match clear {
184            Clear::None => 0.0,
185            Clear::Left => self.bottom_of_floats(&self.left_floats),
186            Clear::Right => self.bottom_of_floats(&self.right_floats),
187            Clear::Both => {
188                let left_bottom = self.bottom_of_floats(&self.left_floats);
189                let right_bottom = self.bottom_of_floats(&self.right_floats);
190                left_bottom.max(right_bottom)
191            }
192        }
193    }
194
195    /// Get the y position below all floats (both left and right).
196    ///
197    /// # Returns
198    /// The maximum bottom edge of all floats, or 0.0 if no floats exist.
199    pub fn clear_all(&self) -> f32 {
200        let left_bottom = self.bottom_of_floats(&self.left_floats);
201        let right_bottom = self.bottom_of_floats(&self.right_floats);
202        left_bottom.max(right_bottom)
203    }
204
205    /// Compute the minimum y position for a new float (cannot be above existing floats).
206    fn compute_min_y_for_float(&self, float_type: &Float, cursor_y: f32) -> f32 {
207        let same_side_bottom = match float_type {
208            Float::Left => self.bottom_of_floats(&self.left_floats),
209            Float::Right => self.bottom_of_floats(&self.right_floats),
210            Float::None => 0.0,
211        };
212        cursor_y.max(same_side_bottom)
213    }
214
215    /// Find the next y position where the available space might change.
216    fn next_y_position(&self, current_y: f32, height: f32) -> f32 {
217        let bottom = current_y + height;
218
219        // Find all floats that overlap the current [y, bottom) range
220        let mut next_positions = Vec::new();
221
222        for float_box in self.left_floats.iter().chain(self.right_floats.iter()) {
223            // If float overlaps, try moving to its bottom edge
224            if float_box.rect.y < bottom && float_box.rect.bottom() > current_y {
225                next_positions.push(float_box.rect.bottom());
226            }
227            // Also try moving to its top edge if it's below us
228            if float_box.rect.y >= current_y {
229                next_positions.push(float_box.rect.y);
230            }
231        }
232
233        // Return the smallest position greater than current_y
234        next_positions
235            .into_iter()
236            .filter(|&y| y > current_y)
237            .min_by(|a, b| a.partial_cmp(b).unwrap_or(std::cmp::Ordering::Equal))
238            .unwrap_or(current_y + 1.0)
239    }
240
241    /// Get the bottom edge of the lowest float in a list.
242    fn bottom_of_floats(&self, floats: &[FloatBox]) -> f32 {
243        floats
244            .iter()
245            .map(|f| f.rect.bottom())
246            .fold(0.0_f32, f32::max)
247    }
248}
249
250#[cfg(test)]
251mod tests {
252    use super::*;
253    use crate::fragment::FragmentKind;
254    use crate::geometry::{Edges, Size};
255    use crate::tree::NodeId;
256
257    fn create_fragment(width: f32, height: f32, margin: f32) -> Fragment {
258        let mut frag = Fragment::new(NodeId(0), FragmentKind::Box);
259        frag.size = Size::new(width, height);
260        frag.margin = Edges::all(margin);
261        frag
262    }
263
264    #[test]
265    fn test_new_float_context() {
266        let ctx = FloatContext::new(800.0);
267        assert_eq!(ctx.containing_width, 800.0);
268        assert_eq!(ctx.left_floats.len(), 0);
269        assert_eq!(ctx.right_floats.len(), 0);
270    }
271
272    #[test]
273    fn test_available_width_no_floats() {
274        let ctx = FloatContext::new(800.0);
275        let (left, width) = ctx.available_width_at(0.0, 100.0);
276        assert_eq!(left, 0.0);
277        assert_eq!(width, 800.0);
278    }
279
280    #[test]
281    fn test_place_left_float() {
282        let mut ctx = FloatContext::new(800.0);
283        let frag = create_fragment(100.0, 50.0, 10.0);
284
285        let placed = ctx.place_float(frag, Float::Left, 0.0);
286
287        // Border box should be at (margin.left, margin.top) = (10, 10)
288        assert_eq!(placed.position.x, 10.0);
289        assert_eq!(placed.position.y, 10.0);
290
291        // Check float was recorded
292        assert_eq!(ctx.left_floats.len(), 1);
293
294        // Margin box should be at (0, 0) with size 120x70
295        let margin_box = placed.margin_box();
296        assert_eq!(margin_box.x, 0.0);
297        assert_eq!(margin_box.y, 0.0);
298        assert_eq!(margin_box.width, 120.0); // 100 + 10*2
299        assert_eq!(margin_box.height, 70.0); // 50 + 10*2
300    }
301
302    #[test]
303    fn test_place_right_float() {
304        let mut ctx = FloatContext::new(800.0);
305        let frag = create_fragment(100.0, 50.0, 10.0);
306
307        let placed = ctx.place_float(frag, Float::Right, 0.0);
308
309        // Margin box should be at (800 - 120, 0) = (680, 0)
310        // Border box should be at (680 + 10, 10) = (690, 10)
311        assert_eq!(placed.position.x, 690.0);
312        assert_eq!(placed.position.y, 10.0);
313
314        assert_eq!(ctx.right_floats.len(), 1);
315    }
316
317    #[test]
318    fn test_two_left_floats_side_by_side() {
319        let mut ctx = FloatContext::new(800.0);
320
321        let frag1 = create_fragment(100.0, 50.0, 0.0);
322        let placed1 = ctx.place_float(frag1, Float::Left, 0.0);
323        assert_eq!(placed1.position.x, 0.0);
324        assert_eq!(placed1.position.y, 0.0);
325
326        let frag2 = create_fragment(100.0, 50.0, 0.0);
327        let placed2 = ctx.place_float(frag2, Float::Left, 0.0);
328
329        // Second left float stacks below the first (cannot be placed above previous same-side float)
330        assert_eq!(placed2.position.x, 0.0);
331        assert_eq!(placed2.position.y, 50.0);
332    }
333
334    #[test]
335    fn test_left_float_wraps_when_no_space() {
336        let mut ctx = FloatContext::new(200.0);
337
338        // Place first float taking 150px
339        let frag1 = create_fragment(150.0, 50.0, 0.0);
340        ctx.place_float(frag1, Float::Left, 0.0);
341
342        // Place second float that needs 100px (won't fit next to first)
343        let frag2 = create_fragment(100.0, 40.0, 0.0);
344        let placed2 = ctx.place_float(frag2, Float::Left, 0.0);
345
346        // Should wrap below first float
347        assert_eq!(placed2.position.x, 0.0);
348        assert_eq!(placed2.position.y, 50.0); // Below first float's 50px height
349    }
350
351    #[test]
352    fn test_available_width_with_left_float() {
353        let mut ctx = FloatContext::new(800.0);
354        let frag = create_fragment(200.0, 100.0, 0.0);
355        ctx.place_float(frag, Float::Left, 0.0);
356
357        let (left, width) = ctx.available_width_at(50.0, 10.0);
358        assert_eq!(left, 200.0); // After the 200px float
359        assert_eq!(width, 600.0); // 800 - 200
360    }
361
362    #[test]
363    fn test_available_width_with_right_float() {
364        let mut ctx = FloatContext::new(800.0);
365        let frag = create_fragment(200.0, 100.0, 0.0);
366        ctx.place_float(frag, Float::Right, 0.0);
367
368        let (left, width) = ctx.available_width_at(50.0, 10.0);
369        assert_eq!(left, 0.0);
370        assert_eq!(width, 600.0); // 800 - 200
371    }
372
373    #[test]
374    fn test_available_width_with_both_floats() {
375        let mut ctx = FloatContext::new(800.0);
376
377        let left_frag = create_fragment(150.0, 100.0, 0.0);
378        ctx.place_float(left_frag, Float::Left, 0.0);
379
380        let right_frag = create_fragment(200.0, 100.0, 0.0);
381        ctx.place_float(right_frag, Float::Right, 0.0);
382
383        let (left, width) = ctx.available_width_at(50.0, 10.0);
384        assert_eq!(left, 150.0); // After left float
385        assert_eq!(width, 450.0); // 600 - 150 (space between floats)
386    }
387
388    #[test]
389    fn test_available_width_below_floats() {
390        let mut ctx = FloatContext::new(800.0);
391        let frag = create_fragment(200.0, 100.0, 0.0);
392        ctx.place_float(frag, Float::Left, 0.0);
393
394        // Query below the float
395        let (left, width) = ctx.available_width_at(150.0, 10.0);
396        assert_eq!(left, 0.0); // Float doesn't affect this region
397        assert_eq!(width, 800.0);
398    }
399
400    #[test]
401    fn test_clear_none() {
402        let mut ctx = FloatContext::new(800.0);
403        let frag = create_fragment(100.0, 50.0, 0.0);
404        ctx.place_float(frag, Float::Left, 0.0);
405
406        assert_eq!(ctx.clear(Clear::None), 0.0);
407    }
408
409    #[test]
410    fn test_clear_left() {
411        let mut ctx = FloatContext::new(800.0);
412        let frag = create_fragment(100.0, 50.0, 0.0);
413        ctx.place_float(frag, Float::Left, 0.0);
414
415        assert_eq!(ctx.clear(Clear::Left), 50.0);
416    }
417
418    #[test]
419    fn test_clear_right() {
420        let mut ctx = FloatContext::new(800.0);
421        let frag = create_fragment(100.0, 60.0, 0.0);
422        ctx.place_float(frag, Float::Right, 0.0);
423
424        assert_eq!(ctx.clear(Clear::Right), 60.0);
425    }
426
427    #[test]
428    fn test_clear_both() {
429        let mut ctx = FloatContext::new(800.0);
430
431        let left_frag = create_fragment(100.0, 50.0, 0.0);
432        ctx.place_float(left_frag, Float::Left, 0.0);
433
434        let right_frag = create_fragment(100.0, 80.0, 0.0);
435        ctx.place_float(right_frag, Float::Right, 0.0);
436
437        // Should return the maximum of both
438        assert_eq!(ctx.clear(Clear::Both), 80.0);
439    }
440
441    #[test]
442    fn test_clear_all() {
443        let mut ctx = FloatContext::new(800.0);
444
445        let left_frag = create_fragment(100.0, 50.0, 0.0);
446        ctx.place_float(left_frag, Float::Left, 0.0);
447
448        let right_frag = create_fragment(100.0, 80.0, 0.0);
449        ctx.place_float(right_frag, Float::Right, 0.0);
450
451        assert_eq!(ctx.clear_all(), 80.0);
452    }
453
454    #[test]
455    fn test_float_respects_cursor_y() {
456        let mut ctx = FloatContext::new(800.0);
457        let frag = create_fragment(100.0, 50.0, 0.0);
458
459        // Place float with cursor at y=100
460        let placed = ctx.place_float(frag, Float::Left, 100.0);
461
462        // Float should not be placed above cursor_y
463        assert!(placed.position.y >= 100.0);
464    }
465
466    #[test]
467    fn test_float_with_margins() {
468        let mut ctx = FloatContext::new(800.0);
469        let frag = create_fragment(100.0, 50.0, 20.0);
470
471        let placed = ctx.place_float(frag, Float::Left, 0.0);
472
473        // Border box position should account for margin
474        assert_eq!(placed.position.x, 20.0);
475        assert_eq!(placed.position.y, 20.0);
476
477        // Margin box should start at 0
478        let margin_box = placed.margin_box();
479        assert_eq!(margin_box.x, 0.0);
480        assert_eq!(margin_box.y, 0.0);
481    }
482
483    #[test]
484    fn test_multiple_left_floats_stacking() {
485        let mut ctx = FloatContext::new(500.0);
486
487        let frag1 = create_fragment(100.0, 50.0, 0.0);
488        let placed1 = ctx.place_float(frag1, Float::Left, 0.0);
489        assert_eq!(placed1.position.x, 0.0);
490        assert_eq!(placed1.position.y, 0.0);
491
492        let frag2 = create_fragment(100.0, 50.0, 0.0);
493        let placed2 = ctx.place_float(frag2, Float::Left, 0.0);
494        // Stacks below first float (same-side floats stack vertically)
495        assert_eq!(placed2.position.x, 0.0);
496        assert_eq!(placed2.position.y, 50.0);
497
498        let frag3 = create_fragment(100.0, 50.0, 0.0);
499        let placed3 = ctx.place_float(frag3, Float::Left, 0.0);
500        // Stacks below second float
501        assert_eq!(placed3.position.x, 0.0);
502        assert_eq!(placed3.position.y, 100.0);
503    }
504
505    #[test]
506    fn test_float_below_previous_same_side() {
507        let mut ctx = FloatContext::new(800.0);
508
509        // Place first left float
510        let frag1 = create_fragment(100.0, 50.0, 0.0);
511        ctx.place_float(frag1, Float::Left, 0.0);
512
513        // Place second left float with cursor_y = 0 (should go below first)
514        let frag2 = create_fragment(100.0, 30.0, 0.0);
515        let placed2 = ctx.place_float(frag2, Float::Left, 0.0);
516
517        // Should be positioned to the right at same y (if space) or below
518        assert!(placed2.position.y >= 0.0);
519    }
520
521    #[test]
522    fn test_narrow_containing_block() {
523        let mut ctx = FloatContext::new(100.0);
524
525        // Try to place a float wider than containing block
526        let frag = create_fragment(150.0, 50.0, 0.0);
527        let placed = ctx.place_float(frag, Float::Left, 0.0);
528
529        // Float wider than containing block triggers safety fallback after searching
530        // Algorithm increments y until safety limit (cursor_y + 100000)
531        assert!(placed.position.y > 100000.0);
532        assert_eq!(placed.position.x, 0.0);
533    }
534
535    #[test]
536    fn test_available_width_partial_overlap() {
537        let mut ctx = FloatContext::new(800.0);
538
539        // Place float from y=50 to y=150
540        let frag = create_fragment(200.0, 100.0, 0.0);
541        ctx.place_float(frag, Float::Left, 50.0);
542
543        // Query at y=0 (before float) with height that overlaps
544        let (left, width) = ctx.available_width_at(0.0, 100.0);
545        assert_eq!(left, 200.0); // Reduced by float
546        assert_eq!(width, 600.0);
547
548        // Query at y=0 with height that doesn't reach float
549        let (left, width) = ctx.available_width_at(0.0, 40.0);
550        assert_eq!(left, 0.0); // No overlap
551        assert_eq!(width, 800.0);
552    }
553
554    #[test]
555    fn test_overlapping_left_and_right_floats() {
556        let mut ctx = FloatContext::new(800.0);
557
558        let left = create_fragment(350.0, 100.0, 0.0);
559        ctx.place_float(left, Float::Left, 0.0);
560
561        let right = create_fragment(350.0, 100.0, 0.0);
562        ctx.place_float(right, Float::Right, 0.0);
563
564        let (left_offset, width) = ctx.available_width_at(50.0, 10.0);
565        assert_eq!(left_offset, 350.0);
566        assert_eq!(width, 100.0); // 800 - 350 - 350
567    }
568
569    #[test]
570    fn test_zero_available_width() {
571        let mut ctx = FloatContext::new(400.0);
572
573        let left = create_fragment(250.0, 100.0, 0.0);
574        ctx.place_float(left, Float::Left, 0.0);
575
576        let right = create_fragment(250.0, 100.0, 0.0);
577        ctx.place_float(right, Float::Right, 0.0);
578
579        let (_, width) = ctx.available_width_at(50.0, 10.0);
580        // Right float doesn't fit next to left float, so wraps below it.
581        // At y=50, only left float (y=0-100) overlaps, so available width is 400-250=150
582        assert_eq!(width, 150.0);
583    }
584}