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The new struct rect_union is designed to make it easier to efficiently accumulate a list of rectangles, and then operate on an exact cover of their union. Using rect_union, the times needed to added t rectangles, and then compute their exact cover will be O(t), and something between Ω(t) and O(t^2), depending on the rectangle arrangement. If one tries to do the same by storing a pixman_region32_t and updating it with pixman_region32_union_rect(), then total time needed would be between Ω(t^2) and O(t^3), depending on the input. Without changing the public API (data structure + rectangle ordering rules) for pixman_region32_t, it is impossible to improve its worst case time.
77 lines
2.5 KiB
C
77 lines
2.5 KiB
C
#ifndef UTIL_RECT_UNION_H
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#define UTIL_RECT_UNION_H
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#include <stdlib.h>
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#include <stdbool.h>
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#include <pixman.h>
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#include <wayland-util.h>
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/**
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* `struct rect_union` is a data structure to efficiently accumulate a number
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* of rectangles and then, when needed, compute a disjoint cover of their union.
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* (That is: produce a list of disjoint rectangles which covers every point
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* that was contained in one of the added rectangles.)
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*
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* Add rectangles to the union with `rect_union_add()`; to compute the disjoint
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* union, run `rect_union_evaluate()`, which will place the result in `.region`.
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* If there were any allocation failures, `.region` will instead contain the
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* bounding box for the entire list of rectangles.
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*
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* Example usage:
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*
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* struct rect_union r;
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* rect_union_init(&r);
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* for (j in ...) {
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* rect_union_add(&r, box[j]);
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* }
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* const pixman_region32_t *reg = rect_union_evaluate(&r);
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* int nboxes;
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* pixman_box32_t *boxes = pixman_region32_rectangles(reg, nboxes);
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* for (int i = 0; i < nboxes; i++) {
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* do_stuff(boxes[i]);
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* }
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* rect_union_destroy(&r);
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*
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*/
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struct rect_union {
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pixman_box32_t bounding_box; // Always up-to-date bounding box
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pixman_region32_t region; // Updated only on _evaluate()
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struct wl_array unsorted; // pixman_box32_t
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bool alloc_failure; // If this is true, fall back to computing a bounding box
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};
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/**
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* Initialize *r, disregarding any previous contents.
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*/
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void rect_union_init(struct rect_union *r);
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/**
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* Free heap data associated with *r; should only be called after rect_union_init.
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* Leaves *r in an invalid state.
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*/
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void rect_union_finish(struct rect_union *r);
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/**
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* Add a rectangle to the union. If `box` is empty or invalid (x2 > x1 || y2 > y1),
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* do nothing.
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*
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* Amortized time: O(1)
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*/
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void rect_union_add(struct rect_union *r, pixman_box32_t box);
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/**
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* Compute an exact cover of the rectangles added so far, and return
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* a pointer to a pixman_region32_t giving that cover. The pointer will
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* remain valid until the next time *r is modified. If there was an allocation
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* failure, this function may return a single-rectangle bounding box instead.
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*
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* This may be called multiple times and interleaved with rect_union_add().
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*
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* Worst case time: O(t^2), where t is the number of rectangles in the list.
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* Best case time: O(t), if rectangles are disjoint and have y-x band structure
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*/
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const pixman_region32_t *rect_union_evaluate(struct rect_union *r);
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#endif
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