edges, resistance, snap: unified resistance and snapping engine

include/common/macros.h

@@ -2,6 +2,8 @@
 #ifndef LABWC_MACROS_H
 #define LABWC_MACROS_H
 
+#include <limits.h>
+
 /**
  * ARRAY_SIZE() - Get the number of elements in array.
  * @arr: array to be sized
@@ -50,4 +52,13 @@
 #define MAX(a, b) (((a) > (b)) ? (a) : (b))
 #endif
 
+/**
+ * BOUNDED_INT() - Returns true if an integer is not INT_MAX or INT_MIN
+ *
+ * @param val Value to test (integer)
+ */
+#ifndef BOUNDED_INT
+#define BOUNDED_INT(a) ((a) < INT_MAX && (a) > INT_MIN)
+#endif
+
 #endif /* LABWC_MACROS_H */

include/edges.h

@@ -0,0 +1,110 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef LABWC_EDGES_H
+#define LABWC_EDGES_H
+
+#include <limits.h>
+#include "common/macros.h"
+
+struct border;
+struct output;
+struct view;
+
+static inline int
+clipped_add(int a, int b)
+{
+	if (b > 0) {
+		return a >= (INT_MAX - b) ? INT_MAX : (a + b);
+	} else if (b < 0) {
+		return a <= (INT_MIN - b) ? INT_MIN : (a + b);
+	}
+
+	return a;
+}
+
+static inline int
+clipped_sub(int a, int b)
+{
+	if (b > 0) {
+		return a <= (INT_MIN + b) ? INT_MIN : (a - b);
+	} else if (b < 0) {
+		return a >= (INT_MAX + b) ? INT_MAX : (a - b);
+	}
+
+	return a;
+}
+
+static inline int
+edge_get_best(int next, int edge, bool decreasing)
+{
+	if (!BOUNDED_INT(next)) {
+		/* Any bounded edge beats an unbounded next */
+		return BOUNDED_INT(edge) ? edge : next;
+	}
+
+	/* No unbounded edge ever beats next */
+	if (!BOUNDED_INT(edge)) {
+		return next;
+	}
+
+	/* Max edge wins for decreasing moves, min edge for increasing */
+	return decreasing ? MAX(next, edge) : MIN(next, edge);
+}
+
+/*
+ * edge_validator_t - edge validator signature
+ * @best: pointer to the current "best" edge
+ * @current: current position of a moving edge
+ * @target: position to which the moving edge will be moved
+ * @oppose: opposing edge of encountered region
+ * @align: aligned edge of encountered region
+ * @lesser: true if moving edge is top or left edge; false otherwise
+ *
+ * This function will be used by edge_find_neighbors and edge_find_outputs to
+ * validate and select the "best" output or neighbor edge against which a
+ * moving edge should be snapped. The moving edge has current position
+ * "current" and desired position "target". The validator should determine
+ * whether motion of the crosses the given opposed and aligned edges of a trial
+ * region and should be considered a snap point. An edge is "lesser" if it
+ * occupies a smaller coordinate than the opposite edge of the view region
+ * (i.e., it is a top or left edge).
+ *
+ * Opposing edges are on the opposite side of the target region from the moving
+ * edge (i.e., left <-> right, top <-> bottom). When the moving edge snaps to
+ * an opposing edge, the view should maintain the configured gap. Aligned edges
+ * are on the same side of the target region from the moving edge (i.e.,
+ * left <-> left, right <-> right, top <-> top, bottom <-> bottom). When the
+ * moving edge snaps to an aligned edge, the view should *not* include a gap.
+ *
+ * If window gaps are configured, all edges will be offset as appropriate to
+ * reflect the desired padding. Thus, the validator should generally compare
+ * the given current or target values directly to the opposing and aligned edge
+ * without regard for rc.gap.
+ *
+ * Any edge may take the values INT_MIN or INT_MAX to indicate that the edge
+ * should be effectively ignored. Should the validator decide that a given
+ * region edge (oppose or align) should be a preferred snap point, it should
+ * update the value of *best accordingly.
+ */
+typedef void (*edge_validator_t)(int *best,
+	int current, int target, int oppose, int align, bool lesser);
+
+void edges_initialize(struct border *edges);
+
+void edges_adjust_geom(struct view *view, struct border edges,
+	uint32_t resize_edges, struct wlr_box *geom);
+
+void edges_find_neighbors(struct border *nearest_edges, struct view *view,
+	struct wlr_box target, struct output *output,
+	edge_validator_t validator, bool use_pending);
+
+void edges_find_outputs(struct border *nearest_edges, struct view *view,
+	struct wlr_box target, struct output *output,
+	edge_validator_t validator, bool use_pending);
+
+void edges_adjust_move_coords(struct view *view, struct border edges,
+	int *x, int *y, bool use_pending);
+
+void edges_adjust_resize_geom(struct view *view, struct border edges,
+	uint32_t resize_edges, struct wlr_box *geom, bool use_pending);
+
+#endif /* LABWC_EDGES_H */

include/snap.h

@@ -7,11 +7,13 @@
 
 struct wlr_box;
 
-struct border snap_get_max_distance(struct view *view);
+void snap_move_to_edge(struct view *view,
+	enum view_edge direction, bool snap_to_windows, int *dx, int *dy);
 
-void snap_vector_to_next_edge(struct view *view, enum view_edge direction, int *dx, int *dy);
-int snap_distance_to_next_edge(struct view *view, enum view_edge direction);
-void snap_grow_to_next_edge(struct view *view, enum view_edge direction, struct wlr_box *geo);
-void snap_shrink_to_next_edge(struct view *view, enum view_edge direction, struct wlr_box *geo);
+void snap_grow_to_next_edge(struct view *view,
+	enum view_edge direction, struct wlr_box *geo);
+
+void snap_shrink_to_next_edge(struct view *view,
+	enum view_edge direction, struct wlr_box *geo);
 
 #endif /* LABWC_SNAP_H */

src/edges.c

@@ -0,0 +1,303 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <assert.h>
+#include <limits.h>
+#include <wlr/util/box.h>
+#include "common/border.h"
+#include "common/macros.h"
+#include "config/rcxml.h"
+#include "edges.h"
+#include "labwc.h"
+#include "view.h"
+
+static void
+edges_for_target_geometry(struct border *edges, struct view *view,
+		struct wlr_box target)
+{
+	struct border border = ssd_get_margin(view->ssd);
+
+	/* Use the effective height to properly handle shaded views */
+	int eff_height = view->shaded ? 0 : target.height;
+
+	edges->left = target.x - border.left - rc.gap;
+	edges->top = target.y - border.top - rc.gap;
+	edges->right = target.x + target.width + border.right + rc.gap;
+	edges->bottom = target.y + eff_height + border.bottom + rc.gap;
+}
+
+void
+edges_initialize(struct border *edges)
+{
+	assert(edges);
+	edges->top = INT_MIN;
+	edges->right = INT_MAX;
+	edges->bottom = INT_MAX;
+	edges->left = INT_MIN;
+}
+
+static void
+validate_edges(struct border *valid_edges,
+		struct border view, struct border target,
+		struct border region, edge_validator_t validator)
+{
+	/* When a view snaps to a region while moving to its target, it can do
+	 * so in two ways: a view edge can snap to an "opposing" edge of the
+	 * region (left <-> right, top <-> bottom) or to an "aligned" edge
+	 * (left <-> left, right <-> right, top <-> top, bottom <-> bottom).
+	 *
+	 * When a view hits the opposing edge of a region, it should be
+	 * separated by a gap; when a view hits the aligned edge, it should not
+	 * be separated. The view and its target already include necessary
+	 * padding to reflect the gap. The region does not. To make sure the
+	 * "aligned" edges are properly aligned, add padding to the region
+	 * borders for aligned edges only.
+	 */
+
+	struct border region_pad = {
+		.top = clipped_sub(region.top, rc.gap),
+		.right = clipped_add(region.right, rc.gap),
+		.bottom = clipped_add(region.bottom, rc.gap),
+		.left = clipped_sub(region.left, rc.gap),
+	};
+
+	/* Check for edges encountered during movement of left edge */
+	validator(&valid_edges->left, view.left, target.left,
+		region.right, region_pad.left, /* lesser */ true);
+
+	/* Check for edges encountered during movement of right edge */
+	validator(&valid_edges->right, view.right, target.right,
+		region.left, region_pad.right, /* lesser */ false);
+
+	/* Check for edges encountered during movement of top edge */
+	validator(&valid_edges->top, view.top, target.top,
+		region.bottom, region_pad.top, /* lesser */ true);
+
+	/* Check for edges encountered during movement of bottom edge */
+	validator(&valid_edges->bottom, view.bottom, target.bottom,
+		region.top, region_pad.bottom, /* lesser */ false);
+}
+
+void
+edges_find_neighbors(struct border *nearest_edges, struct view *view,
+		struct wlr_box target, struct output *output,
+		edge_validator_t validator, bool use_pending)
+{
+	assert(view);
+	assert(validator);
+	assert(nearest_edges);
+
+	struct border view_edges = { 0 };
+	struct border target_edges = { 0 };
+
+	edges_for_target_geometry(&view_edges, view,
+		use_pending ? view->pending : view->current);
+	edges_for_target_geometry(&target_edges, view, target);
+
+	struct view *v;
+	for_each_view(v, &view->server->views, LAB_VIEW_CRITERIA_CURRENT_WORKSPACE) {
+		if (v == view || !output_is_usable(v->output)) {
+			continue;
+		}
+
+		if (output && v->output != output) {
+			continue;
+		}
+
+		struct border border = ssd_get_margin(v->ssd);
+
+		struct border win_edges = {
+			.top = v->current.y - border.top,
+			.left = v->current.x - border.left,
+			.bottom = v->current.y + border.bottom
+				+ view_effective_height(v, /* use_pending */ false),
+			.right = v->current.x + v->current.width + border.right,
+		};
+
+		validate_edges(nearest_edges, view_edges,
+			target_edges, win_edges, validator);
+	}
+}
+
+void
+edges_find_outputs(struct border *nearest_edges, struct view *view,
+		struct wlr_box target, struct output *output,
+		edge_validator_t validator, bool use_pending)
+{
+	assert(view);
+	assert(validator);
+	assert(nearest_edges);
+
+	struct border view_edges = { 0 };
+	struct border target_edges = { 0 };
+
+	struct wlr_box *view_geom =
+		use_pending ? &view->pending : &view->current;
+
+	edges_for_target_geometry(&view_edges, view, *view_geom);
+	edges_for_target_geometry(&target_edges, view, target);
+
+	struct output *o;
+	wl_list_for_each(o, &view->server->outputs, link) {
+		if (!output_is_usable(o)) {
+			continue;
+		}
+
+		if (output && o != output) {
+			continue;
+		}
+
+		struct wlr_box usable =
+			output_usable_area_in_layout_coords(o);
+
+		struct wlr_box ol;
+		if (!wlr_box_intersection(&ol, view_geom, &usable) &&
+				!wlr_box_intersection(&ol, &target, &usable)) {
+			continue;
+		}
+
+		/*
+		 * Split a single "leaving output" problem into four "entering
+		 * complementary region" problems, treating the view, its
+		 * target and the screen boundaries as half planes. This
+		 * prevents unexpected snapping behavior like the bottom of a
+		 * window snapping above the top of an output, where it would
+		 * become invisible.
+		 */
+		struct border screen;
+		struct border view_eff;
+		struct border target_eff;
+
+		/* First problem: view toward upper half-plane */
+		edges_initialize(&screen);
+		edges_initialize(&view_eff);
+		edges_initialize(&target_eff);
+
+		screen.bottom = usable.y;
+		view_eff.top = view_edges.top;
+		target_eff.top = target_edges.top;
+		validate_edges(nearest_edges, view_eff, target_eff, screen, validator);
+
+		/* Second problem: view toward lower half-plane */
+		edges_initialize(&screen);
+		edges_initialize(&view_eff);
+		edges_initialize(&target_eff);
+
+		screen.top = usable.y + usable.height;
+		view_eff.bottom = view_edges.bottom;
+		target_eff.bottom = target_edges.bottom;
+		validate_edges(nearest_edges, view_eff, target_eff, screen, validator);
+
+		/* Third problem: view toward left half-plane */
+		edges_initialize(&screen);
+		edges_initialize(&view_eff);
+		edges_initialize(&target_eff);
+
+		screen.right = usable.x;
+		view_eff.left = view_edges.left;
+		target_eff.left = target_edges.left;
+		validate_edges(nearest_edges, view_eff, target_eff, screen, validator);
+
+		/* Fourth problem: view toward right half-plane */
+		edges_initialize(&screen);
+		edges_initialize(&view_eff);
+		edges_initialize(&target_eff);
+
+		screen.left = usable.x + usable.width;
+		view_eff.right = view_edges.right;
+		target_eff.right = target_edges.right;
+		validate_edges(nearest_edges, view_eff, target_eff, screen, validator);
+	}
+}
+
+static void
+adjust_move_coords_1d(int *edge, int lesser, int lesser_offset,
+		int greater, int greater_offset, bool decreasing)
+{
+	/* Default best candidate is not valid */
+	int best = INT_MAX;
+
+	if (BOUNDED_INT(lesser)) {
+		/* A valid lesser edge is the always the first candidate */
+		best = clipped_add(lesser, lesser_offset);
+	}
+
+	if (BOUNDED_INT(greater)) {
+		/* Check if a valid greater edge is a better candidate */
+		best = edge_get_best(best,
+			clipped_sub(greater, greater_offset), decreasing);
+	}
+
+	if (BOUNDED_INT(best)) {
+		/* Replace the edge if a valid candidate was found */
+		*edge = best;
+	}
+}
+
+void
+edges_adjust_move_coords(struct view *view, struct border edges,
+		int *x, int *y, bool use_pending)
+{
+	assert(view);
+
+	struct border border = ssd_get_margin(view->ssd);
+	struct wlr_box *view_geom =
+		use_pending ? &view->pending : &view->current;
+
+	/* When moving, limit motion to the best valid, intervening edge */
+
+	if (view_geom->x != *x) {
+		int lshift = border.left + rc.gap;
+		int rshift = border.right + rc.gap + view->pending.width;
+
+		adjust_move_coords_1d(x, edges.left, lshift,
+			edges.right, rshift, *x < view_geom->x);
+	}
+
+	if (view_geom->y != *y) {
+		int tshift = border.top + rc.gap;
+		int bshift = border.bottom + rc.gap
+			+ view_effective_height(view, /* use_pending */ true);
+
+		adjust_move_coords_1d(y, edges.top, tshift,
+			edges.bottom, bshift, *y < view_geom->y);
+	}
+}
+
+void
+edges_adjust_resize_geom(struct view *view, struct border edges,
+		uint32_t resize_edges, struct wlr_box *geom, bool use_pending)
+{
+	assert(view);
+
+	struct border border = ssd_get_margin(view->ssd);
+	struct wlr_box *view_geom =
+		use_pending ? &view->pending : &view->current;
+
+	/*
+	 * When resizing along a given edge, limit the motion of that edge to
+	 * any valid nearest edge in the corresponding direction.
+	 */
+
+	if (resize_edges & WLR_EDGE_LEFT) {
+		if (BOUNDED_INT(edges.left)) {
+			geom->x = edges.left + border.left + rc.gap;
+			geom->width = view_geom->width + view_geom->x - geom->x;
+		}
+	} else if (resize_edges & WLR_EDGE_RIGHT) {
+		if (BOUNDED_INT(edges.right)) {
+			geom->width = edges.right
+				- view_geom->x - border.right - rc.gap;
+		}
+	}
+
+	if (resize_edges & WLR_EDGE_TOP) {
+		if (BOUNDED_INT(edges.top)) {
+			geom->y = edges.top + border.top + rc.gap;
+			geom->height = view_geom->height + view_geom->y - geom->y;
+		}
+	} else if (resize_edges & WLR_EDGE_BOTTOM) {
+		if (BOUNDED_INT(edges.bottom)) {
+			geom->height = edges.bottom
+				- view_geom->y - border.bottom - rc.gap;
+		}
+	}
+}

src/meson.build

@@ -4,6 +4,7 @@ labwc_sources = files(
   'debug.c',
   'desktop.c',
   'dnd.c',
+  'edges.c',
   'foreign.c',
   'idle.c',
   'interactive.c',

src/resistance.c

@@ -4,164 +4,84 @@
 #include "common/border.h"
 #include "common/macros.h"
 #include "config/rcxml.h"
+#include "edges.h"
 #include "labwc.h"
 #include "resistance.h"
 #include "view.h"
 
 static void
-is_within_resistance_range(struct border view, struct border target,
-		struct border other, struct border *flags, int strength)
+check_edge(int *next, int current, int target,
+		int oppose, int align, bool lesser, int tolerance)
 {
-	if (view.left >= other.left) {
-		const int lo = other.left - abs(strength);
-		const int hi = other.left - MIN(strength, 0);
-		flags->left = target.left >= lo && target.left < hi;
-	}
-
-	if (!flags->left && view.right <= other.right) {
-		const int lo = other.right + MIN(strength, 0);
-		const int hi = other.right + abs(strength);
-		flags->right = target.right > lo && target.right <= hi;
-	}
-
-	if (view.top >= other.top) {
-		const int lo = other.top - abs(strength);
-		const int hi = other.top - MIN(strength, 0);
-		flags->top = target.top >= lo && target.top < hi;
-	}
-
-	if (!flags->top && view.bottom <= other.bottom) {
-		const int lo = other.bottom + MIN(strength, 0);
-		const int hi = other.bottom + abs(strength);
-		flags->bottom = target.bottom > lo && target.bottom <= hi;
-	}
-}
-
-static void
-build_view_edges(struct view *view, struct wlr_box new_geom,
-		struct border *view_edges, struct border *target_edges, bool move)
-{
-	struct border border = ssd_get_margin(view->ssd);
-
-	/* Use the effective height to properly snap shaded views */
-	int eff_height = view_effective_height(view, /* use_pending */ false);
-
-	view_edges->left = view->current.x - border.left + (move ? 1 : 0);
-	view_edges->top = view->current.y - border.top + (move ? 1 : 0);
-	view_edges->right = view->current.x + view->current.width + border.right;
-	view_edges->bottom = view->current.y + eff_height + border.bottom;
-
-	target_edges->left = new_geom.x - border.left;
-	target_edges->top = new_geom.y - border.top;
-	target_edges->right = new_geom.x + new_geom.width + border.right;
-	target_edges->bottom = new_geom.y + border.bottom +
-		(view->shaded ? 0 : new_geom.height);
-}
-
-static void
-update_nearest_edge(struct border view_edges, struct border target_edges,
-		struct border region_edges, int strength,
-		struct border *next_edges)
-{
-	struct border flags = { 0 };
-	is_within_resistance_range(view_edges,
-		target_edges, region_edges, &flags, strength);
-
-	if (flags.left == 1) {
-		next_edges->left = MAX(region_edges.left, next_edges->left);
-	} else if (flags.right == 1) {
-		next_edges->right = MIN(region_edges.right, next_edges->right);
-	}
-
-	if (flags.top == 1) {
-		next_edges->top = MAX(region_edges.top, next_edges->top);
-	} else if (flags.bottom == 1) {
-		next_edges->bottom = MIN(region_edges.bottom, next_edges->bottom);
-	}
-}
-
-static void
-find_neighbor_edges(struct view *view, struct wlr_box new_geom,
-		struct border *next_edges, bool move)
-{
-	if (rc.window_edge_strength == 0) {
+	/* Ignore non-moving edges */
+	if (current == target) {
 		return;
 	}
 
-	struct border view_edges = { 0 };
-	struct border target_edges = { 0 };
-
-	build_view_edges(view, new_geom, &view_edges, &target_edges, move);
-
-	struct view *v;
-	for_each_view(v, &view->server->views, LAB_VIEW_CRITERIA_CURRENT_WORKSPACE) {
-		if (v == view || !output_is_usable(v->output)) {
-			continue;
-		}
-
-		struct border border = ssd_get_margin(v->ssd);
-
 	/*
-		 * The significance of window edges here is inverted with
-		 * respect to the usual orientation, because the edges of the
-		 * view v of interest are those that would be encountered by a
-		 * change in geometry in view along the named edge of view.
-		 * Hence, when moving or resizing view *left*, it is the
-		 * *right* edge of v that would be encountered, and vice versa;
-		 * when moving or resizing view *down* ("bottom"), it is the
-		 * *top* edge of v that would be encountered, and vice versa.
+	 * The edge defined by current and moving to target may encounter two
+	 * edges of another region: the opposing edge of the region is that in
+	 * the opposite orientation of the moving edge (i.e., left <-> right or
+	 * top <-> bottom); the aligned edge of the region is that in the same
+	 * orientation as the moving edge (i.e., left <->left, top <-> top,
+	 * right <-> right, bottom <-> bottom).
+	 *
+	 * Any opposing or aligned edge of a region is considered "valid" in
+	 * this search if the resist/attract zone (defined by tolerance) of
+	 * that edge contains the target position of the moving edge.
 	 */
-		struct border win_edges = {
-			.top = v->current.y + border.bottom
-				+ view_effective_height(v, /* use_pending */ false),
-			.right = v->current.x - border.left,
-			.bottom = v->current.y - border.top,
-			.left = v->current.x + v->current.width + border.right,
-		};
 
-		update_nearest_edge(view_edges, target_edges,
-			win_edges, rc.window_edge_strength, next_edges);
+	/* Direction of motion for the edge */
+	const bool decreasing = target < current;
+
+	/* Check the opposing edge */
+	bool valid = false;
+	if (decreasing) {
+		const int lo = clipped_sub(oppose, abs(tolerance));
+		const int hi = clipped_sub(oppose, MIN(tolerance, 0));
+		valid = target >= lo && target < hi;
+	} else {
+		/* Check for increasing movement across opposing edge */
+		const int lo = clipped_add(oppose, MIN(tolerance, 0));
+		const int hi = clipped_add(oppose, abs(tolerance));
+		valid = target > lo && target <= hi;
+	}
+
+	if (valid) {
+		*next = edge_get_best(*next, oppose, decreasing);
+	}
+
+	/* Check the aligned edge */
+	valid = false;
+	if (decreasing) {
+		const int lo = clipped_sub(align, abs(tolerance));
+		const int hi = clipped_sub(align, MIN(tolerance, 0));
+		valid = target >= lo && target < hi;
+	} else {
+		const int lo = clipped_add(align, MIN(tolerance, 0));
+		const int hi = clipped_add(align, abs(tolerance));
+		valid = target > lo && target <= hi;
+	}
+
+	if (valid) {
+		*next = edge_get_best(*next, align, decreasing);
 	}
 }
 
 static void
-find_screen_edges(struct view *view, struct wlr_box new_geom,
-		struct border *next_edges, bool move)
+check_edge_output(int *next, int current, int target,
+		int oppose, int align, bool lesser)
 {
-	if (rc.screen_edge_strength == 0) {
-		return;
+	check_edge(next, current, target,
+		oppose, align, lesser, rc.screen_edge_strength);
 }
 
-	struct border view_edges = { 0 };
-	struct border target_edges = { 0 };
-
-	build_view_edges(view, new_geom, &view_edges, &target_edges, move);
-
-	struct output *output;
-	wl_list_for_each(output, &view->server->outputs, link) {
-		if (!output_is_usable(output)) {
-			continue;
-		}
-
-		struct wlr_box mgeom =
-			output_usable_area_in_layout_coords(output);
-
-		struct wlr_box ol;
-		if (!wlr_box_intersection(&ol, &view->current, &mgeom) &&
-				!wlr_box_intersection(&ol, &new_geom, &mgeom)) {
-			continue;
-		}
-
-		struct border screen_edges = {
-			.top = mgeom.y,
-			.right = mgeom.x + mgeom.width,
-			.bottom = mgeom.y + mgeom.height,
-			.left = mgeom.x,
-		};
-
-		update_nearest_edge(view_edges, target_edges,
-			screen_edges, rc.screen_edge_strength, next_edges);
-	}
+static void
+check_edge_window(int *next, int current, int target,
+		int oppose, int align, bool lesser)
+{
+	check_edge(next, current, target,
+		oppose, align, lesser, rc.window_edge_strength);
 }
 
 void
@@ -169,37 +89,34 @@ resistance_move_apply(struct view *view, double *x, double *y)
 {
 	assert(view);
 
-	struct border border = ssd_get_margin(view->ssd);
+	struct border next_edges;
+	edges_initialize(&next_edges);
 
-	struct border next_edges = {
-		.top = INT_MIN,
-		.right = INT_MAX,
-		.bottom = INT_MAX,
-		.left = INT_MIN,
-	};
-
-	struct wlr_box new_geom = {
+	struct wlr_box target = {
 		.x = *x,
 		.y = *y,
 		.width = view->current.width,
 		.height = view->current.height,
 	};
 
-	find_screen_edges(view, new_geom, &next_edges, /* move */ true);
-	find_neighbor_edges(view, new_geom, &next_edges, /* move */ true);
-
-	if (next_edges.left > INT_MIN) {
-		*x = next_edges.left + border.left;
-	} else if (next_edges.right < INT_MAX) {
-		*x = next_edges.right - view->current.width - border.right;
+	if (rc.screen_edge_strength != 0) {
+		/* Find any relevant output edges encountered by this move */
+		edges_find_outputs(&next_edges, view, target, NULL,
+			check_edge_output, /* use_pending */ false);
 	}
 
-	if (next_edges.top > INT_MIN) {
-		*y = next_edges.top + border.top;
-	} else if (next_edges.bottom < INT_MAX) {
-		*y = next_edges.bottom - border.bottom
-			- view_effective_height(view, /* use_pending */ false);
+	if (rc.window_edge_strength != 0) {
+		/* Find any relevant window edges encountered by this move */
+		edges_find_neighbors(&next_edges, view, target, NULL,
+			check_edge_window, /* use_pending */ false);
 	}
+
+	/* If any "best" edges were encountered during this move, snap motion */
+	edges_adjust_move_coords(view, next_edges,
+		&target.x, &target.y, /* use_pending */ false);
+
+	*x = target.x;
+	*y = target.y;
 }
 
 void
@@ -208,41 +125,22 @@ resistance_resize_apply(struct view *view, struct wlr_box *new_geom)
 	assert(view);
 	assert(!view->shaded);
 
-	struct border border = ssd_get_margin(view->ssd);
+	struct border next_edges;
+	edges_initialize(&next_edges);
 
-	struct border next_edges = {
-		.top = INT_MIN,
-		.right = INT_MAX,
-		.bottom = INT_MAX,
-		.left = INT_MIN,
-	};
-
-	find_screen_edges(view, *new_geom, &next_edges, /* move */ false);
-	find_neighbor_edges(view, *new_geom, &next_edges, /* move */ false);
-
-	if (view->server->resize_edges & WLR_EDGE_LEFT) {
-		if (next_edges.left > INT_MIN) {
-			new_geom->x = next_edges.left + border.left;
-			new_geom->width = view->current.width
-				+ view->current.x - new_geom->x;
-		}
-	} else if (view->server->resize_edges & WLR_EDGE_RIGHT) {
-		if (next_edges.right < INT_MAX) {
-			new_geom->width = next_edges.right
-				- view->current.x - border.right;
-		}
+	if (rc.screen_edge_strength != 0) {
+		/* Find any relevant output edges encountered by this move */
+		edges_find_outputs(&next_edges, view, *new_geom, NULL,
+			check_edge_output, /* use_pending */ false);
 	}
 
-	if (view->server->resize_edges & WLR_EDGE_TOP) {
-		if (next_edges.top > INT_MIN) {
-			new_geom->y = next_edges.top + border.top;
-			new_geom->height = view->current.height
-				+ view->current.y - new_geom->y;
-		}
-	} else if (view->server->resize_edges & WLR_EDGE_BOTTOM) {
-		if (next_edges.bottom < INT_MAX) {
-			new_geom->height = next_edges.bottom
-				- view->current.y - border.bottom;
-		}
+	if (rc.window_edge_strength != 0) {
+		/* Find any relevant window edges encountered by this move */
+		edges_find_neighbors(&next_edges, view, *new_geom, NULL,
+			check_edge_window, /* use_pending */ false);
 	}
+
+	/* If any "best" edges were encountered during this move, snap motion */
+	edges_adjust_resize_geom(view, next_edges,
+		view->server->resize_edges, new_geom, /* use_pending */ false);
 }

src/snap.c

@@ -1,285 +1,259 @@
 // SPDX-License-Identifier: GPL-2.0-only
-#include <strings.h>
+#include <assert.h>
+#include <limits.h>
 #include <wlr/util/box.h>
+#include "common/border.h"
+#include "common/macros.h"
+#include "config/rcxml.h"
+#include "edges.h"
 #include "labwc.h"
+#include "resistance.h"
 #include "snap.h"
 #include "view.h"
-#include "workspaces.h"
-
-/* We cannot use MIN/MAX macros, as they may call functions twice, and
- * can be overridden by previous #define.
- */
-static inline int
-min(int a, int b) {
-	return a < b ? a : b;
-}
-
-static inline int
-max(int a, int b) {
-	return a > b ? a : b;
-}
-
-static inline int
-min3(int a, int b, int c) {
-	return min(min(a, b), c);
-}
-
-enum snap_mode {
-	SNAP_MODE_MOVE = 0,
-	SNAP_MODE_GROW,
-	SNAP_MODE_SHRINK,
-};
-
-static struct border
-snap_get_view_edge(struct view *view)
-{
-	struct border margin = ssd_get_margin(view->ssd);
-	struct border edge = {
-		.left   = view->pending.x - margin.left,
-		.top    = view->pending.y - margin.top,
-		.right  = view->pending.x + margin.right + view->pending.width,
-		.bottom = view->pending.y + margin.bottom
-			+ view_effective_height(view, /* use_pending */ true)
-	};
-	return edge;
-}
-
-struct border
-snap_get_max_distance(struct view *view)
-{
-	struct output *output = view->output;
-	struct border margin = ssd_get_margin(view->ssd);
-	struct wlr_box usable = output_usable_area_scaled(output);
-	struct border distance = {
-		.left   = usable.x + margin.left + rc.gap - view->pending.x,
-		.top    = usable.y + margin.top  + rc.gap - view->pending.y,
-		.right  = usable.x + usable.width - view->pending.width
-			- margin.right  - rc.gap - view->pending.x,
-		.bottom = usable.y + usable.height
-			- view_effective_height(view, /* use_pending */ true)
-			- margin.bottom - rc.gap - view->pending.y
-	};
-	return distance;
-}
-
-struct snap_search {
-	const int search_dir; /* -1: left/up, 1: right/down */
-
-	const int add_view_x;
-	const int add_view_y;
-	const int add_view_width;
-	const int add_view_height;
-
-	const int add_margin_left;
-	const int add_margin_top;
-	const int add_margin_right;
-	const int add_margin_bottom;
-};
-
-/* near/far is the left, right, top or bottom border of a window,
- * depending on the search direction:
- *  - near_right: search to the right, snap to left  (near) border of a window.
- *  - far_right:  search to the right, snap to right (far)  border of a window.
- *  - near_left:  search to the left,  snap to right (near) border of a window.
- *  - far_left:   search to the left,  snap to left  (far)  border of a window.
- *
- * structs below define what coordinates and margins to take into
- * account depending near/far, and direction.
- */
-static const struct snap_search near_left  = { -1,   1, 0, 1, 0,    0,  0,  1,  0 };
-static const struct snap_search near_up    = { -1,   0, 1, 0, 1,    0,  0,  0,  1 };
-static const struct snap_search near_right = {  1,   1, 0, 0, 0,   -1,  0,  0,  0 };
-static const struct snap_search near_down  = {  1,   0, 1, 0, 0,    0, -1,  0,  0 };
-static const struct snap_search far_left   = { -1,   1, 0, 0, 0,   -1,  0,  0,  0 };
-static const struct snap_search far_up     = { -1,   0, 1, 0, 0,    0, -1,  0,  0 };
-static const struct snap_search far_right  = {  1,   1, 0, 1, 0,    0,  0,  1,  0 };
-static const struct snap_search far_down   = {  1,   0, 1, 0, 1,    0,  0,  1,  0 };
-
-static inline int
-_snap_next_edge(struct view *view, int start_pos, const struct snap_search def, int max, int gap)
-{
-	struct output *output = view->output;
-	struct server *server = output->server;
-	struct view *v;
-	int p = max;
-	for_each_view(v, &server->views, LAB_VIEW_CRITERIA_CURRENT_WORKSPACE) {
-		if (v == view || v->output != output || v->minimized
-				|| v->maximized == VIEW_AXIS_BOTH) {
-			continue;
-		}
-
-		struct border margin = ssd_get_margin(v->ssd);
-		int vp = -start_pos;
-		vp += def.add_margin_left   * margin.left;
-		vp += def.add_margin_top    * margin.top;
-		vp += def.add_margin_right  * margin.right;
-		vp += def.add_margin_bottom * margin.bottom;
-		vp += def.add_view_x        * v->pending.x;
-		vp += def.add_view_y        * v->pending.y;
-		vp += def.add_view_width    * v->pending.width;
-		vp += def.add_view_height
-			* view_effective_height(v, /* use_pending */ true);
-		vp += gap;
-
-		if (def.search_dir * vp > 0 && def.search_dir * (vp - p) < 0) {
-			p = vp;
-		}
-	}
-	return p;
-}
 
 static void
-_snap_move_resize_to_edge(struct view *view, enum view_edge direction, enum snap_mode mode,
-		struct wlr_box *delta)
+check_edge(int *next, int current, int target, int oppose, int align, bool lesser)
 {
-	struct border edge = snap_get_view_edge(view);
-	struct border dmax;
-
-	if (mode == SNAP_MODE_SHRINK) {
-		/* limit to half of current size */
-		int eff_height =
-			view_effective_height(view, /* use_pending */ true);
-		int width_max_dx = max(view->pending.width - LAB_MIN_VIEW_WIDTH, 0);
-		int height_max_dy = max(eff_height - LAB_MIN_VIEW_HEIGHT, 0);
-		dmax.right  = min(width_max_dx,  view->pending.width  / 2);
-		dmax.bottom = min(height_max_dy, eff_height / 2);
-		dmax.left   = -dmax.right;
-		dmax.top    = -dmax.bottom;
-	} else {
-		dmax = snap_get_max_distance(view);
+	if (current == target) {
+		return;
 	}
 
+	/*
+	 * The edge defined by current and moving to target may encounter two
+	 * edges of another region: the opposing edge of the region is that in
+	 * the opposite orientation of the moving edge (i.e., left <-> right or
+	 * top <-> bottom); the aligned edge of the region is that in the same
+	 * orientation as the moving edge (i.e., left <-> left, top <-> top,
+	 * right <-> right, bottom <-> bottom).
+	 *
+	 * Any opposing or aligned edge of a region is considered "valid" in
+	 * this search if the edge sits between the current and target
+	 * positions of the moving edge (including the target position itself).
+	 */
+
+	/* Direction of motion for the edge */
+	const bool decreasing = target < current;
+
+	/* Check the opposing edge */
+	if ((target <= oppose && oppose < current) ||
+			(current < oppose && oppose <= target)) {
+		*next = edge_get_best(*next, oppose, decreasing);
+	}
+
+	/* Check the aligned edge */
+	if ((target <= align && align < current) ||
+			(current < align && align <= target)) {
+		*next = edge_get_best(*next, align, decreasing);
+	}
+}
+
+void
+snap_move_to_edge(struct view *view, enum view_edge direction,
+		bool snap_to_windows, int *dx, int *dy)
+{
+	assert(view);
+
+	*dx = 0;
+	*dy = 0;
+
+	struct output *output = view->output;
+	if (!output_is_usable(output)) {
+		wlr_log(WLR_ERROR, "view has no output, not snapping to edge");
+		return;
+	}
+
+	struct wlr_box target = view->pending;
+	struct border ssd = ssd_thickness(view);
+	struct wlr_box usable = output_usable_area_in_layout_coords(output);
+
+	/*
+	 * First try to move the view to the relevant edge of its output. If
+	 * the view is off-screen, such a move might actually run contrary to
+	 * the commanded direction (e.g., a view off the screen to the left,
+	 * when moved to the left edge, will actually move rightward). This is
+	 * counter-intuitive, so abandon any such movements.
+	 *
+	 * In addition, any view that is already at the desired screen edge
+	 * needs no further consideration.
+	 */
 	switch (direction) {
 	case VIEW_EDGE_LEFT:
-		if (mode == SNAP_MODE_MOVE) {
-			delta->x += max(
-				/* left edge to left/right edges */
-				_snap_next_edge(view, edge.left, near_left, dmax.left, rc.gap),
-				_snap_next_edge(view, edge.left, far_left,  dmax.left, 0)
-			);
-		} else if (mode == SNAP_MODE_GROW) {
-			int dx = max(
-				/* left edge to left/right edges */
-				_snap_next_edge(view, edge.left, near_left, dmax.left, rc.gap),
-				_snap_next_edge(view, edge.left, far_left,  dmax.left, 0)
-			);
-			delta->x     +=  dx;
-			delta->width += -dx;
-		} else if (mode == SNAP_MODE_SHRINK) {
-			delta->width += max(
-				/* right edge to left/right edges */
-				_snap_next_edge(view, edge.right, near_left, dmax.left, 0),
-				_snap_next_edge(view, edge.right, far_left,  dmax.left, -rc.gap)
-			);
-		}
-		break;
-	case VIEW_EDGE_UP:
-		if (mode == SNAP_MODE_MOVE) {
-			delta->y += max(
-				/* top edge to top/bottom edges */
-				_snap_next_edge(view, edge.top, near_up, dmax.top, rc.gap),
-				_snap_next_edge(view, edge.top, far_up,  dmax.top, 0)
-			);
-		} else if (mode == SNAP_MODE_GROW) {
-			int dy = max(
-				/* top edge to top/bottom edges */
-				_snap_next_edge(view, edge.top, near_up, dmax.top, rc.gap),
-				_snap_next_edge(view, edge.top, far_up,  dmax.top, 0)
-			);
-			delta->y      +=  dy;
-			delta->height += -dy;
-		} else if (mode == SNAP_MODE_SHRINK) {
-			delta->height += max(
-				/* bottom edge to top/bottom edges */
-				_snap_next_edge(view, edge.bottom, near_up, dmax.top, 0),
-				_snap_next_edge(view, edge.bottom, far_up,  dmax.top, -rc.gap)
-			);
+		target.x = usable.x + ssd.left + rc.gap;
+		if (target.x >= view->pending.x) {
+			return;
 		}
 		break;
 	case VIEW_EDGE_RIGHT:
-		if (mode == SNAP_MODE_MOVE) {
-			delta->x += min3(
-				/* left edge to left/right edges */
-				_snap_next_edge(view, edge.left, near_right, dmax.right, 0),
-				_snap_next_edge(view, edge.left, far_right,  dmax.right, rc.gap),
-				/* right edge to left edge */
-				_snap_next_edge(view, edge.right, near_right, dmax.right, -rc.gap)
-			);
-		} else if (mode == SNAP_MODE_GROW) {
-			delta->width += min(
-				/* right edge to left/right edges */
-				_snap_next_edge(view, edge.right, near_right, dmax.right, -rc.gap),
-				_snap_next_edge(view, edge.right, far_right,  dmax.right, 0)
-			);
-		} else if (mode == SNAP_MODE_SHRINK) {
-			delta->x += min(
-				/* left edge to left/right edges */
-				_snap_next_edge(view, edge.left, near_right, dmax.right, 0),
-				_snap_next_edge(view, edge.left, far_right,  dmax.right, rc.gap)
-			);
-			delta->width += -(delta->x);
+		target.x = usable.x + usable.width
+			- rc.gap - target.width - ssd.right;
+		if (target.x <= view->pending.x) {
+			return;
+		}
+		break;
+	case VIEW_EDGE_UP:
+		target.y = usable.y + ssd.top + rc.gap;
+		if (target.y >= view->pending.y) {
+			return;
 		}
 		break;
 	case VIEW_EDGE_DOWN:
-		if (mode == SNAP_MODE_MOVE) {
-			delta->y += min3(
-				/* top edge to top/bottom edges */
-				_snap_next_edge(view, edge.top, near_down, dmax.bottom, 0),
-				_snap_next_edge(view, edge.top, far_down,  dmax.bottom, rc.gap),
-				/* bottom edge to top edge */
-				_snap_next_edge(view, edge.bottom, near_down, dmax.bottom, -rc.gap)
-			);
-		} else if (mode == SNAP_MODE_GROW) {
-			delta->height += min(
-				/* bottom edge to top/bottom edges */
-				_snap_next_edge(view, edge.bottom, near_down, dmax.bottom, -rc.gap),
-				_snap_next_edge(view, edge.bottom, far_down,  dmax.bottom, 0)
-			);
-		} else if (mode == SNAP_MODE_SHRINK) {
-			delta->y += min(
-				/* top edge to top/bottom edges */
-				_snap_next_edge(view, edge.top, near_down, dmax.bottom, 0),
-				_snap_next_edge(view, edge.top, far_down,  dmax.bottom, rc.gap)
-			);
-			delta->height += -(delta->y);
+		target.y = usable.y + usable.height - rc.gap - ssd.bottom
+			- view_effective_height(view, /* use_pending */ true);
+		if (target.y <= view->pending.y) {
+			return;
 		}
 		break;
 	default:
 		return;
 	}
+
+	/*
+	 * Because the target has been updated to put the view at the edge of
+	 * an output, there is no need to check snapping to output edges. If
+	 * snapping to view is desired, check for snapping against any view on
+	 * the same output.
+	 */
+	if (snap_to_windows) {
+		struct border next_edges;
+		edges_initialize(&next_edges);
+
+		edges_find_neighbors(&next_edges, view, target,
+			output, check_edge, /* use_pending */ true);
+
+		/* If any "best" edges were encountered, limit motion */
+		edges_adjust_move_coords(view, next_edges,
+			&target.x, &target.y, /* use_pending */ true);
+	}
+
+	*dx = target.x - view->pending.x;
+	*dy = target.y - view->pending.y;
 }
 
 void
-snap_vector_to_next_edge(struct view *view, enum view_edge direction, int *dx, int *dy)
+snap_grow_to_next_edge(struct view *view, enum view_edge direction,
+		struct wlr_box *geo)
 {
-	struct wlr_box delta = {0};
-	_snap_move_resize_to_edge(view, direction, SNAP_MODE_MOVE, &delta);
-	*dx = delta.x;
-	*dy = delta.y;
+	assert(view);
+	assert(!view->shaded);
+
+	*geo = view->pending;
+
+	struct output *output = view->output;
+	if (!output_is_usable(output)) {
+		wlr_log(WLR_ERROR, "view has no output, not growing to edge");
+		return;
 	}
 
-int
-snap_distance_to_next_edge(struct view *view, enum view_edge direction)
-{
-	struct wlr_box delta = {0};
-	_snap_move_resize_to_edge(view, direction, SNAP_MODE_MOVE, &delta);
+	struct border ssd = ssd_thickness(view);
+	struct wlr_box usable = output_usable_area_in_layout_coords(output);
+	uint32_t resize_edges;
+
+	/* First try to grow the view to the relevant edge of its output. */
 	switch (direction) {
-	case VIEW_EDGE_LEFT:  return -delta.x;
-	case VIEW_EDGE_UP:    return -delta.y;
-	case VIEW_EDGE_RIGHT: return  delta.x;
-	case VIEW_EDGE_DOWN:  return  delta.y;
-	default: return 0;
+	case VIEW_EDGE_LEFT:
+		geo->x = usable.x + ssd.left + rc.gap;
+		geo->width = view->pending.x + view->pending.width - geo->x;
+		resize_edges = WLR_EDGE_LEFT;
+		break;
+	case VIEW_EDGE_RIGHT:
+		geo->width = usable.x + usable.width
+			- rc.gap - ssd.right - view->pending.x;
+		resize_edges = WLR_EDGE_RIGHT;
+		break;
+	case VIEW_EDGE_UP:
+		geo->y = usable.y + ssd.top + rc.gap;
+		geo->height = view->pending.y + view->pending.height - geo->y;
+		resize_edges = WLR_EDGE_TOP;
+		break;
+	case VIEW_EDGE_DOWN:
+		geo->height = usable.y + usable.height
+			- rc.gap - ssd.bottom - view->pending.y;
+		resize_edges = WLR_EDGE_BOTTOM;
+		break;
+	default:
+		return;
 	}
+
+	/* No grow operation should ever shrink the view */
+	if (geo->width < view->pending.width ||
+			geo->height < view->pending.height) {
+		*geo = view->pending;
+		return;
+	}
+
+	/* If the view doesn't change size, there is no need for snap checks */
+	if (geo->width == view->pending.width &&
+			geo->height == view->pending.height) {
+		*geo = view->pending;
+		return;
+	}
+
+	struct border next_edges;
+	edges_initialize(&next_edges);
+
+	/* Limit motion to any intervening edge of other views on this output */
+	edges_find_neighbors(&next_edges, view, *geo,
+		output, check_edge, /* use_pending */ true);
+	edges_adjust_resize_geom(view, next_edges,
+		resize_edges, geo, /* use_pending */ true);
 }
 
 void
-snap_grow_to_next_edge(struct view *view, enum view_edge direction, struct wlr_box *geo)
+snap_shrink_to_next_edge(struct view *view, enum view_edge direction,
+		struct wlr_box *geo)
 {
-	_snap_move_resize_to_edge(view, direction, SNAP_MODE_GROW, geo);
+	assert(view);
+	assert(!view->shaded);
+
+	*geo = view->pending;
+	uint32_t resize_edges;
+
+	/*
+	 * First shrink the view along the relevant edge. The maximum shrink
+	 * allowed is half the current size, but the window must also meet
+	 * minimum size requirements.
+	 */
+	switch (direction) {
+	case VIEW_EDGE_RIGHT:
+		geo->width = MAX(geo->width / 2, LAB_MIN_VIEW_WIDTH);
+		geo->x = view->pending.x + view->pending.width - geo->width;
+		resize_edges = WLR_EDGE_LEFT;
+		break;
+	case VIEW_EDGE_LEFT:
+		geo->width = MAX(geo->width / 2, LAB_MIN_VIEW_WIDTH);
+		resize_edges = WLR_EDGE_RIGHT;
+		break;
+	case VIEW_EDGE_DOWN:
+		geo->height = MAX(geo->height / 2, LAB_MIN_VIEW_HEIGHT);
+		geo->y = view->pending.y + view->pending.height - geo->height;
+		resize_edges = WLR_EDGE_TOP;
+		break;
+	case VIEW_EDGE_UP:
+		geo->height = MAX(geo->height / 2, LAB_MIN_VIEW_HEIGHT);
+		resize_edges = WLR_EDGE_BOTTOM;
+		break;
+	default:
+		return;
 	}
 
-void
-snap_shrink_to_next_edge(struct view *view, enum view_edge direction, struct wlr_box *geo)
-{
-	_snap_move_resize_to_edge(view, direction, SNAP_MODE_SHRINK, geo);
+	/* If the view doesn't change size, abandon the shrink */
+	if (geo->width == view->pending.width &&
+			geo->height == view->pending.height) {
+		*geo = view->pending;
+		return;
+	}
+
+	struct border next_edges;
+	edges_initialize(&next_edges);
+
+	/* Snap to output edges if the moving edge started off-screen */
+	edges_find_outputs(&next_edges, view, *geo,
+		view->output, check_edge, /* use_pending */ true);
+
+	/* Limit motion to any intervening edge of ther views on this output */
+	edges_find_neighbors(&next_edges, view, *geo,
+		view->output, check_edge, /* use_pending */ true);
+
+	edges_adjust_resize_geom(view, next_edges,
+		resize_edges, geo, /* use_pending */ true);
 }

src/view.c

@@ -1550,27 +1550,7 @@ view_move_to_edge(struct view *view, enum view_edge direction, bool snap_to_wind
 	}
 
 	int dx = 0, dy = 0;
-	if (snap_to_windows) {
-		snap_vector_to_next_edge(view, direction, &dx, &dy);
-	} else {
-		struct border distance = snap_get_max_distance(view);
-		switch (direction) {
-		case VIEW_EDGE_LEFT:
-			dx = distance.left;
-			break;
-		case VIEW_EDGE_UP:
-			dy = distance.top;
-			break;
-		case VIEW_EDGE_RIGHT:
-			dx = distance.right;
-			break;
-		case VIEW_EDGE_DOWN:
-			dy = distance.bottom;
-			break;
-		default:
-			return;
-		}
-	}
+	snap_move_to_edge(view, direction, snap_to_windows, &dx, &dy);
 
 	if (dx != 0 || dy != 0) {
 		/* Move the window if a change was discovered */
@@ -1665,7 +1645,7 @@ view_grow_to_edge(struct view *view, enum view_edge direction)
 
 	view_set_shade(view, false);
 
-	struct wlr_box geo = view->pending;
+	struct wlr_box geo;
 	snap_grow_to_next_edge(view, direction, &geo);
 	view_move_resize(view, geo);
 }