introduce workspace_squash

workspace_squash is container_flatten in the reverse
direction. Instead of eliminating redundant splits that are
parents of the target container, it eliminates pairs of
redundant H/V splits that are children of the workspace.

Splits are redundant if a con and its grandchild have the
same layout, and the immediate child has the opposite split.

For example, layouts are transformed like:

    H[V[H[app1 app2]] app3] -> H[app1 app2 app3]

i3 uses this operation to simplify the tree after moving
heavily nested containers to a higher level in the tree via
an orthogonal move.

1 files changed, 19 insertions, 7 deletions

diff --git a/sway/tree/workspace.c b/sway/tree/workspace.c
index e40792ae..62549434 100644
--- a/sway/tree/workspace.c
+++ b/sway/tree/workspace.c
@@ -714,14 +714,8 @@ void workspace_add_floating(struct sway_workspace *workspace,
 	node_set_dirty(&con->node);
 }
 
-struct sway_container *workspace_insert_tiling(struct sway_workspace *workspace,
+void workspace_insert_tiling_direct(struct sway_workspace *workspace,
 		struct sway_container *con, int index) {
-	if (con->workspace) {
-		container_detach(con);
-	}
-	if (config->default_layout != L_NONE) {
-		con = container_split(con, config->default_layout);
-	}
 	list_insert(workspace->tiling, index, con);
 	con->workspace = workspace;
 	container_for_each_child(con, set_workspace, NULL);
@@ -729,6 +723,17 @@ struct sway_container *workspace_insert_tiling(struct sway_workspace *workspace,
 	workspace_update_representation(workspace);
 	node_set_dirty(&workspace->node);
 	node_set_dirty(&con->node);
+}
+
+struct sway_container *workspace_insert_tiling(struct sway_workspace *workspace,
+		struct sway_container *con, int index) {
+	if (con->workspace) {
+		container_detach(con);
+	}
+	if (config->default_layout != L_NONE) {
+		con = container_split(con, config->default_layout);
+	}
+	workspace_insert_tiling_direct(workspace, con, index);
 	return con;
 }
 
@@ -846,3 +851,10 @@ size_t workspace_num_sticky_containers(struct sway_workspace *ws) {
 	workspace_for_each_container(ws, count_sticky_containers, &count);
 	return count;
 }
+
+void workspace_squash(struct sway_workspace *workspace) {
+	for (int i = 0; i < workspace->tiling->length; i++) {
+		struct sway_container *child = workspace->tiling->items[i];
+		i += container_squash(child);
+	}
+}