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, 62 insertions, 0 deletions

diff --git a/sway/tree/container.c b/sway/tree/container.c
index 23b6c997..4c573e83 100644
--- a/sway/tree/container.c
+++ b/sway/tree/container.c
@@ -1630,3 +1630,65 @@ bool container_is_sticky(struct sway_container *con) {
 bool container_is_sticky_or_child(struct sway_container *con) {
         return container_is_sticky(container_toplevel_ancestor(con));
 }
+
+static bool is_parallel(enum sway_container_layout first,
+                enum sway_container_layout second) {
+        switch (first) {
+        case L_TABBED:
+        case L_HORIZ:
+                return second == L_TABBED || second == L_HORIZ;
+        case L_STACKED:
+        case L_VERT:
+                return second == L_STACKED || second == L_VERT;
+        default:
+                return false;
+        }
+}
+
+static bool container_is_squashable(struct sway_container *con,
+                struct sway_container *child) {
+        enum sway_container_layout gp_layout = container_parent_layout(con);
+        return (con->layout == L_HORIZ || con->layout == L_VERT) &&
+                (child->layout == L_HORIZ || child->layout == L_VERT) &&
+                !is_parallel(con->layout, child->layout) &&
+                is_parallel(gp_layout, child->layout);
+}
+
+static void container_squash_children(struct sway_container *con) {
+        for (int i = 0; i < con->children->length; i++) {
+                struct sway_container *child = con->children->items[i];
+                i += container_squash(child);
+        }
+}
+
+int container_squash(struct sway_container *con) {
+        if (!con->children) {
+                return 0;
+        }
+        if (con->children->length != 1) {
+                container_squash_children(con);
+                return 0;
+        }
+        struct sway_container *child = con->children->items[0];
+        int idx = container_sibling_index(con);
+        int change = 0;
+        if (container_is_squashable(con, child)) {
+                // con and child are a redundant H/V pair. Destroy them.
+                while (child->children->length) {
+                        struct sway_container *current = child->children->items[0];
+                        container_detach(current);
+                        if (con->parent) {
+                                container_insert_child(con->parent, current, idx);
+                        } else {
+                                workspace_insert_tiling_direct(con->workspace, current, idx);
+                        }
+                        change++;
+                }
+                // This will also destroy con because child was its only child
+                container_reap_empty(child);
+                change--;
+        } else {
+                container_squash_children(con);
+        }
+        return change;
+}