Make it easy to swap windows from choose-tree

I picked the new keys for this because of the similarity to switching vs
swapping tabs in a browser or other tabbed UI. There, Ctrl+PgUp switches
to one tab previous and Ctrl+Shift+PgUp swaps with the previous tab. So
I figured adding Shift to swap instead of move might be somewhat natural
here.

Some of the swapping code is copied from cmd-swap-window.c

Fixes #4388

mode-tree.c

@@ -58,6 +58,7 @@ struct mode_tree_data {
 	mode_tree_menu_cb         menucb;
 	mode_tree_height_cb       heightcb;
 	mode_tree_key_cb	  keycb;
+	mode_tree_swap_cb	  swapcb;
 
 	struct mode_tree_list	  children;
 	struct mode_tree_list	  saved;
@@ -287,6 +288,35 @@ mode_tree_down(struct mode_tree_data *mtd, int wrap)
 	return (1);
 }
 
+static void
+mode_tree_swap(struct mode_tree_data *mtd, int direction)
+{
+	u_int	current_depth = mtd->line_list[mtd->current].depth;
+	u_int	swap_with, swap_with_depth;
+
+	if (mtd->swapcb == NULL)
+		return;
+
+	/* Find the next line at the same depth with the same parent . */
+	swap_with = mtd->current;
+	do {
+		if (direction < 0 && swap_with < (u_int)-direction)
+			return;
+		if (direction > 0 && swap_with + direction >= mtd->line_size)
+			return;
+		swap_with += direction;
+		swap_with_depth = mtd->line_list[swap_with].depth;
+	} while (swap_with_depth > current_depth);
+	if (swap_with_depth != current_depth)
+		return;
+
+	if (mtd->swapcb(mtd->line_list[mtd->current].item->itemdata,
+	    mtd->line_list[swap_with].item->itemdata)) {
+		mtd->current = swap_with;
+		mode_tree_build(mtd);
+	}
+}
+
 void *
 mode_tree_get_current(struct mode_tree_data *mtd)
 {
@@ -410,9 +440,9 @@ struct mode_tree_data *
 mode_tree_start(struct window_pane *wp, struct args *args,
     mode_tree_build_cb buildcb, mode_tree_draw_cb drawcb,
     mode_tree_search_cb searchcb, mode_tree_menu_cb menucb,
-    mode_tree_height_cb heightcb, mode_tree_key_cb keycb, void *modedata,
-    const struct menu_item *menu, const char **sort_list, u_int sort_size,
-    struct screen **s)
+    mode_tree_height_cb heightcb, mode_tree_key_cb keycb,
+    mode_tree_swap_cb swapcb, void *modedata, const struct menu_item *menu,
+    const char **sort_list, u_int sort_size, struct screen **s)
 {
 	struct mode_tree_data	*mtd;
 	const char		*sort;
@@ -455,6 +485,7 @@ mode_tree_start(struct window_pane *wp, struct args *args,
 	mtd->menucb = menucb;
 	mtd->heightcb = heightcb;
 	mtd->keycb = keycb;
+	mtd->swapcb = swapcb;
 
 	TAILQ_INIT(&mtd->children);
 
@@ -1147,6 +1178,14 @@ mode_tree_key(struct mode_tree_data *mtd, struct client *c, key_code *key,
 	case 'n'|KEYC_CTRL:
 		mode_tree_down(mtd, 1);
 		break;
+	case KEYC_UP|KEYC_SHIFT:
+	case 'K':
+		mode_tree_swap(mtd, -1);
+		break;
+	case KEYC_DOWN|KEYC_SHIFT:
+	case 'J':
+		mode_tree_swap(mtd, 1);
+		break;
 	case KEYC_PPAGE:
 	case 'b'|KEYC_CTRL:
 		for (i = 0; i < mtd->height; i++) {

tmux.1

@@ -2715,6 +2715,8 @@ The following keys may be used in tree mode:
 .It Li "Enter" Ta "Choose selected item"
 .It Li "Up" Ta "Select previous item"
 .It Li "Down" Ta "Select next item"
+.It Li "S-Up" Ta "Swap the current window with the previous one"
+.It Li "S-Down" Ta "Swap the current window with the next one"
 .It Li "+" Ta "Expand selected item"
 .It Li "-" Ta "Collapse selected item"
 .It Li "M-+" Ta "Expand all items"

tmux.h

@@ -3326,6 +3326,7 @@ typedef int (*mode_tree_search_cb)(void *, void *, const char *);
 typedef void (*mode_tree_menu_cb)(void *, struct client *, key_code);
 typedef u_int (*mode_tree_height_cb)(void *, u_int);
 typedef key_code (*mode_tree_key_cb)(void *, void *, u_int);
+typedef int (*mode_tree_swap_cb)(void *, void *);
 typedef void (*mode_tree_each_cb)(void *, void *, struct client *, key_code);
 u_int	 mode_tree_count_tagged(struct mode_tree_data *);
 void	*mode_tree_get_current(struct mode_tree_data *);
@@ -3340,8 +3341,9 @@ void	 mode_tree_up(struct mode_tree_data *, int);
 int	 mode_tree_down(struct mode_tree_data *, int);
 struct mode_tree_data *mode_tree_start(struct window_pane *, struct args *,
 	     mode_tree_build_cb, mode_tree_draw_cb, mode_tree_search_cb,
-	     mode_tree_menu_cb, mode_tree_height_cb, mode_tree_key_cb, void *,
-	     const struct menu_item *, const char **, u_int, struct screen **);
+	     mode_tree_menu_cb, mode_tree_height_cb, mode_tree_key_cb,
+	     mode_tree_swap_cb, void *, const struct menu_item *, const char **,
+	     u_int, struct screen **);
 void	 mode_tree_zoom(struct mode_tree_data *, struct args *);
 void	 mode_tree_build(struct mode_tree_data *);
 void	 mode_tree_free(struct mode_tree_data *);

window-buffer.c

@@ -350,7 +350,7 @@ window_buffer_init(struct window_mode_entry *wme, struct cmd_find_state *fs,
 
 	data->data = mode_tree_start(wp, args, window_buffer_build,
 	    window_buffer_draw, window_buffer_search, window_buffer_menu, NULL,
-	    window_buffer_get_key, data, window_buffer_menu_items,
+	    window_buffer_get_key, NULL, data, window_buffer_menu_items,
 	    window_buffer_sort_list, nitems(window_buffer_sort_list), &s);
 	mode_tree_zoom(data->data, args);
 

window-client.c

@@ -310,7 +310,7 @@ window_client_init(struct window_mode_entry *wme,
 
 	data->data = mode_tree_start(wp, args, window_client_build,
 	    window_client_draw, NULL, window_client_menu, NULL,
-	    window_client_get_key, data, window_client_menu_items,
+	    window_client_get_key, NULL, data, window_client_menu_items,
 	    window_client_sort_list, nitems(window_client_sort_list), &s);
 	mode_tree_zoom(data->data, args);
 

window-customize.c

@@ -891,8 +891,8 @@ window_customize_init(struct window_mode_entry *wme, struct cmd_find_state *fs,
 
 	data->data = mode_tree_start(wp, args, window_customize_build,
 	    window_customize_draw, NULL, window_customize_menu,
-	    window_customize_height, NULL, data, window_customize_menu_items,
-	    NULL, 0, &s);
+	    window_customize_height, NULL, NULL, data,
+	    window_customize_menu_items, NULL, 0, &s);
 	mode_tree_zoom(data->data, args);
 
 	mode_tree_build(data->data);

window-tree.c

@@ -902,6 +902,58 @@ window_tree_get_key(void *modedata, void *itemdata, u_int line)
 	return (key);
 }
 
+static int
+window_tree_swap(void *cur_itemdata, void *other_itemdata)
+{
+	struct window_tree_itemdata	*cur = cur_itemdata;
+	struct window_tree_itemdata	*other = other_itemdata;
+	struct session			*cur_session, *other_session;
+	struct winlink			*cur_winlink, *other_winlink;
+	struct window			*cur_window, *other_window;
+	struct window_pane		*cur_pane, *other_pane;
+
+	if (cur->type != other->type)
+		return (0);
+	if (cur->type != WINDOW_TREE_WINDOW)
+		return (0);
+
+	window_tree_pull_item(cur, &cur_session, &cur_winlink, &cur_pane);
+	window_tree_pull_item(other, &other_session, &other_winlink,
+	    &other_pane);
+
+	if (cur_session != other_session)
+		return (0);
+
+	if (window_tree_sort->field != WINDOW_TREE_BY_INDEX &&
+	    window_tree_cmp_window(&cur_winlink, &other_winlink) != 0) {
+		/*
+		 * Swapping indexes would not swap positions in the tree, so
+		 * prevent swapping to avoid confusing the user.
+		 */
+		return (0);
+	}
+
+	other_window = other_winlink->window;
+	TAILQ_REMOVE(&other_window->winlinks, other_winlink, wentry);
+	cur_window = cur_winlink->window;
+	TAILQ_REMOVE(&cur_window->winlinks, cur_winlink, wentry);
+
+	other_winlink->window = cur_window;
+	TAILQ_INSERT_TAIL(&cur_window->winlinks, other_winlink, wentry);
+	cur_winlink->window = other_window;
+	TAILQ_INSERT_TAIL(&other_window->winlinks, cur_winlink, wentry);
+
+	if (cur_session->curw == cur_winlink)
+		session_set_current(cur_session, other_winlink);
+	else if (cur_session->curw == other_winlink)
+		session_set_current(cur_session, cur_winlink);
+	session_group_synchronize_from(cur_session);
+	server_redraw_session_group(cur_session);
+	recalculate_sizes();
+
+	return (1);
+}
+
 static struct screen *
 window_tree_init(struct window_mode_entry *wme, struct cmd_find_state *fs,
     struct args *args)
@@ -940,7 +992,7 @@ window_tree_init(struct window_mode_entry *wme, struct cmd_find_state *fs,
 
 	data->data = mode_tree_start(wp, args, window_tree_build,
 	    window_tree_draw, window_tree_search, window_tree_menu, NULL,
-	    window_tree_get_key, data, window_tree_menu_items,
+	    window_tree_get_key, window_tree_swap, data, window_tree_menu_items,
 	    window_tree_sort_list, nitems(window_tree_sort_list), &s);
 	mode_tree_zoom(data->data, args);