Add prev_dir and next_dir navigation actions

Many image collections are organized by directory, so it is nice to have
jump-to-adjacent-directory navigation.

e.g. Given the following file hierarchy:

    .
    ├── A
    │   ├── 1.jpg
    │   ├── 2.jpg
    │   └── C
    │       ├── 1.jpg
    │       ├── 2.jpg
    │       └── 3.jpg
    └── B
        ├── 1.jpg
        ├── 2.jpg
        └── 3.jpg

`feh --recursive` creates the following filelist:

    A/1.jpg <---- current_file
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg
    B/2.jpg
    B/3.jpg

If we press [next_dir], we move the current_file pointer to:

    A/1.jpg
    A/2.jpg
    A/C/1.jpg <-- current_file
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg
    B/2.jpg
    B/3.jpg

Pressing [next_dir] again moves the pointer to:

    A/1.jpg
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg <---- current_file
    B/2.jpg
    B/3.jpg

[next_dir] now moves the pointer back to the top of the list:

    A/1.jpg <---- current_file
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg
    B/2.jpg
    B/3.jpg

Pressing [prev_dir] from here moves backwards to the first image of the
previous directory:

    A/1.jpg
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg <---- current_file
    B/2.jpg
    B/3.jpg

When starting from an position that is not the first image of a
directory, [prev_dir] moves the pointer to the first image of the
current directory.

These actions combine well with `--sort dirname` since all regular files
in a directory will be sorted before any subdirectories, avoiding a
filelist like the following:

    A/1.jpg
    A/SUBDIR/2.jpg
    A/SUBDIR/3.jpg
    A/4.jpg

With `--sort dirname` that filelist becomes:

    A/1.jpg
    A/4.jpg
    A/SUBDIR/2.jpg
    A/SUBDIR/3.jpg

1 files changed, 38 insertions, 0 deletions

diff --git a/src/slideshow.c b/src/slideshow.c
index 7636d31..2af78c9 100644
--- a/src/slideshow.c
+++ b/src/slideshow.c
@@ -322,6 +322,44 @@ void slideshow_change_image(winwidget winwid, int change, int render)
 			   try the previous file, not another jmp */
 			change = SLIDE_NEXT;
 			break;
+		case SLIDE_JUMP_NEXT_DIR:
+			{
+				char old_dir[FEH_MAX_DIRNAME_LEN], new_dir[FEH_MAX_DIRNAME_LEN];
+				int j;
+
+				feh_file_dirname(old_dir, FEH_FILE(current_file->data), FEH_MAX_DIRNAME_LEN);
+
+				for (j = 0; j < our_filelist_len; j++) {
+					current_file = feh_list_jump(filelist, current_file, FORWARD, 1);
+					feh_file_dirname(new_dir, FEH_FILE(current_file->data), FEH_MAX_DIRNAME_LEN);
+					if (strcmp(old_dir, new_dir) != 0)
+						break;
+				}
+			}
+			change = SLIDE_NEXT;
+			break;
+		case SLIDE_JUMP_PREV_DIR:
+			{
+				char old_dir[FEH_MAX_DIRNAME_LEN], new_dir[FEH_MAX_DIRNAME_LEN];
+				int j;
+
+				/* Start the search from the previous file in case we are on
+				   the first file of a directory */
+				current_file = feh_list_jump(filelist, current_file, BACK, 1);
+				feh_file_dirname(old_dir, FEH_FILE(current_file->data), FEH_MAX_DIRNAME_LEN);
+
+				for (j = 0; j < our_filelist_len; j++) {
+					current_file = feh_list_jump(filelist, current_file, BACK, 1);
+					feh_file_dirname(new_dir, FEH_FILE(current_file->data), FEH_MAX_DIRNAME_LEN);
+					if (strcmp(old_dir, new_dir) != 0)
+						break;
+				}
+
+				/* Next file is the first entry of prev_dir */
+				current_file = feh_list_jump(filelist, current_file, FORWARD, 1);
+			}
+			change = SLIDE_NEXT;
+			break;
 		default:
 			eprintf("BUG!\n");
 			break;