feat: add the commander mode, F-key map, and two-pane entry

The duet command lays out two side-by-side dired panes, each in the buffer-local duet-mode, which carries the mc/Norton F-key map (F3 view, F4 edit, F5 copy, F6 move, F7 mkdir, F8 delete, F10 quit). The map is a minor-mode keymap, so the single-key actions fire only inside a pane and leave the global F-keys untouched elsewhere. duet-quit restores the window layout from before launch (Phase 4 in the design spec).

duet--other-pane is the explicit other-pane resolution that replaces dired-dwim-target, the dirvish#36 fix. Its logic lives in a pure helper, duet--sibling-pane, that takes the active window and the list of commander panes and returns the sibling, erroring clearly when there are not exactly two. Keeping the decision pure makes the dangerous part testable without a live frame.

DUET does not depend on dirvish. A pane is a plain dired buffer, and when the user has dirvish rendering dired buffers it renders as dirvish with no extra work. That keeps dirvish recommended but never required, and avoids binding the package to a specific dirvish entry point.

The transfer and file actions are stubs that announce themselves until their phases land, so the keys are bound and their precedence is tested now. The pure selector, the keymap, and minor-mode precedence are unit-tested; the live two-pane launch and the F-key feel in a running Emacs are a manual check.

1 files changed, 130 insertions, 3 deletions

diff --git a/duet.el b/duet.el
index b546264..4c49eca 100644
--- a/duet.el
+++ b/duet.el
@@ -721,11 +721,138 @@ ungated."
                       (list :op 'delete :source s :gate 'copy-success)))))
           sources)))
 
+;;; Commander mode, pane layout, and entry
+
+(defcustom duet-default-left-directory nil
+  "Directory shown in DUET's left pane at launch.
+nil means the current directory when `duet' is invoked."
+  :type '(choice (const :tag "Current directory" nil) directory)
+  :group 'duet)
+
+(defcustom duet-default-right-directory nil
+  "Directory shown in DUET's right pane at launch.
+nil means the current directory when `duet' is invoked."
+  :type '(choice (const :tag "Current directory" nil) directory)
+  :group 'duet)
+
+(defvar duet--saved-window-configuration nil
+  "Window configuration saved when DUET launched, restored by `duet-quit'.")
+
+;; The transfer/file actions land with their owning phases; until then they
+;; announce themselves so the key is bound and its precedence is testable.
+
+(defun duet-view ()
+  "View the file under point.  Arrives with the viewer phase."
+  (interactive)
+  (user-error "DUET: view is not implemented yet"))
+
+(defun duet-edit ()
+  "Edit the file under point.  Arrives with the viewer phase."
+  (interactive)
+  (user-error "DUET: edit is not implemented yet"))
+
+(defun duet-copy ()
+  "Copy the marked files to the other pane.  Arrives with transfer execution."
+  (interactive)
+  (user-error "DUET: copy is not implemented yet"))
+
+(defun duet-move ()
+  "Move the marked files to the other pane.  Arrives with transfer execution."
+  (interactive)
+  (user-error "DUET: move is not implemented yet"))
+
+(defun duet-mkdir ()
+  "Make a directory in the active pane.  Arrives with transfer execution."
+  (interactive)
+  (user-error "DUET: mkdir is not implemented yet"))
+
+(defun duet-delete ()
+  "Delete the marked files (to trash).  Arrives with transfer execution."
+  (interactive)
+  (user-error "DUET: delete is not implemented yet"))
+
+(defun duet-quit ()
+  "Close the DUET commander, restoring the window layout from before launch."
+  (interactive)
+  (if duet--saved-window-configuration
+      (progn
+        (set-window-configuration duet--saved-window-configuration)
+        (setq duet--saved-window-configuration nil))
+    (message "DUET: no saved layout to restore")))
+
+(defvar duet-mode-map
+  (let ((map (make-sparse-keymap)))
+    (define-key map (kbd "<f3>") #'duet-view)
+    (define-key map (kbd "<f4>") #'duet-edit)
+    (define-key map (kbd "<f5>") #'duet-copy)
+    (define-key map (kbd "<f6>") #'duet-move)
+    (define-key map (kbd "<f7>") #'duet-mkdir)
+    (define-key map (kbd "<f8>") #'duet-delete)
+    (define-key map (kbd "<f10>") #'duet-quit)
+    map)
+  "Keymap active in DUET commander panes.
+mc/Norton F-keys, taking precedence inside a pane only.  dired's own chords
+\(C, R, D, v, +) keep working as free aliases.")
+
+(define-minor-mode duet-mode
+  "Buffer-local minor mode marking a buffer as a DUET commander pane.
+Carries `duet-mode-map', so the single-key F-key actions fire only inside a
+commander pane and leave the global F-keys untouched elsewhere."
+  :lighter " Duet"
+  :keymap duet-mode-map)
+
+(defun duet--sibling-pane (window panes)
+  "Return the commander pane in PANES that is not WINDOW.
+PANES is the list of commander windows.  Signal a `user-error' unless there
+are exactly two panes and WINDOW is one of them.  This is the explicit
+other-pane resolution that replaces `dired-dwim-target' (the dirvish#36 fix)."
+  (cond
+   ((/= (length panes) 2)
+    (user-error "DUET needs exactly two commander panes (found %d)"
+                (length panes)))
+   ((not (memq window panes))
+    (user-error "Point is not in a DUET commander pane"))
+   (t (car (delq window (copy-sequence panes))))))
+
+(defun duet--commander-windows ()
+  "Return the live windows whose buffer has `duet-mode' enabled."
+  (cl-remove-if-not (lambda (w) (buffer-local-value 'duet-mode (window-buffer w)))
+                    (window-list)))
+
+(defun duet--other-pane (&optional window)
+  "Return the sibling commander pane window to WINDOW (default selected)."
+  (duet--sibling-pane (or window (selected-window)) (duet--commander-windows)))
+
+(defun duet--pane-directory (window)
+  "Return the directory shown in commander pane WINDOW."
+  (with-current-buffer (window-buffer window) default-directory))
+
+(defun duet--open-pane (directory)
+  "Open DIRECTORY in the selected window as a DUET commander pane.
+The pane is a dired buffer with `duet-mode' enabled; when the user has dirvish
+rendering dired buffers, it renders as dirvish with no extra work, which is why
+dirvish is recommended but never required."
+  (let ((buffer (dired-noselect (expand-file-name directory))))
+    (set-window-buffer (selected-window) buffer)
+    (with-current-buffer buffer (duet-mode 1))
+    buffer))
+
 ;;;###autoload
-(defun duet ()
-  "Launch the DUET dual-pane file commander."
+(defun duet (&optional left right)
+  "Launch the DUET dual-pane file commander.
+Lay out two side-by-side commander panes: LEFT (or
+`duet-default-left-directory', else the current directory) on the left and
+RIGHT (or `duet-default-right-directory', else the current directory) on the
+right.  Each pane is a dired buffer with `duet-mode' enabled.  `duet-quit'
+\(F10) restores the window layout from before launch."
   (interactive)
-  (user-error "DUET is not yet implemented; see the design document"))
+  (setq duet--saved-window-configuration (current-window-configuration))
+  (let ((left-dir (or left duet-default-left-directory default-directory))
+        (right-dir (or right duet-default-right-directory default-directory)))
+    (delete-other-windows)
+    (duet--open-pane left-dir)
+    (with-selected-window (split-window-right)
+      (duet--open-pane right-dir))))
 
 (provide 'duet)
 ;;; duet.el ends here