+;;; Handling of tty's ESC event
+
+;; On a tty, an ESC event can either be the user hitting the escape key, or
+;; some element of a byte sequence used to encode for example cursor keys.
+;; So we try to recognize those events that correspond to the escape key and
+;; turn them into `escape' events (same as used under GUIs). The heuristic we
+;; use to distinguish the two cases is based, as usual, on a timeout, and on
+;; the fact that the special ESC=>escape mapping only takes place if the whole
+;; last key-sequence so far is just [?\e], i.e. either we're still in
+;; read-key-sequence, or the last read-key-sequence only read [?\e], which
+;; should ideally never happen because it should have been mapped to [escape].
+
+(defun viper--tty-ESC-filter (map)
+ (if (and (equal (this-single-command-keys) [?\e])
+ (sit-for (/ viper-fast-keyseq-timeout 1000)))
+ [escape] map))
+
+(defun viper--lookup-key (map key)
+ "Kind of like `lookup-key'.
+Two differences:
+- KEY is a single key, not a sequence.
+- the result is the \"raw\" binding, so it can be a `menu-item', rather than the
+ binding contained in that menu item."
+ (catch 'found
+ (map-keymap (lambda (k b) (if (equal key k) (throw 'found b))) map)))
+
+(defun viper-catch-tty-ESC ()
+ "Setup key mappings of current terminal to turn a tty's ESC into `escape'."
+ (when (memq (terminal-live-p (frame-terminal)) '(t pc))
+ (let ((esc-binding (viper-uncatch-tty-ESC)))
+ (define-key input-decode-map
+ [?\e] `(menu-item "" ,esc-binding :filter viper--tty-ESC-filter)))))
+
+(defun viper-uncatch-tty-ESC ()
+ "Don't hack ESC into `escape' any more."
+ (let ((b (viper--lookup-key input-decode-map ?\e)))
+ (and (eq 'menu-item (car-safe b))
+ (eq 'viper--tty-ESC-filter (nth 4 b))
+ (define-key input-decode-map [?\e] (setq b (nth 2 b))))
+ b))
+
+(defun viper-setup-ESC-to-escape (enable)
+ (if enable
+ (add-hook 'tty-setup-hook 'viper-catch-tty-ESC)
+ (remove-hook 'tty-setup-hook 'viper-catch-tty-ESC))
+ (let ((seen ()))
+ (dolist (frame (frame-list))
+ (let ((terminal (frame-terminal frame)))
+ (unless (memq terminal seen)
+ (push terminal seen)
+ (with-selected-frame frame
+ (if enable (viper-catch-tty-ESC) (viper-uncatch-tty-ESC))))))))