Set GC threshold high at start

I did have this in here before and the code to set it back was still
at the end of the file, so not sure what happened there.  I guess I
accidentally removed it at some point.

.gitignore

@@ -13,3 +13,4 @@ session.*
 eshell/*
 *.data
 auto-save-list/*
+eln-cache/*

config.org

@@ -2,6 +2,17 @@
 #+AUTHOR: Camden Dixie O'Brien
 #+ATTR_LATEX: :float t
 
+* Prelude
+  Before doing anything else, set a higher [[help:gc-cons-threshold][gc-cons-threshold]] (I'm
+  using 100 MiB here) so that we're not garbage collecting during
+  initialisation to make startup a little faster.  We'll save the
+  original value to restore it at the end of initialisation.
+
+  #+begin_src emacs-lisp
+    (setq original-gc-cons-threshold gc-cons-threshold)
+    (setq gc-cons-threshold (* 100 1024 1024))
+  #+end_src
+
 * Customize
   Hey, customize, leave my ~/.emacs.d/init.el alone!
 
@@ -826,25 +837,6 @@
                          (paredit-mode))))
     #+end_src
 
-*** Scheme and Racket
-    [[https://www.nongnu.org/geiser/][Geiser]] is a pretty complete collection of Scheme things for
-    Emacs. Only change from the defaults is to open the REPL in the
-    current window instead of creating a new one.
-
-    #+begin_src emacs-lisp
-      (use-package geiser
-        :config
-        (setq geiser-repl-use-other-window nil))
-    #+end_src
-
-    I commonly use Chez, Guile and Racket so we want the packages for those:
-
-    #+begin_src emacs-lisp
-      (use-package geiser-chez)
-      (use-package geiser-guile)
-      (use-package geiser-racket)
-    #+end_src
-
 ** YAML
    I don't really like YAML if I'm honest, but it's used a lot so...
 
@@ -1396,25 +1388,6 @@
           'clang-format-region-or-buffer)))
    #+end_src
 
-** ChatGPT
-   The =chatpt-shell= package provides a shell-like interface for
-   ChatGPT.
-
-   #+begin_src emacs-lisp
-     (use-package chatgpt-shell)
-   #+end_src
-
-   [[help:chatgpt-shell-openai-key][chatgpt-shell-openai-key]] must also be set to a function that
-   returns an OpenAI API key. I have a key (made on [[https://platform.openai.com/account/api-keys][this page]]) stored
-   in =pass(1)= under =openai/api-key=, so this can be retrieved via
-   [[help:process-lines][process-lines]]:
-
-   #+begin_src emacs-lisp
-     (setq chatgpt-shell-openai-key
-           (lambda ()
-             (car (process-lines "pass" "openai/api-key"))))
-   #+end_src
-
 ** GraphViz
    The =graphviz-dot-mode= package provides some integration for the
    GraphViz suite of tools, and a mode for the 'dot' language they
@@ -1435,6 +1408,186 @@
      (setq Man-notify-method 'pushy)
    #+end_src
 
+** Script-Fu Mode
+   GIMP has a scheme-based language -- Script-Fu -- built into it that
+   you can use to script things (based).  Sadly, the built-in console
+   is rather lackluster as a coding environment.  Happily, there /is/
+   an option to run a server which listens for Script-Fu commands on a
+   TCP port, so I can use =comint= to make my own lil interface in
+   Emacs.
+
+   It's things like this that make me really glad I switched to Emacs
+   because this is ridiculously cool.  By my definition of "cool"
+   anyway -- what can I say, I'm a massive nerd.
+
+   I should probably extract this and make a standalone package out of
+   it and stick it on Melpa at some point.
+
+*** REPL Mode
+    The Script-Fu server request format is very simple:
+
+    | Bytes | Description                                   |
+    |-------+-----------------------------------------------|
+    |     0 | 'G' magic byte (47h)                          |
+    |   1-2 | Length of expression (BE 32-bit unsigned int) |
+    |    3+ | Expression                                    |
+
+    Writing an encoder for this is pretty trivial:
+
+    #+begin_src emacs-lisp
+      (defun script-fu-repl-encode-request (input)
+        (let* ((len (length input))
+               (hi (logand (lsh len -8) #xff))
+               (lo (logand len #xff))
+               (hdr (vector ?G hi lo)))
+          (vconcat hdr (encode-coding-string input 'utf-8))))
+    #+end_src
+
+    We then want a sender function to use with [[help:comint-mode][comint-mode]] that
+    applies this encoding.  Unfortunately, it seems that there is no
+    =comint-send-bytes= or similar function to directly send a byte
+    vector to the comint process.  I did try just sending the request
+    as a string with some invalid characters at the start but ran into
+    issues: Emacs would sometimes insert unicode control characters
+    into the data, which GIMP understandably didn't appreciate.
+
+    The method I ended up with is to create a temporary, unibyte
+    buffer, stick the data in there and then use [[help:comint-send-region][comint-send-region]] to
+    send the data.  It's a bit of a kludge but it seems like it should
+    be reasonably robust.
+
+    #+begin_src emacs-lisp
+      (defun script-fu-repl-comint-send-bytes (proc bytes)
+        (let ((temp-buffer (generate-new-buffer "*script-fu-repl-tmp*")))
+          (unwind-protect
+              (with-current-buffer temp-buffer
+                (set-buffer-multibyte nil)
+                (insert (apply #'string (append bytes nil)))
+                (comint-send-region proc (point-min) (point-max)))
+            (kill-buffer temp-buffer))))
+    #+end_src
+
+    With that handled, implementing the sender function itself is nice
+    and easy:
+
+    #+begin_src emacs-lisp
+      (defun script-fu-repl-send (proc input)
+        (let ((request (script-fu-repl-encode-request input)))
+          (script-fu-repl-comint-send-bytes proc request)))
+    #+end_src
+
+    The response format is similarly simple:
+
+    | Bytes | Content                                 |
+    |-------+-----------------------------------------|
+    |     0 | 'G' magic byte (47h)                    |
+    |     1 | Status code -- 0 on success, 1 on error |
+    |   2-3 | Length of response text                 |
+    |     4 | Response text                           |
+
+    For now, we only care about the response text, so all we need to do
+    is skip the first 4 bytes and add a trailing newline.
+
+    #+begin_src emacs-lisp
+      (defun script-fu-repl-decode-response (response)
+        (concat (substring response 4) "\n"))
+    #+end_src
+
+    Another thing is adding a prompt to the comint buffer -- the
+    server doesn't send one, so we have to add it ourselves.
+
+    #+begin_src emacs-lisp
+      (defvar script-fu-repl-prompt "> ")
+      (defun script-fu-repl-insert-prompt (output)
+        (unless (string-blank-p output)
+          (let ((proc (get-buffer-process (current-buffer))))
+            (goto-char (process-mark proc))
+            (unless (looking-back script-fu-repl-prompt)
+              (insert script-fu-repl-prompt)
+              (set-marker (process-mark proc) (point)))))
+        output)
+    #+end_src
+
+    A mode for the client buffer can then be derived from [[help:comint-mode][comint-mode]].
+
+    #+begin_src emacs-lisp
+      (define-derived-mode script-fu-repl-mode comint-mode "Script-Fu REPL"
+        (setq-local comint-prompt-read-only t)
+        (setq-local comint-prompt-regexp nil)
+        (setq-local comint-input-sender #'script-fu-repl-send)
+        (add-hook 'comint-preoutput-filter-functions
+                  'script-fu-repl-decode-response nil t)
+        (add-hook 'comint-output-filter-functions
+                  'script-fu-repl-insert-prompt nil t))
+    #+end_src
+
+    Now, to create a function to create or get the current REPL
+    buffer.  The [[help:comint-check-proc][comint-check-proc]] function can be used to test
+    whether the buffer is already set up.  Rather nicely,
+    [[help:make-comint-in-buffer][make-comint-in-buffer]] supports passing a ~(HOST . SERVICE)~ pair
+    to specify a TCP connection to open (via [[help:open-network-stream][open-network-stream]]) so
+    this is pretty simple.  In both cases, we want to return the
+    client buffer for the caller to use.
+
+    #+begin_src emacs-lisp
+      (defvar script-fu-repl-server '("localhost" . 10008))
+      (defun script-fu-repl ()
+        (interactive)
+        (let ((buffer (get-buffer-create "*Script-Fu REPL*")))
+          (when (not (comint-check-proc buffer))
+            (make-comint-in-buffer "Script-Fu REPL" buffer
+                                   script-fu-repl-server)
+            (with-current-buffer buffer (script-fu-repl-mode)))
+          (pop-to-buffer buffer '((display-buffer-in-direction)
+                                  (direction . below)
+                                  (window-height . 0.3)))
+          buffer))
+    #+end_src
+
+*** Code Editing Mode
+    With the client stuff done, we can define the code editing mode:
+
+    #+begin_src emacs-lisp
+      (define-derived-mode script-fu-mode scheme-mode "Script-Fu")
+    #+end_src
+
+    Now to define something to send an expression or region to the
+    REPL:
+
+    #+begin_src emacs-lisp
+      (defun script-fu-mode-send-region-or-sexp ()
+        (interactive)
+        (let ((code (if (use-region-p)
+                        (let ((start (region-beginning))
+                              (end (region-end)))
+                          (buffer-substring-no-properties start end))
+                      (thing-at-point 'sexp t))))
+          (if (not code) (message "No code to send.")
+            (let* ((repl-buffer (script-fu-repl))
+                   (repl-proc (get-buffer-process repl-buffer)))
+              (script-fu-repl-send repl-proc code)))))
+
+      (define-key script-fu-mode-map (kbd "C-c C-c")
+                  'script-fu-mode-send-region-or-sexp)
+    #+end_src
+
+    And finally a similar thing for the whole file:
+
+    #+begin_src emacs-lisp
+      (defun script-fu-mode-send-file ()
+        (interactive)
+        (let* ((repl-buffer (script-fu-repl))
+               (repl-proc (get-buffer-process repl-buffer))
+               (buffer-contents
+                (buffer-substring-no-properties (point-min)
+                                                (point-max))))
+          (script-fu-repl-send repl-proc buffer-contents)))
+      (define-key script-fu-mode-map (kbd "C-c C-l")
+                  'script-fu-mode-send-file)
+    #+end_src
+
+    I think that's all I need for now!
+
 * Backup and Autosave
 ** Keep $PWD Tidy
    Emacs' default behaviour of dumping temporary files in the current
@@ -1525,104 +1678,10 @@
      (setq god-exempt-predicates nil)
    #+end_src
 
-** Case-flipping
-   Want to be able to toggle the 'shiftedness' of a selected region,
-   that is, map uppercase to lowercase and vice versa, but also map
-   things like '1' to '!' and '[' to '{'.
-
-   I doubt there's anything in Emacs already that has that mapping for
-   non-alphabetic characters, so first thing to do is define that:
-
-   #+begin_src emacs-lisp
-     (defvar non-letter-case-mapping
-       '((?1 . ?!) (?2 . ?\") (?3 . ?£) (?4 . ?$) (?5 . ?%)
-         (?6 . ?^) (?7 . ?&) (?8 . ?*) (?9 . ?\() (?0 . ?\))
-         (?- . ?_) (?= . ?+) (?\` . ?¬) (?\\ . ?\|) (?\[ . ?{)
-         (?\] . ?}) (?\; . ?:) (?\' . ?@) (?\# . ?~) (?\, . ?<)
-         (?\. . ?>) (?/ . ??)))
-   #+end_src
-
-   And then, a function to toggle a non-letter character, using that
-   mapping, defaulting to the identity if there's no entry.
-
-   #+begin_src emacs-lisp
-     (defun flip-non-letter-case (c)
-         (let (value)
-           (dolist (case-pair non-letter-case-mapping value)
-             (cond ((eq (car case-pair) c)
-                    (setq value (cdr case-pair)))
-                   ((eq (cdr case-pair) c)
-                    (setq value (car case-pair)))))
-           (when (eq value nil)
-             (setq value c))
-           value))
-   #+end_src
-
-   A similar function for letters can be easily defined using [[help:upcase][upcase]]
-   and [[help:downcase][downcase]]:
-
-   #+begin_src emacs-lisp
-     (defun upper-case-p (c)
-       (eq (upcase c) c))
-
-     (defun flip-letter-case (c)
-       (if (upper-case-p c)
-           (downcase c)
-         (upcase c)))
-   #+end_src
-
-   These can then be combined into a case-flipping function that will
-   work for both letters and non-letters:
-
-   #+begin_src emacs-lisp
-     (defun letter-p (c)
-       (and (characterp c)
-            (let ((uc (upcase c)))
-              (and (>= uc ?A) (<= uc ?Z)))))
-
-     (defun flip-char-case (c)
-       (if (letter-p c)
-           (flip-letter-case c)
-         (flip-non-letter-case c)))
-   #+end_src
-
-   ~flip-char-case~ can then applied over a whole string:
-
-   #+begin_src emacs-lisp
-     (defun flip-string-case (s)
-       (let ((len (length s))
-             (i 0))
-         (while (< i len)
-           (aset s i (flip-char-case (aref s i)))
-           (setq i (1+ i)))
-         s))
-   #+end_src
-
-   Finally, this can then be applied to the region, if it's active:
-
-   #+begin_src emacs-lisp
-     (defun flip-region-case ()
-       (interactive)
-       (when (region-active-p)
-         (let* ((start (region-beginning))
-                (end (region-end))
-                (text (buffer-substring-no-properties start end)))
-           (delete-region start end)
-           (insert (flip-string-case text)))))
-   #+end_src
-
-   And of course, I need a keybinding for that:
-
-   #+begin_src emacs-lisp
-     (global-set-key (kbd "C-~") 'flip-region-case)
-   #+end_src
-
-* Fin
-  Now that start-up is finished, [[help:gc-cons-threshold][gc-cons-threshold]] and
-  [[help:gc-cons-percentage][gc-cons-percentage]] need to be set back to reasonable values to avoid
-  memory usage getting too high.
+* Coda
+  Now that initialization is finished, [[help:gc-cons-threshold][gc-cons-threshold]] should be set
+  back to its default value:
 
   #+begin_src emacs-lisp
-    (setq gc-cons-threshold 1000000)
-    (setq gc-cons-percentage 0.2)
+    (setq gc-cons-threshold original-gc-cons-threshold)
   #+end_src