feat(ai-rewrite): add directive-picker wrappers around gptel-rewrite

`gptel-rewrite` is the killer feature for the keep-gptel decision,
and it now lives behind two commands instead of the bare call:

- `cj/gptel-rewrite-with-directive` (`C-; a r`, replacing the
  former bare `gptel-rewrite` binding): completing-read on a
  directive name from `cj/gptel-rewrite-directives`, then rewrite
  the active region.
- `cj/gptel-rewrite-redo-with-different-directive` (`C-; a R`):
  replay the prior region with a different directive.  The region
  is preserved via markers stored buffer-local on the first call so
  it survives accept/reject of the prior rewrite.

I picked the hook injection approach over an `:after`-advice +
state-capture pattern.  `gptel-rewrite-directives-hook` is an
abnormal hook gptel-rewrite already consults for a per-call
system message.  Wrapping the call in a one-shot `let`-binding on
that hook gives the directive exactly the lifetime of the rewrite
and leaves nothing to clean up.  Mutating `gptel-directives`
globally would mean either restoring it afterward or living with
the change -- both worse than the hook.

Directives ship inline as a `defcustom` alist with the six names
called out in the task -- `terse`, `fix-grammar`,
`refactor-readability`, `add-docstring`, `explain-as-comment`,
`shorten`.  Customization is a `customize-variable` or `setq`
away.

9 tests cover the defcustom shape (default names present, bodies
non-empty strings), the wrapper (normal path, no-region error,
unknown-directive error, last-state recording), and the redo
(replays the prior region, errors when no previous, excludes the
current directive from the re-pick prompt).  `gptel-rewrite`
stubbed in tests so no rewrite UI fires.

1 files changed, 159 insertions, 0 deletions

diff --git a/tests/test-ai-rewrite.el b/tests/test-ai-rewrite.el
new file mode 100644
index 00000000..ddb83133
--- /dev/null
+++ b/tests/test-ai-rewrite.el
@@ -0,0 +1,159 @@
+;;; test-ai-rewrite.el --- Tests for ai-rewrite.el -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the directive-picker wrappers around `gptel-rewrite'.
+;; `gptel-rewrite' itself is stubbed so the tests verify what the
+;; wrappers do (which directive body lands in the hook, which region
+;; was captured) without touching the real rewrite UI.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(add-to-list 'load-path (expand-file-name "tests" user-emacs-directory))
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+
+(require 'testutil-ai-config)
+
+;; Stub the gptel-rewrite surface so the wrapper can dispatch to it
+;; without loading the real package.  testutil-ai-config provides a
+;; non-interactive stub of `gptel-rewrite'; we override it with an
+;; interactive recorder that captures the hook-derived directive body
+;; and the active region.
+(defvar gptel-rewrite-directives-hook nil)
+(defvar test-ai-rewrite--captured-directive nil
+  "Last system-message body produced by the hook during a stub rewrite.")
+(defvar test-ai-rewrite--captured-region nil
+  "Cons (BEG . END) captured from `mark' and `point' at stub-rewrite time.")
+(defun gptel-rewrite ()
+  "Stub: capture the directive body and the active region."
+  (interactive)
+  (setq test-ai-rewrite--captured-directive
+        (run-hook-with-args-until-success 'gptel-rewrite-directives-hook))
+  (setq test-ai-rewrite--captured-region
+        (cons (region-beginning) (region-end))))
+
+(require 'ai-rewrite)
+
+;; ---------------------------- defcustom shape
+
+(ert-deftest test-ai-rewrite-directives-defcustom-has-named-entries ()
+  "Default directives include the names called out in the spec."
+  (let ((names (mapcar #'car cj/gptel-rewrite-directives)))
+    (dolist (expected '("terse" "fix-grammar" "refactor-readability"
+                        "add-docstring" "explain-as-comment" "shorten"))
+      (should (member expected names)))))
+
+(ert-deftest test-ai-rewrite-directives-bodies-are-strings ()
+  "Every directive body is a non-empty string."
+  (dolist (entry cj/gptel-rewrite-directives)
+    (should (stringp (cdr entry)))
+    (should (> (length (cdr entry)) 0))))
+
+;; ---------------------------- with-directive
+
+(ert-deftest test-ai-rewrite-with-directive-normal ()
+  "Wrapper injects the directive body and runs gptel-rewrite on the region."
+  (with-temp-buffer
+    (insert "first body line\nsecond body line\n")
+    (let ((test-ai-rewrite--captured-directive nil)
+          (test-ai-rewrite--captured-region nil)
+          (cj/gptel-rewrite-directives
+           '(("test" . "BODY FOR TEST DIRECTIVE"))))
+      ;; Activate the region across both lines
+      (set-mark (point-min))
+      (goto-char (point-max))
+      (activate-mark)
+      (cj/gptel-rewrite-with-directive "test")
+      (should (equal test-ai-rewrite--captured-directive
+                     "BODY FOR TEST DIRECTIVE"))
+      (should test-ai-rewrite--captured-region))))
+
+(ert-deftest test-ai-rewrite-with-directive-error-no-region ()
+  "No active region signals."
+  (with-temp-buffer
+    (insert "no region")
+    (deactivate-mark)
+    (should-error (call-interactively #'cj/gptel-rewrite-with-directive))))
+
+(ert-deftest test-ai-rewrite-with-directive-error-unknown-directive ()
+  "Unknown directive name signals."
+  (with-temp-buffer
+    (insert "body")
+    (set-mark (point-min))
+    (goto-char (point-max))
+    (activate-mark)
+    (let ((cj/gptel-rewrite-directives '(("known" . "x"))))
+      (should-error
+       (cj/gptel-rewrite--call-with-directive
+        "unknown" (point-min) (point-max))))))
+
+(ert-deftest test-ai-rewrite-with-directive-records-last-state ()
+  "Wrapper records the region and directive name for later redo."
+  (with-temp-buffer
+    (insert "abc\ndef\n")
+    (let ((cj/gptel-rewrite-directives
+           '(("first" . "FIRST BODY")))
+          (test-ai-rewrite--captured-directive nil))
+      (set-mark (point-min))
+      (goto-char (point-max))
+      (activate-mark)
+      (cj/gptel-rewrite-with-directive "first")
+      (should (equal cj/gptel-rewrite--last-directive "first"))
+      (should (consp cj/gptel-rewrite--last-region))
+      (should (markerp (car cj/gptel-rewrite--last-region)))
+      (should (markerp (cdr cj/gptel-rewrite--last-region))))))
+
+;; ---------------------------- redo
+
+(ert-deftest test-ai-rewrite-redo-normal ()
+  "Redo replays the last region with a new directive."
+  (with-temp-buffer
+    (insert "line1\nline2\nline3\n")
+    (let* ((cj/gptel-rewrite-directives
+            '(("first" . "FIRST BODY")
+              ("second" . "SECOND BODY")))
+           (test-ai-rewrite--captured-directive nil)
+           (test-ai-rewrite--captured-region nil))
+      (set-mark (point-min))
+      (goto-char (point-max))
+      (activate-mark)
+      (cj/gptel-rewrite-with-directive "first")
+      (should (equal test-ai-rewrite--captured-directive "FIRST BODY"))
+      (let ((first-region test-ai-rewrite--captured-region))
+        (setq test-ai-rewrite--captured-directive nil)
+        (setq test-ai-rewrite--captured-region nil)
+        (cl-letf (((symbol-function 'completing-read)
+                   (lambda (_p choices &rest _) (car choices))))
+          (cj/gptel-rewrite-redo-with-different-directive))
+        (should (equal test-ai-rewrite--captured-directive "SECOND BODY"))
+        (should (equal test-ai-rewrite--captured-region first-region))))))
+
+(ert-deftest test-ai-rewrite-redo-error-no-previous ()
+  "Redo without prior rewrite signals."
+  (with-temp-buffer
+    (setq-local cj/gptel-rewrite--last-region nil)
+    (should-error (cj/gptel-rewrite-redo-with-different-directive))))
+
+(ert-deftest test-ai-rewrite-redo-excludes-current-directive ()
+  "Redo's completing-read prompt offers every directive except the last."
+  (with-temp-buffer
+    (insert "body")
+    (let ((cj/gptel-rewrite-directives
+           '(("a" . "A") ("b" . "B") ("c" . "C")))
+          (offered nil))
+      (set-mark (point-min))
+      (goto-char (point-max))
+      (activate-mark)
+      (cj/gptel-rewrite-with-directive "b")
+      (cl-letf (((symbol-function 'completing-read)
+                 (lambda (_p choices &rest _)
+                   (setq offered choices)
+                   (car choices))))
+        (cj/gptel-rewrite-redo-with-different-directive))
+      (should (equal (sort (copy-sequence offered) #'string<)
+                     '("a" "c"))))))
+
+(provide 'test-ai-rewrite)
+;;; test-ai-rewrite.el ends here