feat: implement gloss-drill org-drill export

Two public commands plus a small helper. `gloss-drill-export-all'
walks `gloss-file' via `org-map-entries' and adds `:drill:' tag and
`DRILL_CARD_TYPE: twosided' property to every top-level entry.
Membership and equality guards make the operation idempotent: re-running
adds nothing and writes nothing. `gloss-drill-untag-all' is the
reverse, and intentionally does not require `org-drill' to be installed
(the user might be cleaning up after uninstalling).

The walking logic factors into a single private helper
`gloss-drill--map-entries' that handles file open, modtime
verification, org-mode activation, the level-1 filter, and a
write-only-if-modified save. Both public commands compose it with their
respective per-entry mutators.

`org-drill' presence is checked with `featurep' before any walk so the
file is never touched when the dep is missing. The user-error message
includes the install command.

Folds in a small fix to the idempotency test helper: the original used
`throw' from inside `org-map-entries' but did not return the count to
the caller. Switched to `catch' / `throw' with the count as the throw
value.

98 tests pass in 0.24s — 88 prior plus 10 new across the four scenarios
named in the design doc (tags-untagged, skips-already-tagged,
no-orgdrill-installed, untag-all).

2 files changed, 73 insertions, 13 deletions

diff --git a/gloss-drill.el b/gloss-drill.el
index 6771f4b..08e2198 100644
--- a/gloss-drill.el
+++ b/gloss-drill.el
@@ -7,8 +7,8 @@
 ;;; Commentary:
 
 ;; Spaced-repetition export for `gloss'.  Walks the glossary org file
-;; via `org-element' and ensures every term entry carries a `:drill:'
-;; tag and a `:DRILL_CARD_TYPE: twosided' property — `org-drill' then
+;; via `org-map-entries' and ensures every term entry carries a `:drill:'
+;; tag and a `:DRILL_CARD_TYPE: twosided' property.  `org-drill' then
 ;; runs unmodified against the file.
 ;;
 ;; Public API:
@@ -16,14 +16,77 @@
 ;;   `gloss-drill-untag-all'
 ;;
 ;; Idempotent: running export twice does not double-tag.
-;; Checks `(featurep 'org-drill)' before running; raises a helpful
-;; user-error if `org-drill' isn't installed.
+;; Checks `(featurep \\='org-drill)' before exporting; raises a helpful
+;; user-error if `org-drill' isn't installed.  Untag-all does NOT
+;; require `org-drill' — the user might be removing tags after
+;; uninstalling.
 ;;
 ;; See `docs/design/gloss.org' for the full design.
 
 ;;; Code:
 
-;; Implementation pending.  Track via todo.org.
+(require 'org)
+(require 'gloss-core)
+
+(defconst gloss-drill--card-type "twosided"
+  "Value written to the :DRILL_CARD_TYPE: property by export-all.")
+
+(defconst gloss-drill--tag "drill"
+  "Tag added to every entry by export-all and removed by untag-all.")
+
+(defun gloss-drill--map-entries (fn)
+  "Open `gloss-file', call FN at every top-level entry, and save.
+FN runs with point at the heading line.  The buffer is saved only if
+modified."
+  (let ((buf (find-file-noselect gloss-file)))
+    (with-current-buffer buf
+      (unless (verify-visited-file-modtime buf)
+        (revert-buffer t t t))
+      (unless (derived-mode-p 'org-mode)
+        (let ((org-mode-hook nil))
+          (org-mode)))
+      (save-excursion
+        (org-map-entries
+         (lambda ()
+           (when (= 1 (org-current-level))
+             (funcall fn)))))
+      (when (buffer-modified-p)
+        (save-buffer)))))
+
+(defun gloss-drill--add-drill-tag-and-property ()
+  "Add `:drill:' tag and `DRILL_CARD_TYPE' property at the entry at point."
+  (let ((tags (org-get-tags nil t)))
+    (unless (member gloss-drill--tag tags)
+      (org-set-tags (append tags (list gloss-drill--tag)))))
+  (unless (equal (org-entry-get nil "DRILL_CARD_TYPE") gloss-drill--card-type)
+    (org-entry-put nil "DRILL_CARD_TYPE" gloss-drill--card-type)))
+
+(defun gloss-drill--remove-drill-tag-and-property ()
+  "Remove `:drill:' tag and `DRILL_CARD_TYPE' property at the entry at point."
+  (let ((tags (org-get-tags nil t)))
+    (when (member gloss-drill--tag tags)
+      (org-set-tags (delete gloss-drill--tag tags))))
+  (when (org-entry-get nil "DRILL_CARD_TYPE")
+    (org-entry-delete nil "DRILL_CARD_TYPE")))
+
+;;;; Public API
+
+(defun gloss-drill-export-all ()
+  "Tag every entry in `gloss-file' for `org-drill'.
+Adds `:drill:' tag and `DRILL_CARD_TYPE: twosided' property to each
+top-level heading.  Idempotent.  Signals `user-error' if `org-drill'
+is not installed."
+  (interactive)
+  (unless (featurep 'org-drill)
+    (user-error
+     "gloss-drill: `org-drill' is not installed.  Install it with M-x package-install RET org-drill RET"))
+  (gloss-drill--map-entries #'gloss-drill--add-drill-tag-and-property))
+
+(defun gloss-drill-untag-all ()
+  "Remove the `:drill:' tag and `DRILL_CARD_TYPE' property from every entry.
+Does NOT require `org-drill' to be installed."
+  (interactive)
+  (gloss-drill--map-entries #'gloss-drill--remove-drill-tag-and-property))
 
 (provide 'gloss-drill)
 ;;; gloss-drill.el ends here
diff --git a/tests/test-gloss-drill--export-all-skips-already-tagged.el b/tests/test-gloss-drill--export-all-skips-already-tagged.el
index 52978c6..20fbaea 100644
--- a/tests/test-gloss-drill--export-all-skips-already-tagged.el
+++ b/tests/test-gloss-drill--export-all-skips-already-tagged.el
@@ -21,15 +21,13 @@
 Reads the file fresh from disk."
   (with-current-buffer (find-file-noselect gloss-file)
     (revert-buffer t t t)
-    (let ((count 0))
+    (catch 'done
       (org-map-entries
        (lambda ()
          (when (= 1 (org-current-level))
-           (setq count (length (cl-remove-if-not
-                                (lambda (tag) (equal tag "drill"))
-                                (org-get-tags nil t))))
-           (throw 'done nil))))
-      count)))
+           (throw 'done
+                  (length (seq-filter (lambda (tag) (equal tag "drill"))
+                                      (org-get-tags nil t))))))))))
 
 (ert-deftest test-gloss-drill-export-all-idempotent-tag-not-duplicated ()
   "Boundary: running export-all twice does not duplicate the :drill: tag."
@@ -37,8 +35,7 @@ Reads the file fresh from disk."
     (gloss-test--with-org-drill-feature
       (gloss-drill-export-all)
       (gloss-drill-export-all)
-      (catch 'done
-        (should (= (gloss-test--drill-tag-count-on-first-entry) 1))))))
+      (should (= (gloss-test--drill-tag-count-on-first-entry) 1)))))
 
 (ert-deftest test-gloss-drill-export-all-idempotent-property-unchanged ()
   "Boundary: running export-all twice keeps :DRILL_CARD_TYPE: twosided."