Fix: (--value-at) Clear value cache when buffer changes

Fixes #59.  Thanks to Daniel Hubmann (@hubisan) for reporting.

org-ql.el

@@ -470,7 +470,8 @@ Values compared with `equal'."
             buffer-unmodified-p (eq (buffer-modified-tick) modified-tick))
       (unless (and buffer-cache buffer-unmodified-p)
         ;; Buffer-local node cache empty or invalid: make new one.
-        (setf position-cache (make-hash-table))
+        (setf position-cache (make-hash-table)
+              value-cache (gethash position position-cache))
         (puthash (current-buffer)
                  (cons (buffer-modified-tick) position-cache)
                  org-ql-node-value-cache))

tests/test-org-ql.el

@@ -142,6 +142,25 @@ RESULTS should be a list of strings as returned by
                           (cl-loop while (re-search-forward org-heading-regexp nil t)
                                    sum 1)))))
 
+  (describe "Caching"
+    (it "Clears value cache after buffer changes"
+      ;; See <https://github.com/alphapapa/org-ql/issues/59>.
+      (with-temp-buffer
+        (org-mode)
+        (insert "* Heading 1
+* Heading 2")
+        ;; FIXME: `--value-at' does not actually move point, so we do it here.
+        (goto-char (point-min))
+        (expect (org-ql--value-at (point-min) #'org-get-heading)
+                :to-equal "Heading 1")
+        (erase-buffer)
+        ;; FIXME: See above.
+        (insert "* Heading 2")
+        (goto-char (point-min))
+        (org-ql--value-at (point-min) #'point)
+        (expect (org-ql--value-at (point-min) #'org-get-heading)
+                :to-equal "Heading 2"))))
+
   (describe "Query functions/macros"
 
     (it "org-ql"