Add: Caching

Seems to work well so far.  Hopefully no bugs will be found (ha,
ha)...

Thanks to @yantar92 for suggesting!
<https://www.reddit.com/r/emacs/comments/990swh/querying_org_files_for_due_items_and_showing/e4kduul/>

org-ql.el

@@ -15,6 +15,13 @@

(defvar org-ql--today nil)

(defvar org-ql-cache (make-hash-table :weakness 'key)
  ;; IIUC, setting weakness to `key' means that, when a buffer is closed, its entries will be
  ;; removed from this table at the next GC.
  "Query cache, keyed by buffer.  Each value is a list of the
buffer's modified tick and another hash table, keyed by arguments
passed to `org-ql--select-cached'.")

;;;; Macros

(cl-defmacro org-ql (buffers-or-files pred-body &key sort narrow markers
@@ -110,7 +117,7 @@ a list of defined `org-ql' sorting methods: `date', `deadline',
                                          (user-error "Can't open file: %s" it)))))
                     ;; Filter buffers (i.e. select items)
                     (--map (with-current-buffer it
                              (org-ql--select :predicate predicate :action action :narrow narrow)))
                              (org-ql--select-cached :predicate predicate :action action :narrow narrow)))
                     ;; Flatten items
                     (-flatten-n 1))))
    ;; Sort items
@@ -124,6 +131,32 @@ a list of defined `org-ql' sorting methods: `date', `deadline',
       (org-ql--sort-by items sort))
      (_ (user-error "SORT must be either nil, or one or a list of the defined sorting methods (see documentation)")))))

(define-hash-table-test 'org-ql-hash-test #'equal (lambda (args)
                                                    (sxhash-equal (prin1-to-string args))))

(defun org-ql--select-cached (&rest args)
  "Return results for ARGS and current buffer using cache."
  ;; MAYBE: Timeout cached queries.  Probably not necessarily since they will be removed when a
  ;; buffer is closed, or when a query is run after modifying a buffer.
  (if-let* ((buffer-cache (gethash (current-buffer) org-ql-cache))
            (query-cache (cadr buffer-cache))
            (modified-tick (car buffer-cache))
            (buffer-unmodified-p (eq (buffer-modified-tick) modified-tick))
            (cached-result (gethash args query-cache)))
      (pcase cached-result
        ('org-ql-nil nil)
        (_ cached-result))
    (let ((new-result (apply #'org-ql--select args)))
      (cond ((or (not query-cache)
                 (not buffer-unmodified-p))
             (puthash (current-buffer)
                      (list (buffer-modified-tick)
                            (aprog1 (make-hash-table :test 'org-ql-hash-test)
                              (puthash args (or new-result 'org-ql-nil) it)))
                      org-ql-cache))
            (t (puthash args (or new-result 'org-ql-nil) query-cache)))
      new-result)))

(cl-defun org-ql--select (&key predicate action narrow)
  "Return results of mapping function ACTION across entries in current buffer matching function PREDICATE.
If NARROW is non-nil, buffer will not be widened."