From 3068ad814f9d354bae063c32200fb337756300cb Mon Sep 17 00:00:00 2001
From: Adam Porter <adam@alphapapa.net>
Date: Mon, 18 Jul 2022 10:50:49 -0500
Subject: [PATCH] WIP: Add org-ql-sort.el

---
 org-ql-sort.el | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++
 org-ql.el      | 12 ++++++++++-
 2 files changed, 67 insertions(+), 1 deletion(-)
 create mode 100644 org-ql-sort.el

diff --git a/org-ql-sort.el b/org-ql-sort.el
new file mode 100644
index 0000000..e45962d
--- /dev/null
+++ b/org-ql-sort.el
@@ -0,0 +1,56 @@
+;;; org-ql-sort.el --- Library for sorting org-elements  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2022  Adam Porter
+
+;; Author: Adam Porter <adam@alphapapa.net>
+;; Keywords: 
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; This library provides tools for sorting `org-element' items.
+
+;;; Code:
+
+;;;; Requirements
+
+(require 'cl-lib)
+(require 'map)
+
+(require 'org-element)
+
+;;;; Variables
+
+(defvar org-ql-sorters nil
+  "Alist of sort functions defined with `org-ql-sort-define-sorter'.
+Maps sorters' short names to function symbols.")
+
+;;;; Macros
+
+(defmacro org-ql-sort-define-sorter (name docstring &rest body)
+  "FIXME: Docstring."
+  (let ((fn-symbol (intern (concat "org-ql-sort--sort-by-" (symbol-name name)))))
+    (map-put org-ql-sorters name fn-symbol)
+    `(defun ,fn-symbol (a b)
+       ,docstring
+       ,@body)))
+
+;;;; Functions
+
+
+
+
+(provide 'org-ql-sort)
+;;; org-ql-sort.el ends here
diff --git a/org-ql.el b/org-ql.el
index 0cf6c8a..a0e03eb 100644
--- a/org-ql.el
+++ b/org-ql.el
@@ -427,7 +427,10 @@ each priority the newest items would appear first."
        (org-ql--sort-by items (-list sort)))
       ;; Sort by user-given comparator.
       ((pred functionp) (-sort sort items))
-      (_ (user-error "SORT must be either nil, one or a list of the defined sorting methods (see documentation), or a comparison function of two arguments")))))
+      ;; FIXME: Test more carefully for sorters with arguments.  In the meantime, just try to sort.
+      (_ (org-ql--sort-by items (-list sort)))
+      ;; (_ (user-error "SORT must be either nil, one or a list of the defined sorting methods (see documentation), or a comparison function of two arguments"))
+      )))
 
 ;;;###autoload
 (cl-defun org-ql-query (&key (select 'element-with-markers) from where narrow order-by)
@@ -2296,6 +2299,13 @@ PREDICATES is a list of one or more sorting methods, including:
                                   (= 0 (random 2))))
                        ;; NOTE: reverse and todo are handled below.
                        ;; TODO: Add more.
+		       (`(tags . ,tags) (lambda (a b)
+					  "Return non-nil if A has certain tags and B doesn't."
+					  (cl-flet ((test-entry
+						     (entry) (org-with-point-at (org-element-property :org-hd-marker entry)
+							       (apply #'org-ql--predicate-tags tags))))
+					    (and (test-entry a)
+						 (not (test-entry b))))))
                        (_ (user-error "Invalid sorting predicate: %s" symbol))))
              (sort-by-todo-keyword (items)
                                    (let* ((grouped-items (--group-by (when-let (keyword (org-element-property :todo-keyword it))