Add first version of workload overview report

This report will list the incremental planned workload between a given time
range using a given increment.  This way, one may better at finding time to
schedule new tasks coming in.

The current implementation might be slow, because it's calling a complete
parsing process for each step in the overview report.  One could instead think
about calling it only once and then disecting the individual tasks for when they
are planned specifically.  But before we dive in these complications, let's
frist see whether the report is worth it.

site-lisp/db-org.el

@@ -515,6 +515,106 @@ understood by `org-read-date'."
 
 (org-dynamic-block-define "db/org-workload-report" #'db/org-insert-workload-report)
 
+(defun org-dblock-write:db/org-workload-overview-report (params)
+  "Write an overview workload report based on tasks in `org-agenda-files'.
+
+This overview report will list the amount of work planned for
+increasing intervals of time until a given end date is reached.
+For example, if the amount to increase the intervals is two
+days (+2d) and the report is meant to start from today (.), then
+this report will list the total amount of work planned for the
+days .+2d, .+4d, .+6d, … until the end date is reached.
+
+PARAMS is a property list of the following parameters:
+
+`:start-date':
+
+  Start date for the workload report.  When not provided, will
+  default to today.  When provided, must be in a format
+  understood by `org-read-date'.
+
+`:end-date':
+
+  End date of the workload report.  Must be provided and provided
+  in a format understood by `org-read-date'.  The end date is
+  inclusive.
+
+`:increment':
+
+  Amount of days to increase the intervals.  Defaults to \"+1d\"
+  and must be provided in a format understandable by
+  `org-read-date'.
+
+`:org-ql-match'
+
+  `org-ql' expression (in sexp syntax) to filter the list of
+  tasks to consider.  Defaults to (todo)."
+  (let* ((start-date (or (plist-get params :start-date)
+                         (org-read-date nil nil ".")))
+         (end-date (or (plist-get params :end-date)
+                       (user-error "No end-date provided")))
+         (increment (or (plist-get params :increment)
+                        "+1d"))
+         (org-ql-match (or (plist-get params :org-ql-match)
+                           '(todo)))
+         (current start-date)
+         (date-range nil))
+
+    ;; Check input
+    (unless (string-match-p (org-re-timestamp 'inactive)
+                            (format "[%s]" start-date))
+      (user-error "Invalid start date given: %s" start-date))
+
+    (unless (string-match-p (org-re-timestamp 'inactive)
+                            (format "[%s]" end-date))
+      (user-error "Invalid end date given: %s" end-date))
+
+    (unless (string-match-p (rx bos "+" (+ digit) (in "dwmy") eos)
+                            increment)
+      (user-error "Increment must be of the form +1d, +2m, +3y, …, but it's %s" increment))
+
+    ;; Compute range of dates to check; simple but potentially costly approach
+    ;; taken from https://sachachua.com/blog/2015/08/org-mode-date-arithmetic/;
+    ;; maybe consider `org-read-date-get-relative' as well?
+    (while (or (string< current end-date)
+               (string= current end-date))
+      (push current date-range)
+      (setq current (org-read-date nil
+                                   nil
+                                   ;; Add an extra + to ensure we increase the
+                                   ;; default time string.
+                                   ;; amount of time relative to the given
+                                   (format "+%s" increment)
+                                   nil
+                                   (org-time-string-to-time current))))
+    (setq date-range (nreverse date-range))
+
+    ;; Compute workload report for each date and record the total time; XXX:
+    ;; this might be slow, try to reduce the calls to
+    ;; `db/org-planned-tasks-in-range'.
+    (insert "| Until | Planned Total |\n| <r> | <r> |\n|---|\n")
+    (dolist (interval-end-date (cdr date-range)) ; `cdr' is for ignoring the `start-date' itself
+      (let ((total-time (car (db/org-planned-tasks-in-range start-date
+                                                            interval-end-date
+                                                            org-ql-match))))
+        (insert "| ")
+        (org-insert-time-stamp (org-time-string-to-time interval-end-date) nil 'inactive)
+        (insert (format " | %s |\n" total-time))))
+    (insert "|--|")
+    (org-table-align)))
+
+(defun db/org-insert-workload-overview-report ()
+  "Create dynamic block of planned tasks in given time range."
+  (interactive)
+  (org-create-dblock
+   (list :name "db/org-workload-overview-report"
+         :end-date (org-read-date nil nil nil "End date: ")
+         :increment (read-string "Increment (default: +1d): " nil nil "+1d")))
+  (org-update-dblock))
+
+(org-dynamic-block-define "db/org-workload-overview-report"
+                          #'db/org-insert-workload-overview-report)
+
 
 ;;; Fixes