Add dice tables

A dice table is a direct translation of a gaming table from a book,
where the table has a stated dice expression, and each entry has a
numeric range. Most tables can now be taken as is from published
sources.

Also, if a message from a randomly rolled table contains a dice
expression, like Found 2d8 scrolls, the expression is automatically
substituted.

Finally if a message contains something like:
  You found a [wolf/fox/badger].
The displayed message will only include on of those entries.

rpgdm-tables-dice.el

@@ -1,4 +1,3 @@
-;; [[file:docs/rpgdm-tables-dice.org::+BEGIN_SRC emacs-lisp][No heading:1]]
 ;;; rpgdm-tables-dice.el --- Rolling dice for choosing items from Tables -*- lexical-binding: t; -*-
 ;;
 ;; Copyright (C) 2021 Howard X. Abrams
@@ -11,4 +10,77 @@
 ;;
 ;;
 ;;; Commentary:
-;; No heading:1 ends here
+;;
+;;     This file contains the source code, but the concept behind what I'm
+;;     calling random dice tables is a bit complex, so I recommend looking
+;;     at the original file in `docs/rpgdm-tables-dice.org'.
+
+;;; Code:
+
+(defstruct dice-table dice rows)
+
+(defun rpgdm-tables--choose-dice-table (table)
+  "Choose a string from a random dice table."
+  (let* ((roll (rpgdm-roll-sum (dice-table-dice table)))
+         (rows (dice-table-rows table))
+         (results (rpgdm-tables-dice--choose roll rows)))
+    (if (stringp results)
+        results
+        (seq-random-elt results))))
+
+(defun rpgdm-tables-dice--choose (roll rows)
+  "Given a numeric ROLL, return row that matches.
+This assumes ROWS is a sorted list where the first element (the
+`car') is a numeric level that if ROLL is less than or equal, we
+return the `rest' of the row. Otherwise, we recursively call this
+function with the `rest' of the rows."
+  (let* ((row (first rows))
+         (level (car row))
+         (answer (rest row)))
+
+    (if (<= roll level)
+        answer
+      (rpgdm-tables-dice--choose roll (rest rows)))))
+
+(setq rpgdm-tables-dice-table-regexp (rx "Roll"
+                                           (one-or-more space)
+                                           (optional (or "on" "for"))
+                                           (zero-or-more space)
+                                           "Table:"
+                                           (zero-or-more space)
+                                           (group
+                                            (regexp rpgdm-roll-regexp))))
+
+(defun rpgdm-tables-dice-table? ()
+  "Return non-nil if current buffer contains a dice-table"
+  (goto-char (point-min))
+  (re-search-forward rpgdm-tables-dice-table-regexp nil t))
+
+(defun rpgdm-tables--parse-as-dice-table ()
+  "Return `dice-table' of lines matching `rpgdm-tables-dice-table-rows'."
+  (let ((dice (match-string-no-properties 1))         ; Grab expression before moving on
+        (rows ())                                     ; Modify this with add-to-list
+        (row-splitter (rx (* space) "|" (* space))))  ; Split rest of table row
+
+    (while (re-search-forward rgpdm-tables-dice-table-rows nil t)
+      (let* ((levelstr (match-string-no-properties 1))
+             (level    (string-to-number levelstr))
+             (row      (match-string-no-properties 2))
+             (choices  (split-string row row-splitter t)))
+        (add-to-list 'rows (cons level choices))))
+    (make-dice-table :dice dice
+                     :rows (sort rows (lambda (a b) (< (first a) (first b)))))))
+
+(setq rgpdm-tables-dice-table-rows (rx bol
+                                         (zero-or-more space) "|" (zero-or-more space)
+                                         (optional (one-or-more digit)
+                                                   (one-or-more "-"))
+                                         (group
+                                          (one-or-more digit))
+                                         (zero-or-more space) "|" (zero-or-more space)
+                                         (group (+? any))
+                                         (zero-or-more space) "|" (zero-or-more space)
+                                         eol))
+
+(provide 'rpgdm-tables-dice)
+;;; rpgdm-tables-dice.el ends here