From 6ece68a4a2f5a413f48bb6816b25a41a9376ee3e Mon Sep 17 00:00:00 2001
From: Mario Lang <mlang@delysid.org>
Date: Fri, 28 Mar 2014 11:57:29 +0100
Subject: Make the strength of book replies configurable.

---
 chess-polyglot.el | 36 ++++++++++++++++++++----------------
 chess-uci.el      | 17 ++++++++++++++---
 2 files changed, 34 insertions(+), 19 deletions(-)

diff --git a/chess-polyglot.el b/chess-polyglot.el
index a58ae8d..a9a0551 100644
--- a/chess-polyglot.el
+++ b/chess-polyglot.el
@@ -462,23 +462,27 @@ Use `chess-ply-keyword' on elements of the returned list to retrieve them."
 	(when ply
 	  (setq plies (nconc plies (list ply))))))))
 
-(defun chess-polyglot-book-ply (book position)
-  "If non-nil, a (randomly picked) ply from plies in BOOK for POSITION.
-Random distribution is defined by the relative weights of the found plies."
-  (let* ((plies (chess-polyglot-book-plies book position))
-	 (random-value (random (cl-reduce
-				#'+ (mapcar (lambda (ply)
-					      (chess-ply-keyword
-					       ply :polyglot-book-weight))
-					    plies))))
-	 (max 0) ply)
-    (while plies
-      (let ((min max))
-	(setq max (+ max (chess-ply-keyword (car plies) :polyglot-book-weight)))
-	(if (and (>= random-value min) (< random-value max))
+(defun chess-polyglot-book-ply (book position &optional strength)
+  "If non-nil a (randomly picked) ply from plies in BOOK for POSITION.
+Random distribution is defined by the relative weights of the found plies.
+If non-nil, STRENGTH defines the bias towards better moves.
+A value below 1.0 will penalize known good moves while a value
+above 1.0 will prefer known good moves.  The default is 1.0.
+A strength value of 0.0 will completely ignore move weights and evenly
+distribute the probability that a move gets picked."
+  (unless strength (setq strength 1.0))
+  (cl-assert (and (>= strength 0) (< strength 4)))
+  (cl-flet ((ply-weight (ply)
+	      (round (expt (chess-ply-keyword ply :polyglot-book-weight)
+			   strength))))
+    (let* ((plies (chess-polyglot-book-plies book position))
+	   (random-value (random (cl-reduce #'+ (mapcar #'ply-weight plies))))
+	   (max 0) ply)
+      (while plies
+	(if (< random-value (setq max (+ max (ply-weight (car plies)))))
 	    (setq ply (car plies) plies nil)
-	  (setq plies (cdr plies)))))
-    ply))
+	  (setq plies (cdr plies))))
+      ply)))
 
 (defalias 'chess-polyglot-book-close 'kill-buffer
   "Close a polyglot book.")
diff --git a/chess-uci.el b/chess-uci.el
index ad6852c..e164f72 100644
--- a/chess-uci.el
+++ b/chess-uci.el
@@ -36,7 +36,16 @@
 (defcustom chess-uci-polyglot-book-file nil
   "The path to a polyglot binary opening book file."
   :group 'chess-uci
-  :type '(choice (const :tag "Not specified" nil) file))
+  :type '(choice (const :tag "Not specified" nil) (file :must-match t)))
+
+(defcustom chess-uci-polyglot-book-strength 1.0
+  "Influence random distribution when picking a ply from the book.
+A value above 1.0 means to prefer known good moves while a value below
+1.0 means to penalize known good moves.  0.0 will stop to consider
+move weights and simply pick a move at random.  For simple
+reasons of numerical overflow, this should be strictly less than 4.0."
+  :group 'chess-uci
+  :type '(float :match (lambda (widget value) (and (>= value 0) (< value 4)))))
 
 (defvar chess-uci-book nil
   "A (polyglot) opening book object.
@@ -121,8 +130,10 @@ If conversion fails, this function fired an 'illegal event."
 
       (let ((book-ply (and chess-uci-book (bufferp chess-uci-book)
 			   (buffer-live-p chess-uci-book)
-			   (chess-polyglot-book-ply chess-uci-book
-						    (chess-game-pos game)))))
+			   (chess-polyglot-book-ply
+			    chess-uci-book
+			    (chess-game-pos game)
+			    chess-uci-polyglot-book-strength))))
 	(if book-ply
 	    (let ((chess-display-handling-event nil))
 	      (funcall chess-engine-response-handler 'move book-ply))
-- 
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