From ad1c18be22d33d7dbda5344c801ee62c3e069072 Mon Sep 17 00:00:00 2001
From: "Kevin M. Rosenberg" <kevin@rosenberg.net>
Date: Sun, 1 Feb 2004 06:43:21 +0000
Subject: [PATCH] r8583: add naagum's epsilon function

---
 math.lisp    | 23 +++++++++++++++++++++++
 package.lisp |  1 +
 2 files changed, 24 insertions(+)

diff --git a/math.lisp b/math.lisp
index e911844..fcbab3c 100644
--- a/math.lisp
+++ b/math.lisp
@@ -73,3 +73,26 @@
 
 (defun fixnum-width ()
   (nth-value 0 (truncate (+ (/ (log (1+ most-positive-fixnum)) (log 2)) .5))))
+
+(defun scaled-epsilon (float &optional (operation '+))
+  "Return the smallest number that would return a value different from
+  FLOAT if OPERATION were applied to FLOAT and this number.  OPERATION
+  should be either + or -, and defauls to +."
+  (multiple-value-bind (significand exponent)
+      (decode-float float)
+    (multiple-value-bind (1.0-significand 1.0-exponent)
+	(decode-float (float 1.0 float))
+      (if (and (eq operation '-)
+	       (= significand 1.0-significand))
+	  (scale-float (typecase float
+			 (short-float short-float-negative-epsilon)
+			 (single-float single-float-negative-epsilon)
+			 (double-float double-float-negative-epsilon)
+			 (long-float long-float-negative-epsilon))
+		       (- exponent 1.0-exponent))
+	(scale-float (typecase float
+		       (short-float short-float-epsilon)
+		       (single-float single-float-epsilon)
+		       (double-float double-float-epsilon)
+		       (long-float long-float-epsilon))
+		     (- exponent 1.0-exponent))))))
diff --git a/package.lisp b/package.lisp
index 1ba962f..62bf637 100644
--- a/package.lisp
+++ b/package.lisp
@@ -115,6 +115,7 @@
    #:ensure-integer
    #:histogram
    #:fixnum-width
+   #:scaled-epsilon
    
    ;; macros.lisp
    #:time-iterations
-- 
2.34.1