Changeset 42 for trunk/floating-point

Timestamp:

03/21/08 19:00:24 (8 months ago)

Author:

lhealy

Message:

Enhance #'float-as-integer and #'integer-as-float to produce or take
an IEEE754 integer instead of just a sequence integer. The
IEEE754 integer is the unsigned version of the sequence integer,
with the sign marked in the high order bit.

Files:

• trunk/floating-point/floating-point.lisp (modified) (5 diffs)

trunk/floating-point/floating-point.lisp

@@ -1 @@
-;; Comparison of floating point numbers
+;; Comparison of floating point numbers using sequence integers
 ;; Liam Healy 2008-01-22 19:00:17EST floating-point.lisp
-;; Time-stamp: <2008-01-29 21:34:52EST floating-point.lisp>
+;; Time-stamp: <2008-03-21 15:04:15EDT floating-point.lisp>
 ;; $Id$
+
+;;; All floats can be represented by integers.  There are two slightly
+;;; different ways of doing this.  The sequence integer
+;;; has the same sign as the float; the IEEE754 integer is the
+;;; unsigned version of the sequence integer, with the sign marked in
+;;; the high order bit.  The sequence integer is used for comparing
+;;; numbers, where order preservation is important; the IEEE754 integer
+;;; is used when reading a float storage location as an integer.
 
 (in-package :gsl)
@@ -47 +55 @@
             (if (minusp sign) 1 0) expbits exp sigbits mant)))
 
-(defun float-as-integer (float)
-  "An integer corresponding to the float which satisfies three properties:
+(defun ieee754-sign-bit (float-type)
+  "The bytespec to access the sign bit in the IEEE754 specification."
+  (byte 1 (if (eq float-type 'double-float) 63 31)))
+                  
+(defun float-as-integer (float &optional ieee754)
+  "The sequence integer corresponding to the float 
+   which satisfies three properties: 
    1) For two floats (< a b), then
       (< (float-as-integer a) (float-as-integer b)).
@@ -56 +69 @@
    The absolute value of the integer is the integer of the
    IEEE754 representation without the sign bit, and the
-   sign of the integer agrees with the sign of the float."
+   sign of the integer agrees with the sign of the float.
+   To get the IEEE754 integer, specify 'single-float or 
+   'double-float to ieee754."
   (multiple-value-bind (mant exp sign sigbits expbits)
       (decode-ieee754 float)
     (declare (ignore expbits))
-    (* sign (+ (ash exp sigbits) mant))))
+    (if ieee754
+        (* (dpb 1 (ieee754-sign-bit ieee754) (+ (ash exp sigbits) mant)))
+        (* sign (+ (ash exp sigbits) mant)))))
 
 ;; (float-as-integer most-negative-single-float)
@@ -80 +97 @@
 
 (defun integer-as-float (integer float-type)
-  "Construct the floating point number from its integer representation.
-   Also return the number in rational form."
+  "Construct the floating point number from its integer representation, 
+   either sequence or IEEE754.
+   Also return the number in rational form.
+   The integer may be either a signed integer produced by
+   float-as-integer, or an IEEE754 integer.  Acceptable
+   float-type is either 'double-float or 'single-float."
   (let ((expbits (if (eq float-type 'double-float) 11 8))
         (sigbits (if (eq float-type 'double-float) 52 23)))
@@ -97 +118 @@
                   0)))
       (let ((result
-             (* (signum integer)        ; preserve the sign
+             (* (signum integer)        ; accept negative integer
+                (if                     ; accept IEEE754 negative
+                 (ldb-test
+                  (ieee754-sign-bit float-type)
+                  pinteger)
+                 -1 1)
                 (expt 2 (- bexponent (1- (expt 2 (1- expbits)))))
                 frac)))