--- a/ShortFloat.st	Fri Jun 03 18:15:16 2011 +0100
+++ b/ShortFloat.st	Wed Jun 08 22:53:07 2011 +0100
@@ -928,6 +928,66 @@
     ^ super ~= aNumber
 ! !
 
+!ShortFloat methodsFor:'mathematical functions'!
+
+fastInverseSqrt
+    "return a rough but fast approximation of (1 / self sqrt).
+     The error is some 1%, which is ok for many 3D computations or physics simulations.
+     Do not use this for now: it is non-portable and probably not speeding things up
+     much, unless inlined into the sender code.
+     The code is here as a reminder and might be later used as a hint for the inliner
+     (to speed up 3D computations, for example).
+     see: http://betterexplained.com/articles/understanding-quakes-fast-inverse-square-root/"
+
+%{  /* NOCONTEXT */
+    float x, rslt;
+    OBJ newFloat;
+
+    if (sizeof(float) == 4) {
+        x = __shortFloatVal(self);
+        {
+            float xhalf = 0.5f * x;
+            int i = *(int*)&x; // store floating-point bits in integer
+
+            i = 0x5f3759d5 - (i >> 1); // initial guess for Newton's method
+            x = *(float*)&i; // convert new bits into float
+            x = x*(1.5f - xhalf*x*x); // One round of Newton's method
+            __qMKSFLOAT(newFloat, x);
+            RETURN ( newFloat );
+        }
+    }
+%}.
+    ^ 1 / self sqrt
+
+    "
+     10.0 asShortFloat fastInverseSqrt   
+     (1 / 10.0 asShortFloat sqrt)
+    "
+
+    "
+     |a b t0 t1 t2|
+
+     a := 345 asShortFloat.
+     t0 := Time millisecondsToRun:[
+        1000000 timesRepeat:[
+        ]
+     ].
+     t1 := Time millisecondsToRun:[
+        1000000 timesRepeat:[
+            a fastInverseSqrt
+        ]
+     ].
+     t2 := Time millisecondsToRun:[
+        1000000 timesRepeat:[
+            (1 / a sqrt)
+        ]
+     ].
+     Transcript show:'empty: '; showCR:t0.
+     Transcript show:'fast: '; showCR:t1.
+     Transcript show:'regular: '; showCR:t2.
+    "
+! !
+
 !ShortFloat methodsFor:'printing & storing'!
 
 printString
@@ -1724,15 +1784,15 @@
 !ShortFloat class methodsFor:'documentation'!
 
 version
-    ^ '$Id: ShortFloat.st 10632 2011-04-09 17:19:04Z vranyj1 $'
+    ^ '$Id: ShortFloat.st 10643 2011-06-08 21:53:07Z vranyj1 $'
 !
 
 version_CVS
-    ^ 'Header: /var/local/cvs/stx/libbasic/ShortFloat.st,v 1.104 2010-05-05 13:56:55 stefan Exp '
+    ^ 'Header: /cvs/stx/stx/libbasic/ShortFloat.st,v 1.108 2011/05/05 08:57:20 mb Exp '
 !
 
 version_SVN
-    ^ '$Id: ShortFloat.st 10632 2011-04-09 17:19:04Z vranyj1 $'
+    ^ '$Id: ShortFloat.st 10643 2011-06-08 21:53:07Z vranyj1 $'
 ! !
 
 
@@ -1741,3 +1801,4 @@
 
 
 
+