Delphi version of Fast inverse square root

[Tommi Prami 140]

https://en.wikipedia.org/wiki/Fast_inverse_square_root

 

There was code samples long time ago for Delphi. With different constants, to spread error to positive , negative  or +/-

 

Also pure pascal version so routine could be inlined. It vas pretty fast Used in odl company. No need for me right now, but would be still cool to have it around and put into the Wiki and so on, also Delphi version of the code.

 

-Tee-

[dummzeuch 1613]

Surprisingly difficult to find any Delphi implementation at all.

 

Googling "5F3759DF" "delphi" finally turned up something:

 

http://www.sql.ru/forum/actualutils.aspx?action=gotomsg&tid=190407&msg=20745512

function FastInvSqrt(const Value: Single): Single; inline;
var
  IntCst: Cardinal absolute result;
begin
  result := Value;
  IntCst := ($BE6EB50C - IntCst) shr 1;
  result := 0.5 * result * (3 - Value * sqr(result));
end; 

and

 

function FastInvSqrt(x: Single): Single;
var
  xhalf: Single;
  i: Integer;
begin
  xhalf := 0.5 * x;
  i := $5F3759DF - (PInteger(@x)^ shr 1);
  x := PSingle(@i)^;
  result := x * (1.5 - (xhalf * x * x));
end;

These are using different constants.

(And some more in that thread, unfortunately I don't read Russian, so I don't know what the result of the tests were.)

 

Nothing useful turned up for "5F375A86" "delphi" which is supposedly the optimal constant when using Newtons iteration.

 

And "fastinvsqrt" "delphi" found some more:

https://github.com/itchyny/fastinvsqrt/issues/1

(In Chinese, no idea what they are talking about, it is not part of the official list in that project.)

 

https://sourceforge.net/p/graphics32/mailman/graphics32-commits/?viewmonth=201109&page=0

 

 

 

[Arnaud Bouchez 407]

The github link is interresting - if you go to the project page https://github.com/itchyny/fastinvsqrt you find some explanation in English.

 

See https://en.wikipedia.org/wiki/Fast_inverse_square_root for the theory. 🙂

Edited July 25, 2019 by Arnaud Bouchez

[Boris Novgorodov 10]

5 hours ago, dummzeuch said:

>And some more in that thread, unfortunately I don't read Russian, so I don't know what the result of the tests were.)

That page test says that function with  $BE6EB50C constant (from GR32_Math.pas) is faster 

 

 

Edited July 25, 2019 by Boris Novgorodov