A 64bit XorShift [1] is now used to generate random numbers in DWScript [2], and there is a now a separate random number generator per-execution, which is auto-randomized when an execution is created.
Previously, the RTL random generator was used, this was “okay” when you had only one script using random numbers at a time, but multiple scripts running at the same time would interfere (Randomize calls would affect each others f.i.), and Random isn’t really thread-safe.
Performance fo XorShift is roughly comparable to the Delphi RTL’s linear congruential generator [3], but with much better statistical random properties and a very long period, without the overhead of a Mersenne Twister [4]. For those interested in the mathematical details, see “XorShift RNGs [5]” paper by G. Marsagalia.
As an illustration of the improved random properties, consider filling a bitmap with “random” RGB colors for each pixel:
var x, y : Integer;
for x := 0 to bmp.Width-1 do
for y := 0 to bmp.Height-1 do
bmp.Pixel[x, y] := RandomInt($1000000);
Using the Delphi built-in Random, you’ll get something like the image below (generated at 512×512, then halved and downgraded to 4bpp for web consumption)
[6]Delphi RTL Random
Oooh… the horizontal scratch lines! Not so random after all… I don’t know if the Delphi LCG is as biased as RANDU [7], but visibly, it is probably not something you want to rely upon too much.
And now, the same but with the XorShift implementation now used in DWS:
[8]DWScript XorShift Random
The XorShift implementation is very simple, fast, and doesn’t require much memory: a single 64bit value is enough to get good random, use two if you want longer periods that won’t have a chance to loop before the universe ends.
Last but not least, 64bit XorShift may be fast in 32bit binaries, but it practically walks on water in 64bit binaries 😉