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--- notes_20on_20the_20use_20of_20rnd [2022/07/29 13:14] – Changed sort% to sort%% for compatibility with BBCSDL richardrussell
+++ notes_20on_20the_20use_20of_20rnd [2024/01/05 00:21] (current) – external edit 127.0.0.1
@@ Line 66: / Line 66: @@
 As mentioned above, the sequence-length of BBC BASIC's pseudo-random number generator is 2^33-1 (8589934591), so unless your program calls RND at least that number of times (unlikely!) only part of the sequence will be utilised. Determining the starting point in the sequence is called **seeding** the generator.
-When //BBC BASIC for Windows// is executed the random number generator is seeded from the value of [[http://www.bbcbasic.co.uk/bbcwin/manual/bbcwin7.html#time|TIME]], but since this counts the number of centiseconds since the PC was last restarted it is likely to be quite small compared with the sequence length. Suppose for example the PC has been running for 24 hours, so TIME is approximately 8640000, this corresponds to only about one-thousandth of the overall sequence length! So relying on the automatic seeding will not make good use of the potential performance of RND.
+When //BBC BASIC for Windows// or //BBC BASIC for SDL 2.0// is executed the random number generator is seeded from the CPU Performance Counter: a 64-bit integer value which counts at a rate up to the clock speed of the CPU.
-To improve the performance you should seed the generator yourself, using the **RND(-n)** option. To do better than the automatic seeding you need to choose a value which is highly variable, not predictable and can range over most or all of the available range (-2147483648 to -1). On a Windows PC a suitable source of such a seed is the Performance Counter: a 64-bit integer value which counts at a rate up to the clock speed of the CPU. The following code may be used:
-<code bb4w>
-        DIM pc{l%,h%}
-        SYS "QueryPerformanceCounter", pc{}
-        seed% = RND(-ABS(pc.l%)-1)
-</code>
 It should be noted, however, that the rate at which the Performance Counter increments is extremely variable between systems, and indeed one isn't guaranteed to exist at all. Therefore if you are lucky enough to have a better source of seed available you should use that instead.