Random numbers in OpenInsight

One issue that recently reared it's head again at Sprezz Towers Support Central was the problem of using the Basic+ Rnd() function to generate a random number sequence - apparently the same series of numbers was being produced every time.

On first investigation it appeared that the customer was not using the InitRnd() function to seed the Rnd() generator before asking for the random numbers. This is a documented requirement - Calling InitRnd() is an absolute necessity before using the Rnd() function.

Even so, using InitRnd() as specified in the online help failed to produce sufficiently random results so we decided to take a closer look at the process to see what the problem was and how we could improve it.

Under the hood the Basic+ InitRnd() and Rnd() functions are little more than thin wrappers around the srand() and rand() functions in the Windows C-runtime library. The rand() function actually produces a pseudorandom sequence of numbers, so it is not truly random at all - instead the srand() function is used to seed the starting point for the pseudorandom sequence so it begins in a different place after each initialisation. Also, if srand() is called again with the same seed the same sequence of numbers is generated by rand()!

Going back to the problem, the OpenInsight online help gives an example of using InitRnd() with a seed based on the current time and date (which is what our customer tried next):

0001  /* 0002     The present time and date set the random number  0003     generator with a unique value. 0004  */ 0005   0006     InitRnd Time(): Date()

Unfortunately for this example using the date results in a seed value that doesn't show much variance in it's low order value (it only increments by one per day), and this appears to have the effect of generating a identical Rnd() sequence between each initialisation (especially if called multiple times on the same day which is something that should be avoided). We then thought about swapping the date and time around so that the low order value would hopefully be more varied, but here we find ourselves in danger of generating predictable sequences if we started our application at the same time every day (like at 9:15am for example).

What we needed was some way to fulfil the following criteria:

• Ensure that InitRnd() is only called once per session to avoid the same seed being used more than once.
• Provide InitRnd with a varied seed value that is less predictable and has a greater "spread" than a simple date/time value.

We quickly put together the following function to do this that gave much better results:

0001  subroutine zz_InitRnd( bReset ) 0002  /* 0003     Author  : Mr C, Sprezzatura Ltd 0004     Date    : 29 July 2010 0005     Purpose : Simple function to call initRnd with with a more varied  0006               seed value, and to also ensure we only call it once per  0007               session. 0008                0009     Parameters 0010     

0011      0012     bReset  : If TRUE$ then call initRnd regardless of whether or 0013               not it's been called before. 0014                0015  */ 0016     declare function getTickCount, getEngineWindow 0017     $insert logical    0018      0019     if assigned( bReset ) else bReset = "" 0020      0021     common /_ZZ_INITRND_/ zzInitRndSeed@ 0022      0023     * // Check to see if we've already been called this session. 0024     if zzInitRndSeed@ then 0025        * // We've called it already - are we asking for a reset? 0026        if bReset else 0027           * // Nope! 0028           return 0029        end 0030     end 0031      0032     * // Start off with the tickcount (ms) and the 0033     * // OE hwnd … 0034     seed = getTickCount() + getEngineWindow() 0035      0036     * // Now adjust this slightly to help avoid the time 0037     * // factor 0038     if mod( seed, 2 ) then 0039        b = seed[-1,1] 0040        if b then 0041           seed = int( seed / b ) 0042         end else 0043           seed = int( seed / 3 ) 0044        end 0045     end 0046      0047     * // Right - run it through some more mods to keep it 0048     * // within range of a short int if it's not already… 0049     loop 0050     while ( seed > 0x7FFF ) 0051        if mod( seed, 2 ) then 0052           seed -= 0x7FFF 0053        end else 0054           b = seed[-1,1] 0055           if b then 0056              seed = int( seed / b ) 0057           end else 0058              seed = int( seed / 3 ) 0059           end 0060        end   0061     repeat 0062      0063     initRnd seed 0064         0065     * // Flag that we've already called this… 0066     zzInitRndSeed@ = seed 0067            0068  return

(Note that we have a global variable in place to ensure that initRnd is only called once per session, unless we explicitly override this)

You can download a text version of zz_InitRnd here.

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