PT - This is a package that provides alternative pseudo-random number generators.

**mt_rand()** is based upon the Mersenne Twister PRNG algorithm ([2 ], [1 ]). This PRNG has an enormous period (in fact it is 2**19937-1 which is a Mersenne Prime, hence the name). It is also supposedly faster than the standard C library rand() function. Furthermore, while the stock Tcl rand function generates numbers in the range (0,1), randmt produces numbers in the range [0,1]. If required, it can also produce values within (0,1) as well as integer values (any 32bit value)

**isaac_rand()** uses the ISAAC algorithm [3 ] by Bob Jenkins. ISAAC (Indirection, Shift, Accumulate, Add, and Count) generates 32-bit random numbers. The period is guaranteed to be at least 2**40 and is 2**8295 on average. The results are uniformly distributed, unbiased, and unpredictable unless you know the seed. This PRNG **is** suitable for cryptographic use provided a suitably random seed can be provided.

**sub_rand()** is a subtractive random number generator. See Knuth, D.E. 1981, "Seminumerical Algorithms" 2nd ed. vol 2 of "The Art of Computer Programming" para: 3.2-3.3.

For each of these PRNGs a command in the *random* namespace exists to provide extended access. Currently the subcommands are *integer*, *double*, *seed* and *state* which provide an integer or double random number (the expr function always provides a double). The *seed* subcommand lets you seed the generator with a byte array, for example an array obtained from /dev/random. The *state*subcommand returns a byte array that contains the current state of the generator. This could be used to seed the next instance of a generator.

Currently the code is available at

cvs -d:pserver:[email protected]:/cvsroot/tclsoap co Random

and the distributable files are at http://sourceforge.net/projects/tclsoap in the downloads section.

**Usage**

package require Random # Seed and use the Mersenne Twister PRNG % expr {mt_srand(0)} 0.548813502432 % expr {mt_rand()} 0.592844616527 # Seed and use the ISAAC PRNG % expr {isaac_srand(0)} 0.0943298905842 % expr {isaac_rand()} 0.187672290296 # Subtractive PRNG % expr {sub_srand(0)} 0.53392385 % expr {sub_rand()} 0.323008803 # Extended ISAAC commands # Provide extra seed data (up to 256 values can be used) % ::random::isaac seed "\x01\x02\x03\x04\x05" # Get integers from the PRNG (must have been seeded already) % ::random::isaac integer 1439929434 # Get doubles (equivalent to the expr function) % ::random::isaac double 0.960589176966 # Extended subtractive PRNG commands % random::subtractive seed [string repeat \x00 3000] % random::subtractive integer 533923850 % string length [random::subtractive state] 224

schlenk A little example generating random passwords with the help of the isaac generator:

proc createPassword {{minlen 15}} { set chars [split {abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890?:!%$*} ""] set len [llength $chars] set pw "" while {[string length $pw] < $minlen} { set rand [::random::isaac integer] while {$rand > 0 && $minlen && [string length $pw] < $minlen} { set rest [expr {$rand % $len}] set rand [expr {$rand / $len}] append pw [lindex $chars $rest] } } return $pw }

**random** is also one of TclX Commands.

**random***limit*

Return a random integer that is uniformly distributed over the range from 0 to one less than *limit*.

**random seed**?*seedval*?

Reseed the random number generator with *seedval*, or a value based on the current system time if *seedval* is omitted.

2011-05-05 jima

For those like me who look for another **rand()** function, simpler perhaps than the one described here but readily available through **expr** command without needing to load this extension look in rand.