source [file join [info library] init.tcl]

# Here is a complimentary-multiply-with-carry RNG with k=4097 and a near-record period,
# more than 10^33000 times as long as that of the Mersenne Twister. (2^131104 vs. 2^19937)
 
  set maxint [ expr { 2 ** 63 } ]
  set rmaxint ${maxint}.0
  set millnum 0
  
  proc initmill { { name ""} { seed 0 } } {
    if { $name eq "" } { set name mill$::millnum ; incr ::millnum }
    upvar $name mill
    if { $seed != 0 } { ::mathfunc::srand $seed }
    set mill(Q) {}
    for { set i 0 } { $i < 4096 } { incr i } {
      set init [ expr { wide( rand() * $::rmaxint ) } ]
      lappend mill(Q) $init
    }
    set mill(c) 362436
    return $name
  }

  set prevmill ""
  proc spin { { spinner "" } } {
    if { $spinner eq "" } {
          set spinner $::prevmill
          if { $spinner eq "" } {
            set spinner [ initmill ]
          }
        }
    set ::prevmill $spinner
    upvar $spinner mill
    set t 0
    set a 18782
    set i 4095
    set x 0
    set r 0xfffffffe
    set i [ expr { ($i + 1) & 4095 } ]
    set Qsubi [ lindex $mill(Q) $i ]
    set t [ expr { wide( $a * $Qsubi + $mill(c) ) } ]
    set mill(c) [ expr { wide($t >> 32) } ]
    set x [ expr { wide($t + $mill(c)) } ]
    if {$x < $mill(c)} { incr x ; incr mill(c) }
    lset mill(Q) $i [ expr { wide( $r - $x ) } ]
    set result [ lindex $mill(Q) $i ].0
    return [ expr { abs ($result / $::rmaxint) } ]
  }
  
  # ex: set test [ initmill ] ; set val [spin $test]

To use, you need to create a new ''mill'' to generate random numbers.
  set name [ initmill ]
or
  initmill <name> <seed>

A bare call to initmill will return an identifier for the new mill.
If you pass a name in as the first argument, the new mill will have
that name.  If you also include a seed, the new mill will be initialized
with that seed.

Once you have the mill you can ''spin'' it for the next random number:
  set random [ spin $name ]

If you don't specify a mill name, it reuses the most recently
used mill.

The actual mill is just an array variable in the local scope, so it will
disappear automatically along with any other local variables if your
proc exits.  Note that the name of the non-existent mill will be saved
and (mis)used again if followed by another bare call of spin, so be
careful to call spin with the appropriate mill id next time.


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