/*REXX program  demonstrates  an  evolutionary algorithm  (by using mutation).          */
parse arg  children  MR  seed .                  /*get optional arguments from the C.L. */
if children=='' | children=="," then children=10 /*# children produced each generation. */
if MR      =='' | MR      =="," then MR= "4%"    /*the character Mutation Rate each gen.*/
if right(MR,1)=='%'  then MR= strip(MR,,"%")/100 /*expressed as a percent?  Then adjust.*/
if seed\==''  then call random ,,seed            /*SEED allow the runs to be repeatable.*/
abc   = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ '  ;     Labc= length(abc)
target= 'METHINKS IT IS LIKE A WEASEL' ;     Ltar= length(target)
parent= mutate( left('', Ltar), 1)               /*gen rand string,same length as target*/
say center('target string', Ltar, "─")    'children'        "mutationRate"
say target  center(children, 8)   center((MR*100/1)'%', 12);                      say
say center('new string'    ,Ltar, "─")    "closeness"       'generation'

       do gen=0  until  parent==target;                      close= fitness(parent)
       almost= parent
                        do  children;                        child= mutate(parent,MR)
                        _= fitness(child);                   if _<=close  then iterate
                        close= _;                            almost= child
                        say almost  right(close, 9)   right(gen, 10)
                        end   /*children*/
       parent= almost
       end   /*gen*/
exit                                             /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
fitness: parse arg x; $=0;  do k=1 for Ltar; $= $+(substr(x,k,1)==substr(target,k,1)); end
         return $
/*──────────────────────────────────────────────────────────────────────────────────────*/
mutate:  parse arg x,rate;  $=                   /*set  X  to 1st argument, RATE to 2nd.*/
                   do j=1  for Ltar;       r= random(1, 100000)  /*REXX's max for RANDOM*/
                   if .00001*r<=rate  then $= $  ||  substr(abc, r//Labc+1, 1)
                                      else $= $  ||  substr(x  , j        , 1)
                   end   /*j*/
         return $