Source code in the rexx programming language

/*REXX program finds and displays a number of  cuban  primes  or the  Nth  cuban prime. */
numeric digits 20                                /*ensure enough decimal digits for #s. */
parse arg N .                                    /*obtain optional argument from the CL.*/
if N=='' | N==","  then N= 200                   /*Not specified?  Then use the default.*/
Nth= N<0;               N= abs(N)                /*used for finding the Nth cuban prime.*/
@.=0; @.0=1; @.2=1; @.3=1; @.4=1; @.5=1; @.6=1; @.8=1  /*ending digs that aren't cubans.*/
sw= linesize() - 1;    if sw<1  then sw= 79      /*obtain width of the terminal screen. */
w=12;              #= 1;    $= right(7, w)       /*start with first cuban prime;  count.*/
     do j=1  until #=>N;    x= (j+1)**3 - j**3   /*compute a possible cuban prime.      */
     parse var x '' -1 _;   if @._  then iterate /*check last digit for non─cuban prime.*/
            do k=1  until km*km>x;  km= k*6 + 1  /*cuban primes can't be   ÷   by  6k+1 */
            if x//km==0  then iterate j          /*Divisible?   Then not a cuban prime. */
            end   /*k*/
     #= #+1                                      /*bump the number of cuban primes found*/
     if Nth  then do;  if #==N  then do;  say commas(x);  leave j;  end /*display 1 num.*/
                                else iterate /*j*/                      /*keep searching*/
                  end                            /* [↑]  try to fit as many #s per line.*/
     cx= commas(x);  L= length(cx)               /*insert commas──►X; obtain the length.*/
     cx= right(cx, max(w, L) );   new= $  cx     /*right justify  CX; concat to new list*/
     if length(new)>sw  then do;  say $;  $= cx  /*line is too long, display #'s so far.*/
                             end                 /* [↑]  initialize the (new) next line.*/
                        else              $= new /*start with cuban # that wouldn't fit.*/
     end   /*j*/
                  if \Nth  &  $\==''  then say $ /*check for residual cuban primes in $.*/
exit                                             /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
commas: parse arg _;  do jc=length(_)-3  to 1  by -3; _=insert(',', _, jc); end;  return _