Source code in the rexx programming language

/*REXX program calculates the   "information entropy"   for  ~this~  REXX program.      */
numeric digits length( e() ) % 2   -  length(.)  /*use 1/2 of the decimal digits of  E. */
#= 0;   @.= 0;   $=;   $$=;   recs= sourceline() /*define some handy─dandy REXX vars.   */
            do m=1  for recs; $=$||sourceLine(m) /* [↓]  obtain program source and ──► $*/
            end   /*m*/                          /* [↑]  $ str won't have any meta chars*/
L=length($)                                      /*the byte length of this REXX program.*/
            do j=1  for L;  _= substr($, j, 1)   /*process each character in  $  string.*/
            if @._==0  then do;  #= # + 1        /*¿Character unique?  Bump char counter*/
                                 $$= $$ || _     /*add this character to the  $$  list. */
                            end
            @._= @._ + 1                         /*keep track of this character's count.*/
            end   /*j*/                          /* [↑]  characters are all 8─bit bytes.*/
sum= 0                                           /*calculate info entropy for each char.*/
            do i=1  for #;  _= substr($$, i, 1)  /*obtain a character from unique list. */
            sum= sum -  @._ / L * log2(@._ / L)  /*add {negatively} the char entropies. */
            end   /*i*/
say '    program length: '   L                   /*pgm length doesn't include meta chars*/
say 'program statements: '   recs                /*pgm statements are actually pgm lines*/
say ' unique characters: '   #;           say    /*characters are 8─bit bytes of the pgm*/
say 'The information entropy of this REXX program ──► '       format(sum,,12)
exit                                             /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
e: e= 2.718281828459045235360287471352662497757247093699959574966967627724076630; return e
/*──────────────────────────────────────────────────────────────────────────────────────*/
log2: procedure;  parse arg x 1 ox;     ig= x>1.5;     ii= 0;         is= 1 - 2 * (ig\==1)
      numeric digits digits()+5;        call e   /*the precision of E must be≥digits(). */
        do  while  ig & ox>1.5 | \ig&ox<.5;       _= e;       do j=-1;   iz= ox * _ ** -is
        if j>=0 & (ig & iz<1 | \ig&iz>.5)  then leave;    _= _ * _;    izz= iz;  end /*j*/
        ox=izz;  ii=ii+is*2**j;  end /*while*/;   x= x * e** -ii -1;   z= 0;  _= -1;  p= z
          do k=1;   _= -_ * x;   z= z+_/k;        if z=p  then leave;  p= z;    end  /*k*/
        r= z + ii;  if arg()==2  then return r;   return r / log2(2,.)