A-M baby

Task/Modular-exponentiation/REXX/modular-exponentiation.rexx

@@ -0,0 +1,28 @@
+/*REXX program to show  modular exponentation:    a**b  mod  M          */
+parse arg a b mm                            /*get the arguments (maybe).*/
+if a=='' | a==',' then a=,
+    2988348162058574136915891421498819466320163312926952423791023078876139
+
+if b=='' | b==',' then b=,
+    2351399303373464486466122544523690094744975233415544072992656881240319
+
+if mm=='' then mm=40
+say 'a=' a;           say '        ('length(a) "digits)"
+say 'b=' b;           say '        ('length(b) "digits)"
+
+      do j=1 for words(mm);   m=word(mm,j);   say copies('─',linesize()-1)
+      say 'a**b (mod 10**'m")=" powerModulated(a,b,10**m)
+      end   /*j*/
+exit                                   /*stick a fork in it, we're done.*/
+/*──────────────────────────────────────POWERMODULATED subroutine───────*/
+powerModulated: procedure;  parse arg x,p,n /*fast modular exponentation*/
+if p==0  then return 1                      /*special case.             */
+if p==1  then return x                      /*special case.             */
+if p<0   then do;  say '***error!*** power is negative:' p;  exit 13;  end
+parse value max(x**2,p,n)'E0'  with  "E" e  /*pick biggest of the three.*/
+numeric digits max(20,e*2)                  /*big enough to handle  A²  */
+_=1
+          do while p\==0;   if p//2==1 then _=_*x//n
+          p=p%2;     x=x*x // n
+          end    /*while*/
+return _