Just another update

Task/Modular-exponentiation/D/modular-exponentiation.d

@@ -1,23 +1,30 @@
-import std.stdio, std.bigint;
+module modular_exponentiation;
 
-BigInt powMod(BigInt base, BigInt exponent, BigInt modulus)
-in {
+private import std.bigint;
+
+BigInt powMod(BigInt base, BigInt exponent, in BigInt modulus)
+pure nothrow /*@safe*/ in {
    assert(exponent >= 0);
 } body {
     BigInt result = 1;
-    while (exponent > 0) {
-        if (exponent % 2 == 1)
+
+    while (exponent) {
+        if (exponent & 1)
             result = (result * base) % modulus;
         exponent /= 2;
         base = base ^^ 2 % modulus;
     }
+
     return result;
 }
 
-void main() {
-    powMod(BigInt("29883481620585741369158914214988194" ~
-                  "66320163312926952423791023078876139"),
-           BigInt("235139930337346448646612254452369009" ~
-                  "4744975233415544072992656881240319"),
-           BigInt(10) ^^ 40).writeln();
-}
+version (modular_exponentiation)
+    void main() {
+        import std.stdio;
+
+        powMod(BigInt("29883481620585741369158914214988194" ~
+                      "66320163312926952423791023078876139"),
+               BigInt("235139930337346448646612254452369009" ~
+                      "4744975233415544072992656881240319"),
+               BigInt(10) ^^ 40).writeln;
+    }

Task/Modular-exponentiation/REXX/modular-exponentiation-1.rexx

@@ -0,0 +1,25 @@
+/*REXX program displays  modular exponentation:     a**b  mod  M        */
+parse arg a b mm                            /*get optional args from CL.*/
+if a=='' | a==','  then a=2988348162058574136915891421498819466320163312926952423791023078876139
+if b=='' | b==','  then b=2351399303373464486466122544523690094744975233415544072992656881240319
+if mm='' | mm=','  then mm=40               /*MM specified? Use default.*/
+say 'a=' a;    say '        ('length(a) "digits)"     /*show value of A.*/
+say 'b=' b;    say '        ('length(b) "digits)"     /*  "    "    " B.*/
+
+     do j=1  for words(mm);   m=word(mm,j)  /*use one of the  MM powers.*/
+     say copies('─',linesize()-1)           /*show a nice separator line*/
+     say 'a**b (mod 10**'m")="   powerModulated(a,b,10**m)    /*show ans*/
+     end   /*j*/
+exit                                        /*stick a fork in it;  done.*/
+/*──────────────────────────────────────POWERMODULATED subroutine───────*/
+powerModulated: procedure;  parse arg x,p,n /*fast modular exponentation*/
+if p==0  then return 1                      /*special case of P = zero. */
+if p==1  then return x                      /*   "      "   " " = unity.*/
+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                                         /*use this for the 1st value*/
+         do  while p\==0;   if p//2==1  then _=_*x//n       /*is P odd? */
+         p=p%2;     x=x*x//n                /*halve P;  calc  x² mod n  */
+         end   /*while*/                    /* [↑]  keep moding 'til =0.*/
+return _

Task/Modular-exponentiation/REXX/modular-exponentiation-2.rexx

@@ -0,0 +1,22 @@
+/* Modular exponentiation */
+
+numeric digits 100
+say powerMod(,
+ 2988348162058574136915891421498819466320163312926952423791023078876139,,
+ 2351399303373464486466122544523690094744975233415544072992656881240319,,
+ 1e40)
+exit
+
+powerMod: procedure
+
+parse arg base, exponent, modulus
+
+exponent = strip(x2b(d2x(exponent)), 'L', '0')
+result = 1
+base = base // modulus
+do exponentPos=length(exponent) to 1 by -1
+  if substr(exponent, exponentPos, 1) = '1'
+    then result = (result * base) // modulus
+  base = (base * base) // modulus
+end
+return result

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

@@ -1,28 +0,0 @@
-/*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 _

Task/Modular-exponentiation/Racket/modular-exponentiation.rkt

@@ -0,0 +1,6 @@
+#lang racket
+(require math)
+(define a 2988348162058574136915891421498819466320163312926952423791023078876139)
+(define b 2351399303373464486466122544523690094744975233415544072992656881240319)
+(define m (expt 10 40))
+(modular-expt a b m)