Data commit

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

@@ -0,0 +1,25 @@
+/* REXX  Modular exponentiation */
+
+say powerMod(,
+ 2988348162058574136915891421498819466320163312926952423791023078876139,,
+ 2351399303373464486466122544523690094744975233415544072992656881240319,,
+ 1e40)
+return
+
+powerMod: procedure
+  parse arg base, exponent, modulus
+
+  /* we need a numeric precision of twice the modulus size,    */
+  /* the exponent size, or the base size, whichever is largest */
+  numeric digits max(2 * length(format(modulus, , , 0)),,
+    length(format(exponent, , , 0)), length(format(base, , , 0)))
+
+  result = 1
+  base = base // modulus
+  do while exponent > 0
+    if exponent // 2 = 1 then
+      result = result * base // modulus
+    base = base * base // modulus
+    exponent = exponent % 2
+  end
+  return result