September Morn Update

Task/Modular-exponentiation/Phix/modular-exponentiation.phix

@@ -0,0 +1,32 @@
+include mpfr.e
+procedure mpz_mod_exp(mpz base, exponent, modulus, result)
+    if mpz_cmp_si(exponent,1)=0 then
+        mpz_set(result,base)
+    else
+        mpz _exp = mpz_init_set(exponent) -- (use a copy)
+        bool odd = mpz_odd(_exp)
+        if odd then
+            mpz_sub_ui(_exp,_exp,1)
+        end if
+        mpz_fdiv_q_2exp(_exp,_exp,1)
+        mpz_mod_exp(base,_exp,modulus,result)
+        _exp = mpz_free(_exp)
+        mpz_mul(result,result,result)
+        if odd then
+            mpz_mul(result,result,base)
+        end if
+    end if
+    mpz_mod(result,result,modulus)
+end procedure
+
+mpz base     = mpz_init("2988348162058574136915891421498819466320163312926952423791023078876139"),
+    exponent = mpz_init("2351399303373464486466122544523690094744975233415544072992656881240319"),
+    modulus  = mpz_init("1"&repeat('0',40)),
+    result   = mpz_init()
+
+mpz_mod_exp(base,exponent,modulus,result)
+?mpz_get_str(result)
+
+-- check against the builtin:
+mpz_powm(result,base,exponent,modulus)
+?mpz_get_str(result)