Using a better algorithm based on property of primes.

I recently found out that `p` being prime, while `p > 3`, then
`p = 6*k (+/-) 1`.

Makefile

@@ -22,7 +22,7 @@ LDFLAGS=-lgmp
 DEFS=-DVERSION=\"2.0\" -DAPP_NAME=\"Indivisible\"
 CFLAGS=$(INCFLAGS) $(DEFS) -std=c99 -Wall -Wextra -Wfatal-errors -Werror
 HDRS=src/globals.h src/llist.h
-OBJS=src/main.o src/llist.o
+OBJS=src/llist.o src/main.o src/prime_test.o
 
 ifeq ($(DEBUG),1)
 	CFLAGS+=-g -O0 -DDEBUG

src/main.c

@@ -18,6 +18,7 @@
 
 #include "globals.h"
 #include "llist.h"
+#include "prime_test.h"
 
 #include <signal.h>
 #include <stdio.h>
@@ -35,40 +36,35 @@ int main() {
 
 	signal(SIGINT, quit_signal);
 
-	mpz_t tnum; // number to be tested for prime-ness
-	mpz_t tsqrt; // square root of tnum
-	mpz_t aux; // auxilary number to test against tnum
-	mpz_t r; // remainder from modulus operation
+	mpz_t test_base; // number used to get tested numbers
+	mpz_t p0, p1; // numbers to be tested for prime-ness.
 
-	mpz_inits(tnum, tsqrt, aux, r, NULL);
-	mpz_set_ui(tnum, 3);
-	mpz_sqrt(tsqrt, tnum);
+	mpz_inits(test_base, p0, p1, NULL);
+	mpz_set_ui(test_base, 6);
 
 	puts("2");
+	puts("3");
 
 	while(run)
 	{
-		mpz_set_ui(aux, 3);
-		int is_prime = 1;
-		while(mpz_cmp(aux, tsqrt) <= 0)
-		{
-			mpz_mod(r, tnum, aux);
-			if(mpz_cmp_ui(r, 0) == 0)
-				is_prime = 0;
-			mpz_add_ui(aux, aux, 2);
-		}
+		mpz_sub_ui(p0, test_base, 1); // p0 = test_base - 1
+		mpz_add_ui(p1, test_base, 1); // p0 = test_base + 1
 
-		if(is_prime)
+		if(test_prime(p0))
 		{
-			mpz_out_str(stdout, 10, tnum);
+			mpz_out_str(stdout, 10, p0);
+			printf("\n");
+		}
+		if(test_prime(p1))
+		{
+			mpz_out_str(stdout, 10, p1);
 			printf("\n");
 		}
 
-		mpz_add_ui(tnum, tnum, 2);
-		mpz_sqrt(tsqrt, tnum);
+		mpz_add_ui(test_base, test_base, 6);
 	}
 
-	mpz_clears(tnum, tsqrt, aux, r, NULL);
+	mpz_clears(test_base, p0, p1, NULL);
 
 	return 0;
 }

src/prime_test.c

@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2019  Ortega Froysa, Nicolás <nicolas@ortegas.org>
+ * Author: Ortega Froysa, Nicolás <nicolas@ortegas.org>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "prime_test.h"
+
+int test_prime(const mpz_t p) {
+	mpz_t psqrt;
+	mpz_t aux;
+	mpz_inits(psqrt, aux, NULL);
+	mpz_sqrt(psqrt, p);
+	int is_prime = 1;
+	for(mpz_set_ui(aux, 5); mpz_cmp(aux, psqrt) <= 0; mpz_add_ui(aux, aux, 2))
+	{
+		if(mpz_divisible_p(p, aux))
+		{
+			is_prime = 0;
+			break;
+		}
+	}
+
+	mpz_clears(psqrt, aux, NULL);
+
+	return is_prime;
+}

src/prime_test.h

@@ -0,0 +1,27 @@
+/*
+ * Copyright (C) 2019  Ortega Froysa, Nicolás <nicolas@ortegas.org>
+ * Author: Ortega Froysa, Nicolás <nicolas@ortegas.org>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include <gmp.h>
+
+/*
+ * checks to see if the number `p` is prime, returning 1 if it is and 0
+ * if it is not.
+ */
+int test_prime(const mpz_t p);