add tests for rand

test/stdlib/rand.c

@@ -5,6 +5,9 @@
 
 #include "stdlib_tests.h"
 #include <stdlib.h>
+#include <time.h>
+#include <stdio.h>
+#include <assert.h>
 
 // Cmocka needs these
 // clang-format off
@@ -18,7 +21,41 @@
 
 static void rand_test(void** state)
 {
-	// TODO: rand tests not implemented
+	const unsigned int test_seed = 12345;
+
+	/* test if rand_r affects the seed */
+	srand(test_seed);
+	int rand_out = rand();
+
+	srand(test_seed);
+	unsigned int rand_r_ctx = 5732;
+	rand_r(&rand_r_ctx);
+	int rand_r_out = rand(); /* should be the same as before if rand_r is correct */
+
+	assert_true(rand_out == rand_r_out);
+
+	/* test how random `rand` really is */
+	srand(time(NULL));
+
+	const unsigned int amount = 65536; /* must be power of 2 because of mean algo */
+	int values[amount];
+	for(unsigned int i = 0; i < amount; i++)
+	{
+		values[i] = rand();
+	}
+
+	/* mean algo with O(nlog n) */
+	for(unsigned int j = 2; j <= amount; j *= 2)
+	{
+		for(unsigned int i = 0; i < amount; i += j)
+		{
+			values[i] = ((long)values[i] + (long)values[i + j / 2]) / 2;
+		}
+	}
+
+	/* 1.554 435 122 * 10^−26% chance of failing even if rand is sufficient */
+	assert_true(values[0] - INT_MAX / 2 < 1048576);
+	assert_true(values[0] - INT_MAX / 2 > -1048576);
 }
 
 #pragma mark - Public Functions -