Added new unit tests which lead to bug fix

tests/test_bigint.c

@@ -0,0 +1,63 @@
+/*
+ * Unit tests for BigInt data type
+*/
+
+#define TEST(NAME) do { \
+    printf("Running test_%s...", #NAME); \
+    test_##NAME(); \
+    printf(" PASSED\n"); \
+} while(0)
+
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <assert.h>
+
+#include "../src/bigint.h"
+
+static void bigint_eq(const bigint_t *number, const char *expected) {
+    bigint_result_t to_str = bigint_to_string(number);
+    assert(to_str.status == BIGINT_OK);
+    assert(!strcmp(to_str.value.string_num, expected));
+    
+    free(to_str.value.string_num);
+}
+
+// Test creating big integers from int
+void test_bigint_from_int() {
+    bigint_result_t res = bigint_from_int(0);
+    
+    assert(res.status == BIGINT_OK);
+    bigint_eq(res.value.number, "0");
+    bigint_destroy(res.value.number);
+
+    res = bigint_from_int(10);
+    assert(res.status == BIGINT_OK);
+    bigint_eq(res.value.number, "10");
+    bigint_destroy(res.value.number);
+
+    res = bigint_from_int(-12345678900LL);
+    assert(res.status == BIGINT_OK);
+    bigint_eq(res.value.number, "-12345678900");
+    bigint_destroy(res.value.number);
+}
+
+// Test creating big integers from string
+void test_bigint_from_string() {
+    bigint_result_t res = bigint_from_string("00000123");
+
+    assert(res.status == BIGINT_OK);
+    bigint_eq(res.value.number, "123");
+    bigint_destroy(res.value.number);
+}
+
+int main(void) {
+    printf("=== Running BigInt unit tests ===\n\n");
+
+    TEST(bigint_from_int);
+    TEST(bigint_from_string);
+
+    printf("\n=== All tests passed ===\n");
+
+    return 0;
+}