introduced and used bigint datatype

test/test_big_int.cpp

@@ -0,0 +1,37 @@
+#include "big_int.hpp"
+#include <catch2/catch_test_macros.hpp>
+
+TEST_CASE("BigInt::from_string", "[big_int]") {
+  std::string VALID [] = {
+    "1",
+    "12",
+    "123",
+    "1234",
+    "12345",
+    "123456",
+    "120456",
+    "123056",
+    "120345",
+    "1234560abc",
+    "aabbcc",
+    "abcdef1"
+  };
+
+  for (size_t i= 0; i<sizeof(VALID)/sizeof(VALID[0]);i++) {
+    auto b = BigInt::from_string(VALID[i]);
+    REQUIRE(b->to_string() == VALID[i]);
+    REQUIRE(b.has_value());
+  }
+  
+  auto invalid_hex = "aabbcc..";
+  {
+    auto b = BigInt::from_string(invalid_hex);
+    REQUIRE(!b);
+  }
+  
+}
+
+TEST_CASE("BigInt::from_int", "[big_int]") {
+  auto b = BigInt::from_int(0xaabbccdd);
+  REQUIRE(b.to_string()=="aabbccdd");
+}