write first version of user system

test/hash_collision_test.cpp

@@ -0,0 +1,17 @@
+#include <iostream>
+#include <string>
+#include <unordered_map>
+#include "../src/include/utils.h"
+std::unordered_map<hash_t, std::string> storage;
+int main() {
+  std::string token;
+  while (std::cin >> token) {
+    hash_t hsh = SplitMix64Hash(token);
+    if (storage.find(hsh) == storage.end()) {
+      storage[hsh] = token;
+    } else if (storage[hsh] != token) {
+      std::cerr << "Collision detected: " << storage[hsh] << " " << token << std::endl;
+    }
+  }
+  return 0;
+}