Switch to using a filesystem class for better testing

handler.hpp

@@ -1,15 +1,54 @@
 #pragma once
 
-#include <string>
 #include <exception>
-
+#include <filesystem>
+#include <fstream>
 #include <stdexcept>
+#include <string>
 
-class InvalidInput : public std::runtime_error
-{
+class IFilesystemWrapper {
 public:
-    InvalidInput(const std::string& what = "") : std::runtime_error(what) {}
+  virtual ~IFilesystemWrapper() = default;
+  virtual bool exists(const std::filesystem::path &path) const = 0;
+  virtual std::string read_file(const std::filesystem::path &path) const = 0;
 };
 
+class RealFilesystemWrapper : public IFilesystemWrapper {
+public:
+  bool exists(const std::filesystem::path &path) const override {
+    return std::filesystem::exists(path);
+  }
 
-std::string process(const std::string &username);
+  std::string read_file(const std::filesystem::path &path) const override {
+    std::ifstream file(path);
+    if (!file.is_open()) {
+      return "";
+    }
+
+    std::string content;
+    std::string line;
+    while (std::getline(file, line)) {
+      content += line + "\n";
+    }
+
+    // Return the content with proper line endings
+    if (!content.empty() && content.back() == '\n') {
+      content.pop_back(); // Remove the last newline
+      content += "\r\n";
+      return content;
+    } else if (!content.empty()) {
+      content += "\r\n";
+      return content;
+    }
+
+    return "";
+  }
+};
+
+class InvalidInput : public std::runtime_error {
+public:
+  InvalidInput(const std::string &what = "") : std::runtime_error(what) {}
+};
+
+std::string process(const std::string &username);
+std::string process(const std::string &username, const IFilesystemWrapper &fs);