libconfig/src/config/public/config.h

/* ***********************************************************************
//
//   Copyright (C) 2025 -- The 4D-STAR Collaboration
//   File Author: Emily Boudreaux
//   Last Modified: February 20, 2025
//
//   4DSSE is free software; you can use it and/or modify
//   it under the terms and restrictions the GNU General Library Public
//   License version 3 (GPLv3) as published by the Free Software Foundation.
//
//   4DSSE is distributed in the hope that it will be useful,
//   but WITHOUT ANY WARRANTY; without even the implied warranty of
//   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
//   See the GNU Library General Public License for more details.
//
//   You should have received a copy of the GNU Library General Public License
//   along with this software; if not, write to the Free Software
//   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
//
// *********************************************************************** */
#ifndef CONFIG_H
#define CONFIG_H

#include <string>
#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>
#include <map>
#include <algorithm>
#include <stdexcept>

#include "yaml-cpp/yaml.h"

/**
 * @class Config
 * @brief Singleton class to manage configuration settings loaded from a YAML file.
 */
class Config {
private:
   /**
    * @brief Private constructor to prevent instantiation.
    */
   Config();

   /**
    * @brief Destructor.
    */
   ~Config();

   YAML::Node yamlRoot; ///< Root node of the YAML configuration.
   std::string configFilePath; ///< Path to the configuration file.
   bool debug = false; ///< Flag to enable debug output.
   bool loaded = false; ///< Flag to indicate if the configuration has been loaded.

   std::map<std::string, YAML::Node> configMap; ///< Cache for the location of configuration settings.
   std::vector<std::string> unknownKeys; ///< Cache for the existence of configuration settings.

   /**
    * @brief Get a value from the configuration cache.
    * @tparam T Type of the value to retrieve.
    * @param key Key of the configuration value.
    * @param defaultValue Default value to return if the key does not exist.
    * @return Configuration value of type T.
    */
   template <typename T>
   T getFromCache(const std::string &key, T defaultValue) {
      if (configMap.find(key) != configMap.end()) {
         try {
               return configMap[key].as<T>();
         } catch (const YAML::Exception& e) {
               return defaultValue;
         }
      }
      return defaultValue;
   }


   /**
    * @brief Check if a key exists in the configuration cache.
    * @param key Key to check.
    * @return True if the key exists in the cache, false otherwise.
    */
   bool isKeyInCache(const std::string &key);

   /**
    * @brief Add a key-value pair to the configuration cache.
    * @param key Key of the configuration value.
    * @param node YAML node containing the configuration value.
    */
   void addToCache(const std::string &key, const YAML::Node &node);

   /**
    * @brief Register a key as not found in the configuration.
    * @param key Key that was not found.
    */
   void registerUnknownKey(const std::string &key);

public:
   /**
    * @brief Get the singleton instance of the Config class.
    * @return Reference to the Config instance.
    */
   static Config& getInstance();

   Config (const Config&) = delete;
   Config& operator= (const Config&) = delete;
   Config (Config&&) = delete;
   Config& operator= (Config&&) = delete;

   void setDebug(bool debug) { this->debug = debug; }

   /**
    * @brief Load configuration from a YAML file.
    * @param configFilePath Path to the YAML configuration file.
    * @return True if the configuration was loaded successfully, false otherwise.
    */
   bool loadConfig(const std::string& configFilePath);

   /**
    * @brief Get the input table from the configuration.
    * @return Input table as a string.
    */
   std::string getInputTable() const;

   /**
    * @brief Get a configuration value by key.
    * @tparam T Type of the value to retrieve.
    * @param key Key of the configuration value.
    * @param defaultValue Default value to return if the key does not exist.
    * @return Configuration value of type T.
    * 
    * @example
    * @code
    * Config& config = Config::getInstance();
    * config.loadConfig("example.yaml");
    * int maxIter = config.get<int>("opac:lowTemp:numeric:maxIter", 10);
    */
   template <typename T>
   T get(const std::string &key, T defaultValue) {
      if (!loaded) {
         throw std::runtime_error("Configuration file not loaded! This should be done at the very start of whatever main function you are using (and only done once!)");
      }
      // --- Check if the key has already been checked for existence 
      if (std::find(unknownKeys.begin(), unknownKeys.end(), key) != unknownKeys.end()) {
         return defaultValue; // If the key has already been added to the unknown cache do not traverse the YAML tree or hit the cache
      }

      // --- Check if the key is already in the cache (avoid traversing YAML nodes)
      if (isKeyInCache(key)) {
         return getFromCache<T>(key, defaultValue);
      }
      // --- If the key is not in the cache, check the YAML file
      else {
         YAML::Node node = YAML::Clone(yamlRoot);
         std::istringstream keyStream(key);
         std::string subKey;
         while (std::getline(keyStream, subKey, ':')) {
            if (!node[subKey]) {
                  // Key does not exist
                  registerUnknownKey(key);
                  return defaultValue;
            }
            node = node[subKey]; // go deeper
         }

         try {
            // Key exists and is of the requested type
            addToCache(key, node);
            return node.as<T>();
         } catch (const YAML::Exception& e) {
            // Key is not of the requested type
            registerUnknownKey(key);
            return defaultValue; // return default value if the key does not exist
         }
      }
   }

   /**
    * @brief Print the configuration file path and the YAML root node.
    * @param os Output stream.
    * @param config Config object to print.
    * @return Output stream.
    */
   friend std::ostream& operator<<(std::ostream& os, const Config& config) {
      if (!config.debug) {
         os << "Config file: " << config.configFilePath << std::endl;
      } else{
         // Print entire YAML file from root
         os << "Config file: " << config.configFilePath << std::endl;
         os << config.yamlRoot << std::endl;
      }
      return os;
   }

   // Setup gTest class as a friend
   friend class configTestPrivateAccessor;
};

#endif