[Windows] Avoid using dirent.h

This PR start series of changes which should make possible compilation of nntrainer on windows platform.
Header dirent.h is not available on windows so to make code more portable I've replaced this functionality with std::filesystem

Self-evaluation:

Build test: [X]Passed [ ]Failed [ ]Skipped
Run test: [X]Passed [ ]Failed [ ]Skipped

Signed-off-by: Grzegorz Kisala <[email protected]>

nntrainer/app_context.cpp

@@ -11,8 +11,8 @@
  * @bug	   No known bugs except for NYI items
  *
  */
-#include <dirent.h>
 #include <dlfcn.h>
+#include <filesystem>
 #include <iostream>
 #include <sstream>
 #include <string>
@@ -418,14 +418,11 @@ AppContext &AppContext::Global() {
 }
 
 void AppContext::setWorkingDirectory(const std::string &base) {
-  DIR *dir = opendir(base.c_str());
-
-  if (!dir) {
+  if (!std::filesystem::is_directory(base)) {
     std::stringstream ss;
     ss << func_tag << "path is not directory or has no permission: " << base;
     throw std::invalid_argument(ss.str().c_str());
   }
-  closedir(dir);
 
   char *ret = getRealpath(base.c_str(), nullptr);
 
@@ -571,40 +568,35 @@ int AppContext::registerOptimizer(const std::string &library_path,
 
 std::vector<int>
 AppContext::registerPluggableFromDirectory(const std::string &base_path) {
-  DIR *dir = opendir(base_path.c_str());
+  const auto directory_exist = std::filesystem::is_directory(base_path);
 
-  NNTR_THROW_IF(dir == nullptr, std::invalid_argument)
+  NNTR_THROW_IF(!directory_exist, std::invalid_argument)
     << func_tag << "failed to open the directory: " << base_path;
 
-  struct dirent *entry;
+  std::vector<int> keys = {};
 
-  std::vector<int> keys;
+  for (const auto &entry : std::filesystem::directory_iterator(base_path)) {
+    const auto &entry_name = entry.path().string();
 
-  while ((entry = readdir(dir)) != NULL) {
-    if (endswith(entry->d_name, solib_suffix)) {
-      if (endswith(entry->d_name, layerlib_suffix)) {
+    if (endswith(entry_name, solib_suffix)) {
+      if (endswith(entry_name, layerlib_suffix)) {
         try {
-          int key = registerLayer(entry->d_name, base_path);
+          int key = registerLayer(entry_name, base_path);
           keys.emplace_back(key);
         } catch (std::exception &e) {
-          closedir(dir);
           throw;
         }
-      } else if (endswith(entry->d_name, optimizerlib_suffix)) {
+      } else if (endswith(entry_name, optimizerlib_suffix)) {
         try {
-          int key = registerOptimizer(entry->d_name, base_path);
+          int key = registerOptimizer(entry_name, base_path);
           keys.emplace_back(key);
         } catch (std::exception &e) {
-          closedir(dir);
           throw;
         }
       }
     }
   }
 
-  if (dir != NULL)
-    closedir(dir);
-
   return keys;
 }