Automatically detect dataset/template mode

examples/14_toml_template.cpp

@@ -61,16 +61,22 @@ void write()
 
 void read()
 {
-    std::string config = R"(
-{
-  "iteration_encoding": "variable_based",
-  "toml": {
-    "mode": "template"
-  }
-}
-)";
+    /*
+     * The config is entirely optional, these things are also detected
+     * automatically when reading
+     */
+
+    //     std::string config = R"(
+    // {
+    //   "iteration_encoding": "variable_based",
+    //   "toml": {
+    //     "mode": "template"
+    //   }
+    // }
+    // )";
+
     openPMD::Series read(
-        "../samples/tomlTemplate.toml", openPMD::Access::READ_ONLY, config);
+        "../samples/tomlTemplate.toml", openPMD::Access::READ_ONLY);
 
     std::string jsonConfig = R"(
 {

include/openPMD/IO/JSON/JSONIOHandlerImpl.hpp

@@ -242,6 +242,7 @@ class JSONIOHandlerImpl : public AbstractIOHandlerImpl
     };
 
     IOMode m_mode = IOMode::Dataset;
+    bool m_modeWasManuallySpecified = false;
 
     /*
      * Is set by constructor.

src/IO/JSON/JSONIOHandlerImpl.cpp

@@ -58,6 +58,13 @@ namespace openPMD
             throw std::runtime_error((TEXT));                                  \
     }
 
+namespace JSONDefaults
+{
+    using const_str = char const *const;
+    constexpr const_str openpmd_internal = "__openPMD_internal";
+    constexpr const_str IOMode = "IO_mode";
+} // namespace JSONDefaults
+
 namespace
 {
     struct DefaultValue
@@ -166,6 +173,7 @@ JSONIOHandlerImpl::JSONIOHandlerImpl(
                     "Invalid value: '" + mode +
                         "' (accepted values are 'dataset' and 'template'.");
             }
+            m_modeWasManuallySpecified = true;
         }
         auto shadow = jsonConfig.invertShadow();
         if (shadow.size() > 0)
@@ -1580,6 +1588,55 @@ std::shared_ptr<nlohmann::json> JSONIOHandlerImpl::obtainJsonContents(File file)
         break;
     }
     VERIFY(fh->good(), "[JSON] Failed reading from a file.");
+    if (res->contains(JSONDefaults::openpmd_internal))
+    {
+        auto const &openpmd_internal = res->at(JSONDefaults::openpmd_internal);
+        if (openpmd_internal.contains(JSONDefaults::IOMode))
+        {
+            auto modeOption = openPMD::json::asLowerCaseStringDynamic(
+                openpmd_internal.at(JSONDefaults::IOMode));
+            if (!modeOption.has_value())
+            {
+                std::cerr
+                    << "[JSON/TOML backend] Warning: Invalid value of "
+                       "non-string type at internal meta table for entry '"
+                    << JSONDefaults::IOMode << "'. Will ignore and continue."
+                    << std::endl;
+            }
+            else if (modeOption.value() == "dataset")
+            {
+                if (m_modeWasManuallySpecified && m_mode == IOMode::Template)
+                {
+                    std::cerr
+                        << "[JSON/TOML backend] Warning: IO Mode was manually "
+                           "specified as 'Template', but opened file is in "
+                           "'Dataset' mode. Will continue with dataset mode."
+                        << std::endl;
+                }
+                m_mode = IOMode::Dataset;
+            }
+            else if (modeOption.value() == "template")
+            {
+                if (m_modeWasManuallySpecified && m_mode == IOMode::Dataset)
+                {
+                    std::cerr
+                        << "[JSON/TOML backend] Warning: IO Mode was manually "
+                           "specified as 'Dataset', but opened file is in "
+                           "'Template' mode. Will continue with template mode."
+                        << std::endl;
+                }
+                m_mode = IOMode::Template;
+            }
+            else
+            {
+                std::cerr << "[JSON/TOML backend] Warning: Invalid value '"
+                          << modeOption.value()
+                          << "' at internal meta table for entry '"
+                          << JSONDefaults::IOMode
+                          << "'. Will ignore and continue." << std::endl;
+            }
+        }
+    }
     m_jsonVals.emplace(file, res);
     return res;
 }
@@ -1608,8 +1665,12 @@ void JSONIOHandlerImpl::putJsonContents(
         {
         case IOMode::Dataset:
             (*it->second)["platform_byte_widths"] = platformSpecifics();
+            (*it->second)[JSONDefaults::openpmd_internal]
+                         [JSONDefaults::IOMode] = "dataset";
             break;
         case IOMode::Template:
+            (*it->second)[JSONDefaults::openpmd_internal]
+                         [JSONDefaults::IOMode] = "template";
             break;
         }