Add isUsgsCsmIsd() quick peek to avoid expensive model construction

isUsgsCsmIsd() checks if a string is a USGS JSON ISD using raw string
searches (no JSON parsing). Checks for '{' start, ISD-only keys
(body_rotation, instrument_position), and extracts the model name.

Used in usgscsm_cam_test to skip the plugin iteration loop for ISDs.
Instead of trying all 5 model types (each building and discarding a
full model), the ISD path now peeks for the model name and constructs
only the matching model.

Author: oleg-alexandrov

bin/usgscsm_cam_test.cc

@@ -168,19 +168,31 @@ bool loadCsmCameraModel(std::string const& model_file,
     return true;
   }
 
-  // Try to read the model as an ISD
-  csm::Isd isd(model_file);
-
-  // Read the model in a string, for potentially finding parsing the
-  // model state from it.
+  // Read the file into a string
   std::string model_state;
   if (!readFileInString(model_file, model_state))
     return false;
 
-  // Check if loading the model worked
-  bool success = false;
+  // Quick peek: if this is a JSON ISD, we know the model name without
+  // expensive trial-and-error. Construct only the matching model.
+  std::string isd_model_name;
+  if (isUsgsCsmIsd(model_state, isd_model_name)) {
+    std::cout << "Detected JSON ISD with model: " << isd_model_name << "\n";
+    csm::Isd isd(model_file);
+    UsgsAstroPlugin cameraPlugin;
+    csm::Model *csm = cameraPlugin.constructModelFromISD(isd, isd_model_name, NULL);
+    if (!csm) {
+      std::cerr << "Failed to construct model from ISD: " << model_file << ".\n";
+      return false;
+    }
+    model = std::shared_ptr<csm::RasterGM>(dynamic_cast<csm::RasterGM*>(csm));
+    std::cout << "Loaded a CSM model of type " << isd_model_name
+              << " from ISD file " << model_file << ".\n";
+    return true;
+  }
 
-  // Try all detected plugins and all models for each plugin.
+  // Not an ISD. Try model state path (JSON state, .sup, etc.) via plugin iteration.
+  bool success = false;
   csm::PluginList plugins = csm::Plugin::getList();
   for (auto iter = plugins.begin(); iter != plugins.end(); iter++) {
 
@@ -196,14 +208,7 @@ bool loadCsmCameraModel(std::string const& model_file,
 
       std::string model_name = (*iter)->getModelName(i);
 
-      if (csm_plugin->canModelBeConstructedFromISD(isd, model_name, warnings)) {
-        // Try to construct the model from the ISD
-        csm = csm_plugin->constructModelFromISD(isd, model_name, warnings);
-        std::cout << "Loaded a CSM model of type " << model_name << " from ISD file "
-                  << model_file << ".\n";
-        success = true;
-      } else if (csm_plugin->canModelBeConstructedFromState(model_name, model_state, warnings)) {
-        // Try to construct it from the model state (handles .sup files)
+      if (csm_plugin->canModelBeConstructedFromState(model_name, model_state, warnings)) {
         csm = csm_plugin->constructModelFromState(model_state, warnings);
         std::cout << "Loaded a CSM model of type " << model_name
                   << " from model state file " << model_file << ".\n";

include/usgscsm/UsgsAstroPluginSupport.h

@@ -9,5 +9,6 @@ csm::RasterGM *getUsgsCsmModelFromIsd(const std::string &stringIsd, const std::s
 csm::RasterGM *getUsgsCsmModelFromState(const std::string &stringState, const std::string &modelName, csm::WarningList *warnings);
 csm::RasterGM *getUsgsCsmModelFromJson(const nlohmann::json &j, const std::string &modelName, csm::WarningList *warnings);
 nlohmann::json getUsgsCsmModelJson(csm::RasterGM *model);
+bool isUsgsCsmIsd(const std::string &str, std::string &modelName);
 
 #endif // INCLUDE_USGSCSM_USGSASTROPLUGINSUPPORT_H_

src/UsgsAstroPluginSupport.cpp

@@ -147,3 +147,34 @@ nlohmann::json getUsgsCsmModelJson(csm::RasterGM *model) {
   std::string aFunction = "UsgsAstroPluginSupport::getUsgsCsmModelJson()";
   throw csm::Error(aErrorType, aMessage, aFunction);
 }
+
+// Quick check if the given string is a USGS CSM JSON ISD (not a model state,
+// .sup, etc.). If yes, extract the model name. No JSON parsing is done, just
+// raw string searches.
+bool isUsgsCsmIsd(const std::string &str, std::string &modelName) {
+  modelName.clear();
+
+  // Model state strings have a text prefix before '{'. ISDs start with '{'.
+  auto pos = str.find_first_not_of(" \t\n\r");
+  if (pos == std::string::npos || str[pos] != '{')
+    return false;
+
+  // Check for ISD-only keys (never present in model state)
+  if (str.find("\"body_rotation\"") == std::string::npos)
+    return false;
+  if (str.find("\"instrument_position\"") == std::string::npos)
+    return false;
+
+  // Extract the name_model value. Search for the model name prefix
+  // which immediately follows the "name_model" key's value quote.
+  std::string prefix = "\"USGS_ASTRO_";
+  pos = str.find(prefix);
+  if (pos == std::string::npos)
+    return false;
+  auto end = str.find("\"", pos + 1);
+  if (end == std::string::npos)
+    return false;
+
+  modelName = str.substr(pos + 1, end - pos - 1);
+  return !modelName.empty();
+}