Add regression test for flat input_layers/output_layers format

Newer Keras versions serialize a single input/output as a flat triple
["layer", node_idx, tensor_idx] instead of a list containing one such
triple. The existing readme_example_main test exercises a single-input
model, but only catches this when CI runs against a Keras version that
already emits the flat form. Post-process the readme example JSON into
flat form and load it from a new test so this code path is covered on
any Keras version.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Dobiasd

test/CMakeLists.txt

@@ -58,6 +58,11 @@ add_custom_command ( OUTPUT readme_example_model.json
                      COMMAND bash -c "${Python3_EXECUTABLE} ${FDEEP_TOP_DIR}/keras_export/convert_model.py readme_example_model.keras readme_example_model.json"
                      WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/)
 
+add_custom_command ( OUTPUT readme_example_flat_format_model.json
+                     DEPENDS readme_example_model.json
+                     COMMAND bash -c "${Python3_EXECUTABLE} ${FDEEP_TOP_DIR}/test/readme_example_flat_format_generate.py readme_example_model.json readme_example_flat_format_model.json"
+                     WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/)
+
 hunter_add_package(doctest)
 find_package(doctest CONFIG REQUIRED)
 
@@ -75,6 +80,7 @@ _add_test(test_model_variable_test test_model_variable.json)
 _add_test(test_model_autoencoder_test test_model_autoencoder.json)
 _add_test(test_model_sequential_test test_model_sequential.json)
 _add_test(readme_example_main readme_example_model.json)
+_add_test(readme_example_flat_format_test readme_example_flat_format_model.json)
 
 add_custom_target(unittest
   COMMAND test_model_exhaustive_test
@@ -83,6 +89,7 @@ add_custom_target(unittest
   COMMAND test_model_autoencoder_test
   COMMAND test_model_sequential_test
   COMMAND readme_example_main
+  COMMAND readme_example_flat_format_test
 
   COMMENT "Running unittests\n\n"
   VERBATIM

test/readme_example_flat_format_generate.py

@@ -0,0 +1,32 @@
+#!/usr/bin/env python3
+"""Force the single-input fdeep model JSON into Keras' newer flat
+input_layers/output_layers format so we can regression-test loading it
+on any Keras version. See https://github.com/Dobiasd/frugally-deep/issues/461.
+"""
+
+import json
+import sys
+
+
+def flatten(node_connections):
+    if (
+        isinstance(node_connections, list)
+        and len(node_connections) == 1
+        and isinstance(node_connections[0], list)
+    ):
+        return node_connections[0]
+    return node_connections
+
+
+def main(src, dst):
+    with open(src, 'r') as f:
+        data = json.load(f)
+    config = data['architecture']['config']
+    config['input_layers'] = flatten(config['input_layers'])
+    config['output_layers'] = flatten(config['output_layers'])
+    with open(dst, 'w') as f:
+        json.dump(data, f)
+
+
+if __name__ == '__main__':
+    main(sys.argv[1], sys.argv[2])

test/readme_example_flat_format_test.cpp

@@ -0,0 +1,23 @@
+// Copyright 2016, Tobias Hermann.
+// https://github.com/Dobiasd/frugally-deep
+// Distributed under the MIT License.
+// (See accompanying LICENSE file or at
+//  https://opensource.org/licenses/MIT)
+
+#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
+#include "doctest/doctest.h"
+#include <fdeep/fdeep.hpp>
+
+// Regression test for https://github.com/Dobiasd/frugally-deep/issues/461.
+// Newer Keras versions serialize a single input/output as a flat triple
+// ["layer", node_idx, tensor_idx] instead of a list containing one such
+// triple. The test JSON is post-processed into that flat form so this
+// path is exercised regardless of the Keras version used by CI.
+TEST_CASE("readme_example_flat_format_test, main")
+{
+    const auto model = fdeep::load_model("../readme_example_flat_format_model.json");
+    const auto result = model.predict(
+        { fdeep::tensor(fdeep::tensor_shape(static_cast<std::size_t>(4)),
+            fdeep::float_vec { 1, 2, 3, 4 }) });
+    std::cout << fdeep::show_tensors(result) << std::endl;
+}