fetches Stim directly with FetchContent when libstim is not already available

Signed-off-by: vedika-saravanan <vsaravanan@nvidia.com>

Author: vedika-saravanan

libs/qec/lib/CMakeLists.txt

@@ -8,6 +8,8 @@
 
 set(LIBRARY_NAME cudaq-qec)
 
+include(FetchContent)
+
 add_compile_options(-Wno-attributes) 
 
 find_package(CUDAToolkit REQUIRED)
@@ -59,18 +61,28 @@ list(APPEND QEC_SOURCES
 # FIXME?: This must be a shared library. Trying to build a static one will fail.
 add_library(${LIBRARY_NAME} SHARED ${QEC_SOURCES})
 
-add_subdirectory(decoders/plugins/example)
-add_subdirectory(decoders/plugins/pymatching)
+if(NOT TARGET libstim)
+  FetchContent_Declare(
+    stim
+    GIT_REPOSITORY https://github.com/quantumlib/Stim.git
+    GIT_TAG v1.15.0
+    EXCLUDE_FROM_ALL
+  )
+  FetchContent_MakeAvailable(stim)
+endif()
 
 if(NOT TARGET libstim)
   message(FATAL_ERROR
-    "libstim target not available; required by cudaq-qec for Stim DEM parsing.")
+    "Stim FetchContent did not provide the libstim target.")
 endif()
 target_link_libraries(${LIBRARY_NAME} PRIVATE libstim)
 target_link_options(${LIBRARY_NAME} PRIVATE
   $<$<OR:$<CXX_COMPILER_ID:GNU>,$<CXX_COMPILER_ID:Clang>>:-Wl,--exclude-libs,libstim.a>
 )
 
+add_subdirectory(decoders/plugins/example)
+add_subdirectory(decoders/plugins/pymatching)
+
 # The TRT decoder plugin honors the tri-state `CUDAQ_QEC_BUILD_TRT_DECODER`
 # cache variable (AUTO/ON/OFF) declared in the parent CMakeLists.txt.  Skip
 # descending entirely when the user explicitly opted out; otherwise let the

libs/qec/lib/decoder_stim_dem.cpp

@@ -17,7 +17,7 @@ dem_default_values dem_defaults_for_missing_keys(
     const std::function<bool(const std::string &)> &contains_user_key,
     const detector_error_model &dem) {
   dem_default_values out;
-  if (!contains_user_key("O"))
+  if (!contains_user_key("O") && dem.num_observables() > 0)
     out.O = &dem.observables_flips_matrix;
   if (!contains_user_key("error_rate_vec"))
     out.error_rate_vec = &dem.error_rates;

libs/qec/python/tests/test_decoder.py

@@ -836,5 +836,13 @@ def test_get_decoder_from_stim_dem_user_O_wins_over_dem_derived():
         qec.get_decoder_from_stim_dem("pymatching", dem_text, O=bad_O)
 
 
+def test_get_decoder_from_stim_dem_without_observables_returns_errors():
+    decoder = qec.get_decoder("pymatching", "error(0.1) D0\n")
+
+    result = decoder.decode([1.0])
+    assert result.converged is True
+    assert list(result.result) == [1.0]
+
+
 if __name__ == "__main__":
     pytest.main()

libs/qec/unittests/test_decoders.cpp

@@ -821,6 +821,16 @@ error(0.1) L0 L0
       << "duplicate L0 in error 1 should XOR-cancel to 0";
 }
 
+TEST(StimDemDecoderFactory, DemWithoutObservablesDoesNotAddODefault) {
+  auto dem = cudaq::qec::dem_from_stim_text("error(0.1) D0\n");
+  auto defaults = cudaq::qec::dem_defaults_for_missing_keys(
+      [](const std::string &) { return false; }, dem);
+
+  EXPECT_EQ(defaults.O, nullptr);
+  ASSERT_NE(defaults.error_rate_vec, nullptr);
+  EXPECT_EQ(defaults.error_rate_vec->size(), 1u);
+}
+
 TEST(StimDemDecoderFactory, ThrowsOnProbabilityOutOfRange) {
   const std::string dem_text = "error(1.5) D0\n";
   EXPECT_THROW(