remove require_dem_text helper function

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

Author: vedika-saravanan

libs/qec/include/cudaq/qec/decoder.h

@@ -13,12 +13,14 @@
 #include "cuda-qx/core/tensor.h"
 #include "sparse_binary_matrix.h"
 #include "cudaq/qec/detector_error_model.h"
+#include <algorithm>
 #include <functional>
 #include <future>
 #include <memory>
 #include <optional>
 #include <string>
 #include <string_view>
+#include <tuple>
 #include <variant>
 #include <vector>
 
@@ -31,8 +33,7 @@ using float_t = double;
 #endif
 
 /// Decoder construction input: either a parity-check matrix or raw Stim DEM
-/// text. PCM-based decoders can accept both; DEM-native decoders can require
-/// the string alternative via `require_dem_text`.
+/// text.
 using decoder_init = std::variant<sparse_binary_matrix, std::string>;
 
 /// @brief Validates that all keys in a heterogeneous map are found in a list of
@@ -516,6 +517,7 @@ get_decoder(const std::string &name, std::string_view stim_dem_text,
   return get_decoder(name, decoder_init{std::string{stim_dem_text}}, options);
 }
 
+namespace details {
 /// DEM-derived defaults; pointers alias into the source `dem`.
 struct dem_default_values {
   const cudaqx::tensor<uint8_t> *O = nullptr;
@@ -526,9 +528,7 @@ struct dem_default_values {
 dem_default_values dem_defaults_for_missing_keys(
     const std::function<bool(const std::string &)> &contains_user_key,
     const detector_error_model &dem);
-
-/// Extract raw Stim DEM text for DEM-native decoders.
-std::string_view require_dem_text(const decoder_init &init);
+} // namespace details
 
 /// Build a PCM-based decoder. If `init` holds DEM text, it is parsed into a PCM
 /// and DEM-derived `"O"` / `"error_rate_vec"` defaults are added when absent.
@@ -541,7 +541,7 @@ make_pcm_decoder(const decoder_init &init,
 
   const auto dem = dem_from_stim_text(std::get<std::string>(init));
   cudaqx::heterogeneous_map merged = params;
-  const auto defaults = dem_defaults_for_missing_keys(
+  const auto defaults = details::dem_defaults_for_missing_keys(
       [&](const std::string &key) { return merged.contains(key); }, dem);
   if (defaults.O)
     merged.insert("O", *defaults.O);

libs/qec/include/cudaq/qec/detector_error_model.h

@@ -8,8 +8,11 @@
 #pragma once
 
 #include "cuda-qx/core/tensor.h"
+#include <cstddef>
+#include <cstdint>
 #include <optional>
 #include <string>
+#include <vector>
 
 namespace cudaq::qec {
 

libs/qec/lib/CMakeLists.txt

@@ -41,7 +41,6 @@ endif()
 set(QEC_SOURCES
   code.cpp
   decoder.cpp
-  decoder_stim_dem.cpp
   detector_error_model.cpp
   experiments.cpp
   pcm_utils.cpp

libs/qec/lib/decoder.cpp

@@ -141,6 +141,21 @@ decoder::get(const std::string &name, const decoder_init &init,
   return iter->second(init, param_map);
 }
 
+namespace details {
+
+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") && dem.num_observables() > 0)
+    out.O = &dem.observables_flips_matrix;
+  if (!contains_user_key("error_rate_vec"))
+    out.error_rate_vec = &dem.error_rates;
+  return out;
+}
+
+} // namespace details
+
 static uint32_t calculate_num_msyn_per_decode(
     const std::vector<std::vector<uint32_t>> &D_sparse) {
   uint32_t max_col = 0;

libs/qec/lib/decoder_stim_dem.cpp

@@ -12,6 +12,12 @@
 
 #include "stim.h"
 
+#include <exception>
+#include <stdexcept>
+#include <string>
+#include <utility>
+#include <vector>
+
 namespace cudaq::qec {
 
 detector_error_model dem_from_stim_text(const std::string &dem_text) {

libs/qec/lib/detector_error_model.cpp

@@ -788,7 +788,7 @@ void bindDecoder(nb::module_ &mod) {
     if (PyDecoderRegistry::contains(name)) {
       auto dem = dem_from_stim_text(dem_text);
 
-      auto defaults = dem_defaults_for_missing_keys(
+      auto defaults = details::dem_defaults_for_missing_keys(
           [&](const std::string &key) { return options.contains(key); }, dem);
       if (defaults.O)
         options["O"] = copyToPyArray(*defaults.O);
@@ -849,10 +849,11 @@ void bindDecoder(nb::module_ &mod) {
           raw DEM text via ``decoder_init``; Python-registered decoders receive
           the DEM-derived PCM plus ``O`` and ``error_rate_vec`` defaults.
 
-        For Python-registered decoders (``cudaq.qec.decoder`` decorator), ``H``
-        is passed through to ``__init__`` unchanged (NumPy array or sparse dict).
-        Call ``Decoder.__init__(self, H)`` so nanobind can store the PCM in CSC
-        form when ``H`` is a dict without building a dense ``rows x cols``
+        For Python-registered decoders (``cudaq.qec.decoder`` decorator),
+        NumPy array and sparse dict inputs are passed through to ``__init__``
+        unchanged. DEM string inputs are parsed first as described above. Call
+        ``Decoder.__init__(self, H)`` so nanobind can store the PCM in CSC form
+        when ``H`` is a dict without building a dense ``rows x cols``
         allocation.
       )pbdoc");
 

libs/qec/python/bindings/py_decoder.cpp

@@ -820,7 +820,7 @@ error(0.1) L0 L0
 
 TEST(StimDemGetDecoder, DemWithoutObservablesDoesNotAddODefault) {
   auto dem = cudaq::qec::dem_from_stim_text("error(0.1) D0\n");
-  auto defaults = cudaq::qec::dem_defaults_for_missing_keys(
+  auto defaults = cudaq::qec::details::dem_defaults_for_missing_keys(
       [](const std::string &) { return false; }, dem);
 
   EXPECT_EQ(defaults.O, nullptr);