refactor(mcmc): resolve sampler_type once via resolve_sampler_spec

src/mcmc/execution/chain_runner.cpp

@@ -6,16 +6,26 @@
 #include "mcmc/samplers/metropolis_sampler.h"
 
 
-std::unique_ptr<SamplerBase> create_sampler(const SamplerConfig& config, WarmupSchedule& schedule) {
-    if (config.sampler_type == "nuts") {
-        return std::make_unique<NUTSSampler>(config, schedule);
-    } else if (config.sampler_type == "adaptive-metropolis") {
-        return std::make_unique<MetropolisSampler>(config, schedule);
+SamplerSpec resolve_sampler_spec(const std::string& sampler_type) {
+    if (sampler_type == "nuts") {
+        return SamplerSpec{SamplerKind::NUTS, /*learn_sd=*/true, /*nuts_diag=*/true, /*am_diag=*/false};
+    } else if (sampler_type == "adaptive-metropolis") {
+        return SamplerSpec{SamplerKind::AdaptiveMetropolis, /*learn_sd=*/false, /*nuts_diag=*/false, /*am_diag=*/true};
     } else {
-        Rcpp::stop("Unknown sampler_type: '%s'", config.sampler_type.c_str());
+        Rcpp::stop("Unknown sampler_type: '%s'", sampler_type.c_str());
     }
 }
 
+std::unique_ptr<SamplerBase> create_sampler(SamplerKind kind, const SamplerConfig& config, WarmupSchedule& schedule) {
+    switch (kind) {
+        case SamplerKind::NUTS:
+            return std::make_unique<NUTSSampler>(config, schedule);
+        case SamplerKind::AdaptiveMetropolis:
+            return std::make_unique<MetropolisSampler>(config, schedule);
+    }
+    Rcpp::stop("Unhandled SamplerKind");  // unreachable: kind comes from resolve_sampler_spec
+}
+
 
 void run_mcmc_chain(
     ChainResult& chain_result,
@@ -28,10 +38,10 @@ void run_mcmc_chain(
     chain_result.chain_id = chain_id + 1;
 
     // Construct warmup schedule (shared by runner and sampler)
-    const bool learn_sd = (config.sampler_type == "nuts");
-    WarmupSchedule schedule(config.no_warmup, config.edge_selection, learn_sd);
+    const SamplerSpec spec = resolve_sampler_spec(config.sampler_type);
+    WarmupSchedule schedule(config.no_warmup, config.edge_selection, spec.learn_sd);
 
-    auto sampler = create_sampler(config, schedule);
+    auto sampler = create_sampler(spec.kind, config, schedule);
 
     // Initialize sampler (step-size heuristic) before the main loop
     sampler->initialize(model);
@@ -138,8 +148,9 @@ std::vector<ChainResult> run_mcmc_sampler(
     const int no_threads,
     ProgressManager& pm
 ) {
-    const bool has_nuts_diag = (config.sampler_type == "nuts");
-    const bool has_am_diag = (config.sampler_type == "adaptive-metropolis");
+    const SamplerSpec spec = resolve_sampler_spec(config.sampler_type);
+    const bool has_nuts_diag = spec.nuts_diag;
+    const bool has_am_diag = spec.am_diag;
     const bool has_sbm_alloc = edge_prior.has_allocations() ||
         (config.edge_selection && dynamic_cast<StochasticBlockEdgePrior*>(&edge_prior) != nullptr);
 

src/mcmc/execution/chain_runner.h

@@ -14,14 +14,38 @@
 #include "mcmc/execution/warmup_schedule.h"
 
 
+/** Which concrete sampler a run uses. */
+enum class SamplerKind { NUTS, AdaptiveMetropolis };
+
+/**
+ * Behavioral descriptor for a sampler type. resolve_sampler_spec is the single
+ * place that decodes the config.sampler_type string; all downstream code reads
+ * these fields instead of re-comparing the string.
+ */
+struct SamplerSpec {
+    SamplerKind kind;
+    bool learn_sd;    ///< NUTS dual-averaging step-size adaptation during warmup
+    bool nuts_diag;   ///< reserve/store NUTS per-iteration diagnostics
+    bool am_diag;     ///< reserve/store adaptive-Metropolis diagnostics
+};
+
+/**
+ * Decode a sampler-type string into a SamplerSpec.
+ *
+ * @param sampler_type  "nuts" or "adaptive-metropolis".
+ * @return Descriptor with the concrete kind and its derived behavior flags.
+ */
+SamplerSpec resolve_sampler_spec(const std::string& sampler_type);
+
 /**
- * Create a sampler matching config.sampler_type
+ * Create a sampler of the given kind.
  *
- * @param config   Sampler configuration (type, step size, tree depth, etc.)
+ * @param kind     Concrete sampler kind (from resolve_sampler_spec).
+ * @param config   Sampler configuration (step size, tree depth, etc.)
  * @param schedule Shared warmup schedule for adaptation staging
  * @return Owning pointer to a concrete SamplerBase subclass
  */
-std::unique_ptr<SamplerBase> create_sampler(const SamplerConfig& config, WarmupSchedule& schedule);
+std::unique_ptr<SamplerBase> create_sampler(SamplerKind kind, const SamplerConfig& config, WarmupSchedule& schedule);
 
 /**
  * Run a single MCMC chain (warmup + sampling)