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#ifndef IAEASourceIdRegistry_hh
#define IAEASourceIdRegistry_hh 1

#include <bitset>
#include "G4AutoLock.hh"
#include "globals.hh"

constexpr G4int kIAEA_MaxSources = 30;  // MAX_NUM_SOURCES

class IAEASourceIdRegistry {

public:

  static IAEASourceIdRegistry& Instance() {
    static IAEASourceIdRegistry inst;
    return inst;
  }

  // Reserve the next free ID (lowest or highest; your choice)
  G4int ReserveNextLowest() {
    G4AutoLock lock(&fMutex);
    for (G4int ii = 0; ii < kIAEA_MaxSources; ii++) {
      if (!fUsed.test(ii)) {
	fUsed.set(ii);
	return ii;
      }
    }
    return -1; // none free
  }

  // Reserve a specific ID (returns true if we could mark it)
  bool ReserveExact(G4int id) {
    if (id < 0 || id >= kIAEA_MaxSources)  return false;
    G4AutoLock lock(&fMutex);
    if (fUsed.test(id))  return false;
    fUsed.set(id);
    return true;
  }

  void Release(G4int id) {
    if (id < 0 || id >= kIAEA_MaxSources) return;
    G4AutoLock lock(&fMutex);
    fUsed.reset(id);
  }


private:

  IAEASourceIdRegistry() = default;
  IAEASourceIdRegistry(const IAEASourceIdRegistry&) = delete;
  IAEASourceIdRegistry& operator=(const IAEASourceIdRegistry&) = delete;

  std::bitset<kIAEA_MaxSources> fUsed;
  G4Mutex fMutex = G4MUTEX_INITIALIZER;
};

#endif