Examples

Several examples are included in the directory examples/, including using site dependent fields, compiling without a field, using the same spin type for multiple transformation types, and more.

The simplist, standalone example examples/ising_standalone.cpp of measuring the average cluster size of an Ising model is provided in full below, demonstrating how to define a model, measurement, couplings, and initialize the algorithm.

#include <iostream>
#include <chrono>

#include <wolff.hpp>

using namespace wolff;

class ising_t {
  public:
    int s;

    ising_t() : s(1) {};
    ising_t(int i) : s(i) {};

    ising_t act(const ising_t& x) const {
      return ising_t(s * x.s);
    }

    ising_t act_inverse(const ising_t& x) const {
      return this->act(x);
    }
};

typedef graph<> G_t;
typedef wolff::system<ising_t, ising_t> sys;

class measure_clusters : public measurement<ising_t, ising_t> {
  private:
    unsigned C;

  public:
    double Ctotal;

    measure_clusters() { Ctotal = 0; }

    void pre_cluster(unsigned, unsigned, const sys&, const G_t::vertex&, const ising_t&) override { C = 0; }

    void plain_site_transformed(const sys&, const G_t::vertex&, const ising_t&) override { C++; }

    void post_cluster(unsigned, unsigned, const sys&) override { Ctotal += C; }
};

int main(int argc, char *argv[]) {
  // set defaults
  unsigned N = (unsigned)1e3;
  unsigned D = 2;
  unsigned L = 128;
  double T = 2.26918531421;
  double H = 0.01;

  // define the spin-spin coupling
  std::function <double(const ising_t&, const ising_t&)> Z =
    [](const ising_t& s1, const ising_t& s2) -> double {
      return (double)(s1.s * s2.s);
    };

  // define the spin-field coupling
  std::function <double(const ising_t&)> B =
    [=](const ising_t& s) -> double {
      return H * s.s;
    };

  // initialize the lattice
  G_t G(D, L);

  // initialize the system
  sys S(G, T, Z, B);

  // define function that generates self-inverse rotations
  std::function <ising_t(std::mt19937&, const sys&, const G_t::vertex&)> gen_R =
    [] (std::mt19937&, const sys&, const G_t::vertex&) -> ising_t {
      return ising_t(-1);
    };

  // initailze the measurement object
  measure_clusters A;

  // initialize the random number generator
  auto seed = std::chrono::high_resolution_clock::now().time_since_epoch().count();
  std::mt19937 rng(seed);

  // run wolff N times
  S.run_wolff(N, gen_R, A, rng);

  // print results
  std::cout << "The average cluster size per site was " << (A.Ctotal / N) / S.nv << ".\n";

  // exit
  return 0;
}