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.. default-domain:: cpp

************
Measurements
************

One of the most complicated parts of using this library is setting up measurements to take during a run, but once understood they provide a powerful way to quickly measure arbitrary properties. The class is defined in the header :file:`wolff/measurement.hpp`.

.. class:: template<R_t, X_t> measurement

   This class defines several virtual member functions. To use the library, you must supply your own measurement class that inherits this one and defines those functions, which may be trivial.

   Each member function defines a hook that is run at a specific time during execution. These hooks can be used to modify member objects of your inheritor measurement class, and thereby extract information from the simulation.

   .. function:: void measurement::pre_cluster(N_t n, N_t N, const system<R_t, X_t>& S, v_t i0, const R_t& r)

      This hook is run prior to cluster formation.

      :param N_t n: The number of clusters already flipped.
      :param N_t N: The total number of clusters to flip.
      :param const system<R_t, X_t>& S: The current system state.
      :param v_t i0: The index of the seed vertex for the cluster.
      :param const R_t& r: The transformation by which the cluster will be flipped.

   .. function:: void measurement::plain_bond_visited(const system<R_t, X_t>& S, v_t i1, const X_t& s1_new, v_t i2, double dE)

         This hook is run when an ordinary edge is visited during cluster formation, whether it is activated or not.

         :param const system<R_t, X_t>& S: The current system state.
         :param v_t i1:                    The index of the vertex soon to be flipped.
         :param const X_t& s1_new:         The state vertex :code:`i1` will be flipped to.
         :param v_t i2:                    The other vertex sharing the edge.
         :param double dE:                 The change in energy that will result when the spin at site :code:`i1` is flipped.

   .. cpp:function:: void measurement::plain_site_transformed(const system<R_t, X_t>& S, v_t i, const X_t& si_new)

         This hook is run when an ordinary site is about to be flipped.

         :param const system<R_t, X_t>& S: The current system state.
         :param v_t i:                     The index of the vertex soon to be flipped.
         :param const X_t& si_new:          The state vertex :code:`i` will be flipped to.

   .. cpp:function:: void measurement::ghost_bond_visited(const system<R_t, X_t>& S, v_t i, const X_t& s0si_old, const X_t& s0si_new, double dE)

         This hook is run when an edge containing the ghost site is visited during cluster formation, whether activated or not.

         :param const system<R_t, X_t>& S: The current system state.
         :param v_t i:                     The index of the non-ghost site in this edge.
         :param const X_t& s0si_old:       The state that the spin on the non-ghost site has before the transformation is applied, rotated by the inverse action of the ghost site.
         :param const X_t& s0si_new:       The state that the spin on the non-ghost site will have after the transformation is applied, rotated by the inverse action of the ghost site.
         :param double dE:                 The change in energy that will result when one site is transformed.

   .. cpp:function:: void measurement::ghost_site_transformed(const system<R_t, X_t>& S, const R_t& R_new)

         This hook is run when the ghost site is about to be flipped.

         :param const system<R_t, X_t>& S: The current system state.
         :param const R_t& R_new:          The state the ghost site will be flipped to.

   .. cpp:function:: void measurement::post_cluster(N_t n, N_t N, const system<R_t, X_t>& S) 

         This hook is run after a cluster has been flipped.

         :param N_t n:                     The number of clusters already flipped.
         :param N_t N:                     The total number of clusters to flip.
         :param const system<R_t, X_t>& S: The current system state.