Add equal_weights option to resample, fix Genesis4 bug

pmd_beamphysics/interfaces/genesis.py

@@ -398,7 +398,8 @@ def write_genesis4_distribution(particle_group,
     If particles are at different z, they will be drifted to the same z, 
     because the output should have different times. 
 
-    If any of the weights are different, the bunch will be resampled.
+    If any of the weights are different, the bunch will be resampled
+    to have equal weights.
     Note that this can be very slow for a large number of particles.
 
     """
@@ -423,7 +424,7 @@ def write_genesis4_distribution(particle_group,
         n = len(P)
         if verbose:
             print(f'Resampling {n} weighted particles')      
-        P = P.resample(n)        
+        P = P.resample(n, equal_weights=True)        
 
     for k in ['x', 'xp', 'y', 'yp', 't']:
         h5[k] = P[k]

pmd_beamphysics/particles.py

@@ -1064,8 +1064,8 @@ def copy(self):
         return deepcopy(self)    
 
     @functools.wraps(resample_particles)
-    def resample(self, n=0):
-        data = resample_particles(self, n)
+    def resample(self, n=0, equal_weights=False):
+        data = resample_particles(self, n, equal_weights=equal_weights)
         return ParticleGroup(data=data)
 
     # Internal sorting

pmd_beamphysics/statistics.py

@@ -460,7 +460,7 @@ def slice_statistics(particle_group,  keys=['mean_z'], n_slice=40, slice_key=Non
 
 
 
-def resample_particles(particle_group, n=0):
+def resample_particles(particle_group, n=0, equal_weights=False):
     """
     Resamples a ParticleGroup randomly.
 
@@ -479,6 +479,9 @@ def resample_particles(particle_group, n=0):
         Number to resample. 
         If n = 0, this will use all particles.
 
+    equal_weights: bool, default = False
+        If True, will ensure that all particles have equal weights. 
+
     Returns
     -------
     data: dict of ParticleGroup data
@@ -498,19 +501,21 @@ def resample_particles(particle_group, n=0):
         ixlist = np.random.choice(n_old, n, replace=False)
         weight = np.full(n, particle_group.charge/n)
 
-    # variable weights        
+    # variable weights found      
+    elif equal_weights or n != n_old:
+        # From SciPy example:
+        # https://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.rv_discrete.html#scipy.stats.rv_discrete
+        pk = weight / np.sum(weight) # Probabilities
+        xk = np.arange(len(pk)) # index
+        ixsampler = scipy_stats.rv_discrete(name='ixsampler', values=(xk, pk))        
+        ixlist = ixsampler.rvs(size=n)
+        weight = np.full(n, particle_group.charge/n)       
+
     else:
-        if n == n_old:
-            ixlist = np.random.choice(n_old, n, replace=False)
-            weight = weight[ixlist] #just scramble
-        else:
-            # From SciPy example:
-            # https://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.rv_discrete.html#scipy.stats.rv_discrete
-            pk = weight / np.sum(weight) # Probabilities
-            xk = np.arange(len(pk)) # index
-            ixsampler = scipy_stats.rv_discrete(name='ixsampler', values=(xk, pk))        
-            ixlist = ixsampler.rvs(size=n)
-            weight = np.full(n, particle_group.charge/n)
+        assert n == n_old
+        ixlist = np.random.choice(n_old, n, replace=False)
+        weight = weight[ixlist] #just scramble
+
 
     data = {}
     for key in particle_group._settable_array_keys: