def sort(pp,col):
    ncol,npart,ntime = pp.shape
    assert('id' in col)
    for itime in range(0,ntime):
      idx = pp[col['id'],:,itime].squeeze().argsort()
      pp[:,:,itime] = pp[:,idx,itime]
    return pp

def slice_columns(pp,col,keys):
    idx_col = []
    col_new = {}
    ii = 0
    for key in keys:
        idx_col.append(col[key])
        col_new[key] = ii
        ii+=1
    return pp[idx_col,:,:], col_new

def translate_circular(pp,col,translation,bounds,axis=0):
    '''Translates particles while taking into account
    the bounding box'''
    assert(axis<3)
    L = bounds[2*axis+1]
    keys = ('x','y','z')
    pp[col[keys[axis]],:,:] = (pp[col[keys[axis]],:,:]+translation)%L
    return pp