new_contents: # -*- coding: utf-8 -*-
  # This file is part of beets.
  # Copyright 2019, Carl Suster
  #
  # Permission is hereby granted, free of charge, to any person obtaining
  # a copy of this software and associated documentation files (the
  # "Software"), to deal in the Software without restriction, including
  # without limitation the rights to use, copy, modify, merge, publish,
  # distribute, sublicense, and/or sell copies of the Software, and to
  # permit persons to whom the Software is furnished to do so, subject to
  # the following conditions:
  #
  # The above copyright notice and this permission notice shall be
  # included in all copies or substantial portions of the Software.
  
  """Test the beets.util.random utilities associated with the random plugin.
  """
  
  from __future__ import division, absolute_import, print_function
  
  import unittest
  from test.helper import TestHelper
  
  import math
  from random import Random
  
  from beets.util import random
  
  
  class RandomTest(unittest.TestCase, TestHelper):
      def setUp(self):
          self.lib = None
          self.artist1 = 'Artist 1'
          self.artist2 = 'Artist 2'
          self.item1 = self.create_item(artist=self.artist1)
          self.item2 = self.create_item(artist=self.artist2)
          self.items = [self.item1, self.item2]
          for _ in range(8):
              self.items.append(self.create_item(artist=self.artist2))
          self.random_gen = Random()
          self.random_gen.seed(12345)
  
      def tearDown(self):
          pass
  
      def _stats(self, data):
          mean = sum(data) / len(data)
          stdev = math.sqrt(
                  sum((p - mean) ** 2 for p in data) / (len(data) - 1))
          quot, rem = divmod(len(data), 2)
          if rem:
              median = sorted(data)[quot]
          else:
              median = sum(sorted(data)[quot - 1:quot + 1]) / 2
          return mean, stdev, median
  
      def test_equal_permutation(self):
          """We have a list of items where only one item is from artist1 and the
          rest are from artist2. If we permute weighted by the artist field then
          the solo track will almost always end up near the start. If we use a
          different field then it'll be in the middle on average.
          """
          def experiment(field, histogram=False):
              """Permutes the list of items 500 times and calculates the position
              of self.item1 each time. Returns stats about that position.
              """
              positions = []
              for _ in range(500):
                  shuffled = list(random._equal_chance_permutation(
                      self.items, field=field))
                  positions.append(shuffled.index(self.item1))
              # Print a histogram (useful for debugging).
              if histogram:
                  for i in range(len(self.items)):
                      print('{:2d} {}'.format(i, '*' * positions.count(i)))
              return self._stats(positions)
  
          mean1, stdev1, median1 = experiment('artist')
          mean2, stdev2, median2 = experiment('track')
          self.assertAlmostEqual(0, median1, delta=1)
          self.assertAlmostEqual(len(self.items) // 2, median2, delta=1)
          self.assertGreater(stdev2, stdev1)
  
subject: add test for equal_chance_permutation