def test_read_returns_correct_value(self):
lru = LRUCache(2)
lru.write('a', 10)
lru.write('b', 20)
expected = 20
actual = lru.read('b')
self.assertEqual(actual, expected, 'should return the correct value')
def test_write_evicts_the_oldest_elem_when_cache_is_full(self):
lru = LRUCache(2)
lru.write('a', 10)
lru.write('b', 20)
evicted = lru.write('c', 30)
self.assertEqual(len(lru.data), 2, 'should have only two elements')
self.assertEqual(lru.first['key'], 'c', 'c is the most recent')
self.assertEqual(lru.last['key'], 'b', 'b is the latest addition')
self.assertEqual(evicted['key'], 'a', 'should have evicted key a')
self.assertEqual(evicted['value'], 10,
'should have evicted value for a')
def test_evict_when_cache_has_one_element(self):
lru = LRUCache(2)
lru.write('a', 10)
lru.evict()
self.assertNotIn('a', lru.data, 'cache should be empty')
self.assertIsNone(lru.first, 'double linked list is empty')
self.assertIsNone(lru.last, 'double linked list is empty')
def test_lru_evicts_old_data(self):
lru = LRUCache(2)
lru.write('a', 10)
lru.write('b', 20)
evicted = lru.write('c', 30)
self.assertEqual(lru.read('b'), 20, 'should read the correct data')
with self.assertRaises(Exception) as notFound:
lru.read('a')
self.assertIsNotNone(notFound, 'Cache miss')
self.assertEqual(evicted['key'], 'a', 'should have evicted key a')
self.assertEqual(evicted['value'], 10,
'should have evicted value for a')
def test_write_moves_key_to_the_first_position(self):
lru = LRUCache(2)
lru.write('a', 10)
lru.write('b', 20)
lru.write('a', 30)
self.assertEqual(lru.first['key'], 'a', 'a is the first')
self.assertEqual(lru.last['key'], 'b', 'b is the last')
self.assertEqual(lru.first['next']['key'], 'b', 'b is successor to a')
self.assertEqual(lru.data['a']['value'], 30, 'should have updated val')
def test_lru_remove_correctly_deletes_first_key_from_cache(self):
lru = LRUCache(3)
lru.write('a', 10)
lru.write('b', 20)
lru.write('c', 20)
lru.remove('c')
self.assertIn('a', lru.data, 'a is in the cache')
self.assertIn('b', lru.data, 'b is in the cache')
self.assertNotIn('c', lru.data, 'c was removed')
self.assertEqual(lru.first['key'], 'b', 'b is first pointer')
self.assertIsNone(lru.first['previous'], 'b is head pointer')
self.assertEqual(lru.first['next']['key'], 'a', 'a comes after b')
self.assertEqual(lru.last['key'], 'a', 'a is last pointer')
self.assertIsNone(lru.last['next'], 'a is the end pointer')
self.assertEqual(lru.last['previous']['key'], 'b', 'b comes before a')
def test_evict_when_cache_has_at_least_two_elements(self):
lru = LRUCache(2)
lru.write('a', 10)
lru.write('b', 20)
lru.evict()
self.assertNotIn('a', lru.data, 'cache should not contain key a')
self.assertIn('b', lru.data, 'cache should contain key b')
self.assertEqual(lru.first['key'], 'b', 'cache should only contain b')
self.assertEqual(lru.last['key'], 'b', 'cache should only contain b')
self.assertIsNone(lru.last['previous'], 'no more data in cache')
self.assertIsNone(lru.last['next'], 'no more data in cache')
def test_write_inserts_new_key_in_front_of_cache(self):
lru = LRUCache(3)
lru.write('a', 10)
lru.write('b', 20)
lru.write('c', 30)
self.assertEqual(lru.first['key'], 'c', 'c is the first')
self.assertIsNone(lru.first['previous'], 'a has no predecessor')
self.assertEqual(lru.first['next']['key'], 'b', 'a successor is b')
self.assertEqual(lru.data['b']['previous']['key'], 'c', 'c before b')
self.assertEqual(lru.data['b']['next']['key'], 'a', 'a after b')
self.assertEqual(lru.last['key'], 'a', 'a is the last')
self.assertEqual(lru.last['previous']['key'], 'b', 'b before last')
self.assertIsNone(lru.data['a']['next'], 'a is the last')
def test_read_must_rewire_the_first_pointer(self):
lru = LRUCache(2)
lru.write('a', 10)
lru.write('b', 20)
lru.read('a')
self.assertEqual(lru.first['key'], 'a', 'a should now be the first')
self.assertIsNone(lru.first['previous'],
'first should not have anything before')
self.assertEqual(lru.first['next']['key'], 'b',
'b is not least recently used')
self.assertEqual(lru.last['key'], 'b', 'a should now be the first')
self.assertIsNone(lru.last['next'],
'last should not have anything after it')
self.assertEqual(lru.last['previous']['key'], 'a', 'a is before last')
def test_evict_when_cache_has_no_elements(self):
lru = LRUCache(2)
lru.evict()
self.assertEqual(len(lru.data), 0, 'noop')
def test_read_throws_when_cache_miss(self):
lru = LRUCache(2)
with self.assertRaises(Exception) as notFound:
lru.read('a')
self.assertIsNotNone(notFound, 'Cache miss exception raised')
def test_write_adds_only_element_to_cache(self):
lru = LRUCache(2)
lru.write('a', 10)
self.assertEqual(lru.first['key'], 'a', 'a is the first')
self.assertEqual(lru.last['key'], 'a', 'a is the last')