Improve hash map (#12678)

* Mutable _Item * document falsy item * resize_down: expected test result * resize_down: actual result This is a problem since it causes rapid cycling * improve comment about falsy item Co-authored-by: Andrey <Cjkjvfnby@gmail.com> * fix long line * Update hash_map.py * Update hash_map.py --------- Co-authored-by: Andrey <Cjkjvfnby@gmail.com> Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>

Author: 3 people

data_structures/hashing/hash_map.py

@@ -16,7 +16,7 @@
 VAL = TypeVar("VAL")
 
 
-@dataclass(frozen=True, slots=True)
+@dataclass(slots=True)
 class _Item(Generic[KEY, VAL]):
  key: KEY
  val: VAL
@@ -72,16 +72,17 @@ def _try_set(self, ind: int, key: KEY, val: VAL) -> bool:
 
  If bucket is empty or key is the same, does insert and return True.
 
- If bucket has another key or deleted placeholder,
- that means that we need to check next bucket.
+ If bucket has another key that means that we need to check next bucket.
  """
  stored = self._buckets[ind]
  if not stored:
+ # A falsy item means that bucket was never used (None)
+ # or was deleted (_deleted).
  self._buckets[ind] = _Item(key, val)
  self._len += 1
  return True
  elif stored.key == key:
- self._buckets[ind] = _Item(key, val)
+ stored.val = val
  return True
  else:
  return False
@@ -228,6 +229,27 @@ def __delitem__(self, key: KEY) -> None:
  Traceback (most recent call last):
  ...
  KeyError: 4
+
+ # Test resize down when sparse
+ ## Setup: resize up
+ >>> hm = HashMap(initial_block_size=100, capacity_factor=0.75)
+ >>> len(hm._buckets)
+ 100
+ >>> for i in range(75):
+ ... hm[i] = i
+ >>> len(hm._buckets)
+ 100
+ >>> hm[75] = 75
+ >>> len(hm._buckets)
+ 200
+
+ ## Resize down
+ >>> del hm[75]
+ >>> len(hm._buckets)
+ 200
+ >>> del hm[74]
+ >>> len(hm._buckets)
+ 100
  """
  for ind in self._iterate_buckets(key):
  item = self._buckets[ind]