Refactor bag operations for efficiency

collections_extended/bags.py

@@ -28,9 +28,8 @@ def __init__(self, iterable=None):
 		self._size = 0
 		if iterable:
 			if isinstance(iterable, _basebag):
-				for elem, count in iterable._dict.items():
-					self._dict[elem] = count
-					self._size += count
+				self._dict = iterable._dict.copy()
+				self._size = iterable._size
 			else:
 				for value in iterable:
 					self._dict[value] = self._dict.get(value, 0) + 1
@@ -253,8 +252,6 @@ def isdisjoint(self, other):
 		"""Return if this bag is disjoint with the passed collection.
 
 		This runs in O(len(other))
-
-		TODO move isdisjoint somewhere more appropriate
 		"""
 		for value in other:
 			if value in self:
@@ -290,9 +287,14 @@ def __add__(self, other):
 			other (Iterable): elements to add to self
 		"""
 		out = self.copy()
-		for value in other:
-			out._dict[value] = out._dict.get(value, 0) + 1
-			out._size += 1
+		if isinstance(other, _basebag):
+			for elem, count in other._dict.items():
+				out._dict[elem] = out._dict.get(elem, 0) + count
+				out._size += count
+		else:
+			for elem in other:
+				out._dict[elem] = out._dict.get(elem, 0) + 1
+				out._size += 1
 		return out
 
 	def __sub__(self, other):
@@ -307,16 +309,17 @@ def __sub__(self, other):
 		Args:
 			other (Iterable): elements to remove
 		"""
-		out = self.copy()
-		for value in other:
-			old_count = out._dict.get(value, 0)
-			if old_count == 1:
-				del out._dict[value]
-				out._size -= 1
-			elif old_count > 1:
-				out._dict[value] = old_count - 1
-				out._size -= 1
-		return out
+		if isinstance(other, _basebag):
+			values = dict()
+			for elem, self_count in self._dict.items():
+				values[elem] = max(self_count - other.count(elem), 0)
+		else:
+			values = self._dict.copy()
+			for elem in other:
+				old_count = values.get(elem, 0)
+				if old_count:
+					values[elem] -= 1
+		return self.from_mapping(values)
 
 	def __mul__(self, other):
 		"""Cartesian product of the two sets.
@@ -355,7 +358,19 @@ def __xor__(self, other):
 			m = len(self)
 			n = len(other)
 		"""
-		return (self - other) | (other - self)
+		if isinstance(other, _basebag):
+			values = dict()
+			for elem in self._dict.keys() | other._dict.keys():
+				values[elem] = abs(self.count(elem) - other.count(elem))
+		else:
+			values = self._dict.copy()
+			for elem in other:
+				old_count = values.get(elem, 0)
+				if old_count:
+					values[elem] -= 1
+				else:
+					values[elem] += 1
+		return self.from_mapping(values)
 
 
 class bag(_basebag, MutableSet):
@@ -480,9 +495,14 @@ def __ixor__(self, other):
 		"""
 		if not isinstance(other, _basebag):
 			other = self._from_iterable(other)
-		other_minus_self = other - self
-		self.discard_all(other)
-		self |= other_minus_self
+		for elem, other_count in other._dict.items():
+			self_count = self.count(elem)
+			new_count = abs(self_count - other_count)
+			if new_count:
+				self._dict[elem] = new_count
+			else:
+				del self._dict[elem]
+			self._size -= (self_count - new_count)
 		return self
 
 	def __isub__(self, other):

tests/test_bags.py

@@ -315,9 +315,12 @@ def test_iand():
 
 def test_ixor():
 	"""Test __ixor__."""
-	b = bag('abbc')
-	b ^= bag('bg')
+	b = bag('abbbccd')
+	b ^= bag('bbcdg')
 	assert b == bag('abcg')
+	b = bag('bbcdg')
+	b ^= bag('abbbccd')
+	assert b == bag('acbg')
 	b = bag('abbc')
 	b ^= set('bg')
 	assert b == bag('abcg')