memoization implementation with simple limit. some thoughs [here](http://www.az2000.de/docs/memoization/)

memoization.py

class Entry:
	def __init__(self, x, y):
		self.x = x
		self.y = y
		self.leftEntry = None
		self.rightEntry = None
	def pop(self):
		if self.leftEntry is not None:
			self.leftEntry.rightEntry = self.rightEntry
		if self.rightEntry is not None:
			self.rightEntry.leftEntry = self.leftEntry
		self.leftEntry = None
		self.rightEntry = None

class MemoizedFunction:
	N = 100

	def __init__(self, f):
		self.f = f
		self.entries = {}
		self.topEntry = None
		self.bottomEntry = None

	def pushTop(self, entry):
		entry.rightEntry = self.topEntry
		if self.topEntry is not None:
			self.topEntry.leftEntry = entry
		self.topEntry = entry
		if self.bottomEntry is None:
			self.bottomEntry = entry

	def __call__(self, x):
		if x in self.entries:
			entry = self.entries[x]
			if entry is self.bottomEntry:
				assert entry.rightEntry is None
				self.bottomEntry = entry.leftEntry
			entry.pop()
			self.pushTop(entry)
			return entry.y

		y = self.f(x)
		entry = Entry(x, y)
		self.entries[x] = entry
		self.pushTop(entry)
		
		if len(self.entries) > self.N:
			entry = self.bottomEntry
			assert entry is not None
			assert entry.rightEntry is None
			assert entry.leftEntry is not None # at least for N>=2
			del self.entries[entry.x]
			self.bottomEntry = entry.leftEntry
			self.bottomEntry.rightEntry = None

		return y