Dirty implementation of the second-chance algorithm

secondchance.py

#!/usr/bin/env python

import sys

buffsize = 4
refstring = [0, 1, 2, 3, 1, 0, 4, 5, 1, 0, 1, 2, 6, 5, 2, 1, 0, 1, 2, 5]

if len(sys.argv) >= 3:
	buffsize = int(sys.argv[1])
	refstring = sys.argv[2].split()

class InMemoryPage():
	def __init__(self, id):
		self.id = id
		self.guard = False

class LinkedObject():
	def __init__(self, obj):
		self.o = obj
		self.next = None
		self.prev = None

class FIFO():
	def __init__(self, size):
		self.size = size
		self.head = None
		self.tail = None
		self.length = 0

	def indexof(self, o):
		i = 0
		current = self.head
		while current != None:
			if current.o == o:
				return i 
			i += 1
			current = current.next

	def enqueue(self, o):
		obj = LinkedObject(o)
		if self.head == None:
			self.head = obj
			self.tail = obj
			self.length = 1
		else:
			if self.isfull():
				self.dequeue()
				self.enqueue(o)				
			else:
				obj.next = self.head
				self.head.prev = obj
				self.head = obj
				self.length += 1

	def dequeue(self):
		if self.tail == None:
			return None
		obj = self.tail.o
		if self.tail != self.head:
			self.tail.prev.next = None
		else:
			self.head = None
		self.tail = self.tail.prev
		self.length -= 1
		return obj
	
	def isfull(self):
		return self.length == self.size

class SecondChance(FIFO):
	def __init__(self, size):
		super().__init__(size)

	def findpage(self, i):
		cur = self.head
		while cur != None:
			if cur.o.id == i:
				return cur.o
			cur = cur.next
		return None

	def getpage(self, i):
		page = self.findpage(i)
		if page:
			print(self.tostring())		
			page.guard = True
			return page
		if self.isfull():		
			while self.tail.o.guard:
				page = self.dequeue()
				page.guard = False
				self.enqueue(page)
		page = InMemoryPage(i)
		self.enqueue(page)
		print(self.tostring() + ' F')
		return page

	def tostring(self):
		s = ''
		next = self.head
		while next != None:
			s += str(next.o.id) + ('+ ' if next.o.guard else '  ')
			next = next.next
		s += ''.join([' ' for a in range((self.size - self.length) * 3)])
		return s

sc = SecondChance(buffsize)

for i in refstring:
	sc.getpage(i)