#!/usr/bin/env python
# coding: utf-8

# This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges).

# # Challenge Notebook

# ## Problem: Find the start of a linked list loop.
# 
# * [Constraints](#Constraints)
# * [Test Cases](#Test-Cases)
# * [Algorithm](#Algorithm)
# * [Code](#Code)
# * [Unit Test](#Unit-Test)
# * [Solution Notebook](#Solution-Notebook)

# ## Constraints
# 
# * Is this a singly linked list?
#     * Yes
# * Can we assume we are always passed a circular linked list?
#     * No
# * When we find a loop, do we return the node or the node's data?
#     * The node
# * Can we assume we already have a linked list class that can be used for this problem?
#     * Yes

# ## Test Cases
# 
# * Empty list -> None
# * Not a circular linked list -> None
#     * One element
#     * Two or more elements
# * Circular linked list general case

# ## Algorithm
# 
# Refer to the [Solution Notebook](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/linked_lists/find_loop_start/find_loop_start_solution.ipynb).  If you are stuck and need a hint, the solution notebook's algorithm discussion might be a good place to start.

# ## Code

# In[ ]:


get_ipython().run_line_magic('run', '../linked_list/linked_list.py')
get_ipython().run_line_magic('load', '../linked_list/linked_list.py')


# In[ ]:


class MyLinkedList(LinkedList):

    def find_loop_start(self):
        # TODO: Implement me
        pass


# ## Unit Test

# 
# 
# **The following unit test is expected to fail until you solve the challenge.**

# In[ ]:


# %load test_find_loop_start.py
import unittest


class TestFindLoopStart(unittest.TestCase):

    def test_find_loop_start(self):
        print('Test: Empty list')
        linked_list = MyLinkedList()
        self.assertEqual(linked_list.find_loop_start(), None)

        print('Test: Not a circular linked list: One element')
        head = Node(1)
        linked_list = MyLinkedList(head)
        self.assertEqual(linked_list.find_loop_start(), None)

        print('Test: Not a circular linked list: Two elements')
        linked_list.append(2)
        self.assertEqual(linked_list.find_loop_start(), None)

        print('Test: Not a circular linked list: Three or more elements')
        linked_list.append(3)
        self.assertEqual(linked_list.find_loop_start(), None)

        print('Test: General case: Circular linked list')
        node10 = Node(10)
        node9 = Node(9, node10)
        node8 = Node(8, node9)
        node7 = Node(7, node8)
        node6 = Node(6, node7)
        node5 = Node(5, node6)
        node4 = Node(4, node5)
        node3 = Node(3, node4)
        node2 = Node(2, node3)
        node1 = Node(1, node2)
        node0 = Node(0, node1)
        node10.next = node3
        linked_list = MyLinkedList(node0)
        self.assertEqual(linked_list.find_loop_start(), node3)

        print('Success: test_find_loop_start')


def main():
    test = TestFindLoopStart()
    test.test_find_loop_start()


if __name__ == '__main__':
    main()


# ## Solution Notebook
# 
# Review the [Solution Notebook](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/linked_lists/find_loop_start/find_loop_start_solution.ipynb) for a discussion on algorithms and code solutions.