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

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

# # Solution Notebook

# ## Problem: Find the length of the longest substring with at most k distinct characters.
# 
# * [Constraints](#Constraints)
# * [Test Cases](#Test-Cases)
# * [Algorithm](#Algorithm)
# * [Code](#Code)
# * [Unit Test](#Unit-Test)

# ## Constraints
# 
# * Can we assume the inputs are valid?
#     * No
# * Can we assume the strings are ASCII?
#     * Yes
# * Is this case sensitive?
#     * Yes
# * Is a substring a contiguous block of chars?
#     * Yes
# * Do we expect an int as a result?
#     * Yes
# * Can we assume this fits memory?
#     * Yes

# ## Test Cases
# 
# * None -> TypeError
# * '', k = 3 -> 0
# * 'abcabcdefgghiij', k=3 -> 6
# * 'abcabcdefgghighij', k=3 -> 7

# ## Algorithm
# 
# We'll use a `chars_to_index_map` dictionary: char (key) to index (val) map to maintain a sliding window.
# 
# The index (val) will keep track of the character index in the input string.
# 
# For each character in the string:
# 
# * Add the char (key) and index (value) to the map
# * If the length of our map is greater than k, then we'll need to eliminate one item
#     * Scan the map to find the lowest index and remove it
#     * The new lowest index will therefore be incremented by 1
# * The max length will be the current index minus the lower index + 1
# 
# Complexity:
# * Time: O(n * k), where n is the number of chars, k is the length of the map due to the min() call
# * Space: O(n)

# ## Code

# In[1]:


class Solution(object):

    def longest_substr(self, string, k):
        if string is None:
            raise TypeError('string cannot be None')
        if k is None:
            raise TypeError('k cannot be None')
        low_index = 0
        max_length = 0
        chars_to_index_map = {}
        for index, char in enumerate(string):
            chars_to_index_map[char] = index
            if len(chars_to_index_map) > k:
                low_index = min(chars_to_index_map.values())
                del chars_to_index_map[string[low_index]]
                low_index += 1
            max_length = max(max_length, index - low_index + 1)
        return max_length


# ## Unit Test

# In[2]:


get_ipython().run_cell_magic('writefile', 'test_longest_substr.py', "import unittest\n\n\nclass TestSolution(unittest.TestCase):\n\n    def test_longest_substr(self):\n        solution = Solution()\n        self.assertRaises(TypeError, solution.longest_substr, None)\n        self.assertEqual(solution.longest_substr('', k=3), 0)\n        self.assertEqual(solution.longest_substr('abcabcdefgghiij', k=3), 6)\n        self.assertEqual(solution.longest_substr('abcabcdefgghighij', k=3), 7)\n        print('Success: test_longest_substr')\n\n\ndef main():\n    test = TestSolution()\n    test.test_longest_substr()\n\n\nif __name__ == '__main__':\n    main()\n")


# In[3]:


get_ipython().run_line_magic('run', '-i test_longest_substr.py')