LeetCode 567 - Permutation in String - 2023.10.15 solution

class Solution:
    def checkInclusion(self, s1: str, s2: str) -> bool:
        """ We can't just check if the characters (and their numbers) of s1 is in
        s2 because all the characters need to be adjacent.
        """
        chars_in_s1 = _get_chars_in_s1(s1)
        chars_to_counts = _get_initial_window(s1, s2)

        l = 0
        r = len(s1)

        while r <= len(s2):
            if _chars_are_the_same(chars_in_s1, chars_to_counts):
                return True

            chars_to_counts[s2[l]] -= 1

            if r < len(s2):
                chars_to_counts[s2[r]] += 1

            l += 1
            r += 1

        return False


def _chars_are_the_same(chars_in_s1, chars_to_counts):
    for c in chars_in_s1.keys():
        if chars_to_counts[c] != chars_in_s1[c]:
            return False
    return True


def _get_chars_in_s1(s1):
    chars_in_s1 = defaultdict(int)
    for c in s1:
        chars_in_s1[c] += 1
    return chars_in_s1


def _get_initial_window(s1, s2):
    chars_to_counts = defaultdict(int)
    if len(s2) == 0:
        return chars_to_counts
    for i, c in enumerate(s2):
        if i >= len(s1):
            return chars_to_counts
        chars_to_counts[c] += 1
    return chars_to_counts