Boyer Moore ALgorithm

class Solution:
    def strStr(self, text: str, pat: str) -> int:
        item=[-1]*256
        if not pat:return 0
        #creating bad character array . preprocessing pattern
        for i,j in enumerate(pat):
            item[ord(j)]=i
        m,n=len(text),len(pat)
        i=0#used for text
        while i<=m-n:# if it crosses m-n then you wont get sufficient length of pattern to search
            j=n-1#used for pattern
            while j>=0 and text[i+j]==pat[j]:
                j=j-1
            if j<0:
                return i
            else:
                i=i+max(1,j-item[ord(text[i+j])]) #j-item[ord(text[i+j])] might be zero some times
        return -1