Horspool Boyer Moore ALgorithm Pattern Matching

class Solution:
    def strStr(self, text: str, pat: str) -> int:
        if not pat:
            return 0
        m,n=len(text),len(pat)
        pre=[n]*256 #preprocessing work
        if n>m:return -1

        for i in range(n-1):#vvvv Imp .Here preprocessing need to be done till n-1 th index . If already that pat[n-1] exists already in pat at some other index then it takes its index else it will be length
            pre[ord(pat[i])]=n-i-1

        shift=n-1#shift is pointer to point text . It is constant for each iteration

        while shift<m:
            j=n-1
            i=shift #i,j are used for traversing so shift doesnt affect
            while j>=0 and text[i]==pat[j]:
                i,j=i-1,j-1
            #print((i,j))
            if j<0:#success
                return i+1
            else:# when match does nt happen check the length to be moved by searching in pre array(preprocessing table)
                shift=shift+pre[ord(text[shift])]
        return -1