KMP-algorithm

class Solution:
    """
    @param source:
    @param target:
    @return: return the index
    """
    def strStr(self, source, target):
        # Write your code here
        if len(target)==0:
            return 0
        if len(source) < len(target):
            return -1
        lps_target = self.preprocess(target)
        index_source = 0
        index_target = 0
        while True:
            if index_target == len(target):
                return index_source - len(target)
            if index_source == len(source):
                return -1
            if target[index_target]==source[index_source]:
                index_source+=1
                index_target+=1
            else:
                if index_target!=0:
                    index_target = lps_target[index_target+1]
                else:
                    index_source+=1

    def preprocess(self, target):
        i=1 # this tracks the length of substring we are going to discover
        length=0 #this tracks the length of lps
        lps=[0] * len(target)
        while i<len(target):
            if target[length-1] == target[i]:
                lps[i]=length+1
                print(length)
                length += 1
                i+=1
            else:
                if length == 0:
                    lps[i]=0
                    i+=1
                else:
                    length = lps[length-1]
        return lps