a fabulous KMP tutorial

kmp.cpp

//http://dev-faqs.blogspot.com/2010/05/knuth-morris-pratt-algorithm.html
#include "PatternMatcher.h"

vector<int> PatternMatcher::computeKmpPrefix(const std::string &pattern){
    int patternSize = pattern.size();
    vector<int> kmpPrefix(patternSize);
    size_t prefixPos = 0;
    size_t suffixPos = 1;

    while(suffixPos < patternSize){
        if(pattern[prefixPos] == pattern[suffixPos]){
            kmpPrefix[suffixPos] = prefixPos + 1;
            prefixPos++;
            suffixPos++;
        }
        else if(prefixPos > 0){//found some match
            prefixPos = kmpPrefix[prefixPos -1];//backtrack for matching prefix e.g. aaaaabaaaaaa
        }
        else{
            kmpPrefix[suffixPos] = 0;
            suffixPos++;
        }
    }
    return kmpPrefix;
}

int PatternMatcher::kmpSearch(const std::string &text, const std::string &pattern){

    size_t textSize = text.size();
    size_t patternSize = pattern.size();

    if(patternSize > textSize)
        return -1;

    vector<int> kmpNext = computeKmpPrefix(pattern);
    int tIdx = 0;
    int pIdx = 0;

    while(tIdx < textSize){
        if(pattern[pIdx] == text[tIdx]){
            if(pIdx == patternSize - 1) 
                return tIdx - (patternSize - 1);
            tIdx++;
            pIdx++;
        }
        else if(pIdx > 0){
            pIdx = kmpNext[pIdx - 1];
        }
        else{
            tIdx++;
        }
    }
    return -1;
}