TheAlgorithms-C-Plus-Plus/strings/z_function.cpp

/**
 * @file
 * @brief The [Z function](https://cp-algorithms.com/string/z-function.html) for finding occurences of a pattern 
 * within a piece of text with time and space complexity O(n + m)
 * @details 
 * DESCRIPTION
 * 1. The Z-function for a string is an array of length n where the 
 * i-th element is equal to the greatest number of characters starting 
 * from the position i that coincide with the first characters of s.
 * 2. Eg string : ababb then z[2]=2 as s[2]=s[0] and s[3]=s[1] and s[4]!=s[2]
 * @author [Ritika Gupta](https://github.com/RitikaGupta8734)
 */

#include <iostream>   /// for IO operations
#ifdef _MSC_VER
#include <string>  /// for string (use this for MS Visual C++)
#else
#include <cstring>   /// for string
#endif
#include <vector>     /// for std::vector
#include <cassert>   /// for assert

/**
 * @brief Generate the Z-function for the inputted string.
 * \param[in] pattern text on which to apply the Z-function
 * \returns the Z-function output as a vector array
 */
std::vector<int> Z_function(const std::string &pattern) {
    int pattern_length = pattern.size();
    std::vector<int> z(pattern_length,0);
    
    for (int i = 1,l = 0,r = 0; i < pattern_length; i++) {
        if(i<=r)z[i]=std::min(r-i+1,z[i-l]);
        while(i+z[i]<pattern_length && pattern[z[i]]==pattern[i+z[i]]) z[i]++;
        if(i+z[i]-1>r)r=i+z[i]-1;
    }
    return z;
}

/**
 * @brief Using Z_function to find a pattern in a text
 * \param[in] pattern string pattern to search
 * \param[in] text text in which to search
 * \returns a vector of starting indexes where pattern is found in the text 
 */
std::vector<int> find_pat_in_text(const std::string &pattern, const std::string &text) {
    int text_length = text.size(), pattern_length = pattern.size();
    std::vector<int> z = Z_function(pattern+'#'+text);
    std::vector<int> matching_indexes;

    for (int i = 0; i < text_length; i++) {
        if(z[i+pattern_length+1]==pattern_length) matching_indexes.push_back(i);   
    }
    return matching_indexes;
}  

/**
 * @brief Self-test implementations
 * @returns void
 */
static void test(){
    
    // usual case
    std::string text1 = "alskfjaldsabc1abc1abcbksbcdnsdabcabc";
    std::string pattern1 = "abc";
    
    std::vector<int> matching_indexes1=find_pat_in_text(pattern1,text1);
    assert((matching_indexes1 == std::vector<int>{10,14,18,30,33}));
    
    // corner case
    std::string text2 = "greengrass";
    std::string pattern2 = "abc";
    
    std::vector<int> matching_indexes2=find_pat_in_text(pattern2,text2);
    assert((matching_indexes2 == std::vector<int>{}));
}

/**
 * @brief Main function
 * @returns 0 on exit
 */
int main() {
    
    testing();  
    return 0;
}