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107 lines
3.3 KiB
C++
107 lines
3.3 KiB
C++
/**
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* \file
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* \brief The [Rabin-Karp
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* Algorithm](https://en.wikipedia.org/wiki/Rabin–Karp_algorithm) for finding a
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* pattern within a piece of text with complexity O(n + m)
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*/
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#include <cassert>
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#include <cmath>
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#include <iostream>
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#ifdef _MSC_VER
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#include <string> // use this for MS Visucal C++
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#else
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#include <cstring>
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#endif
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#define PRIME 5 ///< Prime modulus for hash functions
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/**
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* convert a string to an intger - called as hashing function
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* \param[in] s source of string to hash
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* \param[in] n length of substring to hash
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* \returns hash integer
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*/
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int64_t create_hash(const std::string& s, int n) {
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int64_t result = 0;
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for (int i = 0; i < n; ++i) {
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result += (int64_t)(s[i] * (int64_t)pow(PRIME, i));
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}
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return result;
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}
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/**
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* re-hash a string using known existing hash
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* \param[in] s source of string to hash
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* \param[in] old_index previous index of string
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* \param[in] new_index new index of string
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* \param[in] old_hash previous hash of substring
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* \param[in] patLength length of substring to hash
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* \returns new hash integer
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*/
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int64_t recalculate_hash(const std::string& s, int old_index, int new_index,
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int64_t old_hash, int patLength) {
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int64_t new_hash = old_hash - s[old_index];
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new_hash /= PRIME;
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new_hash += (int64_t)(s[new_index] * (int64_t)pow(PRIME, patLength - 1));
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return new_hash;
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}
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/**
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* compare if two sub-strings are equal
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* \param[in] str1 string pattern to search
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* \param[in] str2 text in which to search
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* \param[in] start1,end1 start and end indices for substring in str1
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* \param[in] start2,end2 start and end indices for substring in str2
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* \returns `true` if pattern was found
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* \returns `false` if pattern was not found
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* @note can this be replaced by std::string::compare?
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*/
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bool check_if_equal(const std::string& str1, const std::string& str2,
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int start1, int end1, int start2, int end2) {
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if (end1 - start1 != end2 - start2) {
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return false;
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}
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while (start1 <= end1 && start2 <= end2) {
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if (str1[start1] != str2[start2]) {
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return false;
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}
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start1++;
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start2++;
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}
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return true;
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}
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/**
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* Perform string pattern search using Rabin-Karp algorithm
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* @param[in] str string to search in
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* @param[in] pat pattern to search for
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* @return index of first occurrence of pattern
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* @return -1 if pattern not found
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*/
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int rabin_karp(const std::string& str, const std::string& pat) {
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int64_t pat_hash = create_hash(pat, pat.size());
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int64_t str_hash = create_hash(str, pat.size());
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for (int i = 0; i <= str.size() - pat.size(); ++i) {
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if (pat_hash == str_hash &&
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check_if_equal(str, pat, i, i + pat.size() - 1, 0,
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pat.size() - 1)) {
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return i;
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}
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if (i < str.size() - pat.size()) {
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str_hash =
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recalculate_hash(str, i, i + pat.size(), str_hash, pat.size());
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}
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}
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return -1; // return -1 if given pattern not found
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}
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/** Main function */
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int main(void) {
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assert(rabin_karp("helloWorld", "world") == -1);
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assert(rabin_karp("helloWorld", "World") == 5);
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assert(rabin_karp("this_is_c++", "c++") == 8);
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assert(rabin_karp("happy_coding", "happy") == 0);
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return 0;
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}
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