From 26701b38befee0151c124e4e0ae3702e88ed149a Mon Sep 17 00:00:00 2001 From: Krishna Vedala Date: Fri, 29 May 2020 08:27:33 -0400 Subject: [PATCH] document rabin-karp string search --- strings/rabin_karp.cpp | 108 ++++++++++++++++++++++++++--------------- 1 file changed, 68 insertions(+), 40 deletions(-) diff --git a/strings/rabin_karp.cpp b/strings/rabin_karp.cpp index 62e3691c1..018ff5632 100644 --- a/strings/rabin_karp.cpp +++ b/strings/rabin_karp.cpp @@ -1,39 +1,64 @@ -/* - * file name : rabin_karp.cpp - * author : Amit Kumar - * Copyright : 2020 , Amit Kumar - * version : 1.0 +/** + * \file + * \brief The [Rabin-Karp + * Algorithm](https://en.wikipedia.org/wiki/Rabin–Karp_algorithm) for finding a + * pattern within a piece of text with complexity O(n + m) */ -#include -#include -#include -#include +#include +#include +#include +#ifdef _MSC_VER +#include // use this for MS Visucal C++ +#else +#include +#endif -using std::string; -using std::pow; +#define PRIME 5 ///< Prime modulus for hash functions -#define PRIME 5 - -int64_t create_hash(string s , int n) { +/** + * convert a string to an intger - called as hashing function + * \param[in] s source of string to hash + * \param[in] n length of substring to hash + * \returns hash integer + */ +int64_t create_hash(const std::string& s, int n) { int64_t result = 0; - for ( int i = 0; i < n; ++i ) { - result += (int64_t)(s[i] * (int64_t)pow(PRIME , i)); + for (int i = 0; i < n; ++i) { + result += (int64_t)(s[i] * (int64_t)pow(PRIME, i)); } return result; } -int64_t recalculate_hash(string s , int old_index , - int new_index , int64_t old_hash , int patLength) { +/** + * re-hash a string using known existing hash + * \param[in] s source of string to hash + * \param[in] old_index previous index of string + * \param[in] new_index new index of string + * \param[in] old_hash previous hash of substring + * \param[in] patLength length of substring to hash + * \returns new hash integer + */ +int64_t recalculate_hash(const std::string& s, int old_index, int new_index, + int64_t old_hash, int patLength) { int64_t new_hash = old_hash - s[old_index]; new_hash /= PRIME; - new_hash += (int64_t)(s[new_index]*(int64_t)pow(PRIME, patLength-1)); + new_hash += (int64_t)(s[new_index] * (int64_t)pow(PRIME, patLength - 1)); return new_hash; } -bool check_if_equal(string str1 , string str2 , - int start1 , int end1 , - int start2 , int end2) { - if (end1-start1 != end2-start2) { +/** + * compare if two sub-strings are equal + * \param[in] str1 string pattern to search + * \param[in] str2 text in which to search + * \param[in] start1,end1 start and end indices for substring in str1 + * \param[in] start2,end2 start and end indices for substring in str2 + * \returns `true` if pattern was found + * \returns `false` if pattern was not found + * @note can this be replaced by std::string::compare? + */ +bool check_if_equal(const std::string& str1, const std::string& str2, + int start1, int end1, int start2, int end2) { + if (end1 - start1 != end2 - start2) { return false; } while (start1 <= end1 && start2 <= end2) { @@ -46,33 +71,36 @@ bool check_if_equal(string str1 , string str2 , return true; } -/* - * @description : search pattern in the given text - * @param : string str - * @param : string pat - * @return index of first occurrence of pattern or -1 if pattern not found +/** + * Perform string pattern search using Rabin-Karp algorithm + * @param[in] str string to search in + * @param[in] pat pattern to search for + * @return index of first occurrence of pattern + * @return -1 if pattern not found */ -int rabin_karp(const string &str , const string& pat) { - int64_t pat_hash = create_hash(pat , pat.size()); - int64_t str_hash = create_hash(str , pat.size()); - for (int i=0; i <= str.size()-pat.size(); ++i) { +int rabin_karp(const std::string& str, const std::string& pat) { + int64_t pat_hash = create_hash(pat, pat.size()); + int64_t str_hash = create_hash(str, pat.size()); + for (int i = 0; i <= str.size() - pat.size(); ++i) { if (pat_hash == str_hash && - check_if_equal(str , pat , i , i+pat.size()-1 , 0 , pat.size()-1)) { - return i; + check_if_equal(str, pat, i, i + pat.size() - 1, 0, + pat.size() - 1)) { + return i; } - if (i < str.size()-pat.size()) { + if (i < str.size() - pat.size()) { str_hash = - recalculate_hash(str, i, i+pat.size(), str_hash, pat.size()); + recalculate_hash(str, i, i + pat.size(), str_hash, pat.size()); } } return -1; // return -1 if given pattern not found } +/** Main function */ int main(void) { - assert(rabin_karp("helloWorld", "world") == -1); - assert(rabin_karp("helloWorld", "World") == 5); - assert(rabin_karp("this_is_c++" , "c++") == 8); - assert(rabin_karp("happy_coding", "happy") == 0); + assert(rabin_karp("helloWorld", "world") == -1); + assert(rabin_karp("helloWorld", "World") == 5); + assert(rabin_karp("this_is_c++", "c++") == 8); + assert(rabin_karp("happy_coding", "happy") == 0); return 0; }