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502a953e23
* fix: KMP algorithm (#2712) * feat: let the KMP algorithm return index and add more tests. * feat: format code * Update strings/knuth_morris_pratt.cpp * update knuth_morris_pratt.cpp --------- Co-authored-by: realstealthninja <68815218+realstealthninja@users.noreply.github.com>
99 lines
2.9 KiB
C++
99 lines
2.9 KiB
C++
/**
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* @file
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* @brief The [Knuth-Morris-Pratt
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* Algorithm](https://en.wikipedia.org/wiki/Knuth–Morris–Pratt_algorithm) for
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* finding a pattern within a piece of text with complexity O(n + m)
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* @details
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* 1. Preprocess pattern to identify any suffixes that are identical to
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* prefixes. This tells us where to continue from if we get a mismatch between a
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* character in our pattern and the text.
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* 2. Step through the text one character at a time and compare it to a
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* character in the pattern updating our location within the pattern if
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* necessary
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* @author [Yancey](https://github.com/Yancey2023)
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*/
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#include <cassert> /// for assert
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#include <iostream> /// for IO operations
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#include <string> /// for std::string
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#include <vector> /// for std::vector
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/**
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* @namespace string_search
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* @brief String search algorithms
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*/
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namespace string_search {
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/**
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* @brief Generate the partial match table aka failure function for a pattern to
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* search.
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* @param pattern text for which to create the partial match table
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* @returns the partial match table as a vector array
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*/
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std::vector<size_t> getFailureArray(const std::string &pattern) {
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size_t pattern_length = pattern.size();
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std::vector<size_t> failure(pattern_length + 1);
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failure[0] = std::string::npos;
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size_t j = std::string::npos;
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for (int i = 0; i < pattern_length; i++) {
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while (j != std::string::npos && pattern[j] != pattern[i]) {
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j = failure[j];
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}
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failure[i + 1] = ++j;
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}
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return failure;
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}
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/**
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* @brief KMP algorithm to find a pattern in a text
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* @param pattern string pattern to search
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* @param text text in which to search
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* @returns the starting index of the pattern if found
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* @returns `std::string::npos` if not found
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*/
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size_t kmp(const std::string &pattern, const std::string &text) {
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if (pattern.empty()) {
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return 0;
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}
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std::vector<size_t> failure = getFailureArray(pattern);
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size_t text_length = text.size();
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size_t pattern_length = pattern.size();
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size_t k = 0;
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for (size_t j = 0; j < text_length; j++) {
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while (k != std::string::npos && pattern[k] != text[j]) {
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k = failure[k];
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}
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if (++k == pattern_length) {
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return j - k + 1;
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}
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}
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return std::string::npos;
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}
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} // namespace string_search
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using string_search::kmp;
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/**
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* @brief self-test implementations
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* @returns void
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*/
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static void tests() {
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assert(kmp("abc1abc12l", "alskfjaldsabc1abc1abc12k2") == std::string::npos);
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assert(kmp("bca", "abcabc") == 1);
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assert(kmp("World", "helloWorld") == 5);
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assert(kmp("c++", "his_is_c++") == 7);
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assert(kmp("happy", "happy_coding") == 0);
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assert(kmp("", "pattern is empty") == 0);
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// this lets the user know that the tests have passed
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std::cout << "All KMP algorithm tests have successfully passed!\n";
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}
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/*
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* @brief Main function
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* @returns 0 on exit
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*/
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int main() {
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tests();
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return 0;
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}
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