string_search Namespace Reference

String search algorithms. More...

Functions
int	brute_force (const std::string &text, const std::string &pattern)
std::vector< int >	getFailureArray (const std::string &pattern)
bool	kmp (const std::string &pattern, const std::string &text)
int64_t	create_hash (const std::string &s, int n)
int64_t	recalculate_hash (const std::string &s, int old_index, int new_index, int64_t old_hash, int patLength)
bool	check_if_equal (const std::string &str1, const std::string &str2, int start1, int end1, int start2, int end2)
int	rabin_karp (const std::string &str, const std::string &pat)

Detailed Description

String search algorithms.

Function Documentation

brute_force()

int string_search::brute_force	(	const std::string &	text,
		const std::string &	pattern
	)

Find a pattern in a string by comparing the pattern to every substring.

Parameters

text	Any string that might contain the pattern.
pattern	String that we are searching for.

Returns

Index where the pattern starts in the text

-1 if the pattern was not found.

                                                                 {
    size_t pat_l = pattern.length();
    size_t txt_l = text.length();
    int index = -1;
    if (pat_l <= txt_l) {
        for (size_t i = 0; i < txt_l - pat_l + 1; i++) {
            std::string s = text.substr(i, pat_l);
            if (s == pattern) {
                index = i;
                break;
            }
        }
    }
    return index;
}

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check_if_equal()

bool string_search::check_if_equal	(	const std::string &	str1,
		const std::string &	str2,
		int	start1,
		int	end1,
		int	start2,
		int	end2
	)

compare if two sub-strings are equal

Parameters

[in]	str1	string pattern to search
[in]	str2	text in which to search
[in]	start1,end1	start and end indices for substring in str1
[in]	start2,end2	start and end indices for substring in str2

Returns

true if pattern was found

false if pattern was not found

Note

can this be replaced by std::string::compare?

                                                                {
    if (end1 - start1 != end2 - start2) {
        return false;
    }
    while (start1 <= end1 && start2 <= end2) {
        if (str1[start1] != str2[start2]) {
            return false;
        }
        start1++;
        start2++;
    }
    return true;
}

create_hash()

int64_t string_search::create_hash	(	const std::string &	s,
		int	n
	)

convert a string to an intger - called as hashing function

Parameters

[in]	s	source of string to hash
[in]	n	length of substring to hash

Returns

hash integer

                                               {
    int64_t result = 0;
    for (int i = 0; i < n; ++i) {
        result += (int64_t)(s[i] * (int64_t)pow(PRIME, i));
    }
    return result;
}

getFailureArray()

std::vector<int> string_search::getFailureArray

(

const std::string &

pattern

)

Generate the partial match table aka failure function for a pattern to search.

Parameters

[in]

pattern

text for which to create the partial match table

Returns

the partial match table as a vector array

                                                         {
    int pattern_length = pattern.size();
    std::vector<int> failure(pattern_length + 1);
    failure[0] = -1;
    int j = -1;
 
    for (int i = 0; i < pattern_length; i++) {
        while (j != -1 && pattern[j] != pattern[i]) {
            j = failure[j];
        }
        j++;
        failure[i + 1] = j;
    }
    return failure;
}

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kmp()

bool string_search::kmp	(	const std::string &	pattern,
		const std::string &	text
	)

KMP algorithm to find a pattern in a text

Parameters

[in]	pattern	string pattern to search
[in]	text	text in which to search

Returns

true if pattern was found

false if pattern was not found

                                                          {
    int text_length = text.size(), pattern_length = pattern.size();
    std::vector<int> failure = getFailureArray(pattern);
 
    int k = 0;
    for (int j = 0; j < text_length; j++) {
        while (k != -1 && pattern[k] != text[j]) {
            k = failure[k];
        }
        k++;
        if (k == pattern_length)
            return true;
    }
    return false;
}

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rabin_karp()

int string_search::rabin_karp	(	const std::string &	str,
		const std::string &	pat
	)

Perform string pattern search using Rabin-Karp algorithm

Parameters

[in]	str	string to search in
[in]	pat	pattern to search for

Returns

index of first occurrence of pattern

-1 if pattern not found

                                                           {
    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;
        }
        if (i < str.size() - pat.size()) {
            str_hash =
                recalculate_hash(str, i, i + pat.size(), str_hash, pat.size());
        }
    }
    return -1;  // return -1 if given pattern not found
}

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recalculate_hash()

int64_t string_search::recalculate_hash	(	const std::string &	s,
		int	old_index,
		int	new_index,
		int64_t	old_hash,
		int	patLength
	)

re-hash a string using known existing hash

Parameters

[in]	s	source of string to hash
[in]	old_index	previous index of string
[in]	new_index	new index of string
[in]	old_hash	previous hash of substring
[in]	patLength	length of substring to hash

Returns

new hash integer

                                                          {
    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));
    return new_hash;
}