boyer_moore.cpp File Reference

The Boyer–Moore algorithm searches for occurrences of pattern P in text T by performing explicit character comparisons at different alignments. Instead of a brute-force search of all alignments (of which there are n - m + 1), Boyer–Moore uses information gained by preprocessing P to skip as many alignments as possible. More...

#include <cassert>
#include <climits>
#include <cstring>
#include <iostream>
#include <string>
#include <vector>

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Classes
struct	strings::boyer_moore::pattern
	A structure representing all the data we need to search the preprocessed pattern in text. More...

Namespaces
namespace	strings
	Algorithms with strings.

Macros
#define	APLHABET_SIZE   CHAR_MAX
	number of symbols in the alphabet we use

Functions
void	strings::boyer_moore::init_good_suffix (const std::string &str, std::vector< size_t > &arg)
	A function that preprocess the good suffix thable.
void	strings::boyer_moore::init_bad_char (const std::string &str, std::vector< size_t > &arg)
	A function that preprocess the bad char table.
void	strings::boyer_moore::init_pattern (const std::string &str, pattern &arg)
	A function that initializes pattern.
std::vector< size_t >	strings::boyer_moore::search (const std::string &str, const pattern &arg)
	A function that implements Boyer-Moore's algorithm.
bool	strings::boyer_moore::is_prefix (const char *str, const char *pat, size_t len)
	Check if pat is prefix of str.
void	and_test (const char *text)
	A test case in which we search for every appearance of the word 'and'.
void	pat_test (const char *text)
	A test case in which we search for every appearance of the word 'pat'.
static void	tests ()
	Self-test implementations.
int	main ()
	Main function.

Detailed Description

The Boyer–Moore algorithm searches for occurrences of pattern P in text T by performing explicit character comparisons at different alignments. Instead of a brute-force search of all alignments (of which there are n - m + 1), Boyer–Moore uses information gained by preprocessing P to skip as many alignments as possible.

The key insight in this algorithm is that if the end of the pattern is compared to the text, then jumps along the text can be made rather than checking every character of the text. The reason that this works is that in lining up the pattern against the text, the last character of the pattern is compared to the character in the text.

If the characters do not match, there is no need to continue searching backwards along the text. This leaves us with two cases.

Case 1: If the character in the text does not match any of the characters in the pattern, then the next character in the text to check is located m characters farther along the text, where m is the length of the pattern.

Case 2: If the character in the text is in the pattern, then a partial shift of the pattern along the text is done to line up along the matching character and the process is repeated.

There are two shift rules:

[The bad character rule] (https://en.wikipedia.org/wiki/Boyer%E2%80%93Moore_string-search_algorithm#The_bad_character_rule)

[The good suffix rule] (https://en.wikipedia.org/wiki/Boyer%E2%80%93Moore_string-search_algorithm#The_good_suffix_rule)

The shift rules are implemented as constant-time table lookups, using tables generated during the preprocessing of P.

Author

Stoycho Kyosev

Macro Definition Documentation

APLHABET_SIZE

#define APLHABET_SIZE   CHAR_MAX

number of symbols in the alphabet we use

for assert for CHAR_MAX macro for strlen for IO operations for std::string for std::vector

Function Documentation

and_test()

void and_test

(

const char *

text

)

A test case in which we search for every appearance of the word 'and'.

Parameters

text	The text in which we search for appearance of the word 'and'

Returns

void

                                {
    strings::boyer_moore::pattern ands;
    strings::boyer_moore::init_pattern("and", ands);
    std::vector<size_t> indexes = strings::boyer_moore::search(text, ands);
 
    assert(indexes.size() == 2);
    assert(strings::boyer_moore::is_prefix(text + indexes[0], "and", 3));
    assert(strings::boyer_moore::is_prefix(text + indexes[1], "and", 3));
}

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init_bad_char()

void strings::boyer_moore::init_bad_char	(	const std::string &	str,
		std::vector< size_t > &	arg
	)

A function that preprocess the bad char table.

Parameters

str	The string being preprocessed
arg	The bad char table

Returns

void

                                                                 {
    arg.resize(APLHABET_SIZE, str.length());
 
    for (size_t i = 0; i < str.length(); i++) {
        arg[str[i]] = str.length() - i - 1;
    }
}

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init_good_suffix()

void strings::boyer_moore::init_good_suffix	(	const std::string &	str,
		std::vector< size_t > &	arg
	)

A function that preprocess the good suffix thable.

Parameters

str	The string being preprocessed
arg	The good suffix table

Returns

void

                                                                    {
    arg.resize(str.size() + 1, 0);
 
    // border_pos[i] - the index of the longest proper suffix of str[i..] which
    // is also a proper prefix.
    std::vector<size_t> border_pos(str.size() + 1, 0);
 
    size_t current_char = str.length();
 
    size_t border_index = str.length() + 1;
 
    border_pos[current_char] = border_index;
 
    while (current_char > 0) {
        while (border_index <= str.length() &&
               str[current_char - 1] != str[border_index - 1]) {
            if (arg[border_index] == 0) {
                arg[border_index] = border_index - current_char;
            }
 
            border_index = border_pos[border_index];
        }
 
        current_char--;
        border_index--;
        border_pos[current_char] = border_index;
    }
 
    size_t largest_border_index = border_pos[0];
 
    for (size_t i = 0; i < str.size(); i++) {
        if (arg[i] == 0) {
            arg[i] = largest_border_index;
        }
 
        // If we go pass the largest border we find the next one as we iterate
        if (i == largest_border_index) {
            largest_border_index = border_pos[largest_border_index];
        }
    }
}

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init_pattern()

void strings::boyer_moore::init_pattern	(	const std::string &	str,
		pattern &	arg
	)

A function that initializes pattern.

Parameters

str	Text used for initialization
arg	Initialized structure

Returns

void

                                                      {
    arg.pat = str;
    init_bad_char(str, arg.bad_char);
    init_good_suffix(str, arg.good_suffix);
}

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is_prefix()

bool strings::boyer_moore::is_prefix	(	const char *	str,
		const char *	pat,
		size_t	len
	)

Check if pat is prefix of str.

Parameters

str	pointer to some part of the input text.
pat	the searched pattern.
len	length of the searched pattern

Returns

true if pat IS prefix of str.

false if pat is NOT a prefix of str.

                                                             {
    if (strlen(str) < len) {
        return false;
    }
 
    for (size_t i = 0; i < len; i++) {
        if (str[i] != pat[i]) {
            return false;
        }
    }
 
    return true;
}

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main()

int main

(

void

)

Main function.

Returns

0 on exit

           {
    tests();  // run self-test implementations
    return 0;
}

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pat_test()

void pat_test

(

const char *

text

)

A test case in which we search for every appearance of the word 'pat'.

Parameters

text	The text in which we search for appearance of the word 'pat'

Returns

void

                                {
    strings::boyer_moore::pattern pat;
    strings::boyer_moore::init_pattern("pat", pat);
    std::vector<size_t> indexes = strings::boyer_moore::search(text, pat);
 
    assert(indexes.size() == 6);
 
    for (const auto& currentIndex : indexes) {
        assert(strings::boyer_moore::is_prefix(text + currentIndex, "pat", 3));
    }
}

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search()

std::vector< size_t > strings::boyer_moore::search	(	const std::string &	str,
		const pattern &	arg
	)

A function that implements Boyer-Moore's algorithm.

Parameters

str	Text we are seatching in.
arg	pattern structure containing the preprocessed pattern

Returns

Vector of indexes of the occurrences of pattern in text

                                                                   {
    size_t index_position = arg.pat.size() - 1;
    std::vector<size_t> index_storage;
 
    while (index_position < str.length()) {
        size_t index_string = index_position;
        int index_pattern = static_cast<int>(arg.pat.size()) - 1;
 
        while (index_pattern >= 0 &&
               str[index_string] == arg.pat[index_pattern]) {
            --index_pattern;
            --index_string;
        }
 
        if (index_pattern < 0) {
            index_storage.push_back(index_position - arg.pat.length() + 1);
            index_position += arg.good_suffix[0];
        } else {
            index_position += std::max(arg.bad_char[str[index_string]],
                                       arg.good_suffix[index_pattern + 1]);
        }
    }
 
    return index_storage;
}

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tests()

static void tests ( )

static

Self-test implementations.

Returns

void

                    {
    const char* text =
        "When pat Mr. and Mrs. pat Dursley woke up on the dull, gray  \
                            Tuesday our story starts, \
                there was nothing about pat the cloudy sky outside to pat suggest that\
                        strange and \
                    mysterious things would pat soon be happening all pat over the \
                        country.";
 
    and_test(text);
    pat_test(text);
 
    std::cout << "All tests have successfully passed!\n";
}

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