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C++ Program to Implement Bitap Algorithm for String Matching
This is a C++ Program to Implement Bitap Algorithm for String Matching. The algorithm tells whether a given text contains a substring which is “approximately equal” to a given pattern, where approximate equality is defined in terms of Levenshtein distance — if the substring and pattern are within a given distance k of each other, then according to the algorithm they are equal. It begins by precomputing a set of bitmasks containing one bit for each element of the pattern. So we are able to do most of the work with bitwise operations, which are extremely fast.
Algorithm
Begin Take the string and pattern as input. function bitmap_search() and it takes argument string text t and string pattern p : Initialize the bit array A. Initialize the pattern bitmasks, p_mask[300] Update the bit array. for i = 0 to 299 p_mask[i] = ~0 for i = 0 to m-1 p_mask[p[i]] and= ~(1L left shift i); for i = 0 to t.length()-1 A |= p_mask[t[i]]; A <<= 1; if ((A and (1L left shift m)) == 0 return i - m + 1 return -1 End
Example Code
#include <string>
#include <map>
#include <iostream>
using namespace std;
int bitmap_search(string t, string p) {
int m = p.length();
long p_mask[300];
long A = ~1;
if (m == 0)
return -1;
if (m >63) {
cout<<"Pattern is too long!";//if pattern is too long
return -1;
}
for (int i = 0; i <= 299; ++i)
p_mask[i] = ~0;
for (int i = 0; i < m; ++i)
p_mask[p[i]] &= ~(1L << i);
for (int i = 0; i < t.length(); ++i) {
A |= p_mask[t[i]];
A <<= 1;
if ((A & (1L << m)) == 0)
return i - m + 1;
}
return -1;
}
void findPattern(string t, string p) {
int position = bitmap_search(t, p);//initialize the position with the function bitmap_search
if (position == -1)
cout << "\nNo Match\n";
else
cout << "\nPattern found at position : " << position;
}
int main(int argc, char **argv) {
cout << "Enter Text:\n";
string t;
cin >>t;
cout << "Enter Pattern:\n";
string p;
cin >>p;
findPattern(t, p);
}Output
Enter Text: Tutorialspoint Enter Pattern: point Pattern found at position : 9
Updated on: 30-Jul-2019
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