79 lines
1.9 KiB
C++
79 lines
1.9 KiB
C++
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#include <bits/stdc++.h>
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using namespace std;
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// d is the number of characters in the input alphabet
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#define d 256
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/* pat -> pattern
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txt -> text
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q -> A prime number
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*/
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void search(char pat[], char txt[], int q)
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{
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int M = strlen(pat);
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int N = strlen(txt);
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int i, j;
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int p = 0; // hash value for pattern
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int t = 0; // hash value for txt
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int h = 1;
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// The value of h would be "pow(d, M-1)%q"
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for (i = 0; i < M - 1; i++)
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h = (h * d) % q;
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// Calculate the hash value of pattern and first
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// window of text
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for (i = 0; i < M; i++)
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{
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p = (d * p + pat[i]) % q;
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t = (d * t + txt[i]) % q;
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}
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// Slide the pattern over text one by one
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for (i = 0; i <= N - M; i++)
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{
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// Check the hash values of current window of text
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// and pattern. If the hash values match then only
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// check for characters on by one
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if ( p == t )
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{
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/* Check for characters one by one */
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for (j = 0; j < M; j++)
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{
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if (txt[i+j] != pat[j])
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break;
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}
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// if p == t and pat[0...M-1] = txt[i, i+1, ...i+M-1]
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if (j == M)
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cout<<"Pattern found at index "<< i<<endl;
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}
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// Calculate hash value for next window of text: Remove
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// leading digit, add trailing digit
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if ( i < N-M )
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{
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t = (d*(t - txt[i]*h) + txt[i+M])%q;
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// We might get negative value of t, converting it
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// to positive
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if (t < 0)
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t = (t + q);
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}
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}
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}
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/* Driver code */
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int main()
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{
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char txt[] = "ABCAMCABAMMAM";
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char pat[] = "AM";
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// A prime number
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int q = 101;
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// Function Call
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search(pat, txt, q);
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return 0;
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}
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