txt -> length N (string s)
pat -> length M (pattern)
N>=M
//To calculate hash from previous hash
// hash( txt[s+1 .. s+m] ) = ( d ( hash( txt[s .. s+m-1]) – txt[s]*h ) + txt[s + m] ) mod q
// hash( txt[s .. s+m-1] ) : Hash value at shift s.
// hash( txt[s+1 .. s+m] ) : Hash value at next shift (or shift s+1)
// d: Number of characters in the alphabet (256)
// q: A prime number (INT_MAX) //Used for modulo
// h: d^(m-1)
// d is the number of characters in the input alphabet
#define d 256
void search(char pat[], char txt[], int q)
{
int M = strlen(pat);
int N = strlen(txt);
int i, j;
int p = 0; // hash value for pattern
int t = 0; // hash value for txt
int h = 1;
// The value of h would be "pow(d, M-1)%q"
for (i = 0; i < M - 1; i++)
h = (h * d) % q;
// Calculate the hash value of pattern and first
// window of text
for (i = 0; i < M; i++)
{
p = (d * p + pat[i]) % q;
t = (d * t + txt[i]) % q;
}
// Slide the pattern over text one by one
for (i = 0; i <= N - M; i++)
{
// Check the hash values of current window of text
// and pattern. If the hash values match then only
// check for characters one by one
if ( p == t )
{
/* Check for characters one by one */
for (j = 0; j < M; j++)
{
if (txt[i+j] != pat[j])
{
break;
}
}
// if p == t and pat[0...M-1] = txt[i, i+1, ...i+M-1]
if (j == M)
cout<<"Pattern found at index "<< i<<endl;
}
// Calculate hash value for next window of text: Remove
// leading digit, add trailing digit
//As we always remove first character, so we multiply it by h (d^m-1)
if ( i < N-M )
{
t = (d*(t - txt[i]*h) + txt[i+M])%q;
// We might get negative value of t, converting it
// to positive
if (t < 0)
t = (t + q);
}
}
}