卡普·拉宾的素数和块长度

我从那个站点找到了一个rabin karp代码，并更改为尝试。更改后的代码如下。您可以在hashtable.txt中看到单词及其散列值。对于下面的示例，hashtable.txt似乎是正确的。

但是当我将M(块长度)更改为150时，我得到了错误的结果。例如，在hashtable.txt中，第一行和第6行必须相同，但它们的哈希值不同。

或者当我将q(素数)更改为683303时，它也会得到错误的结果。

rabin karp算法中质数与块长度的关系是什么，产生错误结果的原因是什么?

#include<stdio.h>
#include<string.h>
#include <fstream>
#include <iostream>
// d is the number of characters in input alphabet
#define d 256 
int M = 80;
/*  
txt  -> text
q    -> A prime number
*/
using namespace std;
void writeTable(char *txt, int q)
{
ofstream myfile;
myfile.open ("hashtable.txt");
int N = strlen(txt);
int i, j;
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++)
{
    t = (d*t + txt[i])%q;
}
// Slide the pattern over text one by one 
for (i = 0; i <= N - M; i++)
{
    myfile << t <<" ";
    for (long z = i; z < M+i; z++){myfile<<txt[z];}myfile<<"n";
    // Calulate hash value for next window of text: Remove leading digit, 
    // add trailing digit           
    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); 
    }
}
myfile.close();
}
/* Driver program to test above function */
int main()
{
    char *txt ="abcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcde";
int q = 683303;  // A prime number
writeTable(txt, q);
printf("finish");
getchar();
return 0;
}