字符串hash模板题(Oulipo

字符串hash模板题(Oulipo)

题目poj-3461

The French author Georges Perec (1936–1982) once wrote a book, La disparition, without the letter 'e'. He was a member of the Oulipo group. A quote from the book:

Tout avait Pair normal, mais tout s’affirmait faux. Tout avait Fair normal, d’abord, puis surgissait l’inhumain, l’affolant. Il aurait voulu savoir où s’articulait l’association qui l’unissait au roman : stir son tapis, assaillant à tout instant son imagination, l’intuition d’un tabou, la vision d’un mal obscur, d’un quoi vacant, d’un non-dit : la vision, l’avision d’un oubli commandant tout, où s’abolissait la raison : tout avait l’air normal mais…

Perec would probably have scored high (or rather, low) in the following contest. People are asked to write a perhaps even meaningful text on some subject with as few occurrences of a given “word” as possible. Our task is to provide the jury with a program that counts these occurrences, in order to obtain a ranking of the competitors. These competitors often write very long texts with nonsense meaning; a sequence of 500,000 consecutive 'T's is not unusual. And they never use spaces.

So we want to quickly find out how often a word, i.e., a given string, occurs in a text. More formally: given the alphabet {'A', 'B', 'C', …, 'Z'} and two finite strings over that alphabet, a word W and a text T, count the number of occurrences of W in T. All the consecutive characters of W must exactly match consecutive characters of T. Occurrences may overlap.

Input
The first line of the input file contains a single number: the number of test cases to follow. Each test case has the following format:

One line with the word W, a string over {'A', 'B', 'C', …, 'Z'}, with 1 ≤ |W| ≤ 10,000 (here |W| denotes the length of the string W).
One line with the text T, a string over {'A', 'B', 'C', …, 'Z'}, with |W| ≤ |T| ≤ 1,000,000.
Output
For every test case in the input file, the output should contain a single number, on a single line: the number of occurrences of the word W in the text T.

Sample Input
3
BAPC
BAPC
AZA
AZAZAZA
VERDI
AVERDXIVYERDIAN
Sample Output
1
3
0

题意

给你两个字符串，求第一个字符串在第二个字符串中出现的次数。

代码模板

#include 
#include 
#include 
#include 
using namespace std;
#define base 161
typedef unsigned long long ull;

ull p[10000000];
ull ash[10000000];
ull a1;
ull gethash(int l, int r)
{
    return ash[r] - ash[l - 1] * p[r - l + 1];
}

int main()
{
    ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
    int T;
    cin >> T;
    p[0] = 1;
    for (int i = 1; i <= 1000000; i++)
        p[i] = p[i - 1] * base;
    while (T--)
    {
        ash[0] = 0;
        a1 = 0;
        //string 可换成char[]
        string a, b;
        int lena, lenb;
        cin >> a >> b;
        lena = a.size(), lenb = b.size();
        for (int i = 1; i <= lena; i++)
            a1 = a1 * base + a[i - 1];

        for (int i = 1; i <= lenb; i++)
            ash[i] = ash[i - 1] * base + b[i - 1];
        int res = 0;
        for (int i = 1; i + lena - 1 <= lenb; i++)
        {

            if (a1 == gethash(i, i + lena - 1))
                res++;
        }
        cout << res << endl;
    }
}