是否可以窥视下一个兰特值

Question

假设生成器已播种,可以在不更改的情况下查看下一个随机值？

即给出:

#include <stdlib.h>

int r;
r = rand(); // say this is 99
r = rand(); // say this is 80

这可能吗

#include <stdlib.h>

int r;
r = peekAtRand(); // this will give 99
r = rand(); // but this still gives 99

r = peekAtRand(); // this will give 80
r = rand(); // but this still gives 80

此外,这可以扩展到下一个n号码偷看吗？

Answer 1

对于大多数随机数生成器的当前实现,这是不可能的.但是有两个解决方案.

解决方案1

如果将使用该srand()函数的随机数生成器的起始值设置为相同的值,则始终会获得相同的数字序列.这样,您可以轻松预测第二遍中的数字.

解决方案2

简单地为数字写一个小缓冲区.

const int randBufferSize = 1024;
int randBuffer[randBufferSize];
int randBufferPosition = 0;

// Initialise the buffer with random data.
void initRandBuffer() {
    for (int i = 0; i < randBufferSize; ++i) {
        randBuffer[i] = rand();
    }
}

// Peek at the n'th random number (starting from 0).
int peekAtRand(int n) {
    int peekIndex = randBufferPosition + n;
    if (peekIndex >= randBufferSize) {
        peekIndex -= randBufferSize;
    }
    return randBuffer[peekIndex];
}

// Get the next random number.
int getRand() {
    int result = randBuffer[randBufferPosition];
    randBuffer[randBufferPosition] = rand();
    ++randBufferPosition;
    if (randBufferPosition >= randBufferPosition) {
        randBufferPosition = 0;
    }
}