性能评估

题目描述：Amtel has announced that it will release a 128-bit computer chip by 2010, a 256-bit computer by 2020, and so on, continuing its strategy of doubling the word-size every ten years. (Amtel released a 64-bit computer in 2000, a 32-bit computer in 1990, a 16-bit computer in 1980, an 8-bit computer in 1970, and a 4-bit computer, its first, in 1960.)

Amtel will use a new benchmark - the Factstone - to advertise the vastly improved capacity of its new chips. The Factstone rating is defined to be the largest integer n such that n! can be represented as an unsigned integer in a computer word.

输入：Given a year 1960 ≤ y ≤ 2160, what will be the Factstone rating of Amtel’s most recently released chip?

输出：For test, there is one line of input containing y. For each test case, output a line giving the Factstone rating.

示例：

输入： 1960

输出： 3

（解释：因为1960年计算机是支持4位，其中的最大的无符号数为（全为1）：15；3的阶乘为6，4的阶乘为24，故这里的n=3）

思考：

1、阶乘数据太大，2的高次方数据也太大。

所以考虑使用对数简化计算。

2、float比double有更大精度误差，所以使用double类型。

浮点的作差规范做法：

#define eps 1e-8

double a, b;
while(a - b > eps) {//两个浮点数作差小于某个很小的数，就可以认为他们相等了。

}

#include <stdio.h>
#include <math.h>
int main() {
int y;
scanf("%d", &y);
double bit = (y - 1960) / 10;
bit = pow(2, bit + 2);
int i = 1;
while (bit > log2(i)) {
bit -= log2(i);
i++;
}
i--;
printf("%d\n", i);
return 0;
}