我正在寻找最快的方法来获得π的值，作为一个个人挑战。更具体地说，我使用的方法不涉及使用#define常量M_PI，或硬编码的数字。
下面的程序测试了我所知道的各种方法。从理论上讲，内联汇编版本是最快的选择，尽管显然不能移植。我将它作为一个基准，与其他版本进行比较。在我的测试中，使用内置函数，4 * atan(1)版本在GCC 4.2上是最快的，因为它自动将atan(1)折叠成一个常量。通过指定-fno-builtin, atan2(0， -1)版本是最快的。
下面是主要的测试程序(pitimes.c):
#include <math.h>
#include <stdio.h>
#include <time.h>
#define ITERS 10000000
#define TESTWITH(x) { \
diff = 0.0; \
time1 = clock(); \
for (i = 0; i < ITERS; ++i) \
diff += (x) - M_PI; \
time2 = clock(); \
printf("%s\t=> %e, time => %f\n", #x, diff, diffclock(time2, time1)); \
}
static inline double
diffclock(clock_t time1, clock_t time0)
{
return (double) (time1 - time0) / CLOCKS_PER_SEC;
}
int
main()
{
int i;
clock_t time1, time2;
double diff;
/* Warmup. The atan2 case catches GCC's atan folding (which would
* optimise the ``4 * atan(1) - M_PI'' to a no-op), if -fno-builtin
* is not used. */
TESTWITH(4 * atan(1))
TESTWITH(4 * atan2(1, 1))
#if defined(__GNUC__) && (defined(__i386__) || defined(__amd64__))
extern double fldpi();
TESTWITH(fldpi())
#endif
/* Actual tests start here. */
TESTWITH(atan2(0, -1))
TESTWITH(acos(-1))
TESTWITH(2 * asin(1))
TESTWITH(4 * atan2(1, 1))
TESTWITH(4 * atan(1))
return 0;
}
内联汇编的东西(fldpi.c)只适用于x86和x64系统:
double
fldpi()
{
double pi;
asm("fldpi" : "=t" (pi));
return pi;
}
和一个构建脚本，构建我正在测试的所有配置(build.sh):
#!/bin/sh
gcc -O3 -Wall -c -m32 -o fldpi-32.o fldpi.c
gcc -O3 -Wall -c -m64 -o fldpi-64.o fldpi.c
gcc -O3 -Wall -ffast-math -m32 -o pitimes1-32 pitimes.c fldpi-32.o
gcc -O3 -Wall -m32 -o pitimes2-32 pitimes.c fldpi-32.o -lm
gcc -O3 -Wall -fno-builtin -m32 -o pitimes3-32 pitimes.c fldpi-32.o -lm
gcc -O3 -Wall -ffast-math -m64 -o pitimes1-64 pitimes.c fldpi-64.o -lm
gcc -O3 -Wall -m64 -o pitimes2-64 pitimes.c fldpi-64.o -lm
gcc -O3 -Wall -fno-builtin -m64 -o pitimes3-64 pitimes.c fldpi-64.o -lm
除了在各种编译器标志之间进行测试(我也比较了32位和64位，因为优化是不同的)，我还尝试切换测试的顺序。但是，atan2(0， -1)版本在每次测试中仍然名列前茅。
蒙特卡罗方法，如前所述，应用了一些伟大的概念，但很明显，它不是最快的，不是从任何合理的标准来看。此外，这完全取决于你想要什么样的准确性。我所知道的最快的π是数字硬编码的π。看看圆周率和圆周率，有很多公式。
Here is a method that converges quickly — about 14 digits per iteration. PiFast, the current fastest application, uses this formula with the FFT. I'll just write the formula, since the code is straightforward. This formula was almost found by Ramanujan and discovered by Chudnovsky. It is actually how he calculated several billion digits of the number — so it isn't a method to disregard. The formula will overflow quickly and, since we are dividing factorials, it would be advantageous then to delay such calculations to remove terms.
在那里,
下面是Brent-Salamin算法。维基百科提到，当a和b“足够接近”时，(a + b)²/ 4t将是π的近似值。我不确定“足够接近”是什么意思，但从我的测试来看，一次迭代得到2位数字，两次得到7位，3次得到15位，当然这是双精度，所以它可能会有一个基于它的表示的错误，真实的计算可能会更准确。
let pi_2 iters =
let rec loop_ a b t p i =
if i = 0 then a,b,t,p
else
let a_n = (a +. b) /. 2.0
and b_n = sqrt (a*.b)
and p_n = 2.0 *. p in
let t_n = t -. (p *. (a -. a_n) *. (a -. a_n)) in
loop_ a_n b_n t_n p_n (i - 1)
in
let a,b,t,p = loop_ (1.0) (1.0 /. (sqrt 2.0)) (1.0/.4.0) (1.0) iters in
(a +. b) *. (a +. b) /. (4.0 *. t)
最后，来点圆周率高尔夫(800位数字)怎么样?160个字符!
int a=10000,b,c=2800,d,e,f[2801],g;main(){for(;b-c;)f[b++]=a/5;for(;d=0,g=c*2;c-=14,printf("%.4d",e+d/a),e=d%a)for(b=c;d+=f[b]*a,f[b]=d%--g,d/=g--,--b;d*=b);}
基本上是C版本的回形针优化器的答案，并且更加简化:
#include <stdio.h>
#include <math.h>
double calc_PI(int K) {
static const int A = 545140134;
static const int B = 13591409;
static const int D = 640320;
const double ID3 = 1.0 / ((double) D * (double) D * (double) D);
double sum = 0.0;
double b = sqrt(ID3);
long long int p = 1;
long long int a = B;
sum += (double) p * (double) a * b;
for (int k = 1; k < K; ++k) {
a += A;
b *= ID3;
p *= (6 * k) * (6 * k - 1) * (6 * k - 2) * (6 * k - 3) * (6 * k - 4) * (6 * k - 5);
p /= (3 * k) * (3 * k - 1) * (3 * k - 2) * k * k * k;
p = -p;
sum += (double) p * (double) a * b;
}
return 1.0 / (12 * sum);
}
int main() {
for (int k = 1; k <= 5; ++k) {
printf("k = %i, PI = %.16f\n", k, calc_PI(k));
}
}
但为了更简化，这个算法采用Chudnovsky公式，如果你不太理解代码，我可以完全简化这个公式。
Summary: We will get a number from 1 to 5 and add it in to a function we will use to get PI. Then 3 numbers are given to you: 545140134 (A), 13591409 (B), 640320 (D). Then we will use D as a double multiplying itself 3 times into another double (ID3). We will then take the square root of ID3 into another double (b) and assign 2 numbers: 1 (p), the value of B (a). Take note that C is case-insensitive. Then a double (sum) will be created by multiplying the value's of p, a and b, all in doubles. Then a loop up until the number given for the function will start and add up A's value to a, b's value gets multiplied by ID3, p's value will be multiplied by multiple values that I hope you can understand and also gets divided by multiple values as well. The sum will add up by p, a and b once again and the loop will repeat until the value of the loop's number is greater or equal to 5. Later, the sum is multiplied by 12 and returned by the function giving us the result of PI.
好吧，这很长，但我想你会理解的……
