仿matlab的gamma函数的做法，做的 。在 x >16后 x为整数时 计算误差偏大 。
public class ComputeGamma {
static double x = 0;
static double[] p = {-1.71618513886549492533811e+0,2.47656508055759199108314e+1,
-3.79804256470945635097577e+2,6.29331155312818442661052e+2,
8.66966202790413211295064e+2,-3.14512729688483675254357e+4,
-3.61444134186911729807069e+4,6.64561438202405440627855e+4};
static double[] q = {-3.08402300119738975254353e+1,3.15350626979604161529144e+2,
-1.01515636749021914166146e+3,-3.10777167157231109440444e+3,
2.25381184209801510330112e+4,4.75584627752788110767815e+3,
-1.34659959864969306392456e+5,-1.15132259675553483497211e+5};
static double[] c = {-1.910444077728e-03,8.4171387781295e-04,
-5.952379913043012e-04,7.93650793500350248e-04,
-2.777777777777681622553e-03, 8.333333333333333331554247e-02,
5.7083835261e-03};
public static double getValue(double a){
x = a;
double xp = x;
double res = 0;
double xn = 0;
double z = 0;
double xnum = 0;
double xden = 0;
double ysq = 0;
double sum = 0;
if(x == 0){
return 1;
}
if(x <= 1){
xp = xp + 1;
}
if(x <= 12){
xn = Math.floor(xp) - 1;
xp = xp - xn;
z = xp - 1;
xnum = 0;
xden = xnum + 1;
for(int i = 0;i < p.length;i++){
xnum = (xnum + p[i]) * z;
xden = xden * z + q[i];
}
res = xnum / xden + 1;
}
if(x <= 1){
res = res / x;
}
if(x > 2 && x <= 12){
for(int i = 0;i < xn; i++ ){
res = res * xp;
xp = xp + 1;
}
}
if(x > 12){
ysq = xp * xp;
sum = c[6];
for(int i = 0;i < c.length-1;i++){
sum = sum / ysq + c[i];
}
double spi = 0.9189385332046727417803297;
sum = sum / xp - xp + spi;
sum = sum + (xp-0.5) * Math.log(xp);
res = Math.exp(sum);
}
return res;
}
}