#include<iostream.h>
#include<math.h>


#define f(x) (1/x)  //舉例函數
#define epsilon 0.000001  //精度

double Romberg(double aa, double bb,int MAXREPT)
{ //aa,bb 積分上下限
    int m, n;//m控制迭代次數, 而n控制復化梯形積分的分點數. n=2^m
    double h, x;
    double s, q;
    double ep; //精度要求
    double *y = new double[4];//為節省空間,只需一維數組
    //每次循環依次存儲Romberg計算表的每行元素,以供計算下一行,算完后更新
    double p;//p總是指示待計算元素的前一個元素(同一行)
    //迭代初值
    h = bb - aa;
    y[0] = h*(f(aa) + f(bb))/2.0;
    m = 1; 
    n = 1; 
    ep = epsilon + 1.0;
    //迭代計算
    while ((ep >= epsilon) && (m <= MAXREPT))
    { 
        //復化積分公式求T2n(Romberg計算表中的第一列),n初始為1,以后倍增
        p = 0.0;
        for (int i=0; i<n; i++)//求Hn
        { 
            x = aa + (i+0.5)*h;
            p = p + f(x);
        }
        p = (y[0] + h*p)/2.0;//求T2n = 1/2(Tn+Hn),用p指示
        //求第m行元素,根據Romberg計算表本行的前一個元素(p指示),
        //和上一行左上角元素(y[k-1]指示)求得.        
        s = 1.0;
	    int k;
        for (k=1; k<=m; k++)
        { 
			if(k==4)
				break;
            s = 4.0*s;
            q = (s*p - y[k-1])/(s - 1.0);
            y[k-1] = p; 
            p = q;
        }
	    y[k-1] = q; 
        ep = fabs(q - y[k-2]);
        m = m + 1; 
        n = n + n; h = h/2.0;
    }
    return (q);
}


void main()
{
    double a,b;
	int MAXREPT;
    cout<<"Romberg積分,請輸入積分范圍a,b:"<<endl;
    cin>>a>>b;
    cout<<"Romberg積分迭代次數:"<<endl;
	cin>>MAXREPT;
    cout<<"積分結果:"<<Romberg(a, b, MAXREPT)<<endl;
}