#include <iostream>
#include <math.h>
using namespace std;

enum state { ITERATING, SUCCESS, WONTSTOP, NEARZERO, SAMESIGN };
typedef double (*func)(double);
state interval(func, double, double, double&, int&);
state regfalsi(func, double, double, double&);


const double delta = 5.0E-7;
const double eps = 5.0E-8;
const int maxiter = 100;

state interval(func f, double a, double b, double &x, int &iter)
{  
       if (fabs(a - b) < delta) return SUCCESS;
       if (++iter == maxiter) return WONTSTOP;
       x = a - f(a)*(b - a)/(f(b) - f(a));
       if (fabs(f(x)) < eps) return SUCCESS;
       cout << iter << "\t" << a << "\t" << b << "\t" << x << "\n";
       f(x)*f(a) > 0 ? a = x: b = x;
       return interval(f, a, b, x, iter);
}

state regfalsi(func f, double a, double b, double &x)
{
       if (f(a)*f(b) > 0) return  SAMESIGN;
       int iter = 0;
       return interval(f, a, b, x, iter);
}



inline double F(double x) { return x - exp(-x);}

main()
{
       double a, b, root;
       cout << "lower estimate = ";
       cin >> a;
       cout << "upper estimate = ";
       cin >> b;
       state s = regfalsi(F, a, b, root);
       switch(s){
       case SUCCESS: cout << "root = " << root << "\n"; break;
       case WONTSTOP: cout << "not convergent!\n"; break;
       case SAMESIGN: cout << "unsuitable interval!\n"; break;
       default: cout << "that's very peculiar!\n"; break;
       }

}