#include<stdio.h>
#include<stdlib.h>
#include<math.h>

#define Trial 1000000
#define Number_of_Bins 2001 
#define KbT 1.0

FILE *fp;

void simple_PRNG_function(int *idum_, double *random_);

int main()
{
    int i, idum, Bin;
    double random, Initial_x, Former_x, Latter_x;
    double Delta_E, Boltzmann_Factor, Checker;
    int y_axis[Number_of_Bins+1];
    double x_axis[Number_of_Bins+1];
	char fname[80];
 
    for(i=0; i<Number_of_Bins; i++)
    {
        y_axis[i] = 0;
        x_axis[i] = 0.;
    }

    idum = 5981437;

    simple_PRNG_function(&idum, &random);

    Initial_x = ((10.0*random)-5.0);

    Former_x = Initial_x;

    for(i=1; i<=Trial; i++)
    {
        simple_PRNG_function(&idum, &random);

        Latter_x = Former_x + ((2.0*random)-1.0);

        Delta_E = (Latter_x*Latter_x) - (Former_x*Former_x);

        Boltzmann_Factor = exp(-Delta_E/KbT);

        simple_PRNG_function(&idum, &random);

        Checker = random;

        if(Checker<=Boltzmann_Factor)
        {
            Bin = (int)((Latter_x+10.005)*100);

            if(0<=Bin<Number_of_Bins)
                y_axis[Bin]++;

            Former_x = Latter_x;
        }
        else
            i--;

    } /////* end of for i */////

    for(i=0; i<Number_of_Bins; i++)
        x_axis[i] = ((double)i/100.0) - 10.0;

	sprintf(fname, "histogram_kbt%2.2f.dat", KbT);
    fp=fopen(fname, "w");
  
    for(i=0; i<Number_of_Bins; i++)
        fprintf(fp, "%f\t%d\n", x_axis[i], y_axis[i]); 
  
    fclose(fp);
	return 0;

} /////* end of main */////

void simple_PRNG_function(int *idum_, double *random_)
{
    *idum_ = (*idum_)*1812433253 + 1;
    *random_ = 2.3283064e-10*(*idum_) + 0.5;
} /////* end of simple_PRNG_function() */////