#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define MAX_NUM 10000
#define MAX_BINS 100
int defineInput(double buf[], int max);
void fillDist(double *data, int *bin, double min, double max, 
              int n, int nData);
double chiSquare(int *obs, double *exp, int n);
main() {
  int k, bins, entries, content[MAX_BINS] = {0};
  int seed;
  double xmin, xmax;
  double average = 0., chi2, data[MAX_NUM]; 
  double expected[MAX_BINS] = {0.};
  seed = time(0);
  srand48(seed);
  entries = defineInput(data, MAX_NUM);
  printf("Dare limiti della distribuzione\n xmin = ");
  scanf("%lf", &xmin);
  printf(" xmax = ");
  scanf("%lf", &xmax);
  do {
    printf("Dare numero di intervalli\n bins = ");
    scanf("%d", &bins);
  } while (bins < 0 || bins > MAX_BINS);
  fillDist(data, content, xmin, xmax, bins, entries);
  for (k = 0; k < bins; k++) {
    average += (double)content[k];
  }
  average /= (double)bins;
  for (k = 0; k < bins; k++) {
    expected[k] = average;
  }  
  chi2 = chiSquare(content, expected, bins);
  printf(" Chi2 = %f\n", chi2);
  exit(EXIT_SUCCESS);   
}
double chiSquare(int *obs, double *exp, int n) {
  double S = 0., t1, t2;
  int k;
  for (k = 0; k < n; k++) {
    t1 = (double) *(obs + k);
    t2 = *(exp + k);
    S += (t1 - t2) * (t1 - t2) / t2;
  }
  return S;
}

void fillDist(double *data, int *bin, double min, double max, int n, int nData) {
  int i, j;
  double invWidth = (double) n / (max - min);
  for (i = 0; i < nData; i++) {    
    j =  ( (*(data + i) - min) * invWidth);
    (*(bin+j))++;    
  }
}

int defineInput(double buf[], int max) {
  int k, N;
  do {
    printf("Numero di eventi nell'istogramma = ");
    scanf("%d", &N);
  } while (N < 0 || N > max);
  for(k=0; k < N; k++) {
    buf[k] = (double) lrand48() / (RAND_MAX + 1.);
    if(k<10)printf("TOST %lf\n",buf[k]);
  }
  return N;
}