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

// preferred method for defining array size:
#define MAXDICE 1000000

void Roll(int* array_, int size_);
int Stat(int* array_, int size_, double* average_, double* empdev_);

int main(void) {
  int *dice;  // array to store dice rolls
  double av;  // average 
  double dev; // standard deviation (estimation of)
              // Homework: what is the exact value?
  
  // memory allocation for the array 'dice':
  dice = (int*)calloc(MAXDICE, sizeof(int));
  if (!dice) {
    fprintf(stderr, "Not enough memory or memory error... Exit...\n");
    return 0;
  }
  
  srand(time(0));  // initialize with system time (not the best)

  // generate data:
  Roll(dice, MAXDICE);
  
  // call the statistics function:
  if (Stat(dice, MAXDICE, &av, &dev)) {
    printf("%lf +/- %lf\n", av, dev);
  }
  else {
    fprintf(stderr, "Stat function failed. Return 0...\n");
    free(dice);
    return 0;
  }
  
  free(dice);
  return 1;
}

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

void Roll(int* array_, int size_) {
  int i;
  for (i=0; i<size_; array_[i++] = rand() % 6 +1); // Too compact.
                                                   // Not recommended but still does work.
}

int Stat(int* array_, int size_, double* average_, double* empdev_) {
  // Statistics function
  // calculates the avergae and estimation of standard dev.
  // see: http://en.wikipedia.org/wiki/Standard_deviation#With_sample_standard_deviation
  // return value: 0 - error, 1 - OK.
  
  int i;
  long int avrg = 0;
  double ssum = 0.0;
    
  // check the range of the data
  // (only for demonstration)
  for (i=0; i<size_; i++) {
    if ( (array_[i] < 1) || (array_[i] > 6) ) {
      fprintf(stderr, "Invalid data: array_[%d]=%d... Return 0...\n", i, array_[i]);
      return 0; // something went wrong
    }
  }
  
  // calculate average:
  avrg = 0;
  for (i=0; i<size_; i++) {
    avrg += array_[i];
  }
  *average_ = (double)avrg / (double)size_;
  
  // estimate standard deviation:
  if (size_ < 2) {
    *empdev_ = -1.0;
    fprintf(stderr, "size_ = %d too small... Return 0...\n", size_);
    return 0;
  }
  ssum = 0.0;
  for (i=0; i<size_; i++) {
    ssum += (array_[i] - *average_) * (array_[i] - *average_);
  }
  *empdev_ = sqrt(ssum / (size_-1));
  
  // here, all well done:
  return 1;
}