//
// Program to compute the variance of a set of numbers. 
//
// Command-line arguments are:
//    number of threads (P) -- required; number of threads to use
//    number of numbers (N) -- optional; see below
//
// If only one command-line argument is given, the program reads
//    input numbers (floating-point okay) from standard input until 
//    end-of-file is reached.
// If two command-line arguments are given, the program generates
//    N random numbers to use as input.
//
// Program output:
//    computed variance
//    elapsed time needed for computation, starting when all the
//      input numbers have been read in / generated, and ending
//      when the variance has been computed.
// 
#include <iostream.h>
#include <iomanip>		// has setprecision()
#include <stdlib.h>		// has exit(), etc.
#include <vector>
#include <algorithm>		// has generate()
#include <numeric>		// has accumulate()
#include "threads-timer.h"      // has timer()

// Function to compute a random number between 0 and 1.
double dRand() { return double(rand())/double(RAND_MAX); }

// ---- Main program ----

int main(int argc, char* argv[]) {

  int P;			// number of threads
  int N;			// number of input numbers
  double * nums;		// will point to array of input numbers

  if (argc < 2) {
    cerr << "Usage:  " << argv[0] << " numThreads [numNumbers]\n";
    exit(EXIT_FAILURE);
  }

  P = atoi(argv[1]);

  // Read/generate input.
  if (argc == 2) {
    // read input
    vector<double> tempNums;
    double temp;
    cout << "Enter input numbers, control-D to end:\n";
    while (cin >> temp)
      tempNums.push_back(temp);
    N = tempNums.size();
    nums = new double[N];
    copy(tempNums.begin(), tempNums.end(), nums);
  }
  else {
    // generate input
    N = atoi(argv[2]);
    nums = new double[N];
    generate(nums, nums+N, dRand);
#ifdef DEBUG
    cerr << "Input:  \n";
    for (int i = 0; i < N; ++i)
      cerr << nums[i] << endl;
#endif
  }

  // Print some useful info.
  cout << "Computing the variance of " << N << " numbers using "
       << P << " threads\n";
  
  // Start timing.
  double startTime = timer();

  // Compute variance.
  // ==== START OF CODE TO REPLACE ====
  double avg = (accumulate(nums, nums+N, 0.0))/double(N);
#ifdef DEBUG
  cerr << "Average = " << avg << endl;
#endif
  double variance = 0.0;
  for (int i = 0; i < N; ++i) 
    variance += (nums[i] - avg)*(nums[i] - avg);
  // ==== END OF CODE TO REPLACE ====

  // Stop timing.
  double endTime = timer();

  // Print results.
  cout << "Variance = " << variance << endl;
  cout << "Time to compute (seconds) = " << setprecision(4)
       << endTime - startTime << endl;
  return EXIT_SUCCESS;
}