//====================================================== file = as3_1ec.c =====
//=  Simulation of booting-up multiple server with failure probability        =
//=============================================================================
//=  Notes: 1) This is assignment #1 for CIS 4930 (Simulation)                =
//=             - This is the extra credit solution                           =
//=         2) The theoretical solution for 10 + 1 servers is:                =
//=              Percent on time = 100.0 *                                    =
//=                (Pr[0 of 10 fail] + Pr[1 of 10 fail] * (1 - Pr[boot fail)) =
//=              where Pr[0 of 10 fail] = (1 - Pr[boot fail])^10 and          =
//=              Pr[1 of 10 fail] = 10*(1 - Pr[boot fail])^9 * Pr[boot fail]  =
//=              for Pr[boot fail] = 0.01 the answer is, 99.482%              =
//=---------------------------------------------------------------------------=
//=  Build: bcc32 as3_1ec.c, gcc -o as3_1ec as3_1ec.c                         =
//=---------------------------------------------------------------------------=
//=  Execute: as3_1ec                                                         =
//=---------------------------------------------------------------------------=
//=  History: KJC (12/16/98) - Genesis                                        =
//=           KJC (12/16/98) - Modified as3_1sol.c for the extra credit       =
//=                            requirement                                    =
//=============================================================================

//----- Include files ---------------------------------------------------------
#include <stdio.h>              // Needed for printf()
#include <stdlib.h>             // Needed for rand() and RAND_MAX

//----- Constants -------------------------------------------------------------
#define FALSE              0    // Boolean false
#define TRUE               1    // Boolean true
#define NUM_DAYS       10000    // Number of days to simulate
#define NUM_SERVERS       11    // Number of servers
#define NUM_ALLOW_FAIL     1    // Number of allowed failures
#define FAIL_PERCENT      1.0   // Percent of time a boot-up fails

//----- Function prototypes ---------------------------------------------------
double rand_val(void);          // Generate random value between 0.0 and 1.0

//===== Main program ==========================================================
void main(void)
{
  int i, j;         // Loop counters
  int sum_fail;     // Counter for "opening late" days
  int fail;         // Failure flag (set to TRUE or FALSE)
  int num_fail;     // Number of failed servers in a given morning
  double on_time;   // Percentage of days on which mall opens on time

  // Initialize sum_fail to zero
  sum_fail = 0;

  // Output a "running" banner
  printf(">>> The simulation is running for %d days... \n", NUM_DAYS);

  // Run the simulation for NUM_DAYS
  for (i=0; i<NUM_DAYS; i++)
  {
    // Loop through a boot-up of NUM_SERVERS setting fail to TRUE
    // if more than NUM_ALLOW_FAIL servers fail their boot-up
    fail = FALSE;
    num_fail = 0;
    for (j=0; j<NUM_SERVERS; j++)
    {
      if ((100.0 * rand_val()) < FAIL_PERCENT)
      {
        num_fail++;
        if (num_fail > NUM_ALLOW_FAIL) fail = TRUE;
      }
    }

    // Increment the number of days with a failure
    if (fail == TRUE) sum_fail++;
  }

  // Compute percent of days on which we open on time
  on_time = 100.0 * ((double) (NUM_DAYS - sum_fail) / NUM_DAYS);

  // Output results
  printf("============================================================= \n");
  printf("==     *** Results from server boot-up simulation ***     == \n");
  printf("============================================================= \n");
  printf("=  PROBLEM INPUT:                                             \n");
  printf("=    Number of days simulated = %d days    \n", NUM_DAYS);
  printf("=    Number of servers        = %d servers \n", NUM_SERVERS);
  printf("=    Number of allowed fails  = %d servers \n", NUM_ALLOW_FAIL);
  printf("=    Boot-up failure percent  = %6.3f %%   \n", FAIL_PERCENT);
  printf("============================================================= \n");
  printf("=  PROBLEM OUTPUT:                                            \n");
  printf("=    Days opening on time     = %6.3f %%   \n", on_time);
  printf("============================================================= \n");
}

//=============================================================================
//= Function to return a random value between 0.0 and 1.0                     =
//=============================================================================
double rand_val(void)
{
  double z;   // Random value to be generated and returned

  // Use rand() and RAND_MAX defined in stdlib()
  z = (double) rand() / RAND_MAX;

  return(z);
}