// Source file: prime.c 
//     Purpose: generate prime numbers and test if prime 
//     Authors: Scott Wenger & Mark Wenger, 1997 
//              Box 802
//              Stevens Point, WI 54481
//              panther@wctc.net
//   
//       Notes: will compile under DOS; the body of code should 
//              also compile under UNIX  
             

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

#define MAX 200000000  // two hundred million 

// function prototypes 
void finished(long amount);
void list_primes(void);
void pcls(void);
int prime(long number);
int prime_numbers(long amount);
void single_test(void);

int pause = 0;   // flag 

//--------------------------------------------------------------------

main()
{
  int ch;
  int valid;

  pcls();
  fprintf(stderr, "\n        PRIME Number program by Scott Wenger & Mark Wenger");
  fprintf(stderr, "\n        --------------------------------------------------\n\n");

  valid = 0;
  while (!valid) {
    fprintf(stderr, "\n          --------------- M E N U ------------ \n");
    fprintf(stderr, "\n            1. Show a list of prime numbers.");
    fprintf(stderr, "\n            2. Test a single number to see if prime.");
    fprintf(stderr, "\n            3. Quit this program.\n");
    fprintf(stderr, "\n            Choose 1, 2, or 3 above [ ]\b\b");
    ch = getch();   
    switch(ch) {
      case '1' : list_primes(); 
                 break;
      case '2' : single_test();
                 break;
      case '3' : pcls();
                 exit(0);
      default  : break;                 
    } // end of switch 
  }  // end of while 
}

//--------------------------------------------------------------------

void list_primes(void)
{  
  long amount;
  int valid;
  int ch;
  
  pcls();
  fprintf(stderr, "\n\nHow many prime numbers do you want to see?  ");
  scanf("%ld", &amount);
  if (amount > 20L)  {
    fprintf(stderr, "\nPause after each screen of prime numbers? (y/n) [ ]\b\b");
    valid = 0;
    while (!valid) {
      ch = getch();
      if (ch == 'y' || ch == 'Y') {
        pause = 1;
        valid = 1;
      }	    
      if (ch == 'n' || ch == 'N') {
        valid = 1;
      }
    } // end of while 
    printf("\n");
  }  // end of outer if 
  prime_numbers(amount);
}

//--------------------------------------------------------------------

void single_test(void)
{
  long test;
  int valid = 0;
  int p_number = 0;
  
  pcls();
  while (!valid) {
    fprintf(stderr, "\nNumber to test: ");
    scanf("%ld", &test);
    if (test > 0 && test <= MAX) valid = 1;
    if (test < 1 || test >  MAX) {
      fprintf(stderr, "\nNumber to test must be in range of 1 to %ld\n", MAX);  
    }
  }

  p_number = prime(test);

  if (p_number) fprintf(stderr, "\n%ld is a prime number. ", test);
  if (!p_number) fprintf(stderr, "\n%ld is NOT a prime number.", test);

  fprintf(stderr, "\n\nPress any key to continue [ ]\b\b");
  getch();
  pcls();
}

//--------------------------------------------------------------------

int prime(long number)
{
  long i;
  int not_prime = 0;
  
  if (number == 1L || number == 2L || number == 3L) return(1); // prime 
  
  for (i = 2L; i < number - 1L; i++) {
    if (number % i == 0L) {
      not_prime = 1;
      break;
    }
    if (not_prime == 0) {
      if (i * i > number)
        break;
    }
  }  // end of for loop 

  if (not_prime == 0)  return(1);   // a prime number 
  else return(0);   // not prime 
}

//--------------------------------------------------------------------

// pcls is a primitive clear screen (yet highly portable) 

void pcls(void)
{
  register int x;
  for (x = 0; x < 40; x++)
    printf("\n");
}

//--------------------------------------------------------------------

int prime_numbers(long amount)
{
  long counter = 0L;
  int done = 0;
  int lcount = 0;   // line counter 
  int ch;
  long i;
  long number = 1;
  int not_prime = 0;  // a flag 

  // show the first 3 prime numbers "manually" 
  for (i = 0; i < 3; i++) {
    printf("    %16ld\n", number);  // a prime number 
    counter++;
    if (amount == counter) {
      finished(amount);
      return(1);
    }
    number++;
  }
  
  // print the rest of the prime numbers here   
  while (!done) {
    number++;
    
    for (i = 2L; i < number - 1L; i++) {
      if (number % i == 0) {
        not_prime = 1;
        break;
      }
      if (not_prime == 0) {
        if (i * i > number) {
          break;
        }
      }
    }	            

    if (not_prime == 0) {
     
      printf("    %16ld\n", number);  // a prime number 

      counter++;
      if (counter == amount) {
        finished(amount); 
        return(1);
      }

      if (pause) {
        lcount++;
        if (lcount == 20) {
          fprintf(stderr, "\nPress space bar to continue or 'q' to quit.");
          ch = getch();
          if (ch == 'q' || ch == 'Q') {
            pcls();
            exit(0);
          }
          else {
            lcount = 0;
            pcls();
          }  
        }	      
      }  // end of outer if 
    }

    not_prime = 0;  // reset flag 

  } // end of outer while loop 	  
  return(0);
}  // end of prime_numbers function 

//--------------------------------------------------------------------

void finished(long amount)
{
  fprintf(stderr, "\n\nDone. Displayed %ld prime numbers.", amount);
  fprintf(stderr, "\nPress any key to return to menu [ ]\b\b");
  getch();
  pcls();
}

//--------------------------------------------------------------------