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

#define g_si 9.80665;  //m/s^2   -   global constants
#define g_us 32.1740; //f/s^2
float airdensity, tvelocity, drag, surfacearea, mass, k, dragchoice;  // global variables
char quit;

void si();
void us();  //one will be SI, one will be US units;

void main() {  //unit selection
    int unit=0;
    printf("This program will calculate the terminal velocity of an object on EARTH.\n\n\n\n");
    printf("Select the corresponding number to the type of unit you wish to use:\n");
    printf("1.  SI Units\n");
    printf("2.  US Units\n");
    scanf("%d", &unit);
    if(unit=1) si();  // calls the corresponding function via user input
    else us();
} // end MAIN function


void si() {
    dragchoice = 0;
    drag = 1;  // a good base value would be 1, if it were 0 the terminal velocity would be undefined, meaning the slope would be a verticle line meaning there would be no terminal velocity meaning and it would accelerate until the end of time which is crazy...
    printf("\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n");
    printf("First, we need to choose the object.  Below are some basic categories.\n");
    printf("Choose the corresponding number to the category that best suits your object,\n");
    printf("and you will be supplied with a list of objects.  Please, then, type in the coefficient of\n");
    printf("drag exactly as it appears.  This list will be updated as often as possible.\n\n\n");
    // CATEGORIES LISTED BELOW
    printf("1.  Basic Shaped Objects, Miscellaneous (including sporting equipment)\n");
    printf("2.  Human Positions (including bicycle with a cyclist on it\n");
    printf("3.  Vehicles (general/specific vehicles included)\n");
    printf("4.  Enter your own coefficient of drag.\n");
    scanf("%f", &dragchoice);
    printf("\n\n\n\n\n");
 if (dragchoice = 1) {
     printf("BASIC SHAPED OBJECTS, MISCELLANEOUS\n\n");
     printf("Cone (Air hitting pointy side)\t0.5\n");
     printf("Cube (Air Hitting pointy side)\t0.8\n");
     printf("Cube (Air Hitting straight side)\t1.05\n");
     printf("Golf Ball\t0.3\n");
     printf("Half-Sphere (Air hitting rounded side)\t0.42\n");
     printf("Ping-Pong Ball\t0.5\n");
     printf("Smooth Brick\t2.1\n");
     printf("Sphere\t0.47\n");
     printf("Tennis Ball\t0.3\n");
     printf("Very Large Man, Upright Position\t1.3\n");
     printf("To go back to main menu, type 0.  Otherwise type in the coefficient.\n");
     scanf("%f", &drag);
   if (drag = 0) {
       si();  // if the user fucked up, they type 0 and it sends them to the si() { function (the beginning of SI)
   } // end SI if
     } // end if
 else if (dragchoice = 2) {
     printf("HUMAN POSITIONS\n\n");
     printf("Large Man, Upright Position\t1.2\n");
     printf("Medium Sized Man, Upright Position\t1.1\n");
     printf("Parachutist, Speed Position\t1.05\n");
     printf("Parachutist, Sprawled Out\t1.35\n");
     printf("Parachutist, Straight Position\t1.25\n");
     printf("Ski Jumper1.25\n");
     printf("Skier\t1.05\n");
     printf("Small Man, Upright Position\t1.0\n");
     printf("Typical Bicycle w/Cyclist\t0.9\n");
     printf("To go back to main menu, type 0.  Otherwise type in the coefficient.\n");
     scanf("%f", &drag);
   if (drag = 0) {
       si();
   }
 } // end else if
 else if (dragchoice = 3) {
     printf("VEHICLES (SPECIFIC AND GENERAL\n\n");
     printf("Standard Economy Car\t0.45\n");
     printf("Standard Pickup Truck\t0.5\n");
     printf("Standard Medium Sized Truck (Larger than pickup)\t0.8\n");
     printf("Standard Sports Car\t0.35\n");
     printf("Audi 100, 1983\t0.3\n");
     printf("Audi A3, 2006\t0.33\n");
     printf("BMW 8-Series, 1989\t0.29\n");
     printf("Chevrolet Camaro, 1995\t0.338\n");
     printf("Chevrolet Caprice, 1994-1996\t0.34\n");
     printf("Citroën 2CV\t0.51\n");
     printf("Citroën CX, 1974\t0.36\n");
     printf("Citroën DS, 1955\t0.36\n");
     printf("Citroën GSA, 1980\t0.31\n");
     printf("Dodge Charger, 2006\t0.33\n");
     printf("Dodge Durango, 2004\t0.39\n");
     printf("Ferrari F40, 1987\t0.34\n");
     printf("Ferrari F50, 1996\t0.372\n");
     printf("Ferrari Testarossa, 1986\t0.36\n");
     printf("Please type 0 if you want to go to the next page.\n");
     scanf("%f", &drag);
       if (drag = 0) {
       drag = 1;
       printf("Ford Probe V prototype, 1985\t0.137\n");
       printf("Ford Sierra, 1982\t0.34\n");
       printf("General Motors EV1, 1996\t0.195\n");
       printf("Honda Accord Hybrid, 2005\t0.29\n");
       printf("Honda Insight, 1999\t0.25\n");
       printf("Hummer H2, 2003\t0.57\n");
       printf("Infiniti G35, 2002\t0.27\n");
       printf("Lamborghini Countach, 1974\t0.42\n");
       printf("Lotus Elite, 1958\t0.29\n");
       printf("Mercedes-Benz 'Bionic Car' Concept, 2005    0.19\n");
       printf("Porsche 996, 1997\t0.3\n");
       printf("Porsche 997, 2004\t0.28\n");
       printf("Porsche Boxster, 2005\t0.29\n");
       printf("Saab 92, 1947\t0.3\n");
       printf("Subaru Impreza WRX STi, 2004\t0.33\n");
       printf("Tatra T77, 1938\t0.212\n");
       printf("Toyota Camry or Lexus ES, 2005\t0.28\n");
       printf("Toyota Celica,1995-2005\t0.32\n");
       printf("Toyota Prius, 2004\t0.26\n");
       printf("Volkswagen Beetle\t0.38\n");
       printf("To go back to main menu, type 0.  Otherwise type in the coefficient.\n");
       scanf("%f", &drag);
     if (drag = 0) {
         si();
     }
       }
   }
 else if (dragchoice = 4) {
     drag = 1;
     printf("Enter in the drag coefficient you wish to use:  ");
     scanf("%f", &drag);
   if (drag = 0) {
       si();
   }
 }
 else {
     printf("Error, please retry.\n");
     si();
 }
} // end SI function

void us() {
    dragchoice = 0;
    drag = 1;  // a good base value would be 1, if it were 0 the terminal velocity would be undefined, meaning the slope would be a verticle line meaning there would be no terminal velocity meaning and it would accelerate until the end of time which is crazy...
    printf("\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n");
    printf("First, we need to choose the object.  Below are some basic categories.\n");
    printf("Choose the corresponding number to the category that best suits your object,\n");
    printf("and you will be supplied with a list of objects.  Please, then, type in the coefficient of\n");
    printf("drag exactly as it appears.  This list will be updated as often as possible.\n\n\n");
    // CATEGORIES LISTED BELOW
    printf("1.  Basic Shaped Objects, Miscellaneous (including sporting equipment)\n");
    printf("2.  Human Positions (including bicycle with a cyclist on it\n");
    printf("3.  Vehicles (general/specific vehicles included)\n");
    printf("4.  Enter your own coefficient of drag.\n");
    scanf("%f", &dragchoice);
    printf("\n\n\n\n\n\n");
 if (dragchoice = 1) {
     printf("BASIC SHAPED OBJECTS, MISCELLANEOUS\n\n");
     printf("Cone (Air hitting pointy side)\t0.5\n");
     printf("Cube (Air Hitting pointy side)\t0.8\n");
     printf("Cube (Air Hitting straight side)\t1.05\n");
     printf("Golf Ball\t0.3\n");
     printf("Half-Sphere (Air hitting rounded side)\t0.42\n");
     printf("Ping-Pong Ball\t0.5\n");
     printf("Smooth Brick\t2.1\n");
     printf("Sphere\t0.47\n");
     printf("Tennis Ball\t0.3\n");
     printf("Very Large Man, Upright Position\t1.3\n");
     printf("To go back to main menu, type 0.  Otherwise type in the coefficient.\n");
     scanf("%f", &drag);
   if (drag = 0) {
       us();  // if the user fucked up, they type 0 and it sends them to the si() { function (the beginning of SI)
   } // end SI if
     } // end if
 else if (dragchoice = 2) {
     printf("HUMAN POSITIONS\n\n");
     printf("Large Man, Upright Position\t1.2\n");
     printf("Medium Sized Man, Upright Position\t1.1\n");
     printf("Parachutist, Speed Position\t1.05\n");
     printf("Parachutist, Sprawled Out\t1.35\n");
     printf("Parachutist, Straight Position\t1.25\n");
     printf("Ski Jumper1.25\n");
     printf("Skier\t1.05\n");
     printf("Small Man, Upright Position\t1.0\n");
     printf("Typical Bicycle w/Cyclist\t0.9\n");
     printf("To go back to main menu, type 0.  Otherwise type in the coefficient.\n");
     scanf("%f", &drag);
   if (drag = 0) {
       us();
   }
 } // end 2nd else if
 else if (dragchoice=3) {
     printf("VEHICLES (SPECIFIC AND GENERAL\n\n");
     printf("Standard Economy Car\t0.45\n");
     printf("Standard Pickup Truck\t0.5\n");
     printf("Standard Medium Sized Truck (Larger than pickup)\t0.8\n");
     printf("Standard Sports Car\t0.35\n");
     printf("Audi 100, 1983\t0.3\n");
     printf("Audi A3, 2006\t0.33\n");
     printf("BMW 8-Series, 1989\t0.29\n");
     printf("Chevrolet Camaro, 1995\t0.338\n");
     printf("Chevrolet Caprice, 1994-1996\t0.34\n");
     printf("Citroën 2CV\t0.51\n");
     printf("Citroën CX, 1974\t0.36\n");
     printf("Citroën DS, 1955\t0.36\n");
     printf("Citroën GSA, 1980\t0.31\n");
     printf("Dodge Charger, 2006\t0.33\n");
     printf("Dodge Durango, 2004\t0.39\n");
     printf("Ferrari F40, 1987\t0.34\n");
     printf("Ferrari F50, 1996\t0.372\n");
     printf("Ferrari Testarossa, 1986\t0.36\n");
     printf("Please type 0 if you want to go to the next page.\n");
     scanf("%f", &drag);
       if (drag = 0) {
       drag = 1;
       printf("Ford Probe V prototype, 1985\t0.137\n");
       printf("Ford Sierra, 1982\t0.34\n");
       printf("General Motors EV1, 1996\t0.195\n");
       printf("Honda Accord Hybrid, 2005\t0.29\n");
       printf("Honda Insight, 1999\t0.25\n");
       printf("Hummer H2, 2003\t0.57\n");
       printf("Infiniti G35, 2002\t0.27\n");
       printf("Lamborghini Countach, 1974\t0.42\n");
       printf("Lotus Elite, 1958\t0.29\n");
       printf("Mercedes-Benz 'Bionic Car' Concept, 2005    0.19\n");
       printf("Porsche 996, 1997\t0.3\n");
       printf("Porsche 997, 2004\t0.28\n");
       printf("Porsche Boxster, 2005\t0.29\n");
       printf("Saab 92, 1947\t0.3\n");
       printf("Subaru Impreza WRX STi, 2004\t0.33\n");
       printf("Tatra T77, 1938\t0.212\n");
       printf("Toyota Camry or Lexus ES, 2005\t0.28\n");
       printf("Toyota Celica,1995-2005\t0.32\n");
       printf("Toyota Prius, 2004\t0.26\n");
       printf("Volkswagen Beetle\t0.38\n");
       printf("To go back to main menu, type 0.  Otherwise type in the coefficient.\n");
       scanf("%f", &drag);
     if (drag = 0) {
         us();
     }
       }
   }
 else if (dragchoice = 4) {
     drag = 1;
     printf("Enter in the drag coefficient you wish to use:  ");
     scanf("%f", &drag);
   if (drag = 0) {
       us();
   }
 }
 else {
     printf("Error, please retry.\n");
     us();
 }
} // end US function

   if (drag = 0) {
       us();
   }

else if (dragchoice == 3)