// PROGRAM free_fall
// no air resistance

#include "TrueBASIC.h"
int main();
void initial(double *y, double *v, double *a, double *g, double *t, double *dt);
void Euler(double *y, double *v, double *a, double g, double *t, double dt);
void print_table(double y, double v, double a, double t, int *nshow);

int main()
{
    int counter, nshow;
    double a, dt, g, t, v, y;

    initial(&y, &v, &a, &g, &t, &dt);       // initial conditions and parameters
    print_table(y, v, a, t, &nshow);        // print initial conditions
    counter = 0;
    while(y >= 0)
    {
        Euler(&y, &v, &a, g, &t, dt);
        if((++counter % nshow) == 0)
            print_table(y, v, a, t, &nshow);
    }
    print_table(y, v, a, t, &nshow);        // print values at surface

    return 0;
}

void initial(double *y, double *v, double *a, double *g, double *t, double *dt)
{
    char Stmp1_[_LBUFF_];

    (*t) = 0;                               // initial time (sec)
    (*y) = 10;                              // initial height (m)
    (*v) = 0;                               // initial velocity
    (*g) = 9.8;                             // (magnitude) of accel due to gravity
    (*a) = -(*g);
    printf("time step dt = ");
    fgets(Stmp1_, _LBUFF_, stdin);
    sscanf(Stmp1_, "%lg", dt);
}

void Euler(double *y, double *v, double *a, double g, double *t, double dt)
{
    (*y) += (*v)*dt;                  // use velocity at beginning of interval
    // following included to remind us that acceleration is constant
    (*a) = -g;                              // y positive upward
    (*v) += (*a)*dt;
    (*t) += dt;
}

void print_table(double y, double v, double a, double t, int *nshow)
{
    char Stmp1_[_LBUFF_];

    if(t == 0)
    {
        printf("number of time steps between output = ");
        fgets(Stmp1_, _LBUFF_, stdin);
        sscanf(Stmp1_, "%d", nshow);
        printf("\n    time (s)        y (m) velocity (m/s) accel (m/s^2)\n\n");
    }
    printf("%12f %12f %12f %12f\n", t, y, v, a);
}