// PROGRAM plot_function
// plot analytical solution

#include "TrueBASIC.h"
int main();
void initial(double *t, double *tmax, double *r, double *Ts, double *T0);
void set_up_window(float xmin, float xmax, float ymin, float ymax);
void show_plot(double *t, double tmax, double r, double Ts, double T0);
double f(double t, double r, double Ts, double T0);

int main()
{
    double T0, Ts, r, t, tmax;

    GWopen(0);

    initial(&t, &tmax, &r, &Ts, &T0);
    set_up_window((float)t, (float)tmax, (float)Ts, (float)T0);
    show_plot(&t, tmax, r, Ts, T0);

    GWquit();
    return 0;
}

void initial(double *t, double *tmax, double *r, double *Ts, double *T0)
{
    char Stmp1_[_LBUFF_];

    (*t) = 0;
    (*T0) = 83;                             // initial coffee temperature (C)
    (*Ts) = 22;                             // room temperature (C)
    printf("cooling constant r = ");
    fgets(Stmp1_, _LBUFF_, stdin);
    sscanf(Stmp1_, "%lg", r);               // cooling constant
    printf("duration = ");
    fgets(Stmp1_, _LBUFF_, stdin);
    sscanf(Stmp1_, "%lg", tmax);            // time (minutes)
}

void set_up_window(float xmin, float xmax, float ymin, float ymax)
{
    float mx, my;

    mx = 0.01f*(xmax - xmin);               // margin
    my = 0.01f*(ymax - ymin);
    GWindow(xmin - mx, ymin - my, xmax + mx, ymax + my);
//  default background color black on all computers except Macintosh
    GWmove2(xmin, ymin);                    // abbreviation for PLOT LINES:
    GWline2(xmax, ymin);
    GWline2(xmin, ymin);
    GWline2(xmin, ymax);
}

void show_plot(double *t, double tmax, double r, double Ts, double T0)
{
    GWsetpen(-1, -1, -1, -1);
    GWsetpen(RED, -1, -1, -1);
    while((*t) <= tmax)
    {
        GWline2((float)((*t)), (float)(f(*t, r, Ts, T0)));
        (*t) += 0.1;
    }
}

double f(double t, double r, double Ts, double T0)
{
    double f_;

    f_ = Ts - (Ts - T0)*exp(-r*t);
    return f_;
}