// PROGRAM cool
// ニュートンの冷却の法則の数値解

#include "TrueBASIC.h"
int main();
void initial(double *t, double *T_init, double *T_room, double *r, 
             double *delta_t, double *tmax, double *nshow);
void Euler(double *t, double *T_coffee, double T_room, double r, double delta_t);
void output(double t, double T_coffee, double T_room);

int main()
{
    double T_coffee, T_room, counter, delta_t, nshow, r, t, tmax;

    initial(&t, &T_coffee, &T_room, &r, &delta_t, &tmax, &nshow);
    counter = 0;                            // 変数 counter の初期化
    while(t <= tmax)
    {
        Euler(&t, &T_coffee, T_room, r, delta_t);
        ++counter;                          // 反復回数
        if(fmod(counter, nshow) == 0)
            output(t, T_coffee, T_room);
    }

    return 0;
}

void initial(double *t, double *T_init, double *T_room, double *r, 
             double *delta_t, double *tmax, double *nshow)
{
    double tshow;
    char Stmp1_[_LBUFF_];

    // time は True BASIC の予約語なので，変数名には使えない
    (*t) = 0;                               // 時刻
    (*T_init) = 82.3;                       // コーヒーの初期温度(C)
    (*T_room) = 17;                         // 室温(C)
    printf("cooling constant r = ");
    fgets(Stmp1_, _LBUFF_, stdin);
    sscanf(Stmp1_, "%lg", r);
    printf("time step dt = ");
    fgets(Stmp1_, _LBUFF_, stdin);
    sscanf(Stmp1_, "%lg", delta_t);
    printf("duration of run = ");
    fgets(Stmp1_, _LBUFF_, stdin);
    sscanf(Stmp1_, "%lg", tmax);            // 分
    printf("time between output of data = ");
    fgets(Stmp1_, _LBUFF_, stdin);
    sscanf(Stmp1_, "%lg", &tshow);
    (*nshow) = (int)(tshow/(*delta_t) + 0.5);
    output(*t, *T_init, *T_room);
}

void Euler(double *t, double *T_coffee, double T_room, double r, double delta_t)
{
    double change;

    change = -r*((*T_coffee) - T_room)*delta_t;
    (*T_coffee) += change;
    (*t) += delta_t;
}

void output(double t, double T_coffee, double T_room)
{
    if(t == 0)
    {
        printf("\n");
        printf("  time (min)     T_coffee T_coffee - T_room\n");
        printf("\n");
    }
    printf("%12f %12f %12f\n", t, T_coffee, T_coffee - T_room);
                                            // 結果をスクリーンに表示
}