//* Using functions - revisited to demo structures *
#include <stdio.h>
#include <math.h>

typedef struct point_t {  // a point_t on plains
    double x, y;
};

/*
typedef struct triangle_t {  // a point_t on plains
    struct point_t A, B, C;
};
*/

double DIST(const struct point_t * a, const struct point_t * b);
double AREA(double a, double b, double c);

int main() {
    struct point_t p, q, r;
    double d1, d2, d3, area;

    printf("Please enter the coordinates of the triangle vertices and press ENTER.\n");
    scanf("%lf %lf %lf %lf %lf %lf", &p.x, &p.y, &q.x, &q.y, &r.x, &r.y);

    d1 = DIST(&p, &q);
    d2 = DIST(&p, &r);
    d3 = DIST(&q, &r);
    area = AREA(d1, d2, d3);
    area = AREA(d1, d2, d3);

    printf("The area of the triangle is %lf\n", area);
    return(0);
}

inline double sqr(double x) {
    return(x * x);
}

double DIST(const struct point_t * a, const struct point_t * b) {
    return(sqrt(sqr(a->x - b->x) + sqr(a->y - b->y)));
    //  return( sqrt( sqr( (*a).x - (*b).x) + sqr( (*a).y - (*b).y) ) );
}

double AREA(double a, double b, double c) {
    // Hero's formula for the triangle area
    double p = 0.5 * (a + b + c);
    double ar = sqrt(p * (p - a) * (p - b) * (p - c));
    return(ar);
}