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

// Here's a basic interface for geometric shapes.
typedef struct {
    double (*area)(void *);
    double (*perim)(void *);
} geometry;

// For our example we'll implement this interface on
// `rect` and `circle` types.
typedef struct {
    double width, height;
} rect;

typedef struct {
    double radius;
} circle;

// To implement an interface in C, we need to define functions
// that match the function pointers in the interface.
// Here we implement `geometry` on `rect`s.
double rect_area(void *r) {
    rect *rect_ptr = (rect *)r;
    return rect_ptr->width * rect_ptr->height;
}

double rect_perim(void *r) {
    rect *rect_ptr = (rect *)r;
    return 2 * rect_ptr->width + 2 * rect_ptr->height;
}

// The implementation for `circle`s.
double circle_area(void *c) {
    circle *circle_ptr = (circle *)c;
    return M_PI * circle_ptr->radius * circle_ptr->radius;
}

double circle_perim(void *c) {
    circle *circle_ptr = (circle *)c;
    return 2 * M_PI * circle_ptr->radius;
}

// If a variable has an interface type, then we can call
// methods that are in the named interface. Here's a
// generic `measure` function taking advantage of this
// to work on any `geometry`.
void measure(void *g, geometry *geom) {
    printf("Area: %f\n", geom->area(g));
    printf("Perimeter: %f\n", geom->perim(g));
}

int main() {
    rect   r = {.width = 3, .height = 4};
    circle c = {.radius = 5};

    // The `circle` and `rect` struct types both
    // implement the `geometry` interface so we can use
    // instances of these structs as arguments to `measure`.
    geometry rect_geometry = {rect_area, rect_perim};
    geometry circle_geometry = {circle_area, circle_perim};

    printf("Rectangle:\n");
    measure(&r, &rect_geometry);

    printf("\nCircle:\n");
    measure(&c, &circle_geometry);

    return 0;
}