shape.h

#ifndef SHAPE_H
#define SHAPE_H

#include <stdbool.h>
#include <math.h>
#include <raylib.h>

enum Shape {
    S_SQUARE, S_TRIANGLE, S_CIRCLE
};
typedef int Shape;

typedef struct Triangle {
    Vector2 p1, p2, p3;
} Triangle;

typedef struct Circle {
    Vector2 center;
    float   radius;
} Circle;

/* ... */
static Triangle triangle(Vector2 p1, Vector2 p2, Vector2 p3) {
    return (Triangle) { p1, p2, p3 };
}

/* Calculates the area of any triangle. */
static float triangle_area(Triangle tr) {
    return 0.5f * fabs(
        tr.p1.x * (tr.p2.y - tr.p3.y) +
        tr.p2.x * (tr.p3.y - tr.p1.y) +
        tr.p3.x * (tr.p1.y - tr.p2.y)
    );
}

/* Returns true if `point` is within `tr`. */
static bool triangle_point(Triangle tr, Vector2 point) {
    float
        areaABC = triangle_area(tr),
        areaPAB = triangle_area(triangle(point, tr.p1, tr.p2)),
        areaPBC = triangle_area(triangle(point, tr.p2, tr.p3)),
        areaPCA = triangle_area(triangle(point, tr.p3, tr.p1));
    return (areaABC == areaPAB + areaPBC + areaPCA);
}

/* ... */
static Circle circle(Vector2 center, float radius) {
    return (Circle) { .center = center, .radius = radius };
}

/* Calculates the area of a circle. */
static float circle_area(Circle cl) {
    float t = PI * (cl.radius);
    return t * t;
}

/* Returns true if `point` is within `cl`. */
static bool circle_point(Circle cl, Vector2 point) {
    float dist_p2 =
        (point.x - cl.center.x) * (point.x - cl.center.x)
        + (point.y - cl.center.y) * (point.y - cl.center.y);
    return dist_p2 <= cl.radius * cl.radius;
}

// Vector2, Vector3, etc.

static float vec2_cross(Vector2 v1, Vector2 v2) {
    return v1.x * v2.y - v1.y * v2.x;
}

static Vector2 vec2_add(Vector2 v1, Vector2 v2) {
    return (Vector2) { v1.x + v2.x, v1.y + v2.y };
}

static Vector2 vec2_sub(Vector2 v1, Vector2 v2) {
    return (Vector2) { v1.x - v2.x, v1.y - v2.y };
}

static float vec2_dot(Vector2 v1, Vector2 v2) {
    return v1.x * v2.x + v1.y * v2.y;
}

#endif