Shape Collision

collision.cpp

#include "stdio.h"
#include "math.h"
#include "assert.h"
#define static_assert _Static_assert

typedef float f32;
typedef struct Vec2 { f32 X, Y; } Vec2;

float Vec2_Distance(Vec2 a, Vec2 b){
    f32 dx = a.X - b.X;
    f32 dy = a.Y - b.Y;
    return sqrt(dx*dx + dy*dy);
}

enum ShapeType {
    Shape_Circle,
    Shape_AABB,
    Shape_VerticalLine,
    Shape_HorizontalLine,
    ShapeCount
};

//TODO: ensure that this uses pow of 2
#define ShapePair(AType, BType) ((AType * ShapeCount) | BType)

struct Circle {
    ShapeType Type;
    Vec2      Center;
    f32       Radius;
};

struct AABB {
    ShapeType Type;
    Vec2      Min, Max;
};

struct VerticalLine {
    ShapeType Type;
    Vec2      Start;
    f32       Radius;
    f32       Height;
};

struct HorizontalLine {
    ShapeType Type;
    Vec2      Start;
    f32       Radius;
    f32       Width;
};

struct Shape {
    union {
        struct {
            ShapeType Type;
            Vec2      Position;
        };
        struct Circle         Circle;
        struct AABB           AABB;
        struct VerticalLine   VerticalLine;
        struct HorizontalLine HorizontalLine;
    };
};

bool Shapes_Colliding(Shape *a, Shape *b){
    if(a->Type > b->Type){
        Shape *t = b; b = a; a = t;
    }

    #define case_pair(aname, AType, bname, BType)     \
        case ShapePair(Shape_##AType, Shape_##BType): \
            Shape *tmp_a = a; Shape *tmp_b = b;         \
            static_assert(Shape_##AType <= Shape_##BType, "order differently"); \
            AType* aname = (AType*)tmp_a;                 \
            BType* bname = (BType*)tmp_b;

    switch ShapePair(a->Type, b->Type) {
    {case_pair(a, Circle, b, Circle)
            return Vec2_Distance(a->Center, b->Center) < a->Radius + b->Radius;
    }
    {case_pair(circle, Circle, aabb, AABB)
        return Vec2_Distance(circle->Center, aabb->Min) < circle->Radius;
    }
    }
    return false;
}