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+# Game Math/C++/Calculating The Future
+
+Sara Gerretsen
+
+https://git.saragerretsen.nl/Sara/kernmodule-cpp
+
+# Game Style
+
+Speed racer (with full 2d motion).
+
+# Math
+
+Other than all the basic binary vector operations, my `Vecf` 2d vector class implements the following:
+
+Only thing particularly of-note is the SDL_FPoint conversion operator. Used to simplify interaction with the `SDL_Render` API. Most of this was copied from my [previous 2d vector implementation]([cutes/vmath.h at main - cutes - Sara's Gitea](https://git.saragerretsen.nl/Sara/cutes/src/branch/main/vmath.h), with some minor modifications and updates as well as translating it to be C++ rather than C.
+
+```cpp
+struct Vecf {
+    float x{0.f}, y{0.f};
+
+    operator SDL_FPoint();
+    //! squared distance between two vectors as points. Use for comparing distances efficiently.
+    static float sqr_distance(Vecf const &from, Vecf const &to);
+    //! distance between two vectors as points
+    static float distance(Vecf const &from, Vecf const &to);
+    //! compare within an epsilon
+    static bool equals_approximate(Vecf const &lhs, Vecf const &rhs);
+    //! scalar member-wise multiplication product
+    static float dot(Vecf const &lhs, Vecf const &rhs);
+    //! unsigned angle difference in radians between lhs and rhs
+    static float angle_between(Vecf const &lhs, Vecf const &rhs);
+    //! interpolate linearly between two points
+    static Vecf lerp(Vecf const &from, Vecf const &to, float t);
+    //! move towards a point by a set unit distance
+    static Vecf move_towards(Vecf const &from, Vecf const &to, float delta);
+    //! calculate the outgoing velocity of an object reflecting against a surface
+    static Vecf reflect(Vecf const &in, Vecf const &normal);
+    //! magnitude (a.k.a length or absolute) of this vector
+    float magnitude() const;
+    //! square of the magnitude, use for comparing lengths of vectors efficiently
+    float sqr_magnitude() const;
+    //! vector perpendicular to this one
+    Vecf perpendicular() const;
+    //! rotate vector by t Radians
+    Vecf rotated(float t) const;
+    //! vector pointing in the same direction with a length of 1. Or, vector divided by it's magnitude
+    Vecf normalized() const;
+    //! reverse scale of vector
+    Vecf reciprocal() const;
+    //! returns true if either the x or y element is NaN
+    bool is_nan() const;
+
+    Vecf clamp_magnitude(float max) const;
+    //! scale vector member-wise
+    void scale(float x, float y);
+    //! scale vector member-wise
+    void scale(Vecf const &factors);
+    Vecf scaled(Vecf const &factor) const;
+
+    static Vecf const RIGHT;
+    static Vecf const UP;
+    static Vecf const ONE;
+    static Vecf const POSITIVE_INFINITY;
+    static Vecf const ZERO;
+};
+
+```
+
+# General Structure:
+
+The core systems of the project make extensive use of C++ smart pointers for memory management. There are no *owning* raw pointers. Though many raw pointers are used to access smart-pointer-managed memory. Basically everything in the `core/` directory will be part of the `ce` namespace, short for `CanvasEngine`.
+
+I decided to try writing purely C++20 as standardised. (Hence no `#pragma`, oops)
+
+## CanvasEngine
+
+Named after the (now-discontinued) "engine" used for teaching C# basics at GLU.
+
+Represents the main application state and provides an interface to the engine subsystems.
+
+```cpp
+void CanvasEngine::run(std::unique_ptr<Level> &level) {
+    if(!stay_open)
+        return;
+    assert(CanvasEngine::singleton_instance == nullptr && "Engine singleton instance already assigned, starting another instance is invalid");
+    // register as singleton
+    CanvasEngine::singleton_instance = this;
+    // take ownership of and instantiate level
+    this->level = std::move(level);
+    if(!this->level->is_instantiated())
+        this->level->instantiate();
+    // start tracking time
+    this->frame_start_time = this->last_frame_start_time = SDL_GetTicks64();
+    SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "CANVAS: starting");
+    // main application loop
+    while(stay_open) {
+        // track frame time
+        this->frame_start_time = SDL_GetTicks64();
+        this->delta_time = double(this->frame_start_time - this->last_frame_start_time) / 1000.f;
+        // process events
+        this->process_events();
+        // update application implementation
+        if(this->delta_time > this->target_delta_time) {
+            this->tick(this->delta_time);
+            this->last_frame_start_time = this->frame_start_time;
+            this->draw(render);
+            SDL_RenderPresent(this->render);
+        } else {
+            SDL_Delay(2);
+        }
+        if(this->next_level) {
+            this->level = std::move(this->next_level);
+            this->level->instantiate();
+        }
+    }
+    assert(CanvasEngine::singleton_instance == this && "Engine singleton instance changed while game was running");
+    CanvasEngine::singleton_instance = nullptr;
+}
+
+```
+
+
+
+## Hierarchy
+
+The world is made up of instances of, or inheriting from, `Node`. `Node` defines the interface required for gameplay functionality and rendering, without implementing it.
+
+The majority of actual functionality in `Node` is in the `propagate_` functions. As they're responsible for pushing notifications like _tick and _added to child objects.
+
+```cpp
+class Node {
+public:
+friend class Level;
+typedef std::unique_ptr<Node> OwnedPtr;
+typedef std::vector<std::pair<std::string, Node::OwnedPtr>> ChildrenVector;
+private:
+    Node *parent{nullptr};
+    bool inside_tree{false};
+    bool request_deletion{false};
+    bool has_ticked{false};
+    std::string name{};
+    ChildrenVector children{};
+    bool visible{true};
+    bool tick{true};
+    ce::Level *level{nullptr};
+public:
+    Signal<> destroyed{}; //!< Signal invoked by the destructor
+    Signal<Node*> child_removed{}; //!< Signal invoked when a child is removed.
+    Signal<Node*> child_added{}; //!< Signal invoked when a child is added.
+public:
+    Node(std::string name);
+    virtual ~Node();
+public:
+    virtual void _added() {} //!< called the moment after the object is added as a child to another node
+    virtual void _first_tick() {} //!< called the first frame this object is active
+    virtual void _tick(double delta_time [[maybe_unused]]) {} //!< called every frame
+    virtual void _removed() {} //!< called the moment before the object is removed as a child to another node
+    virtual void _draw(SDL_Renderer *render [[maybe_unused]], ce::Transform const &view_transform [[maybe_unused]]) {}
+    virtual void _draw_ui(SDL_Renderer *render [[maybe_unused]], ce::Transform const &ui_transform [[maybe_unused]]) {}
+    virtual void _update_transform() {}
+public:
+    template <class TNode> TNode *get_child(std::string const &name); //!< get a non-owning pointer to a child
+    template <class TNode, typename... Args> TNode *create_child(Args... cargs);
+    void add_child(Node::OwnedPtr &child); //!< add a child, the caller must own the pointer
+    void set_parent(Node *new_parent);
+    Node *get_parent() const; //!< get the parent.
+    std::string const &get_name() const; //!< get the name of this node.
+    void set_name(std::string const &name); //!< change the name of this node.
+    void flag_for_deletion(); //!< request deletion at the end of the frame.
+    bool requests_deletion() const; //!< returns whether or not the node is flagged for deletion.
+    void set_visible(bool value); //!< enable or disable _draw for this object and all children
+    bool is_visible() const;
+    void set_tick(bool value);
+    bool is_ticking() const;
+    ce::Level *get_level() const;
+    bool is_inside_tree() const;
+    ChildrenVector &get_children();
+private:
+    void set_level(ce::Level *level);
+    void set_is_inside_tree(bool value);
+    std::optional<Node::OwnedPtr> remove_child(Node *child); //!< remove a child, the caller now owns the pointer
+    void propagate_tick(double delta_time);
+    void propagate_post_tick();
+    void propagate_added();
+    void propagate_removed();
+    void propagate_draw(SDL_Renderer *render, ce::Transform const &view_transform);
+    void propagate_draw_ui(SDL_Renderer *renderer, ce::Transform const &ui_transform);
+    bool rename_child(std::string const &old_name, std::string const &new_name);
+};
+```
+
+ `Node2D` provides transformation and transform hierarchy through `Transform`. `Transform` is a simple struct with a `position`, `rotation` and `scale`. As SDL_render (the media library i used instead of SFML) doesn't support using matrix transformations.
+
+core/node2d.cpp:
+
+```cpp
+void Node2D::_update_transform() {
+    if(this->parent_node2d != nullptr)
+        this->global_transform = this->transform * this->parent_node2d->get_global_transform();
+    else
+        this->global_transform = this->transform;
+    for(ChildrenVector::value_type &pair : this->get_children())
+        pair.second->_update_transform();
+}
+
+void Node2D::set_transform(Transform const &transform) {
+    this->transform = transform;
+    this->_update_transform();
+}
+
+...
+
+void Node2D::set_global_transform(Transform transform) {
+    this->global_transform = transform;
+    if(this->parent_node2d) {
+        Transform parent = this->parent_node2d->get_global_transform();
+        transform.position -= parent.position.rotated(-parent.rotation);
+        transform.scale_by(parent.scale.reciprocal());
+        assert(transform.scale.x != 0.f || transform.scale.y != 0.f);
+        transform.rotation -= parent.rotation;
+    }
+    this->transform = transform;
+    this->_update_transform();
+}
+
+```
+
+## Collision Subsystem
+
+Accessed through the `CollisionWorld` object, available on the `CanvasEngine` singleton through `ce::CanvasEngine::get_singleton()->get_collision_world()`. The collision world is a simple list of all currently active `CollidableNodes`. It loops over every pair, every tick. And notifies them if they overlapped. `RigidBody` inherits from `CollidableNode` and implements very rudimentary collision solving. As well as velocity, forces, and drag.
+
+core/colldiable_node.hpp
+
+```cpp
+class CollidableNode : public Node2D {
+    CollisionMask mask{~0x0u /* all layers by default */};
+    CollisionMask layers{0x1u /* only the first layer is enabled by default */};
+public:
+    Signal<CollisionShape*, CollidableNode*, CollisionShape*> overlap_enter{};
+public:
+    CollidableNode(std::string const &name, CollisionMask layers, CollisionMask mask);
+    void add_overlap(CollisionShape *shape, CollisionShape *other);
+
+    void set_mask(CollisionMask mask);
+    CollisionMask get_mask() const;
+    void set_layers(CollisionMask layers);
+    CollisionMask get_layers() const;
+};
+```
+
+`CollidableNode`s on their own cannot collide with anything. They need one or more shapes as their children to function. `CollisionShape`s are also nodes that will automatically register as part of a parent `CollidableNode`. `CollisionShape` is just a circle with a transform. Of the transform, only position applies, as rotation doesn't matter, and scaling would require implementing actually complicated overlap and escape vector detection.
+
+```cpp
+class CollisionShape : public Node2D {
+    CollidableNode *owner{nullptr};
+    float radius;
+    CollisionWorld &world;
+    bool is_registered{false};
+    float bounce{0.5f};
+public:
+    CollisionShape(std::string const &name, float radius, float bounce);
+
+    virtual void _added() override;
+    virtual void _removed() override;
+    virtual void _draw(SDL_Renderer *render, ce::Transform const &view_transform) override;
+
+    // return true if either of these objects could receive a notification for colliding with the other
+    static bool can_collide(CollisionShape const *lhs, CollisionShape const *rhs);
+    static bool shapes_overlap(CollisionShape const *lhs, CollisionShape const *rhs);
+
+    // get the motion lhs will have to make to fully disconnect from rhs.
+    static Vecf get_escape_vector(CollisionShape const *lhs, CollisionShape const *rhs);
+
+
+    CollidableNode *get_owner() const;
+    float get_radius() const;
+    CollisionMask get_layers() const;
+    CollisionMask get_mask() const;
+    float get_bounce() const;
+private:
+    void register_with_world();
+    void deregister_with_world();
+};
+
+```
+
+core/rigidbody.cpp:
+
+```cpp
+void RigidBody::_tick(double delta) {
+    Vecf next_velocity{
+        this->linear_velocity * (1.f - drag * delta)
+        + this->acceleration * delta
+        + this->impulse
+    };
+    this->set_global_transform(this->get_global_transform()
+        .translated((next_velocity + this->linear_velocity) * 0.5f * delta + this->motion)
+    );
+    this->linear_velocity = next_velocity;
+    this->motion = this->acceleration = this->impulse = Vecf::ZERO;
+}
+
+void RigidBody::on_collision(CollisionShape *shape, CollidableNode *other, CollisionShape *other_shape) {
+    Transform const shape_transform{shape->get_global_transform()};
+    Transform const other_transform{other_shape->get_global_transform()};
+    Vecf escape_vector{CollisionShape::get_escape_vector(shape, other_shape)};
+    // we do this because there's no collision object to maintain a static physics state for different body's resolution steps.
+    // (if we were to directly modify the transform here, the escape vector for the other body would be completely off)
+    this->motion = escape_vector;
+    float const bounce{shape->get_bounce() * other_shape->get_bounce()};
+    if(RigidBody *other_rb{dynamic_cast<RigidBody*>(other)}) {
+        Vecf const relative_velocity{other_rb->linear_velocity - this->linear_velocity};
+        float const relative_mass{other_rb->mass / (other_rb->mass + this->mass)};
+        this->add_impulse(escape_vector.normalized() * relative_velocity.magnitude() * bounce * relative_mass);
+    } else {
+        this->add_impulse(Vecf::reflect(this->linear_velocity, escape_vector.normalized()) * bounce);
+    }
+}
+```
+
+core/collision.hpp
+
+```cpp
+void CollisionWorld::check_collisions() {
+    for(int i{0}; i < this->shapes.size(); ++i) {
+        CollisionShape *shape{this->shapes[i]};
+        this->check_collisions_for(shape, i);
+    }
+}
+
+void CollisionWorld::check_collisions_for(CollisionShape *shape, size_t begin) {
+    // check all shapes *after* this one in the shapes list.
+    // (As the shapes *before* have already been checked, guaranteeing that each pair will only be checked once)
+    for(size_t i{begin+1}; i < this->shapes.size(); ++i) {
+        CollisionShape *other{this->shapes[i]};
+        if(other != shape && other->get_owner() != shape->get_owner() && CollisionShape::shapes_overlap(shape, other)) {
+            if((shape->get_mask() & other->get_layers()) != 0x0u)
+                shape->get_owner()->add_overlap(shape, other);
+            if((shape->get_layers() & other->get_mask()) != 0x0u)
+                other->get_owner()->add_overlap(other, shape);
+        }
+    }
+}
+
+```
+
+Collision detection uses masks to minimize checks. Though I didn't end up using those in the final version. It uses sqr_magnitude instead of magnitude to avoid a sqrt. Though it's a minor optimization that isn't really needed for a project this size.
+
+```cpp
+bool CollisionShape::can_collide(CollisionShape const *lhs, CollisionShape const *rhs) {
+    return lhs->owner != nullptr && rhs->owner != nullptr
+        && lhs->owner != rhs->owner
+        && ((lhs->get_mask() & rhs->get_layers()) != 0x0u
+            || (rhs->get_mask() & lhs->get_layers()) != 0x0u);
+}
+
+bool CollisionShape::shapes_overlap(CollisionShape const *lhs, CollisionShape const *rhs) {
+    float const rad_sum{lhs->radius + rhs->radius};
+    return (lhs->get_global_transform().position - rhs->get_global_transform().position).sqr_magnitude() < rad_sum * rad_sum;
+}
+
+Vecf CollisionShape::get_escape_vector(CollisionShape const *lhs, CollisionShape const *rhs) {
+    Vecf const difference{lhs->get_global_transform().position - rhs->get_global_transform().position};
+    float const diff_mag{difference.magnitude()};
+    return diff_mag < (lhs->radius + rhs->radius)
+        ? (difference / diff_mag) * (lhs->radius + rhs->radius - diff_mag)
+        : Vecf::ZERO;
+}
+```
+
+# Signals
+
+Notifications are carried between objects using Signals. An implementation of the Observer pattern intended to provide generic, typesafe, anonymous callbacks.
+
+core/signal.hpp:
+
+```cpp
+
+/*! Observer-listener implementation
+ */
+template <class... Args>
+class Signal {
+    std::vector<Callable<void, Args...>> listeners{};
+public:
+    void connect(Callable<void, Args...> callable);
+    void disconnect(Callable<void, Args...> callable);
+    void invoke(Args...);
+};
+
+template <class... Args> void Signal<Args...>::connect(Callable<void, Args...> callable) {
+    this->listeners.push_back(callable);
+}
+
+template <class... Args> void Signal<Args...>::disconnect(Callable<void, Args...> callable) {
+    std::erase_if(this->listeners,
+    [&callable](Callable<void, Args...> &listener) -> bool {
+        return listener == callable;
+    });
+}
+
+template <class... Args> void Signal<Args...>::invoke(Args... args) {
+    for(Callable<void, Args...> &listener : this->listeners) {
+        listener.call(args...);
+    }
+}
+
+#ifdef DEBUG
+static inline
+void TEST_signals() {
+    struct A {inline void f(int val) {
+        std::printf("A: %d\n", val);
+    }};
+    struct B { inline void f(int val) {
+        std::printf("B: %d\n", val);
+    }};
+
+    A a_object;
+    B b_object;
+
+    Signal<int> signal;
+
+    Callable<void, int> a_callable{Callable<void, int>::make(&a_object, &A::f)};
+    Callable<void, int> b_callable{Callable<void, int>::make(&b_object, &B::f)};
+
+    signal.connect(a_callable);
+    signal.invoke(5);
+    signal.connect(b_callable);
+    signal.invoke(12);
+    signal.disconnect(a_callable);
+    signal.invoke(10);
+}
+#endif
+```
+
+This is achieved using the custom Callable template class. It's made up of an end-programmer-facing `Callable` wrapper. And a `HiddenCallable` to enable the end-programmer to use a very simple stack-allocated interface, while behind the scenes it can do polymorphism as required to make `Callable` anonymous (so not requiring the caller to know the type of the target).
+
+core/callable.hpp:
+
+```cpp
+/* Base interface template for a member function pointer object
+ */
+template <class Return, class... Args>
+struct HiddenCallableBase {
+    virtual ~HiddenCallableBase() = default;
+    virtual Return call(Args... args) const = 0;
+    virtual bool equals(HiddenCallableBase<Return, Args...> const &other) const;
+};
+
+template <class Return, class... Args> bool HiddenCallableBase<Return, Args...>::equals(HiddenCallableBase<Return, Args...> const &other) const {
+    return &other == this;
+}
+
+/* Hidden component of a Callable, specialized for a specific target object
+ */
+template <class Target, class Return, class... Args>
+struct HiddenCallable : public HiddenCallableBase<Return, Args...> {
+typedef Return (Target::*Signature)(Args...);
+    HiddenCallable(Target *target, Signature function);
+    Target *target{nullptr};
+    Signature function{nullptr};
+    virtual void call(Args...) const override;
+    virtual bool equals(HiddenCallableBase<Return, Args...> const &other) const override;
+};
+
+template <class Target, class Return, class... Args> bool HiddenCallable<Target, Return, Args...>::equals(HiddenCallableBase<Return, Args...> const &other) const {
+    HiddenCallable<Target, Return, Args...> const *cast{dynamic_cast<HiddenCallable<Target, Return, Args...> const *>(&other)};
+    return cast != nullptr && &other == this && cast->target == this->target && cast->function == this->function;
+}
+
+template <class Target, class Return, class... Args> HiddenCallable<Target, Return, Args...>::HiddenCallable(Target *target, Signature function) : target{target}, function{function} {}
+template <class Target, class Return, class... Args> void HiddenCallable<Target, Return, Args...>::call(Args... args) const {
+    std::invoke(this->function, this->target, args...);
+}
+
+/* Class for referring to a callable pointer to a member function.
+ */
+template <class Return, class... Args>
+class Callable {
+public:
+    std::shared_ptr<HiddenCallableBase<Return, Args...> const> hidden;
+    template <class Target> static Callable<Return, Args...> make(Target *target, Return (Target::*function)(Args...));
+    Return call(Args... args);
+};
+
+template <class Return, class... Args> template <class Target> Callable<Return, Args...>
+Callable<Return, Args...>::make(Target *target, Return (Target::*function)(Args...)) {
+    Callable<Return, Args...> callable;
+    callable.hidden = std::make_unique<HiddenCallable<Target, Return, Args...>>(target, function);
+    return callable;
+};
+
+template <class Return, class... Args> Return Callable<Return, Args...>::call(Args... args) {
+    return this->hidden->call(args...);
+}
+
+template <class Return, class... Args> bool operator==(Callable<Return, Args...> const &lhs, Callable<Return, Args...> const &rhs) {
+    return lhs.hidden->equals(*rhs.hidden.get());
+}
+```
+
+The general concept of this is based on Godot's concepts of the same names (the exposed/hidden layers and function pointers). Though heavily modified, as I don't need to support a custom scripting language. Which then allowed me to use compile-time type checking. Rather than having to do that using a variant type at runtime. It also lets me avoid `void*` sorcery. However much I may love myself a `void*`, it's best to avoid them where needed.
+
+These are then exposed in classes as such:
+
+core/collidable_node.hpp:
+
+```cpp
+class CollidableNode : public Node2D {
+...
+public:
+    Signal<CollisionShape*, CollidableNode*, CollisionShape*> overlap_enter{};
+...
+};
+```
+
+and listened to like this:
+
+```cpp
+Player::Player()
+...
+    this->overlap_enter.connect(ce::Callable<void, ce::CollisionShape *, ce::CollidableNode *, ce::CollisionShape *>::make(
+        this, &Player::_on_overlap_enter
+    ));
+...
+}
+```
+
+# Input
+
+The input subsystem is rather simple to use. It has three concepts. The `InputMap` being the simplest.
+
+```cpp
+/*! Map input bindings to functions.
+ */
+class InputMap {
+    std::map<std::string, InputAction> bindings{};
+public:
+    InputAction &bind_input(std::string const &name, std::vector<InputEffect*> &&binds);
+    InputAction &get_action(std::string const &action_id);
+    void process_event(SDL_Event const &evt);
+};
+```
+
+The input map can be acquired from the `CanvasEngine`. Input actions are then registered. Which expose a `changed` `Signal`. That can be listened for as described in the relevant section.
+
+An input Action is built of `InputEffect`s. Which are objects that receive SDL events, filter them, and transform them, to produce a signal whenever they want. Containing whatever data they want.
+
+For example a `KeyboardKey` effect will wait for a specific key to be pressed. And mark itself as changed. So that the `InputAction` will notify it's listeners.
+
+More complex effects are also possible. For example a `ButtonAxis` combines two `InputEffects` and maps one to -1 and one to +1. Giving an easy 2-directional axis.
+
+```cpp
+/*! Represents a collection of bindings.
+ */
+class InputAction {
+private:
+    std::vector<InputEffect::Ptr> effects{}; //!< list of all effects used to produce final result
+public:
+    Signal<InputValue> changed{};
+    InputAction() = default;
+    InputAction(InputEffect::Ptr &effect);
+    InputAction(std::vector<InputEffect*> &effects);
+
+    void process_event(SDL_Event const &evt); //!< process an incoming OS event
+
+    template <class Effect> Effect *get_effect_of_type(); //!< get the first effect from the effects list that is of type Effect
+};
+
+```
+
+The part of the `Player` constructor that sets up input looks like this:
+
+```cpp
+    ce::InputMap &map{ce::CanvasEngine::get_singleton()->get_input_map()};
+    map.bind_input("horizontal", {
+        new ce::ButtonAxis(
+            new ce::KeyboardScancode(SDL_SCANCODE_A),
+            new ce::KeyboardScancode(SDL_SCANCODE_D)
+        )
+    }).changed.connect(ce::Callable<void, ce::InputValue>::make(
+        this, &Player::_input_horizontal_movement)
+    );
+    map.bind_input("vertical", {
+        new ce::ButtonAxis(
+            new ce::KeyboardScancode(SDL_SCANCODE_W),
+            new ce::KeyboardScancode(SDL_SCANCODE_S)
+        )
+    }).changed.connect(ce::Callable<void, ce::InputValue>::make(
+        this, &Player::_input_vertical_movement)
+    );
+
+```
+
+It uses a `ButtonAxis` to combine two scancodes into an axis with a range of `-1` to `1`. Making input tracking slightly easier.
+
+# Levels
+
+Levels are relatively tiny objects. Only really existing to have somewhere to construct the node hierarchy. Though also useful for storing level-local data.
+
+```cpp
+class Level1 : public ce::Level {
+public:
+    unsigned score{0};
+    unsigned lives{3};
+    ce::Signal<unsigned> score_added{};
+    ce::Signal<unsigned> life_lost{};
+public:
+    void add_score();
+    void lose_life();
+    virtual ce::Node::OwnedPtr construct() override;
+};
+```
+
+Although they can be constructed anywhere. Only the `CanvasEngine` should call `instantiate` to actually construct and instantiate the scene when the level becomes the new primary level.
+
+# External resources used:
+
+Game Engine Architecture by Jason Gregory
+
+Game Physics by David H. Eberly
+
+Many hours on [Desmos.com](desmos.com) to check my math.
+
+So so so much [cppreference.com](cppreference.com) (though i downloaded it so i could check it on the train).
diff --git a/premake5.lua b/premake5.lua
index 88faef4..07b4707 100644
--- a/premake5.lua
+++ b/premake5.lua
@@ -12,7 +12,7 @@ project "game"
     postbuildcommands "{COPYDIR} ../resources %{cfg.targetdir}"
     files { "src/**.cpp" }
     includedirs { "src/" }
-    links { "SDL2", "SDL2_image", "m" }
+    links { "SDL2", "SDL2_image", "m", "SDL2_ttf" }
     targetdir "bin/"
     exceptionhandling "Off"
     filter "configurations:debug"
diff --git a/src/core/collidable_node.hpp b/src/core/collidable_node.hpp
index a5695a9..a64df88 100644
--- a/src/core/collidable_node.hpp
+++ b/src/core/collidable_node.hpp
@@ -14,7 +14,6 @@ class CollidableNode : public Node2D {
     CollisionMask mask{~0x0u /* all layers by default */};
     CollisionMask layers{0x1u /* only the first layer is enabled by default */};
 public:
-    // collision(local_shape, other_node, other_shape)
     Signal<CollisionShape*, CollidableNode*, CollisionShape*> overlap_enter{};
 public:
     CollidableNode(std::string const &name, CollisionMask layers, CollisionMask mask);
diff --git a/src/core/collision_shape.cpp b/src/core/collision_shape.cpp
index 0ff62bb..06fd929 100644
--- a/src/core/collision_shape.cpp
+++ b/src/core/collision_shape.cpp
@@ -40,6 +40,7 @@ void CollisionShape::_removed() {
 }
 
 void CollisionShape::_draw(SDL_Renderer *render, ce::Transform const &view_transform) {
+#if 0
     static const size_t points_c{17};
     SDL_FPoint points[points_c];
     Vecf point{this->radius, 0.f};
@@ -53,6 +54,7 @@ void CollisionShape::_draw(SDL_Renderer *render, ce::Transform const &view_trans
     }
     points[points_c-1] = points[0];
     SDL_RenderDrawLinesF(render, points, points_c);
+#endif
 }
 
 bool CollisionShape::can_collide(CollisionShape const *lhs, CollisionShape const *rhs) {
diff --git a/src/core/collision_shape.hpp b/src/core/collision_shape.hpp
index 8dd25c2..a5cabae 100644
--- a/src/core/collision_shape.hpp
+++ b/src/core/collision_shape.hpp
@@ -21,9 +21,11 @@ public:
     virtual void _removed() override;
     virtual void _draw(SDL_Renderer *render, ce::Transform const &view_transform) override;
 
+    // return true if either of these objects could receive a notification for colliding with the other
     static bool can_collide(CollisionShape const *lhs, CollisionShape const *rhs);
     static bool shapes_overlap(CollisionShape const *lhs, CollisionShape const *rhs);
 
+    // get the motion lhs will have to make to fully disconnect from rhs.
     static Vecf get_escape_vector(CollisionShape const *lhs, CollisionShape const *rhs);