From 5273b8017e7b412c6b889e32cef168ab7f402996 Mon Sep 17 00:00:00 2001
From: Benjamin Palko <benjaminpalko@hotmail.com>
Date: Wed, 9 Oct 2024 11:26:03 -0400
Subject: [PATCH] implement ECS library

---
 libs/component.hpp        | 13 ++++++++
 libs/componentarray.cpp   | 47 +++++++++++++++++++++++++++++
 libs/componentarray.hpp   | 37 +++++++++++++++++++++++
 libs/componentmanager.cpp | 63 +++++++++++++++++++++++++++++++++++++++
 libs/componentmanager.hpp | 35 ++++++++++++++++++++++
 libs/coordinator.cpp      | 63 +++++++++++++++++++++++++++++++++++++++
 libs/coordinator.hpp      | 30 +++++++++++++++++++
 libs/entity.cpp           | 42 ++++++++++++++++++++++++++
 libs/entity.hpp           | 31 +++++++++++++++++++
 libs/system.cpp           | 54 +++++++++++++++++++++++++++++++++
 libs/system.hpp           | 29 ++++++++++++++++++
 11 files changed, 444 insertions(+)
 create mode 100644 libs/component.hpp
 create mode 100644 libs/componentarray.cpp
 create mode 100644 libs/componentarray.hpp
 create mode 100644 libs/componentmanager.cpp
 create mode 100644 libs/componentmanager.hpp
 create mode 100644 libs/coordinator.cpp
 create mode 100644 libs/coordinator.hpp
 create mode 100644 libs/entity.cpp
 create mode 100644 libs/entity.hpp
 create mode 100644 libs/system.cpp
 create mode 100644 libs/system.hpp

diff --git a/libs/component.hpp b/libs/component.hpp
new file mode 100644
index 0000000..323866e
--- /dev/null
+++ b/libs/component.hpp
@@ -0,0 +1,13 @@
+#ifndef COMPONENT_HPP
+#define COMPONENT_HPP
+
+#include <bitset>
+#include <cstdint>
+
+using ComponentType = std::uint8_t;
+
+const ComponentType MAX_COMPONENTS = 32;
+
+using Signature = std::bitset<MAX_COMPONENTS>;
+
+#endif // COMPONENT_HPP
diff --git a/libs/componentarray.cpp b/libs/componentarray.cpp
new file mode 100644
index 0000000..3b3b45b
--- /dev/null
+++ b/libs/componentarray.cpp
@@ -0,0 +1,47 @@
+#include <cassert>
+#include "componentarray.hpp"
+
+template <typename T>
+void ComponentArray<T>::InsertData(Entity entity, T component) {
+  assert(mEntityToIndexMap.find(entity) == mEntityToIndexMap.end() &&
+         "Component added to same entity more than once.");
+
+  // Put new entry at end and update the maps
+  size_t newIndex = mSize;
+  mEntityToIndexMap[entity] = newIndex;
+  mIndexToEntityMap[newIndex] = entity;
+  mComponentArray[newIndex] = component;
+  ++mSize;
+};
+template <typename T> void ComponentArray<T>::RemoveData(Entity entity) {
+  assert(mEntityToIndexMap.find(entity) != mEntityToIndexMap.end() &&
+         "Removing non-existent component.");
+
+  // Copy element at end into deleted element's place to maintain density
+  size_t indexOfRemovedEntity = mEntityToIndexMap[entity];
+  size_t indexOfLastElement = mSize - 1;
+  mComponentArray[indexOfRemovedEntity] = mComponentArray[indexOfLastElement];
+
+  // Update map to point to moved spot
+  Entity entityOfLastElement = mIndexToEntityMap[indexOfLastElement];
+  mEntityToIndexMap[entityOfLastElement] = indexOfRemovedEntity;
+  mIndexToEntityMap[indexOfRemovedEntity] = entityOfLastElement;
+
+  mEntityToIndexMap.erase(entity);
+  mIndexToEntityMap.erase(indexOfLastElement);
+
+  --mSize;
+};
+template <typename T> T &ComponentArray<T>::GetData(Entity entity) {
+  assert(mEntityToIndexMap.find(entity) != mEntityToIndexMap.end() &&
+         "Retrieving non-existent component.");
+
+  // Return a reference to the entity's component
+  return mComponentArray[mEntityToIndexMap[entity]];
+};
+template <typename T> void ComponentArray<T>::EntityDestroyed(Entity entity) {
+  if (mEntityToIndexMap.find(entity) != mEntityToIndexMap.end()) {
+    // Remove the entity's component if it existed
+    RemoveData(entity);
+  }
+};
diff --git a/libs/componentarray.hpp b/libs/componentarray.hpp
new file mode 100644
index 0000000..f8b10c4
--- /dev/null
+++ b/libs/componentarray.hpp
@@ -0,0 +1,37 @@
+#ifndef COMPONENTARRAY_HPP
+#define COMPONENTARRAY_HPP
+
+#include <unordered_map>
+#include <array>
+#include "entity.hpp"
+
+class IComponentArray {
+public:
+  virtual ~IComponentArray() = default;
+  virtual void EntityDestroyed(Entity entity) = 0;
+};
+
+template <typename T> class ComponentArray : public IComponentArray {
+  // The packed array of components (of generic type T),
+  // set to a specified maximum amount, matching the maximum number
+  // of entities allowed to exist simultaneously, so that each entity
+  // has a unique spot.
+  std::array<T, MAX_ENTITIES> mComponentArray;
+
+  // Map from an entity ID to an array index.
+  std::unordered_map<Entity, size_t> mEntityToIndexMap;
+
+  // Map from an array index to an entity ID.
+  std::unordered_map<size_t, Entity> mIndexToEntityMap;
+
+  // Total size of valid entries in the array.
+  size_t mSize;
+
+public:
+  void InsertData(Entity entity, T component);
+  void RemoveData(Entity entity);
+  T &GetData(Entity entity);
+  void EntityDestroyed(Entity entity) override;
+};
+
+#endif // COMPONENTARRAY_HPP
diff --git a/libs/componentmanager.cpp b/libs/componentmanager.cpp
new file mode 100644
index 0000000..24584db
--- /dev/null
+++ b/libs/componentmanager.cpp
@@ -0,0 +1,63 @@
+#include "componentmanager.hpp"
+#include "componentarray.hpp"
+#include <memory>
+
+//  Private functions
+template <typename T>
+std::shared_ptr<ComponentArray<T>> ComponentManager::GetComponentArray() {
+  const char *typeName = typeid(T).name();
+
+  assert(mComponentTypes.find(typeName) != mComponentTypes.end() &&
+         "Component not registered before use.");
+
+  return std::static_pointer_cast<ComponentArray<T>>(
+      mComponentArrays[typeName]);
+}
+
+//  Public functions
+template <typename T> void ComponentManager::RegisterComponent() {
+  const char *typeName = typeid(T).name();
+
+  assert(mComponentTypes.find(typeName) == mComponentTypes.end() &&
+         "Registering component type more than once.");
+
+  // Add this component type to the component type map
+  mComponentTypes.insert({typeName, mNextComponentType});
+
+  // Create a ComponentArray pointer and add it to the component arrays map
+  mComponentArrays.insert({typeName, std::make_shared<ComponentArray<T>>()});
+
+  // Increment the value so that the next component registered will be different
+  ++mNextComponentType;
+};
+template <typename T> ComponentType ComponentManager::GetComponentType() {
+  const char *typeName = typeid(T).name();
+
+  assert(mComponentTypes.find(typeName) != mComponentTypes.end() &&
+         "Component not registered before use.");
+
+  // Return this component's type - used for creating signatures
+  return mComponentTypes[typeName];
+}
+template <typename T>
+void ComponentManager::AddComponent(Entity entity, T component) {
+  // Add a component to the array for an entity
+  GetComponentArray<T>()->InsertData(entity, component);
+}
+template <typename T> void ComponentManager::RemoveComponent(Entity entity) {
+  // Remove a component from the array for an entity
+  GetComponentArray<T>()->RemoveData(entity);
+}
+template <typename T> T &ComponentManager::GetComponent(Entity entity) {
+  // Get a reference to a component from the array for an entity
+  return GetComponentArray<T>()->GetData(entity);
+}
+void ComponentManager::EntityDestroyed(Entity entity) {
+  // Notify each component array that an entity has been destroyed
+  // If it has a component for that entity, it will remove it
+  for (auto const &pair : mComponentArrays) {
+    auto const &component = pair.second;
+
+    component->EntityDestroyed(entity);
+  }
+};
diff --git a/libs/componentmanager.hpp b/libs/componentmanager.hpp
new file mode 100644
index 0000000..16ab368
--- /dev/null
+++ b/libs/componentmanager.hpp
@@ -0,0 +1,35 @@
+#ifndef COMPONENTMANAGER_HPP
+#define COMPONENTMANAGER_HPP
+
+#include <unordered_map>
+#include <memory>
+#include <cassert>
+#include "component.hpp"
+#include "componentarray.hpp"
+
+class ComponentManager {
+  // Map from type string pointer to a component type
+  std::unordered_map<const char *, ComponentType> mComponentTypes{};
+
+  // Map from type string pointer to a component array
+  std::unordered_map<const char *, std::shared_ptr<IComponentArray>>
+      mComponentArrays{};
+
+  // The component type to be assigned to the next registered component -
+  // starting at 0
+  ComponentType mNextComponentType{};
+
+  // Convenience function to get the statically casted pointer to the
+  // ComponentArray of type T.
+  template <typename T> std::shared_ptr<ComponentArray<T>> GetComponentArray();
+
+public:
+  template <typename T> void RegisterComponent();
+  template <typename T> ComponentType GetComponentType();
+  template <typename T> void AddComponent(Entity entity, T component);
+  template <typename T> void RemoveComponent(Entity entity);
+  template <typename T> T &GetComponent(Entity entity);
+  void EntityDestroyed(Entity entity);
+};
+
+#endif // COMPONENTMANAGER_HPP
diff --git a/libs/coordinator.cpp b/libs/coordinator.cpp
new file mode 100644
index 0000000..af6f545
--- /dev/null
+++ b/libs/coordinator.cpp
@@ -0,0 +1,63 @@
+#include "coordinator.hpp"
+
+void Coordinator::Init() {
+  // Create pointers to each manager
+  mComponentManager = std::make_unique<ComponentManager>();
+  mEntityManager = std::make_unique<EntityManager>();
+  mSystemManager = std::make_unique<SystemManager>();
+}
+
+// Entity methods
+Entity Coordinator::CreateEntity() { return mEntityManager->CreateEntity(); }
+
+void Coordinator::DestroyEntity(Entity entity) {
+  mEntityManager->DestroyEntity(entity);
+
+  mComponentManager->EntityDestroyed(entity);
+
+  mSystemManager->EntityDestroyed(entity);
+}
+
+// Component methods
+template <typename T> void Coordinator::RegisterComponent() {
+  mComponentManager->RegisterComponent<T>();
+}
+
+template <typename T>
+void Coordinator::AddComponent(Entity entity, T component) {
+  mComponentManager->AddComponent<T>(entity, component);
+
+  auto signature = mEntityManager->GetSignature(entity);
+  signature.set(mComponentManager->GetComponentType<T>(), true);
+  mEntityManager->SetSignature(entity, signature);
+
+  mSystemManager->EntitySignatureChanged(entity, signature);
+}
+
+template <typename T> void Coordinator::RemoveComponent(Entity entity) {
+  mComponentManager->RemoveComponent<T>(entity);
+
+  auto signature = mEntityManager->GetSignature(entity);
+  signature.set(mComponentManager->GetComponentType<T>(), false);
+  mEntityManager->SetSignature(entity, signature);
+
+  mSystemManager->EntitySignatureChanged(entity, signature);
+}
+
+template <typename T> T &Coordinator::GetComponent(Entity entity) {
+  return mComponentManager->GetComponent<T>(entity);
+}
+
+template <typename T> ComponentType Coordinator::GetComponentType() {
+  return mComponentManager->GetComponentType<T>();
+}
+
+// System methods
+template <typename T> std::shared_ptr<T> Coordinator::RegisterSystem() {
+  return mSystemManager->RegisterSystem<T>();
+}
+
+template <typename T>
+void Coordinator::SetSystemSignature(Signature signature) {
+  mSystemManager->SetSignature<T>(signature);
+}
diff --git a/libs/coordinator.hpp b/libs/coordinator.hpp
new file mode 100644
index 0000000..73f5a36
--- /dev/null
+++ b/libs/coordinator.hpp
@@ -0,0 +1,30 @@
+#ifndef COORDINATOR_HPP
+#define COORDINATOR_HPP
+
+#include <memory>
+#include "componentmanager.hpp"
+#include "entity.hpp"
+#include "system.hpp"
+
+class Coordinator {
+  std::unique_ptr<ComponentManager> mComponentManager;
+  std::unique_ptr<EntityManager> mEntityManager;
+  std::unique_ptr<SystemManager> mSystemManager;
+
+public:
+  void Init();
+  // Entity methods
+  Entity CreateEntity();
+  void DestroyEntity(Entity entity);
+  // Component methods
+  template <typename T> void RegisterComponent();
+  template <typename T> void AddComponent(Entity entity, T component);
+  template <typename T> void RemoveComponent(Entity entity);
+  template <typename T> T &GetComponent(Entity entity);
+  template <typename T> ComponentType GetComponentType();
+  // System methods
+  template <typename T> std::shared_ptr<T> RegisterSystem();
+  template <typename T> void SetSystemSignature(Signature signature);
+};
+
+#endif // COORDINATOR_HPP
diff --git a/libs/entity.cpp b/libs/entity.cpp
new file mode 100644
index 0000000..83f7262
--- /dev/null
+++ b/libs/entity.cpp
@@ -0,0 +1,42 @@
+#include "entity.hpp"
+#include <cassert>
+
+EntityManager::EntityManager() {
+  // Initialize the queue with all possible entity IDs
+  for (Entity entity = 0; entity < MAX_ENTITIES; ++entity) {
+    mAvailableEntities.push(entity);
+  }
+}
+Entity EntityManager::CreateEntity() {
+  assert(mLivingEntityCount < MAX_ENTITIES &&
+         "Too many entities in existence.");
+
+  // Take an ID from the front of the queue
+  Entity id = mAvailableEntities.front();
+  mAvailableEntities.pop();
+  ++mLivingEntityCount;
+
+  return id;
+}
+void EntityManager::DestroyEntity(Entity entity) {
+  assert(entity < MAX_ENTITIES && "Entity out of range.");
+
+  // Invalidate the destroyed entity's signature
+  mSignatures[entity].reset();
+
+  // Put the destroyed ID at the back of the queue
+  mAvailableEntities.push(entity);
+  --mLivingEntityCount;
+}
+void EntityManager::SetSignature(Entity entity, Signature signature) {
+  assert(entity < MAX_ENTITIES && "Entity out of range.");
+
+  // Put this entity's signature into the array
+  mSignatures[entity] = signature;
+}
+Signature EntityManager::GetSignature(Entity entity) {
+  assert(entity < MAX_ENTITIES && "Entity out of range.");
+
+  // Get this entity's signature from the array
+  return mSignatures[entity];
+}
diff --git a/libs/entity.hpp b/libs/entity.hpp
new file mode 100644
index 0000000..e2f7a15
--- /dev/null
+++ b/libs/entity.hpp
@@ -0,0 +1,31 @@
+#ifndef ENTITY_HPP
+#define ENTITY_HPP
+
+#include <cstdint>
+#include <queue>
+#include <array>
+#include "component.hpp"
+
+using Entity = std::uint32_t;
+
+const Entity MAX_ENTITIES = 5000;
+
+class EntityManager {
+  // Queue of unused entity IDs
+  std::queue<Entity> mAvailableEntities{};
+
+  // Array of signatures where the index corresponds to the entity ID
+  std::array<Signature, MAX_ENTITIES> mSignatures{};
+
+  // Total living entities - used to keep limits on how many exist
+  uint32_t mLivingEntityCount{};
+
+public:
+  EntityManager();
+  Entity CreateEntity();
+  void DestroyEntity(Entity entity);
+  void SetSignature(Entity entity, Signature signature);
+  Signature GetSignature(Entity entity);
+};
+
+#endif // ENTITY_HPP
diff --git a/libs/system.cpp b/libs/system.cpp
new file mode 100644
index 0000000..a7e4ef4
--- /dev/null
+++ b/libs/system.cpp
@@ -0,0 +1,54 @@
+#include "system.hpp"
+
+#include <cassert>
+
+template <typename T> std::shared_ptr<T> SystemManager::RegisterSystem() {
+  const char *typeName = typeid(T).name();
+
+  assert(mSystems.find(typeName) == mSystems.end() &&
+         "Registering system more than once.");
+
+  // Create a pointer to the system and return it so it can be used externally
+  auto system = std::make_shared<T>();
+  mSystems.insert({typeName, system});
+  return system;
+}
+
+template <typename T> void SystemManager::SetSignature(Signature signature) {
+  const char *typeName = typeid(T).name();
+
+  assert(mSystems.find(typeName) != mSystems.end() &&
+         "System used before registered.");
+
+  // Set the signature for this system
+  mSignatures.insert({typeName, signature});
+}
+
+void SystemManager::EntityDestroyed(Entity entity) {
+  // Erase a destroyed entity from all system lists
+  // mEntities is a set so no check needed
+  for (auto const &pair : mSystems) {
+    auto const &system = pair.second;
+
+    system->mEntities.erase(entity);
+  }
+}
+
+void SystemManager::EntitySignatureChanged(Entity entity,
+                                           Signature entitySignature) {
+  // Notify each system that an entity's signature changed
+  for (auto const &pair : mSystems) {
+    auto const &type = pair.first;
+    auto const &system = pair.second;
+    auto const &systemSignature = mSignatures[type];
+
+    // Entity signature matches system signature - insert into set
+    if ((entitySignature & systemSignature) == systemSignature) {
+      system->mEntities.insert(entity);
+    }
+    // Entity signature does not match system signature - erase from set
+    else {
+      system->mEntities.erase(entity);
+    }
+  }
+}
diff --git a/libs/system.hpp b/libs/system.hpp
new file mode 100644
index 0000000..af7e831
--- /dev/null
+++ b/libs/system.hpp
@@ -0,0 +1,29 @@
+#ifndef SYSTEM_HPP
+#define SYSTEM_HPP
+
+#include <set>
+#include <memory>
+#include <unordered_map>
+#include "component.hpp"
+#include "entity.hpp"
+
+class System {
+public:
+  std::set<Entity> mEntities;
+};
+
+class SystemManager {
+  // Map from system type string pointer to a signature
+  std::unordered_map<const char *, Signature> mSignatures{};
+
+  // Map from system type string pointer to a system pointer
+  std::unordered_map<const char *, std::shared_ptr<System>> mSystems{};
+
+public:
+  template <typename T> std::shared_ptr<T> RegisterSystem();
+  template <typename T> void SetSignature(Signature signature);
+  void EntityDestroyed(Entity entity);
+  void EntitySignatureChanged(Entity entity, Signature entitySignature);
+};
+
+#endif // !SYSTEM_HPP