text: #pragma once
  
  #include "ExternalLibraries.h"
  
  #include "mesh.h"
  #include "shader.h"
  
  
  using namespace std;
  
  unsigned int TextureFromFile(const char* path, const string& directory, bool gamma = false);
  
  class Model
  {
  	public:
  		// model data 
  	vector<Texture> textures_loaded;	// stores all the textures loaded so far, optimization to make sure textures aren't loaded more than once.
  	vector<Mesh>    meshes;
  	string directory;
  	bool gammaCorrection;
  
  	Model()
  	{
  	}
  
  	// constructor, expects a filepath to a 3D model.
  	Model(string const& path, bool gamma = false) : gammaCorrection(gamma)
  	{
  		loadModel(path);
  	}
  
  	// draws the model, and thus all its meshes
  	void Draw(GLuint shaderID)
  	{
  		for (unsigned int i = 0; i < meshes.size(); i++)
  			meshes[i].Draw(shaderID);
  	}
  
  	// loads a model with supported ASSIMP extensions from file and stores the resulting meshes in the meshes vector.
  	void loadModel(string const& path)
  	{
  // read file via ASSIMP
  		Assimp::Importer importer;
  		const aiScene* scene = importer.ReadFile(path, aiProcess_Triangulate | aiProcess_GenSmoothNormals | aiProcess_CalcTangentSpace);
  		// check for errors
  		if (!scene || scene->mFlags & AI_SCENE_FLAGS_INCOMPLETE || !scene->mRootNode) // if is Not Zero
  		{
  			cout << "ERROR::ASSIMP:: " << importer.GetErrorString() << endl;
  			return;
  		}
  		// retrieve the directory path of the filepath
  		directory = path.substr(0, path.find_last_of('/'));
  
  		// process ASSIMP's root node recursively
  		processNode(scene->mRootNode, scene);
  	}
  
  	// processes a node in a recursive fashion. Processes each individual mesh located at the node and repeats this process on its children nodes (if any).
  	void processNode(aiNode* node, const aiScene* scene)
  	{
  // process each mesh located at the current node
  		for (unsigned int i = 0; i < node->mNumMeshes; i++)
  		{
  // the node object only contains indices to index the actual objects in the scene. 
  // the scene contains all the data, node is just to keep stuff organized (like relations between nodes).
  			aiMesh* mesh = scene->mMeshes[node->mMeshes[i]];
  			meshes.push_back(processMesh(mesh, scene));
  		}
  		// after we've processed all of the meshes (if any) we then recursively process each of the children nodes
  		for (unsigned int i = 0; i < node->mNumChildren; i++)
  		{
  			processNode(node->mChildren[i], scene);
  		}
  
  	}
  
  	Mesh processMesh(aiMesh* mesh, const aiScene* scene)
  	{
  // data to fill
  		vector<Vertex> vertices;
  		vector<unsigned int> indices;
  		vector<Texture> textures;
  
  		// walk through each of the mesh's vertices
  		for (unsigned int i = 0; i < mesh->mNumVertices; i++)
  		{
  			Vertex vertex;
  			glm::vec3 vector; // we declare a placeholder vector since assimp uses its own vector class that doesn't directly convert to glm's vec3 class so we transfer the data to this placeholder glm::vec3 first.
  			// positions
  			vector.x = mesh->mVertices[i].x;
  			vector.y = mesh->mVertices[i].y;
  			vector.z = mesh->mVertices[i].z;
  			vertex.Position = vector;
  			// normals
  			if (mesh->HasNormals())
  			{
  				vector.x = mesh->mNormals[i].x;
  				vector.y = mesh->mNormals[i].y;
  				vector.z = mesh->mNormals[i].z;
  				vertex.Normal = vector;
  			}
  			// texture coordinates
  			if (mesh->mTextureCoords[0]) // does the mesh contain texture coordinates?
  			{
  				glm::vec2 vec;
  				// a vertex can contain up to 8 different texture coordinates. We thus make the assumption that we won't 
  				// use models where a vertex can have multiple texture coordinates so we always take the first set (0).
  				vec.x = mesh->mTextureCoords[0][i].x;
  				vec.y = mesh->mTextureCoords[0][i].y;
  				vertex.TexCoords = vec;
  				// tangent
  				vector.x = mesh->mTangents[i].x;
  				vector.y = mesh->mTangents[i].y;
  				vector.z = mesh->mTangents[i].z;
  				vertex.Tangent = vector;
  				// bitangent
  				vector.x = mesh->mBitangents[i].x;
  				vector.y = mesh->mBitangents[i].y;
  				vector.z = mesh->mBitangents[i].z;
  				vertex.Bitangent = vector;
  			}
  			else
  				vertex.TexCoords = glm::vec2(0.0f, 0.0f);
  
  			vertices.push_back(vertex);
  		}
  		// now wak through each of the mesh's faces (a face is a mesh its triangle) and retrieve the corresponding vertex indices.
  		for (unsigned int i = 0; i < mesh->mNumFaces; i++)
  		{
  			aiFace face = mesh->mFaces[i];
  			// retrieve all indices of the face and store them in the indices vector
  			for (unsigned int j = 0; j < face.mNumIndices; j++)
  				indices.push_back(face.mIndices[j]);
  		}
  		// process materials
  		aiMaterial* material = scene->mMaterials[mesh->mMaterialIndex];
  		// we assume a convention for sampler names in the shaders. Each diffuse texture should be named
  		// as 'texture_diffuseN' where N is a sequential number ranging from 1 to MAX_SAMPLER_NUMBER. 
  		// Same applies to other texture as the following list summarizes:
  		// diffuse: texture_diffuseN
  		// specular: texture_specularN
  		// normal: texture_normalN
  
  		// 1. diffuse maps
  		vector<Texture> diffuseMaps = loadMaterialTextures(material, aiTextureType_DIFFUSE, "texture_diffuse");
  		textures.insert(textures.end(), diffuseMaps.begin(), diffuseMaps.end());
  		// 2. specular maps
  		vector<Texture> specularMaps = loadMaterialTextures(material, aiTextureType_SPECULAR, "texture_specular");
  		textures.insert(textures.end(), specularMaps.begin(), specularMaps.end());
  		// 3. normal maps
  		std::vector<Texture> normalMaps = loadMaterialTextures(material, aiTextureType_HEIGHT, "texture_normal");
  		textures.insert(textures.end(), normalMaps.begin(), normalMaps.end());
  		// 4. height maps
  		std::vector<Texture> heightMaps = loadMaterialTextures(material, aiTextureType_AMBIENT, "texture_height");
  		textures.insert(textures.end(), heightMaps.begin(), heightMaps.end());
  
  		// return a mesh object created from the extracted mesh data
  		return Mesh(vertices, indices, textures);
  	}
  
  	// checks all material textures of a given type and loads the textures if they're not loaded yet.
  	// the required info is returned as a Texture struct.
  	vector<Texture> loadMaterialTextures(aiMaterial* mat, aiTextureType type, string typeName)
  	{
  		vector<Texture> textures;
  		for (unsigned int i = 0; i < mat->GetTextureCount(type); i++)
  		{
  			aiString str;
  			mat->GetTexture(type, i, &str);
  			// check if texture was loaded before and if so, continue to next iteration: skip loading a new texture
  			bool skip = false;
  			for (unsigned int j = 0; j < textures_loaded.size(); j++)
  			{
  				if (std::strcmp(textures_loaded[j].path.data(), str.C_Str()) == 0)
  				{
  					textures.push_back(textures_loaded[j]);
  					skip = true; // a texture with the same filepath has already been loaded, continue to next one. (optimization)
  					break;
  				}
  			}
  			if (!skip)
  			{   // if texture hasn't been loaded already, load it
  				Texture texture;
  				texture.id = TextureFromFile(str.C_Str(), this->directory);
  				texture.type = typeName;
  				texture.path = str.C_Str();
  				textures.push_back(texture);
  				textures_loaded.push_back(texture);  // store it as texture loaded for entire model, to ensure we won't unnecesery load duplicate textures.
  			}
  		}
  		return textures;
  	}
  };
  
  
  unsigned int TextureFromFile(const char* path, const string& directory, bool gamma)
  {
  	string filename = string(path);
  	filename = directory + '/' + filename;
  
  	unsigned int textureID;
  	glGenTextures(1, &textureID);
  
  	int width, height, nrComponents;
  	unsigned char* data = stbi_load(filename.c_str(), &width, &height, &nrComponents, 0);
  	if (data)
  	{
  		GLenum format;
  		if (nrComponents == 1)
  			format = GL_RED;
  		else if (nrComponents == 3)
  			format = GL_RGB;
  		else if (nrComponents == 4)
  			format = GL_RGBA;
  
  		glBindTexture(GL_TEXTURE_2D, textureID);
  		glTexImage2D(GL_TEXTURE_2D, 0, format, width, height, 0, format, GL_UNSIGNED_BYTE, data);
  		glGenerateMipmap(GL_TEXTURE_2D);
  
  		glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
  		glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
  		glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
  		glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
  
  		stbi_image_free(data);
  	}
  	else
  	{
  		std::cout << "Texture failed to load at path: " << path << std::endl;
  		stbi_image_free(data);
  	}
  
  	return textureID;
  }
  
  