// To compile on Windows

// Install GLFW 3.3.8
// https://www.glfw.org/download.html
// Install GLEW 2.2.0
// https://github.com/nigels-com/glew/releases/tag/glew-2.2.0
//
// extract the downloaded .zip files to "C:/libs"; this is currently expected
// by our CMakeLists.txt.

// Install CMake
// https://cmake.org/download
// Add to PATH for all users
// from project root:
// mkdir build
// cd build
// cmake ..
// cmake --build .
// The last step compiles the executable - this can also be done from Visual
// Studio

// To run in VS
// Set startup project in Solution Explorer
// Press F5 to run
//
// To compile on Arch Linux
//
// Install dependencies
// sudo pacman -S glfw mesa glew
//
// Build
// cmake ..
// cmake --build .

#include <GL/glew.h>
#include <GLFW/glfw3.h>

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>

#include <chrono>

std::string readFile(const std::string& filePath)
{
    std::ifstream fileStream(filePath, std::ios::in);

    if (!fileStream.is_open())
    {
        std::cerr << "Could not read file " << filePath <<
            ". File does not exist." << std::endl;
        return "";
    }

    std::stringstream sstr;
    sstr << fileStream.rdbuf();
    fileStream.close();

    return sstr.str();
}

GLuint compileShader(const std::string& shaderPath, GLenum shaderType)
{
    GLuint shader;
    GLint success;

    std::string shaderSource = readFile(shaderPath);
    const char* source = shaderSource.c_str();

    shader = glCreateShader(shaderType);
    glShaderSource(shader, 1, &source, NULL);
    glCompileShader(shader);

    glGetShaderiv(shader, GL_COMPILE_STATUS, &success);
    if (!success)
    {
        GLchar infoLog[512];
        glGetShaderInfoLog(shader, 512, NULL, infoLog);
        std::cerr << "shader compilation failed" << std::endl
            << infoLog << std::endl;
    }

    return shader;
}

std::chrono::steady_clock::time_point startTime = std::chrono::steady_clock::now();
float getTime()
{
    std::chrono::steady_clock::time_point now = std::chrono::steady_clock::now();
    std::chrono::duration<float> timeSpan = std::chrono::duration_cast<std::chrono::duration<float>>(now - startTime);
    return timeSpan.count();
}

int main()
{
    // Set up GLFW, OpenGL and GLEW.
    if (!glfwInit())
        return -1;

    GLFWwindow* window = glfwCreateWindow(640, 480, "Hello Triangle", NULL, NULL);
    if (!window)
    {
        glfwTerminate();
        return -1;
    }

    glfwMakeContextCurrent(window);

    glewExperimental = GL_TRUE;
    if (glewInit() != GLEW_OK)
    {
        std::cerr << "Failed to initialize GLEW" << std::endl;
        return -1;
    }

    // VAO, VBO
    float vertices[] =
    {
        -0.5, -0.5, 0.0,    // left
         0.5, -0.5, 0.0,    // right
         0.0,  0.5, 0.0     // top
    };

    GLuint VBO, VAO;
    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);

    glBindVertexArray(VAO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindVertexArray(0);

    // Shaders
    GLuint vertShader = compileShader("./vert.glsl", GL_VERTEX_SHADER);
    GLuint fragShader = compileShader("./frag.glsl", GL_FRAGMENT_SHADER);

    GLuint shaderProgram = glCreateProgram();
    glAttachShader(shaderProgram, vertShader);
    glAttachShader(shaderProgram, fragShader);
    glLinkProgram(shaderProgram);

    GLint success;
    GLchar infoLog[512];
    glGetProgramiv(shaderProgram, GL_LINK_STATUS, &success);
    if (!success)
    {
        glGetProgramInfoLog(shaderProgram, 512, NULL, infoLog);
        std::cerr << "shader linking failed" << std::endl
            << infoLog << std::endl;
    }

    // We no longer need the individual shaders
    glDeleteShader(vertShader);
    glDeleteShader(fragShader);

    // Main loop
    while (!glfwWindowShouldClose(window))
    {
        glClearColor(0.2, 0.3, 0.3, 1.0);
        glClear(GL_COLOR_BUFFER_BIT);

        glUseProgram(shaderProgram);

        GLint timeLocation = glGetUniformLocation(shaderProgram, "_Time");
        if (timeLocation == -1)
        {
            std::cerr << "Could not find uniform: _Time" << std::endl;
        }
        float timeValue = getTime();
        glUniform1f(timeLocation, timeValue);

        glBindVertexArray(VAO);
        glDrawArrays(GL_TRIANGLES, 0, 3);

        glfwSwapBuffers(window);
        glfwPollEvents();
    }

    glDeleteVertexArrays(1, &VAO);
    glDeleteBuffers(1, &VBO);

    glfwTerminate();
    return 0;
}