//========================================================================
// Heightmap example program using OpenGL 3 core profile
// Copyright (c) 2010 Olivier Delannoy
//
// This software is provided 'as-is', without any express or implied
// warranty. In no event will the authors be held liable for any damages
// arising from the use of this software.
//
// Permission is granted to anyone to use this software for any purpose,
// including commercial applications, and to alter it and redistribute it
// freely, subject to the following restrictions:
//
// 1. The origin of this software must not be misrepresented; you must not
//    claim that you wrote the original software. If you use this software
//    in a product, an acknowledgment in the product documentation would
//    be appreciated but is not required.
//
// 2. Altered source versions must be plainly marked as such, and must not
//    be misrepresented as being the original software.
//
// 3. This notice may not be removed or altered from any source
//    distribution.
//
//========================================================================

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <assert.h>
#include <stddef.h>

#include <glad/glad.h>
#include <GLFW/glfw3.h>

/* Map height updates */
#define MAX_CIRCLE_SIZE (5.0f)
#define MAX_DISPLACEMENT (1.0f)
#define DISPLACEMENT_SIGN_LIMIT (0.3f)
#define MAX_ITER (200)
#define NUM_ITER_AT_A_TIME (1)

/* Map general information */
#define MAP_SIZE (10.0f)
#define MAP_NUM_VERTICES (80)
#define MAP_NUM_TOTAL_VERTICES (MAP_NUM_VERTICES*MAP_NUM_VERTICES)
#define MAP_NUM_LINES (3* (MAP_NUM_VERTICES - 1) * (MAP_NUM_VERTICES - 1) + \
               2 * (MAP_NUM_VERTICES - 1))


/**********************************************************************
 * Default shader programs
 *********************************************************************/

static const char* vertex_shader_text =
"#version 150\n"
"uniform mat4 project;\n"
"uniform mat4 modelview;\n"
"in float x;\n"
"in float y;\n"
"in float z;\n"
"\n"
"void main()\n"
"{\n"
"   gl_Position = project * modelview * vec4(x, y, z, 1.0);\n"
"}\n";

static const char* fragment_shader_text =
"#version 150\n"
"out vec4 color;\n"
"void main()\n"
"{\n"
"    color = vec4(0.2, 1.0, 0.2, 1.0); \n"
"}\n";

/**********************************************************************
 * Values for shader uniforms
 *********************************************************************/

/* Frustum configuration */
static GLfloat view_angle = 45.0f;
static GLfloat aspect_ratio = 4.0f/3.0f;
static GLfloat z_near = 1.0f;
static GLfloat z_far = 100.f;

/* Projection matrix */
static GLfloat projection_matrix[16] = {
    1.0f, 0.0f, 0.0f, 0.0f,
    0.0f, 1.0f, 0.0f, 0.0f,
    0.0f, 0.0f, 1.0f, 0.0f,
    0.0f, 0.0f, 0.0f, 1.0f
};

/* Model view matrix */
static GLfloat modelview_matrix[16] = {
    1.0f, 0.0f, 0.0f, 0.0f,
    0.0f, 1.0f, 0.0f, 0.0f,
    0.0f, 0.0f, 1.0f, 0.0f,
    0.0f, 0.0f, 0.0f, 1.0f
};

/**********************************************************************
 * Heightmap vertex and index data
 *********************************************************************/

static GLfloat map_vertices[3][MAP_NUM_TOTAL_VERTICES];
static GLuint  map_line_indices[2*MAP_NUM_LINES];

/* Store uniform location for the shaders
 * Those values are setup as part of the process of creating
 * the shader program. They should not be used before creating
 * the program.
 */
static GLuint mesh;
static GLuint mesh_vbo[4];

/**********************************************************************
 * OpenGL helper functions
 *********************************************************************/

/* Creates a shader object of the specified type using the specified text
 */
static GLuint make_shader(GLenum type, const char* text)
{
    GLuint shader;
    GLint shader_ok;
    GLsizei log_length;
    char info_log[8192];

    shader = glCreateShader(type);
    if (shader != 0)
    {
        glShaderSource(shader, 1, (const GLchar**)&text, NULL);
        glCompileShader(shader);
        glGetShaderiv(shader, GL_COMPILE_STATUS, &shader_ok);
        if (shader_ok != GL_TRUE)
        {
            fprintf(stderr, "ERROR: Failed to compile %s shader\n", (type == GL_FRAGMENT_SHADER) ? "fragment" : "vertex" );
            glGetShaderInfoLog(shader, 8192, &log_length,info_log);
            fprintf(stderr, "ERROR: \n%s\n\n", info_log);
            glDeleteShader(shader);
            shader = 0;
        }
    }
    return shader;
}

/* Creates a program object using the specified vertex and fragment text
 */
static GLuint make_shader_program(const char* vs_text, const char* fs_text)
{
    GLuint program = 0u;
    GLint program_ok;
    GLuint vertex_shader = 0u;
    GLuint fragment_shader = 0u;
    GLsizei log_length;
    char info_log[8192];

    vertex_shader = make_shader(GL_VERTEX_SHADER, vs_text);
    if (vertex_shader != 0u)
    {
        fragment_shader = make_shader(GL_FRAGMENT_SHADER, fs_text);
        if (fragment_shader != 0u)
        {
            /* make the program that connect the two shader and link it */
            program = glCreateProgram();
            if (program != 0u)
            {
                /* attach both shader and link */
                glAttachShader(program, vertex_shader);
                glAttachShader(program, fragment_shader);
                glLinkProgram(program);
                glGetProgramiv(program, GL_LINK_STATUS, &program_ok);

                if (program_ok != GL_TRUE)
                {
                    fprintf(stderr, "ERROR, failed to link shader program\n");
                    glGetProgramInfoLog(program, 8192, &log_length, info_log);
                    fprintf(stderr, "ERROR: \n%s\n\n", info_log);
                    glDeleteProgram(program);
                    glDeleteShader(fragment_shader);
                    glDeleteShader(vertex_shader);
                    program = 0u;
                }
            }
        }
        else
        {
            fprintf(stderr, "ERROR: Unable to load fragment shader\n");
            glDeleteShader(vertex_shader);
        }
    }
    else
    {
        fprintf(stderr, "ERROR: Unable to load vertex shader\n");
    }
    return program;
}

/**********************************************************************
 * Geometry creation functions
 *********************************************************************/

/* Generate vertices and indices for the heightmap
 */
static void init_map(void)
{
    int i;
    int j;
    int k;
    GLfloat step = MAP_SIZE / (MAP_NUM_VERTICES - 1);
    GLfloat x = 0.0f;
    GLfloat z = 0.0f;
    /* Create a flat grid */
    k = 0;
    for (i = 0 ; i < MAP_NUM_VERTICES ; ++i)
    {
        for (j = 0 ; j < MAP_NUM_VERTICES ; ++j)
        {
            map_vertices[0][k] = x;
            map_vertices[1][k] = 0.0f;
            map_vertices[2][k] = z;
            z += step;
            ++k;
        }
        x += step;
        z = 0.0f;
    }
#if DEBUG_ENABLED
    for (i = 0 ; i < MAP_NUM_TOTAL_VERTICES ; ++i)
    {
        printf ("Vertice %d (%f, %f, %f)\n",
                i, map_vertices[0][i], map_vertices[1][i], map_vertices[2][i]);

    }
#endif
    /* create indices */
    /* line fan based on i
     * i+1
     * |  / i + n + 1
     * | /
     * |/
     * i --- i + n
     */

    /* close the top of the square */
    k = 0;
    for (i = 0 ; i < MAP_NUM_VERTICES  -1 ; ++i)
    {
        map_line_indices[k++] = (i + 1) * MAP_NUM_VERTICES -1;
        map_line_indices[k++] = (i + 2) * MAP_NUM_VERTICES -1;
    }
    /* close the right of the square */
    for (i = 0 ; i < MAP_NUM_VERTICES -1 ; ++i)
    {
        map_line_indices[k++] = (MAP_NUM_VERTICES - 1) * MAP_NUM_VERTICES + i;
        map_line_indices[k++] = (MAP_NUM_VERTICES - 1) * MAP_NUM_VERTICES + i + 1;
    }

    for (i = 0 ; i < (MAP_NUM_VERTICES - 1) ; ++i)
    {
        for (j = 0 ; j < (MAP_NUM_VERTICES - 1) ; ++j)
        {
            int ref = i * (MAP_NUM_VERTICES) + j;
            map_line_indices[k++] = ref;
            map_line_indices[k++] = ref + 1;

            map_line_indices[k++] = ref;
            map_line_indices[k++] = ref + MAP_NUM_VERTICES;

            map_line_indices[k++] = ref;
            map_line_indices[k++] = ref + MAP_NUM_VERTICES + 1;
        }
    }

#ifdef DEBUG_ENABLED
    for (k = 0 ; k < 2 * MAP_NUM_LINES ; k += 2)
    {
        int beg, end;
        beg = map_line_indices[k];
        end = map_line_indices[k+1];
        printf ("Line %d: %d -> %d (%f, %f, %f) -> (%f, %f, %f)\n",
                k / 2, beg, end,
                map_vertices[0][beg], map_vertices[1][beg], map_vertices[2][beg],
                map_vertices[0][end], map_vertices[1][end], map_vertices[2][end]);
    }
#endif
}

static void generate_heightmap__circle(float* center_x, float* center_y,
        float* size, float* displacement)
{
    float sign;
    /* random value for element in between [0-1.0] */
    *center_x = (MAP_SIZE * rand()) / (1.0f * RAND_MAX);
    *center_y = (MAP_SIZE * rand()) / (1.0f * RAND_MAX);
    *size = (MAX_CIRCLE_SIZE * rand()) / (1.0f * RAND_MAX);
    sign = (1.0f * rand()) / (1.0f * RAND_MAX);
    sign = (sign < DISPLACEMENT_SIGN_LIMIT) ? -1.0f : 1.0f;
    *displacement = (sign * (MAX_DISPLACEMENT * rand())) / (1.0f * RAND_MAX);
}

/* Run the specified number of iterations of the generation process for the
 * heightmap
 */
static void update_map(int num_iter)
{
    assert(num_iter > 0);
    while(num_iter)
    {
        /* center of the circle */
        float center_x;
        float center_z;
        float circle_size;
        float disp;
        size_t ii;
        generate_heightmap__circle(&center_x, &center_z, &circle_size, &disp);
        disp = disp / 2.0f;
        for (ii = 0u ; ii < MAP_NUM_TOTAL_VERTICES ; ++ii)
        {
            GLfloat dx = center_x - map_vertices[0][ii];
            GLfloat dz = center_z - map_vertices[2][ii];
            GLfloat pd = (2.0f * sqrtf((dx * dx) + (dz * dz))) / circle_size;
            if (fabs(pd) <= 1.0f)
            {
                /* tx,tz is within the circle */
                GLfloat new_height = disp + (float) (cos(pd*3.14f)*disp);
                map_vertices[1][ii] += new_height;
            }
        }
        --num_iter;
    }
}

/**********************************************************************
 * OpenGL helper functions
 *********************************************************************/

/* Create VBO, IBO and VAO objects for the heightmap geometry and bind them to
 * the specified program object
 */
static void make_mesh(GLuint program)
{
    GLuint attrloc;

    glGenVertexArrays(1, &mesh);
    glGenBuffers(4, mesh_vbo);
    glBindVertexArray(mesh);
    /* Prepare the data for drawing through a buffer inidices */
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, mesh_vbo[3]);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(GLuint)* MAP_NUM_LINES * 2, map_line_indices, GL_STATIC_DRAW);

    /* Prepare the attributes for rendering */
    attrloc = glGetAttribLocation(program, "x");
    glBindBuffer(GL_ARRAY_BUFFER, mesh_vbo[0]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(GLfloat) * MAP_NUM_TOTAL_VERTICES, &map_vertices[0][0], GL_STATIC_DRAW);
    glEnableVertexAttribArray(attrloc);
    glVertexAttribPointer(attrloc, 1, GL_FLOAT, GL_FALSE, 0, 0);

    attrloc = glGetAttribLocation(program, "z");
    glBindBuffer(GL_ARRAY_BUFFER, mesh_vbo[2]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(GLfloat) * MAP_NUM_TOTAL_VERTICES, &map_vertices[2][0], GL_STATIC_DRAW);
    glEnableVertexAttribArray(attrloc);
    glVertexAttribPointer(attrloc, 1, GL_FLOAT, GL_FALSE, 0, 0);

    attrloc = glGetAttribLocation(program, "y");
    glBindBuffer(GL_ARRAY_BUFFER, mesh_vbo[1]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(GLfloat) * MAP_NUM_TOTAL_VERTICES, &map_vertices[1][0], GL_DYNAMIC_DRAW);
    glEnableVertexAttribArray(attrloc);
    glVertexAttribPointer(attrloc, 1, GL_FLOAT, GL_FALSE, 0, 0);
}

/* Update VBO vertices from source data
 */
static void update_mesh(void)
{
    glBufferSubData(GL_ARRAY_BUFFER, 0, sizeof(GLfloat) * MAP_NUM_TOTAL_VERTICES, &map_vertices[1][0]);
}

/**********************************************************************
 * GLFW callback functions
 *********************************************************************/

static void key_callback(GLFWwindow* window, int key, int scancode, int action, int mods)
{
    switch(key)
    {
        case GLFW_KEY_ESCAPE:
            /* Exit program on Escape */
            glfwSetWindowShouldClose(window, GL_TRUE);
            break;
    }
}

static void error_callback(int error, const char* description)
{
    fprintf(stderr, "Error: %s\n", description);
}

int main(int argc, char** argv)
{
    GLFWwindow* window;
    int iter;
    double dt;
    double last_update_time;
    int frame;
    float f;
    GLint uloc_modelview;
    GLint uloc_project;

    GLuint shader_program;

    glfwSetErrorCallback(error_callback);

    if (!glfwInit())
        exit(EXIT_FAILURE);

    glfwWindowHint(GLFW_RESIZABLE, GL_FALSE);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 2);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
    glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);

    window = glfwCreateWindow(800, 600, "GLFW OpenGL3 Heightmap demo", NULL, NULL);
    if (! window )
    {
        glfwTerminate();
        exit(EXIT_FAILURE);
    }

    /* Register events callback */
    glfwSetKeyCallback(window, key_callback);

    glfwMakeContextCurrent(window);
    gladLoadGLLoader((GLADloadproc) glfwGetProcAddress);

    /* Prepare opengl resources for rendering */
    shader_program = make_shader_program(vertex_shader_text, fragment_shader_text);

    if (shader_program == 0u)
    {
        glfwTerminate();
        exit(EXIT_FAILURE);
    }

    glUseProgram(shader_program);
    uloc_project   = glGetUniformLocation(shader_program, "project");
    uloc_modelview = glGetUniformLocation(shader_program, "modelview");

    /* Compute the projection matrix */
    f = 1.0f / tanf(view_angle / 2.0f);
    projection_matrix[0]  = f / aspect_ratio;
    projection_matrix[5]  = f;
    projection_matrix[10] = (z_far + z_near)/ (z_near - z_far);
    projection_matrix[11] = -1.0f;
    projection_matrix[14] = 2.0f * (z_far * z_near) / (z_near - z_far);
    glUniformMatrix4fv(uloc_project, 1, GL_FALSE, projection_matrix);

    /* Set the camera position */
    modelview_matrix[12]  = -5.0f;
    modelview_matrix[13]  = -5.0f;
    modelview_matrix[14]  = -20.0f;
    glUniformMatrix4fv(uloc_modelview, 1, GL_FALSE, modelview_matrix);

    /* Create mesh data */
    init_map();
    make_mesh(shader_program);

    /* Create vao + vbo to store the mesh */
    /* Create the vbo to store all the information for the grid and the height */

    /* setup the scene ready for rendering */
    glViewport(0, 0, 800, 600);
    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

    /* main loop */
    frame = 0;
    iter = 0;
    last_update_time = glfwGetTime();

    while (!glfwWindowShouldClose(window))
    {
        ++frame;
        /* render the next frame */
        glClear(GL_COLOR_BUFFER_BIT);
        glDrawElements(GL_LINES, 2* MAP_NUM_LINES , GL_UNSIGNED_INT, 0);

        /* display and process events through callbacks */
        glfwSwapBuffers(window);
        glfwPollEvents();
        /* Check the frame rate and update the heightmap if needed */
        dt = glfwGetTime();
        if ((dt - last_update_time) > 0.2)
        {
            /* generate the next iteration of the heightmap */
            if (iter < MAX_ITER)
            {
                update_map(NUM_ITER_AT_A_TIME);
                update_mesh();
                iter += NUM_ITER_AT_A_TIME;
            }
            last_update_time = dt;
            frame = 0;
        }
    }

    glfwTerminate();
    exit(EXIT_SUCCESS);
}