#define GL_GLEXT_PROTOTYPES
#include <sys/stat.h>
#include <fcntl.h>
#include <SDL/SDL.h>
#include <GL/gl.h>
#include <GL/glu.h>
#include <iostream>

using namespace std;

#define WIDTH   800
#define HEIGHT  600

#define STRIDE(a, b) ((unsigned long)(b) - (unsigned long)(a))
#define OFFSET(a, b) ((const GLvoid *)STRIDE((a), (b)))

const GLfloat data[][2][3] = {
    {{-1.0, -0.6, 0.0}, {1, 0, 0}},
    {{-0.2, -0.6, 0.0}, {0, 1, 0}},
    {{-0.6, 0.6, 0.0}, {0, 0, 1}},
    {{0.2, -0.6, 0.0}, {1, 0, 0}},
    {{1.0, -0.6, 0.0}, {0, 1, 0}},
    {{0.6, 0.6, -500.0}, {0, 0, 1}}
};
const GLushort indices[] = {
    0, 1, 2, 3, 4, 5
};
GLuint program, vs, fs, data_vbo, index_vbo;
enum Locations {
    LOC_POSITION,
    LOC_COLOR
};

char * loadFile(const char *fname)
{
    struct stat st;
    if (stat(fname, &st) != 0)
        return NULL;
    if (st.st_size <= 0)
        return NULL;
    char * buff = new char[st.st_size + 1];
    int fd = open(fname, O_RDONLY);
    read(fd, buff, st.st_size);
    close(fd);
    buff[st.st_size] = '\0';
    return buff;
}

char *getShaderLog(GLuint id)
{
    GLint log_length;
    glGetShaderiv(id, GL_INFO_LOG_LENGTH, &log_length);
    if (log_length > 0)
    {
        char *log = new char[log_length];
        glGetShaderInfoLog(id, log_length, &log_length, log);
        return log;
    }
    return NULL;
}

char *getProgramLog(GLuint id)
{
    GLint log_length;
    glGetProgramiv(id, GL_INFO_LOG_LENGTH, &log_length);
    if (log_length > 0)
    {
        char *log = new char[log_length];
        glGetProgramInfoLog(id, log_length, &log_length, log);
        return log;
    }
    return NULL;
}

GLuint makeShader(GLenum shaderType, const char *fname)
{
    GLuint id;
    const char *source;
    GLint status;

    id = glCreateShader(shaderType);
    if (id <= 0)
    {
        cerr << "Error creating shader object" << endl;
        goto out;
    }

    source = loadFile(fname);
    if (source == NULL)
    {
        fprintf(stderr, "Error reading file '%s'\n", fname);
        goto cleanup_shader;
    }

    glShaderSource(id, 1, &source, NULL);
    delete[] source;

    glCompileShader(id);

    glGetShaderiv(id, GL_COMPILE_STATUS, &status);
    if (status != GL_TRUE)
    {
        cerr << "Error compiling shader" << endl;
        char *log = getShaderLog(id);
        cerr << "Shader Log:" << endl << log << endl;
        delete[] log;
        goto cleanup_shader;
    }

    return id;

cleanup_shader:
    glDeleteShader(id);
out:
    return 0;
}

GLuint makeBuffer(GLenum target, const void *ptr, size_t sz)
{
    GLuint id;
    glGenBuffers(1, &id);
    glBindBuffer(target, id);
    glBufferData(target, sz, ptr, GL_STATIC_DRAW);
    return id;
}

bool init(int width, int height)
{
    glClearColor (0.0, 0.0, 0.0, 0.0);
    glEnable(GL_DEPTH_TEST);
    glViewport(0, 0, WIDTH, HEIGHT);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho((float)-width/(float)height, (float)width/(float)height,
            -1, 1, 1000, -1000);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    vs = makeShader(GL_VERTEX_SHADER, "v_shader.glsl");
    fs = makeShader(GL_FRAGMENT_SHADER, "f_shader.glsl");

    if (vs <= 0 || fs <= 0)
    {
        return false;
    }

    program = glCreateProgram();
    glAttachShader(program, vs);
    glAttachShader(program, fs);

    glBindAttribLocation(program, LOC_POSITION, "pos");
    glBindAttribLocation(program, LOC_COLOR, "color");

    glLinkProgram(program);

    GLint link_status;
    glGetProgramiv(program, GL_LINK_STATUS, &link_status);
    if (link_status != GL_TRUE)
    {
        char *log = getProgramLog(program);
        cerr << "Program log:" << endl << log << endl;
        delete[] log;
        glDeleteShader(vs);
        glDeleteShader(fs);
        return false;
    }

    data_vbo = makeBuffer(GL_ARRAY_BUFFER, data, sizeof(data));
    index_vbo = makeBuffer(GL_ELEMENT_ARRAY_BUFFER, indices, sizeof(indices));

    return true;
}

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glUseProgram(program);
    glBindBuffer(GL_ARRAY_BUFFER, data_vbo);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, index_vbo);
    glEnableVertexAttribArray(LOC_POSITION);
    glEnableVertexAttribArray(LOC_COLOR);
    glVertexAttribPointer(LOC_POSITION, 3, GL_FLOAT, GL_FALSE,
            STRIDE(&data[0], &data[1]), OFFSET(&data[0], &data[0][0]));
    glVertexAttribPointer(LOC_COLOR, 3, GL_FLOAT, GL_FALSE,
            STRIDE(&data[0], &data[1]), OFFSET(&data[0], &data[0][1]));
    glDrawElements(GL_TRIANGLES, sizeof(indices)/sizeof(indices[0]),
            GL_UNSIGNED_SHORT, OFFSET(&indices, &indices[0]));
    SDL_GL_SwapBuffers();
}

int main(int argc, char *argv[])
{
    if (SDL_Init(SDL_INIT_VIDEO))
    {
        printf("Failed to initialize SDL!\n");
        return 1;
    }

    atexit(SDL_Quit);

    SDL_Surface *screen;
    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
    if (!(screen = SDL_SetVideoMode(WIDTH, HEIGHT, 16, SDL_OPENGL)))
    {
        printf("Failed to set video mode!\n");
        SDL_Quit();
        return 2;
    }
    SDL_WM_SetCaption(argv[0], argv[0]);

    if (!init(WIDTH, HEIGHT))
        return 2;

    display();
    SDL_Event event;
    while (SDL_WaitEvent(&event))
    {
        if (event.type == SDL_QUIT)
            break;
        else if (event.type == SDL_KEYDOWN)
        {
            if (event.key.keysym.sym == SDLK_ESCAPE)
                break;
        }
    }
}