// Draws a simple white triangle
// based on the example from:
// https://github.com/brendanzab/gl-rs/blob/master/gl/examples/triangle.rs

use egui_glfw_gl::gl;
use egui_glfw_gl::gl::types::*;
use std::ffi::CString;
use std::mem;
use std::ptr;
use std::str;

const VS_SRC: &'static str = "
#version 150
in vec2 position;

void main() {
    gl_Position = vec4(position, 0.0, 1.0);
}";

const FS_SRC: &'static str = "
#version 150
out vec4 out_color;

void main() {
    out_color = vec4(1.0, 1.0, 1.0, 1.0);
}";

static VERTEX_DATA: [GLfloat; 6] = [0.0, 0.5, 0.5, -0.5, -0.5, -0.5];

pub struct Triangle {
    pub vs: GLuint,
    pub fs: GLuint,
    pub program: GLuint,
    pub vao: GLuint,
    pub vbo: GLuint,
}

pub fn compile_shader(src: &str, ty: GLenum) -> GLuint {
    let shader;
    unsafe {
        shader = gl::CreateShader(ty);
        // Attempt to compile the shader
        let c_str = CString::new(src.as_bytes()).unwrap();
        gl::ShaderSource(shader, 1, &c_str.as_ptr(), ptr::null());
        gl::CompileShader(shader);

        // Get the compile status
        let mut status = gl::FALSE as GLint;
        gl::GetShaderiv(shader, gl::COMPILE_STATUS, &mut status);

        // Fail on error
        if status != (gl::TRUE as GLint) {
            let mut len = 0;
            gl::GetShaderiv(shader, gl::INFO_LOG_LENGTH, &mut len);
            let mut buf = Vec::with_capacity(len as usize);
            buf.set_len((len as usize) - 1); // subtract 1 to skip the trailing null character
            gl::GetShaderInfoLog(
                shader,
                len,
                ptr::null_mut(),
                buf.as_mut_ptr() as *mut GLchar,
            );
            panic!(
                "{}",
                str::from_utf8(&buf).expect("ShaderInfoLog not valid utf8")
            );
        }
    }
    shader
}

pub fn link_program(vs: GLuint, fs: GLuint) -> GLuint {
    unsafe {
        let program = gl::CreateProgram();
        gl::AttachShader(program, vs);
        gl::AttachShader(program, fs);
        gl::LinkProgram(program);
        // Get the link status
        let mut status = gl::FALSE as GLint;
        gl::GetProgramiv(program, gl::LINK_STATUS, &mut status);

        // Fail on error
        if status != (gl::TRUE as GLint) {
            let mut len: GLint = 0;
            gl::GetProgramiv(program, gl::INFO_LOG_LENGTH, &mut len);
            let mut buf = Vec::with_capacity(len as usize);
            buf.set_len((len as usize) - 1); // subtract 1 to skip the trailing null character
            gl::GetProgramInfoLog(
                program,
                len,
                ptr::null_mut(),
                buf.as_mut_ptr() as *mut GLchar,
            );
            panic!(
                "{}",
                str::from_utf8(&buf).expect("ProgramInfoLog not valid utf8")
            );
        }
        program
    }
}

impl Triangle {
    pub fn new() -> Self {
        // Create Vertex Array Object
        let mut vao = 0;
        let mut vbo = 0;
        let vs = compile_shader(VS_SRC, gl::VERTEX_SHADER);
        let fs = compile_shader(FS_SRC, gl::FRAGMENT_SHADER);
        let program = link_program(vs, fs);
        unsafe {
            gl::GenVertexArrays(1, &mut vao);
            gl::GenBuffers(1, &mut vbo);
        }
        Triangle {
            // Create GLSL shaders
            vs,
            fs,
            program,
            vao,
            vbo,
        }
    }
    pub fn draw(&self) {
        unsafe {
            gl::BindVertexArray(self.vao);

            // Create a Vertex Buffer Object and copy the vertex data to it

            gl::BindBuffer(gl::ARRAY_BUFFER, self.vbo);
            gl::BufferData(
                gl::ARRAY_BUFFER,
                (VERTEX_DATA.len() * mem::size_of::<GLfloat>()) as GLsizeiptr,
                mem::transmute(&VERTEX_DATA[0]),
                gl::STATIC_DRAW,
            );

            // Use shader program
            gl::UseProgram(self.program);
            let c_out_color = CString::new("out_color").unwrap();
            gl::BindFragDataLocation(self.program, 0, c_out_color.as_ptr());

            // Specify the layout of the vertex data
            let c_position = CString::new("position").unwrap();
            let pos_attr = gl::GetAttribLocation(self.program, c_position.as_ptr());
            gl::EnableVertexAttribArray(pos_attr as GLuint);
            gl::VertexAttribPointer(
                pos_attr as GLuint,
                2,
                gl::FLOAT,
                gl::FALSE as GLboolean,
                0,
                ptr::null(),
            );

            // Draw a triangle from the 3 vertices
            gl::DrawArrays(gl::TRIANGLES, 0, 3);
        }
    }
}

impl Drop for Triangle {
    fn drop(&mut self) {
        unsafe {
            gl::DeleteProgram(self.program);
            gl::DeleteShader(self.fs);
            gl::DeleteShader(self.vs);
            gl::DeleteBuffers(1, &self.vbo);
            gl::DeleteVertexArrays(1, &self.vao);
        }
    }
}