// Modern OpenGL (3.3 core): a spinning, colorful triangle drawn with shaders.
// Shows structs of numbers going to the GPU, C out-parameters with addr(), and
// pointers made from numbers with ptr(...). Needs GLFW and OpenGL (in shell.nix).
import "opengl.h"
link "glfw"
link "GL"

fn compileShader(u32 kind, str source) -> u32 {
    shader := glCreateShader(kind)
    src := source.ptr
    glShaderSource(shader, 1, addr(src), null)
    glCompileShader(shader)
    i32 ok
    glGetShaderiv(shader, GL_COMPILE_STATUS, addr(ok))
    if ok == 0 {
        log := " ".repeat(1024)
        glGetShaderInfoLog(shader, 1024, null, log)
        print("shader error:", str(log.ptr))
        exit(1)
    }
    return shader
}

fn makeProgram() -> u32 {
    vertex := "#version 330 core\n" +
        "layout(location = 0) in vec2 pos;\n" +
        "layout(location = 1) in vec3 color;\n" +
        "uniform float time;\n" +
        "out vec3 vColor;\n" +
        "void main() {\n" +
        "    float c = cos(time), s = sin(time);\n" +
        "    gl_Position = vec4(c * pos.x - s * pos.y, s * pos.x + c * pos.y, 0.0, 1.0);\n" +
        "    vColor = color;\n" +
        "}\n"
    fragment := "#version 330 core\n" +
        "in vec3 vColor;\n" +
        "out vec4 outColor;\n" +
        "void main() { outColor = vec4(vColor, 1.0); }\n"

    vs := compileShader(GL_VERTEX_SHADER, vertex)
    fs := compileShader(GL_FRAGMENT_SHADER, fragment)
    program := glCreateProgram()
    glAttachShader(program, vs)
    glAttachShader(program, fs)
    glLinkProgram(program)
    glDeleteShader(vs)
    glDeleteShader(fs)
    return program
}

fn main() {
    if glfwInit() == 0 {
        print("could not start GLFW")
        return
    }
    defer glfwTerminate()
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3)
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3)
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE)

    window := glfwCreateWindow(800, 600, "Finch + OpenGL", null, null)
    if window == null {
        print("could not open a window (does your GPU support OpenGL 3.3?)")
        return
    }
    glfwMakeContextCurrent(window)
    glfwSwapInterval(1)
    print("OpenGL", str(glGetString(GL_VERSION)), "on", str(glGetString(GL_RENDERER)))

    // x, y, r, g, b for each corner
    []f32 vertices = [
         0.0,  0.6,   1.0, 0.48, 0.1,
        -0.6, -0.4,   0.2, 0.6,  1.0,
         0.6, -0.4,   0.6, 1.0,  0.4,
    ]
    u32 vao
    u32 vbo
    glGenVertexArrays(1, addr(vao))
    glGenBuffers(1, addr(vbo))
    glBindVertexArray(vao)
    glBindBuffer(GL_ARRAY_BUFFER, vbo)
    glBufferData(GL_ARRAY_BUFFER, vertices.len * 4, vertices.ptr, GL_STATIC_DRAW)
    i32 stride = 5 * 4
    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, stride, ptr(0))
    glEnableVertexAttribArray(0)
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, stride, ptr(2 * 4))
    glEnableVertexAttribArray(1)

    program := makeProgram()
    timeLoc := glGetUniformLocation(program, "time")

    while glfwWindowShouldClose(window) == 0 {
        glClearColor(0.106, 0.122, 0.165, 1.0)
        glClear(GL_COLOR_BUFFER_BIT)
        glUseProgram(program)
        glUniform1f(timeLoc, f32(glfwGetTime()))
        glDrawArrays(GL_TRIANGLES, 0, 3)
        glfwSwapBuffers(window)
        glfwPollEvents()
    }

    glDeleteBuffers(1, addr(vbo))
    glDeleteVertexArrays(1, addr(vao))
    glDeleteProgram(program)
}