@[has_globals]
module main

import gg
import rand

__global app = App{}

struct Ball {
mut:
    x     f32
    y     f32
    dx    f32
    dy    f32
    color gg.Color
}

struct App {
mut:
    size  gg.Size = gg.Size{1024, 768}
    balls []Ball
}

fn main() {
    init_app()
    gg.start(
        window_title: 'Bouncing balls'
        width:        app.size.width
        height:       app.size.height
        frame_fn:     on_frame
    )
}

fn on_frame(ctx &gg.Context) {
    app.size = gg.window_size()
    if ctx.frame % 20000 == 0 {
        init_app()
    }
    for mut ball in app.balls {
        move_ball(mut ball)
    }
    ctx.begin()
    for ball in app.balls {
        ctx.draw_circle_filled(ball.x, app.size.height - ball.y, 6, ball.color)
    }
    ctx.draw_text(10, 20, 'Frame: ${ctx.frame:06}', color: gg.Color{255, 255, 255, 255}, size: 20)
    ctx.end()
}

fn init_app() {
    app.balls.clear()
    for _ in 0 .. 2500 {
        app.balls << Ball{
            x:     rand.f32() * app.size.width
            y:     3 * app.size.height / 4
            dx:    5 * rand.f32()
            dy:    5 * rand.f32()
            color: gg.Color{rand.u8(), rand.u8(), rand.u8(), 255}
        }
    }
}

@[inline]
fn move_ball(mut ball Ball) {
    ball.x += ball.dx
    ball.y += ball.dy
    ball.dy -= 9.81 * 0.01
    if ball.x <= 0 || ball.x >= app.size.width - 1 {
        ball.dx = -ball.dx
        apply_friction(mut ball)
    }
    if ball.y <= 0 || ball.y >= app.size.height - 1 {
        ball.dy = -ball.dy
        apply_friction(mut ball)
    }
    // apply some clipping:
    ball.x = f32_max(-5, f32_min(f32(app.size.width) * 1.3, ball.x))
    ball.y = f32_max(-5, f32_min(f32(app.size.height) * 1.3, ball.y))
    ball.dx = f32_max(-50, f32_min(30, ball.dx))
    ball.dy = f32_max(-50, f32_min(30, ball.dy))
}

@[inline]
fn apply_friction(mut ball Ball) {
    // apply some random friction on each bounce:
    ball.dy *= (1 - rand.f32() * 0.1)
    ball.dx *= (1 - rand.f32() * 0.01)
}