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use rand::{Rng, SeedableRng};
use rand_xorshift::XorShiftRng;

use geom::{Circle, Distance, Duration, PolyLine, Pt2D, Time};
use widgetry::{Color, Drawable, EventCtx, GeomBatch, GfxCtx};

pub struct Animator {
    active: Vec<Animation>,
    draw_current: Drawable,
}

struct Animation {
    start: Time,
    end: Time,
    effect: Effect,
}

pub enum Effect {
    Scale {
        orig: GeomBatch,
        center: Pt2D,
        lerp_scale: (f64, f64),
    },
    FollowPath {
        color: Color,
        width: Distance,
        pl: PolyLine,
    },
}

impl Animator {
    pub fn new(ctx: &EventCtx) -> Animator {
        Animator {
            active: Vec::new(),
            draw_current: Drawable::empty(ctx),
        }
    }

    /// Pass in a future value for `now` to schedule a delayed effect
    pub fn add(&mut self, now: Time, duration: Duration, effect: Effect) {
        self.active.push(Animation {
            start: now,
            end: now + duration,
            effect,
        });
    }

    pub fn event(&mut self, ctx: &mut EventCtx, now: Time) {
        if self.active.is_empty() {
            return;
        }
        let mut batch = GeomBatch::new();
        self.active.retain(|anim| {
            let pct = (now - anim.start) / (anim.end - anim.start);
            if pct < 0.0 {
                // Hasn't started yet
                true
            } else if pct > 1.0 {
                false
            } else {
                anim.effect.render(pct, &mut batch);
                true
            }
        });
        self.draw_current = ctx.upload(batch);
    }

    pub fn draw(&self, g: &mut GfxCtx) {
        g.redraw(&self.draw_current);
    }
}

impl Effect {
    fn render(&self, pct: f64, batch: &mut GeomBatch) {
        match self {
            Effect::Scale {
                ref orig,
                center,
                lerp_scale,
            } => {
                let scale = lerp_scale.0 + pct * (lerp_scale.1 - lerp_scale.0);
                batch.append(orig.clone().scale(scale).centered_on(*center));
            }
            Effect::FollowPath {
                color,
                width,
                ref pl,
            } => {
                if let Ok(pl) = pl.maybe_exact_slice(Distance::ZERO, pct * pl.length()) {
                    batch.push(*color, pl.make_polygons(*width));
                }
            }
        }
    }
}

pub struct SnowEffect {
    rng: XorShiftRng,
    flakes: Vec<Snowflake>,

    draw: Drawable,
}

struct Snowflake {
    start: Time,
    top_left: Pt2D,
}

impl SnowEffect {
    pub fn new(ctx: &mut EventCtx) -> SnowEffect {
        let mut snow = SnowEffect {
            rng: XorShiftRng::seed_from_u64(42),
            flakes: Vec::new(),
            draw: Drawable::empty(ctx),
        };

        let now = Time::START_OF_DAY;
        for _ in 0..60 {
            snow.spawn_new(ctx, now);
        }
        snow.event(ctx, now);

        snow
    }

    fn spawn_new(&mut self, ctx: &EventCtx, now: Time) {
        let top_left = Pt2D::new(
            self.rng.gen_range(0.0, ctx.canvas.window_width),
            self.rng.gen_range(0.0, ctx.canvas.window_height),
        );
        self.flakes.push(Snowflake {
            start: now,
            top_left,
        });
    }

    pub fn event(&mut self, ctx: &mut EventCtx, now: Time) {
        let shape = Circle::new(Pt2D::new(0.0, 0.0), Distance::meters(50.0)).to_polygon();

        let lifetime = Duration::seconds(5.0);
        let lerp_scale = (1.0, 0.1);

        let mut batch = GeomBatch::new();
        self.flakes.retain(|flake| {
            let pct = (now - flake.start) / lifetime;
            if pct > 1.0 {
                false
            } else {
                let scale = lerp_scale.0 + pct * (lerp_scale.1 - lerp_scale.0);
                batch.push(
                    Color::WHITE.alpha(0.5),
                    shape
                        .scale(scale)
                        .translate(flake.top_left.x(), flake.top_left.y()),
                );
                true
            }
        });
        self.draw = ctx.upload(batch);
    }

    pub fn draw(&self, g: &mut GfxCtx) {
        g.fork_screenspace();
        g.redraw(&self.draw);
        g.unfork();
    }
}