use crate::prelude::*;

pub struct Scroll {
    inner: WidgetHandle,
    axis: Axis,
    amt: f32,
    snap_end: bool,
    container_len: f32,
    content_len: f32,
}

impl Widget for Scroll {
    fn draw(&mut self, painter: &mut Painter) {
        let output_len = painter.output_size().axis(self.axis);
        let container_len = painter.region().axis(self.axis).len();
        let content_len = painter
            .len_axis(&self.inner, self.axis)
            .apply_rest()
            .within_len(container_len)
            .to_abs(output_len);
        self.container_len = container_len.to_abs(output_len);
        self.content_len = content_len;

        if self.snap_end {
            self.amt = self.content_len - self.container_len;
        }
        self.update_amt();

        let mut region = UiRegion::FULL.offset(Vec2::from_axis(self.axis, -self.amt, 0.0));
        region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
        painter.widget_within(&self.inner, region);
    }

    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
        ctx.width(&self.inner)
    }

    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
        ctx.height(&self.inner)
    }
}

impl Scroll {
    pub fn new(inner: WidgetHandle, axis: Axis) -> Self {
        Self {
            inner,
            axis,
            amt: 0.0,
            snap_end: true,
            container_len: 0.0,
            content_len: 0.0,
        }
    }

    pub fn update_amt(&mut self) {
        self.amt = self.amt.max(0.0);
        let len = (self.content_len - self.container_len).max(0.0);
        self.amt = self.amt.min(len);
        self.snap_end = self.amt == len;
    }

    pub fn scroll(&mut self, amt: f32) {
        self.amt -= amt;
        self.update_amt();
    }
}