use crate::prelude::*;

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

impl Widget for Scroll {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        let output_len = painter.output_len(self.axis);
        // Its size is its content's, whatever box that is scrolled within.
        let container_len = UiScalar::px(painter.px_len_for_draw(self.axis));
        // Draw in the whole container only when its scrolling-axis length is
        // not already known, then place it at the scrolled offset.
        let known_len = painter.known_len(&self.inner, self.axis, UiRegion::FULL);
        let measured = known_len.is_none();
        let child = measured.then(|| painter.place(&self.inner, UiRegion::FULL).size());
        let content_len = known_len
            .unwrap_or_else(|| child.unwrap().axis(self.axis))
            .apply_rest()
            .within_len(container_len)
            .to_px(output_len);
        self.container_len = container_len.to_px(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);
        let placed = painter.place(&self.inner, region).size();
        child.unwrap_or(placed)
    }
}

impl Scroll {
    pub fn new(inner: StrongWidget, 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();
    }
}