use crate::prelude::*;

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

impl Widget for Scroll {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        let container_len = painter.px_len(self.axis);
        // Measured in the whole viewport, then drawn at the scrolled offset.
        let whole = UiRegion::FULL;
        let answer_len = painter
            .widget_at(&self.inner, whole, [Place::Fill(Part::All); 2])
            .len(self.axis);
        let fixed = Len::from_parts(answer_len.rel, answer_len.px).to_px(container_len);
        self.container_len = container_len;
        self.content_len = fixed.max(container_len);

        if self.snap_end {
            self.amt = self.content_len - self.container_len;
        }
        self.update_amt();
        let align = painter.alignment().axis(self.axis);
        // Content of a fixed length that fits sits at the start of any box it
        // fits in -- but only anchored there. Anywhere else it is a part of
        // the room left over, so it moves with every length the box takes and
        // the drawing holds for that length alone. One scrolled part way sits
        // where it is until the box shrinks past what is left of it. Kept to
        // the end, it moves with every length.
        let fixed_len = answer_len.rel == Rel::ZERO && answer_len.leftover == Weight::ZERO;
        if fixed_len && self.content_len <= self.container_len && align == AxisAlign::NEG {
            painter.holds(self.axis, fixed..=Px::MAX);
        } else if fixed_len && !self.snap_end {
            let left = self.content_len - self.amt;
            painter.holds(self.axis, Px::MIN..=left);
        }

        // Content shorter than the viewport has room to sit in, and where it
        // sits is this widget's own alignment -- the same property that would
        // have placed the whole scroll in a box longer than it.
        let slack = (self.container_len - self.content_len).max(Px::ZERO);
        let anchor = slack.mul(align.rel());
        let mut content = UiSpan::FULL;
        // Content that fills the viewport and has not been scrolled is the
        // viewport, and is handed back as it came. Writing the same box as
        // its own length in pixels is the same box in another form, and the
        // two do not round alike: a part centred in `rel 1` lands a step from
        // one centred in `px 900`, since halving a difference is not halving
        // each part of it.
        let moved = anchor != Px::ZERO || self.amt != Px::ZERO;
        if moved || self.content_len != self.container_len {
            let start = Len::from_parts(Rel::ZERO, anchor - self.amt);
            content = UiSpan::new(start, start.offset(self.content_len));
        }
        // The viewport is the inner's frame, so a fraction it declares or
        // reports is a fraction of what is on screen rather than of the
        // content box its own answer decided. Where it is drawn is the
        // content box, scrolled.
        painter.widget_at(
            &self.inner,
            whole,
            self.axis
                .pair(Place::Fill(Part::From(content)), Place::Fill(Part::All)),
        );
        // What it occupies is its box, on both axes: it clips its content to
        // that box, so it can neither take less of one nor honestly ask for
        // more. The content's length is what it scrolls through, not what it
        // is.
        Size::LEFTOVER
    }
}

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

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

    /// Scrolled by a distance the platform measures, which is the last place
    /// a wheel notch or a finger is a float.
    pub fn scroll(&mut self, amt: f32) {
        self.amt -= Px::from_f32(amt);
        self.update_amt();
    }
}