use gpui::prelude::FluentBuilder as _;
use gpui::{
    App, Bounds, DispatchPhase, ElementId, InteractiveElement as _, IntoElement, MouseDownEvent,
    MouseMoveEvent, ParentElement, Pixels, RenderOnce, ScrollHandle, SharedString,
    StatefulInteractiveElement as _, Styled, Window, canvas, div, fill, point,
};
use gpui_native_theme::ActiveTheme as _;

use crate::common::StyledExt;

/// Which directions the content scrolls in.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Default)]
pub enum ScrollAxes {
    /// Vertical only; the default.
    #[default]
    Vertical,
    /// Horizontal only.
    Horizontal,
    /// Both directions.
    Both,
}

/// A scrollable container with an overlay scrollbar.
#[derive(IntoElement)]
pub struct ScrollArea {
    id: ElementId,
    handle: Option<ScrollHandle>,
    axes: ScrollAxes,
    a11y_label: Option<SharedString>,
    style: gpui::StyleRefinement,
    children: Vec<gpui::AnyElement>,
}

impl ParentElement for ScrollArea {
    fn extend(&mut self, children: impl IntoIterator<Item = gpui::AnyElement>) {
        self.children.extend(children);
    }
}

impl Styled for ScrollArea {
    fn style(&mut self) -> &mut gpui::StyleRefinement {
        &mut self.style
    }
}

impl ScrollArea {
    /// Creates a vertically scrolling area.
    pub fn new<I>(id: I) -> Self
    where
        I: Into<ElementId>,
    {
        Self {
            id: id.into(),
            handle: None,
            axes: ScrollAxes::Vertical,
            a11y_label: None,
            style: gpui::StyleRefinement::default(),
            children: Vec::new(),
        }
    }

    /// Sets which directions the content scrolls in.
    pub fn axes(mut self, axes: ScrollAxes) -> Self {
        self.axes = axes;
        self
    }

    /// Scrolls with a handle held on the owning view.
    pub fn handle(mut self, handle: ScrollHandle) -> Self {
        self.handle = Some(handle);
        self
    }

    /// Overrides the accessibility label of the container.
    pub fn accessibility_label<L>(mut self, label: L) -> Self
    where
        L: Into<SharedString>,
    {
        self.a11y_label = Some(label.into());
        self
    }
}

/// Pure thumb geometry for an overlay scrollbar.
pub(crate) fn thumb_geometry(
    viewport: Pixels,
    content: Pixels,
    offset: Pixels,
    track: Pixels,
    min_thumb: Pixels,
) -> (Pixels, Pixels) {
    if content <= viewport || content <= Pixels::ZERO {
        return (track, Pixels::ZERO);
    }

    let length = (viewport / content * track).max(min_thumb).min(track);
    let overflow = content - viewport;
    let progress = (-offset / overflow).clamp(0., 1.);
    let travel = track - length;
    (length, travel * progress)
}

impl RenderOnce for ScrollArea {
    fn render(self, _window: &mut Window, _cx: &mut App) -> impl IntoElement {
        let mut pane = div()
            .id(self.id.clone())
            .overflow_hidden()
            .when(self.axes != ScrollAxes::Horizontal, |area| {
                area.overflow_y_scroll()
            })
            .when(self.axes != ScrollAxes::Vertical, |area| {
                area.overflow_x_scroll()
            })
            .when_some(self.handle.clone(), |area, handle| {
                area.track_scroll(&handle)
            })
            .when_some(self.a11y_label, |area, label| area.aria_label(label))
            .on_scroll(|_, _, cx| {
                // Keep wheel events from scrolling an outer scroll area too.
                cx.stop_propagation();
            })
            .refine_style(&self.style);

        // Extend with children
        pane.extend(self.children);

        let mut wrapper = div().relative().refine_style(&self.style);
        wrapper.extend(std::iter::once(pane.into_any_element()));

        if let Some(handle) = self.handle {
            wrapper.extend(std::iter::once(
                canvas(|_, _, _| {}, {
                    let handle = handle.clone();
                    move |_, _, window, cx| {
                        paint_scrollbar(&handle, window, cx);
                    }
                })
                .absolute()
                .inset_0()
                .into_any_element(),
            ));
        }

        wrapper
    }
}

/// Paints the overlay scrollbar thumb.
fn paint_scrollbar(handle: &ScrollHandle, window: &mut Window, cx: &mut App) {
    let theme = cx.theme();
    let color = theme.colors.scrollbar_thumb;
    let width = theme.scrollbar.width;
    let inset = theme.scrollbar.inset;
    let min_thumb = theme.scrollbar.min_thumb;

    let max_offset = handle.max_offset().y;
    let bounds = handle.bounds();
    if max_offset <= Pixels::ZERO || bounds.size.height <= Pixels::ZERO {
        return;
    }

    let track = bounds.size.height - inset * 2.0;
    let (thumb_length, thumb_top) = thumb_geometry(
        bounds.size.height,
        bounds.size.height + max_offset,
        handle.offset().y,
        track,
        min_thumb,
    );

    let thumb_x = bounds.right() - width - inset;
    let thumb_bounds = Bounds::from_corners(
        point(thumb_x, bounds.top() + inset + thumb_top),
        point(
            thumb_x + width,
            bounds.top() + inset + thumb_top + thumb_length,
        ),
    );

    window.paint_quad(fill(thumb_bounds, color).corner_radii(width / 2.0));

    let drag = window.use_keyed_state("scrollbar-thumb-drag", cx, |_, _| {
        Option::<(Pixels, Pixels)>::None
    });

    window.on_mouse_event({
        let handle = handle.clone();
        let drag = drag.clone();
        move |event: &MouseDownEvent, phase, _window, cx| {
            if phase != DispatchPhase::Bubble
                || event.button != gpui::MouseButton::Left
                || !thumb_bounds.contains(&event.position)
            {
                return;
            }

            drag.update(cx, |drag, _| {
                *drag = Some((event.position.y, handle.offset().y));
            });
            cx.stop_propagation();
        }
    });

    window.on_mouse_event({
        let handle = handle.clone();
        let drag = drag.clone();
        move |event: &MouseMoveEvent, phase, _window, cx| {
            if phase != DispatchPhase::Bubble
                || event.pressed_button != Some(gpui::MouseButton::Left)
            {
                return;
            }

            let Some((start_y, start_offset)) = *drag.read(cx) else {
                return;
            };

            if max_offset > Pixels::ZERO && track > thumb_length {
                let scale = max_offset / (track - thumb_length);
                let offset = (start_offset - (event.position.y - start_y) * scale)
                    .clamp(-max_offset, Pixels::ZERO);
                handle.set_offset(point(handle.offset().x, offset));
            }
        }
    });
}