use std::{any::Any, cell::Cell, fmt::Debug, ops::Range, rc::Rc, sync::Arc};

use crate::{IntoElement, prelude::*, px, relative};
use gpui::{
    Along, App, Axis as ScrollbarAxis, BorderStyle, Bounds, ContentMask, Corners, CursorStyle,
    Edges, Element, ElementId, Entity, EntityId, GlobalElementId, Hitbox, HitboxBehavior, Hsla,
    IsZero, LayoutId, ListState, MouseDownEvent, MouseMoveEvent, MouseUpEvent, Pixels, Point,
    ScrollHandle, ScrollWheelEvent, Size, Style, UniformListScrollHandle, Window, quad,
};

pub struct Scrollbar {
    thumb: Range<f32>,
    state: ScrollbarState,
    kind: ScrollbarAxis,
}

#[derive(Default, Debug, Clone, Copy)]
enum ThumbState {
    #[default]
    Inactive,
    Hover,
    Dragging(Pixels),
}

impl ThumbState {
    fn is_dragging(&self) -> bool {
        matches!(*self, ThumbState::Dragging(_))
    }
}

impl ScrollableHandle for UniformListScrollHandle {
    fn max_offset(&self) -> Size<Pixels> {
        self.0.borrow().base_handle.max_offset()
    }

    fn set_offset(&self, point: Point<Pixels>) {
        self.0.borrow().base_handle.set_offset(point);
    }

    fn offset(&self) -> Point<Pixels> {
        self.0.borrow().base_handle.offset()
    }

    fn viewport(&self) -> Bounds<Pixels> {
        self.0.borrow().base_handle.bounds()
    }
}

impl ScrollableHandle for ListState {
    fn max_offset(&self) -> Size<Pixels> {
        self.max_offset_for_scrollbar()
    }

    fn set_offset(&self, point: Point<Pixels>) {
        self.set_offset_from_scrollbar(point);
    }

    fn offset(&self) -> Point<Pixels> {
        self.scroll_px_offset_for_scrollbar()
    }

    fn drag_started(&self) {
        self.scrollbar_drag_started();
    }

    fn drag_ended(&self) {
        self.scrollbar_drag_ended();
    }

    fn viewport(&self) -> Bounds<Pixels> {
        self.viewport_bounds()
    }
}

impl ScrollableHandle for ScrollHandle {
    fn max_offset(&self) -> Size<Pixels> {
        self.max_offset()
    }

    fn set_offset(&self, point: Point<Pixels>) {
        self.set_offset(point);
    }

    fn offset(&self) -> Point<Pixels> {
        self.offset()
    }

    fn viewport(&self) -> Bounds<Pixels> {
        self.bounds()
    }
}

pub trait ScrollableHandle: Any + Debug {
    fn content_size(&self) -> Size<Pixels> {
        self.viewport().size + self.max_offset()
    }
    fn max_offset(&self) -> Size<Pixels>;
    fn set_offset(&self, point: Point<Pixels>);
    fn offset(&self) -> Point<Pixels>;
    fn viewport(&self) -> Bounds<Pixels>;
    fn drag_started(&self) {}
    fn drag_ended(&self) {}
}

/// A scrollbar state that should be persisted across frames.
#[derive(Clone, Debug)]
pub struct ScrollbarState {
    thumb_state: Rc<Cell<ThumbState>>,
    parent_id: Option<EntityId>,
    scroll_handle: Arc<dyn ScrollableHandle>,
}

impl ScrollbarState {
    pub fn new(scroll: impl ScrollableHandle) -> Self {
        Self {
            thumb_state: Default::default(),
            parent_id: None,
            scroll_handle: Arc::new(scroll),
        }
    }

    /// Set a parent model which should be notified whenever this Scrollbar gets a scroll event.
    pub fn parent_entity<V: 'static>(mut self, v: &Entity<V>) -> Self {
        self.parent_id = Some(v.entity_id());
        self
    }

    pub fn scroll_handle(&self) -> &Arc<dyn ScrollableHandle> {
        &self.scroll_handle
    }

    pub fn is_dragging(&self) -> bool {
        matches!(self.thumb_state.get(), ThumbState::Dragging(_))
    }

    fn set_dragging(&self, drag_offset: Pixels) {
        self.set_thumb_state(ThumbState::Dragging(drag_offset));
        self.scroll_handle.drag_started();
    }

    fn set_thumb_hovered(&self, hovered: bool) {
        self.set_thumb_state(if hovered {
            ThumbState::Hover
        } else {
            ThumbState::Inactive
        });
    }

    fn set_thumb_state(&self, state: ThumbState) {
        self.thumb_state.set(state);
    }

    fn thumb_range(&self, axis: ScrollbarAxis) -> Option<Range<f32>> {
        const MINIMUM_THUMB_SIZE: Pixels = px(25.);
        let max_offset = self.scroll_handle.max_offset().along(axis);
        let viewport_size = self.scroll_handle.viewport().size.along(axis);
        if max_offset.is_zero() || viewport_size.is_zero() {
            return None;
        }
        let content_size = viewport_size + max_offset;
        let visible_percentage = viewport_size / content_size;
        let thumb_size = MINIMUM_THUMB_SIZE.max(viewport_size * visible_percentage);
        if thumb_size > viewport_size {
            return None;
        }
        let current_offset = self
            .scroll_handle
            .offset()
            .along(axis)
            .clamp(-max_offset, Pixels::ZERO)
            .abs();
        let start_offset = (current_offset / max_offset) * (viewport_size - thumb_size);
        let thumb_percentage_start = start_offset / viewport_size;
        let thumb_percentage_end = (start_offset + thumb_size) / viewport_size;
        Some(thumb_percentage_start..thumb_percentage_end)
    }
}

impl Scrollbar {
    pub fn vertical(state: ScrollbarState) -> Option<Self> {
        Self::new(state, ScrollbarAxis::Vertical)
    }

    pub fn horizontal(state: ScrollbarState) -> Option<Self> {
        Self::new(state, ScrollbarAxis::Horizontal)
    }

    fn new(state: ScrollbarState, kind: ScrollbarAxis) -> Option<Self> {
        let thumb = state.thumb_range(kind)?;
        Some(Self { thumb, state, kind })
    }
}

impl Element for Scrollbar {
    type RequestLayoutState = ();
    type PrepaintState = Hitbox;

    fn id(&self) -> Option<ElementId> {
        None
    }

    fn source_location(&self) -> Option<&'static core::panic::Location<'static>> {
        None
    }

    fn request_layout(
        &mut self,
        _id: Option<&GlobalElementId>,
        _inspector_id: Option<&gpui::InspectorElementId>,
        window: &mut Window,
        cx: &mut App,
    ) -> (LayoutId, Self::RequestLayoutState) {
        let mut style = Style::default();
        style.flex_grow = 1.;
        style.flex_shrink = 1.;

        if self.kind == ScrollbarAxis::Vertical {
            style.size.width = px(12.).into();
            style.size.height = relative(1.).into();
        } else {
            style.size.width = relative(1.).into();
            style.size.height = px(12.).into();
        }

        (window.request_layout(style, None, cx), ())
    }

    fn prepaint(
        &mut self,
        _id: Option<&GlobalElementId>,
        _inspector_id: Option<&gpui::InspectorElementId>,
        bounds: Bounds<Pixels>,
        _request_layout: &mut Self::RequestLayoutState,
        window: &mut Window,
        _: &mut App,
    ) -> Self::PrepaintState {
        window.with_content_mask(Some(ContentMask { bounds }), |window| {
            window.insert_hitbox(bounds, HitboxBehavior::Normal)
        })
    }

    fn paint(
        &mut self,
        _id: Option<&GlobalElementId>,
        _inspector_id: Option<&gpui::InspectorElementId>,
        bounds: Bounds<Pixels>,
        _request_layout: &mut Self::RequestLayoutState,
        hitbox: &mut Self::PrepaintState,
        window: &mut Window,
        cx: &mut App,
    ) {
        const EXTRA_PADDING: Pixels = px(5.0);
        window.with_content_mask(Some(ContentMask { bounds }), |window| {
            let axis = self.kind;
            let colors = cx.theme().colors();
            let thumb_state = self.state.thumb_state.get();
            let thumb_base_color = match thumb_state {
                ThumbState::Dragging(_) => colors.scrollbar_thumb_active_background,
                ThumbState::Hover => colors.scrollbar_thumb_hover_background,
                ThumbState::Inactive => colors.scrollbar_thumb_background,
            };

            let thumb_background = colors.surface_background.blend(thumb_base_color);

            let padded_bounds = Bounds::from_corners(
                bounds
                    .origin
                    .apply_along(axis, |origin| origin + EXTRA_PADDING),
                bounds
                    .bottom_right()
                    .apply_along(axis, |track_end| track_end - 3.0 * EXTRA_PADDING),
            );

            let thumb_offset = self.thumb.start * padded_bounds.size.along(axis);
            let thumb_end = self.thumb.end * padded_bounds.size.along(axis);

            let thumb_bounds = Bounds::new(
                padded_bounds
                    .origin
                    .apply_along(axis, |origin| origin + thumb_offset),
                padded_bounds
                    .size
                    .apply_along(axis, |_| thumb_end - thumb_offset)
                    .apply_along(axis.invert(), |width| width / 1.5),
            );

            let corners = Corners::all(thumb_bounds.size.along(axis.invert()) / 2.0);

            window.paint_quad(quad(
                thumb_bounds,
                corners,
                thumb_background,
                Edges::default(),
                Hsla::transparent_black(),
                BorderStyle::default(),
            ));

            if thumb_state.is_dragging() {
                window.set_window_cursor_style(CursorStyle::Arrow);
            } else {
                window.set_cursor_style(CursorStyle::Arrow, hitbox);
            }

            let scroll = self.state.scroll_handle.clone();

            enum ScrollbarMouseEvent {
                GutterClick,
                ThumbDrag(Pixels),
            }

            let compute_click_offset =
                move |event_position: Point<Pixels>,
                      max_offset: Size<Pixels>,
                      event_type: ScrollbarMouseEvent| {
                    let viewport_size = padded_bounds.size.along(axis);

                    let thumb_size = thumb_bounds.size.along(axis);

                    let thumb_offset = match event_type {
                        ScrollbarMouseEvent::GutterClick => thumb_size / 2.,
                        ScrollbarMouseEvent::ThumbDrag(thumb_offset) => thumb_offset,
                    };

                    let thumb_start = (event_position.along(axis)
                        - padded_bounds.origin.along(axis)
                        - thumb_offset)
                        .clamp(px(0.), viewport_size - thumb_size);

                    let max_offset = max_offset.along(axis);
                    let percentage = if viewport_size > thumb_size {
                        thumb_start / (viewport_size - thumb_size)
                    } else {
                        0.
                    };

                    -max_offset * percentage
                };

            window.on_mouse_event({
                let scroll = scroll.clone();
                let state = self.state.clone();
                move |event: &MouseDownEvent, phase, _, _| {
                    if !(phase.bubble() && bounds.contains(&event.position)) {
                        return;
                    }

                    if thumb_bounds.contains(&event.position) {
                        let offset = event.position.along(axis) - thumb_bounds.origin.along(axis);
                        state.set_dragging(offset);
                    } else {
                        let click_offset = compute_click_offset(
                            event.position,
                            scroll.max_offset(),
                            ScrollbarMouseEvent::GutterClick,
                        );
                        scroll.set_offset(scroll.offset().apply_along(axis, |_| click_offset));
                    }
                }
            });

            window.on_mouse_event({
                let scroll = scroll.clone();
                move |event: &ScrollWheelEvent, phase, window, _| {
                    if phase.bubble() && bounds.contains(&event.position) {
                        let current_offset = scroll.offset();
                        scroll.set_offset(
                            current_offset + event.delta.pixel_delta(window.line_height()),
                        );
                    }
                }
            });

            let state = self.state.clone();
            window.on_mouse_event(move |event: &MouseMoveEvent, _, window, cx| {
                match state.thumb_state.get() {
                    ThumbState::Dragging(drag_state) if event.dragging() => {
                        let drag_offset = compute_click_offset(
                            event.position,
                            scroll.max_offset(),
                            ScrollbarMouseEvent::ThumbDrag(drag_state),
                        );
                        scroll.set_offset(scroll.offset().apply_along(axis, |_| drag_offset));
                        window.refresh();
                        if let Some(id) = state.parent_id {
                            cx.notify(id);
                        }
                    }
                    _ => state.set_thumb_hovered(thumb_bounds.contains(&event.position)),
                }
            });
            let state = self.state.clone();
            let scroll = self.state.scroll_handle.clone();
            window.on_mouse_event(move |event: &MouseUpEvent, phase, _, cx| {
                if phase.bubble() {
                    if state.is_dragging() {
                        state.set_thumb_hovered(thumb_bounds.contains(&event.position));
                    }
                    scroll.drag_ended();
                    if let Some(id) = state.parent_id {
                        cx.notify(id);
                    }
                }
            });
        })
    }
}

impl IntoElement for Scrollbar {
    type Element = Self;

    fn into_element(self) -> Self::Element {
        self
    }
}