use std::sync::Arc;
use std::time::Duration;
use std::time::Instant;

use gpui::prelude::FluentBuilder as _;
use gpui::{
    App, Bounds, DispatchPhase, ElementId, Entity, EntityId, InteractiveElement as _, IntoElement,
    MouseDownEvent, MouseMoveEvent, MouseUpEvent, ParentElement, Pixels, RenderOnce, ScrollEvent,
    ScrollHandle, SharedString, StatefulInteractiveElement as _, Styled, Task, 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,
}

/// When an overlay scrollbar shows.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub enum ScrollbarVisibility {
    /// The running platform's behavior, from the theme.
    #[default]
    Platform,
    Always,
    Autohide,
}

/// How long an autohide thumb stays fully visible before its fade begins.
const AUTOHIDE_DELAY: Duration = Duration::from_millis(700);
/// How long the autohide fade runs.
const AUTOHIDE_FADE: Duration = Duration::from_millis(250);
/// How often the fade repaints while it runs.
const AUTOHIDE_FADE_STEP: Duration = Duration::from_millis(50);

/// The shared autohide timeline for an overlay scrollbar.
#[derive(Clone)]
pub(crate) struct Autohide {
    view: EntityId,
    state: Entity<AutohideState>,
}

struct AutohideState {
    last_activity: Option<Instant>,
    hide_task: Option<Task<()>>,
}

impl Autohide {
    pub(crate) fn new(key: ElementId, window: &mut Window, cx: &mut App) -> Self {
        let view = window.current_view();
        let state = window.use_keyed_state(key, cx, |_, _| AutohideState {
            last_activity: None,
            hide_task: None,
        });
        Self { view, state }
    }

    /// Marks interaction.
    pub(crate) fn touch(&self, window: &mut Window, cx: &mut App) {
        let view = self.view;
        let reduce_motion = cx.theme().accessibility.reduce_motion;

        let task = window.spawn(cx, async move |cx| {
            if reduce_motion {
                cx.background_executor()
                    .timer(AUTOHIDE_DELAY + AUTOHIDE_FADE)
                    .await;
                cx.update(|_, cx| cx.notify(view)).ok();
                return;
            }

            let steps = (AUTOHIDE_FADE.as_millis() / AUTOHIDE_FADE_STEP.as_millis()).max(1);
            cx.background_executor().timer(AUTOHIDE_DELAY).await;

            for _ in 0..steps {
                cx.update(|_, cx| cx.notify(view)).ok();
                cx.background_executor().timer(AUTOHIDE_FADE_STEP).await;
            }

            cx.update(|_, cx| cx.notify(view)).ok();
        });

        self.state.update(cx, |state, _| {
            state.last_activity = Some(Instant::now());
            state.hide_task = Some(task);
        });
    }

    /// The thumb's opacity.
    pub(crate) fn alpha(&self, cx: &App, dragging: bool) -> f32 {
        if dragging {
            return 1.0;
        }

        let Some(activity) = self.state.read(cx).last_activity else {
            return 0.0;
        };

        let idle = activity.elapsed();
        if idle <= AUTOHIDE_DELAY {
            return 1.0;
        }

        let fade = idle.saturating_sub(AUTOHIDE_DELAY);
        if fade >= AUTOHIDE_FADE {
            return 0.0;
        }

        1.0 - fade.as_secs_f32() / AUTOHIDE_FADE.as_secs_f32()
    }
}

/// An overlay scrollbar thumb's style for one paint: `None` hides it.
pub(crate) fn thumb_style(color: gpui::Hsla, alpha: f32) -> Option<gpui::Hsla> {
    (alpha > 0.0).then_some(gpui::Hsla {
        a: color.a * alpha,
        ..color
    })
}

/// A scrollable container with an overlay scrollbar.
#[derive(IntoElement)]
pub struct ScrollArea {
    id: ElementId,
    handle: Option<ScrollHandle>,
    axes: ScrollAxes,
    visibility: ScrollbarVisibility,
    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,
            visibility: ScrollbarVisibility::default(),
            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
    }

    /// Overrides when the scrollbar shows, instead of the platform default.
    pub fn visibility(mut self, visibility: ScrollbarVisibility) -> Self {
        self.visibility = visibility;
        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
    }
}

/// Names keyed state under the scrollbar's own id.
pub fn scrollbar_state_key(name: &'static str, id: &ElementId) -> ElementId {
    ElementId::NamedChild(Arc::new(id.clone()), SharedString::from(name))
}

/// 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 {
            let autohide = match self.visibility {
                ScrollbarVisibility::Platform => cx.theme().scrollbar.autohide,
                ScrollbarVisibility::Always => false,
                ScrollbarVisibility::Autohide => true,
            };
            let id = self.id.clone();
            wrapper.extend(std::iter::once(
                canvas(|_, _, _| {}, {
                    let handle = handle.clone();
                    move |_, _, window, cx| {
                        paint_scrollbar(&handle, &id, autohide, window, cx);
                    }
                })
                .absolute()
                .inset_0()
                .into_any_element(),
            ));
        }

        wrapper
    }
}

/// Paints the overlay scrollbar thumb.
fn paint_scrollbar(
    handle: &ScrollHandle,
    key: &ElementId,
    autohide: bool,
    window: &mut Window,
    cx: &mut App,
) {
    let theme = cx.theme();
    let view = window.current_view();

    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 autohide_state =
        autohide.then(|| Autohide::new(scrollbar_state_key("autohide", key), window, cx));
    let drag = window.use_keyed_state(scrollbar_state_key("thumb-drag", key), cx, |_, _| {
        Option::<(Pixels, Pixels)>::None
    });
    let dragging = drag.read(cx).is_some();

    let alpha = match (&autohide_state, autohide) {
        (Some(state), true) => state.alpha(cx, dragging),
        _ => 1.0,
    };

    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,
        ),
    );

    if let Some(color) = thumb_style(color, alpha) {
        window.paint_quad(fill(thumb_bounds, color).corner_radii(width / 2.0));
    }

    if let Some(state) = &autohide_state {
        window.on_mouse_event({
            let state = state.clone();
            move |event: &ScrollEvent, phase, window, cx| {
                if phase != DispatchPhase::Bubble || !bounds.contains(&event.position) {
                    return;
                }
                state.touch(window, cx);
            }
        });
    }

    window.on_mouse_event({
        let handle = handle.clone();
        let drag = drag.clone();
        let autohide_state = autohide_state.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));
            });

            if let Some(state) = &autohide_state {
                state.touch(window, cx);
            }

            cx.stop_propagation();
        }
    });

    window.on_mouse_event({
        let drag = drag.clone();
        let autohide_state = autohide_state.clone();
        move |event: &MouseUpEvent, phase, window, cx| {
            if phase != DispatchPhase::Bubble || event.button != gpui::MouseButton::Left {
                return;
            }

            if drag.read(cx).is_none() {
                return;
            }

            drag.update(cx, |drag, _| {
                *drag = None;
            });

            if let Some(state) = &autohide_state {
                state.touch(window, cx);
            }
        }
    });

    window.on_mouse_event({
        let handle = handle.clone();
        let drag = drag.clone();
        let autohide_state = autohide_state.clone();
        move |event: &MouseMoveEvent, phase, window, cx| {
            if phase != DispatchPhase::Bubble {
                return;
            }

            if let Some(state) = &autohide_state
                && drag.read(cx).is_some()
            {
                state.touch(window, cx);
            }

            if 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));
                cx.notify(view);
            }
        }
    });
}