use smallvec::SmallVec;
use taffy::style::{Display, Position};

use crate::{
    point, AnyElement, Bounds, Edges, Element, GlobalElementId, IntoElement, LayoutId,
    ParentElement, Pixels, Point, Size, Style, WindowContext,
};

/// The state that the anchored element element uses to track its children.
pub struct AnchoredState {
    child_layout_ids: SmallVec<[LayoutId; 4]>,
}

/// An anchored element that can be used to display UI that
/// will avoid overflowing the window bounds.
pub struct Anchored {
    children: SmallVec<[AnyElement; 2]>,
    anchor_corner: AnchorCorner,
    fit_mode: AnchoredFitMode,
    anchor_position: Option<Point<Pixels>>,
    position_mode: AnchoredPositionMode,
}

/// anchored gives you an element that will avoid overflowing the window bounds.
/// Its children should have no margin to avoid measurement issues.
pub fn anchored() -> Anchored {
    Anchored {
        children: SmallVec::new(),
        anchor_corner: AnchorCorner::TopLeft,
        fit_mode: AnchoredFitMode::SwitchAnchor,
        anchor_position: None,
        position_mode: AnchoredPositionMode::Window,
    }
}

impl Anchored {
    /// Sets which corner of the anchored element should be anchored to the current position.
    pub fn anchor(mut self, anchor: AnchorCorner) -> Self {
        self.anchor_corner = anchor;
        self
    }

    /// Sets the position in window coordinates
    /// (otherwise the location the anchored element is rendered is used)
    pub fn position(mut self, anchor: Point<Pixels>) -> Self {
        self.anchor_position = Some(anchor);
        self
    }

    /// Sets the position mode for this anchored element. Local will have this
    /// interpret its [`Anchored::position`] as relative to the parent element.
    /// While Window will have it interpret the position as relative to the window.
    pub fn position_mode(mut self, mode: AnchoredPositionMode) -> Self {
        self.position_mode = mode;
        self
    }

    /// Snap to window edge instead of switching anchor corner when an overflow would occur.
    pub fn snap_to_window(mut self) -> Self {
        self.fit_mode = AnchoredFitMode::SnapToWindow;
        self
    }

    /// Snap to window edge and leave some margins.
    pub fn snap_to_window_with_margin(mut self, edges: impl Into<Edges<Pixels>>) -> Self {
        self.fit_mode = AnchoredFitMode::SnapToWindowWithMargin(edges.into());
        self
    }
}

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

impl Element for Anchored {
    type RequestLayoutState = AnchoredState;
    type PrepaintState = ();

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

    fn request_layout(
        &mut self,
        _id: Option<&GlobalElementId>,
        cx: &mut WindowContext,
    ) -> (crate::LayoutId, Self::RequestLayoutState) {
        let child_layout_ids = self
            .children
            .iter_mut()
            .map(|child| child.request_layout(cx))
            .collect::<SmallVec<_>>();

        let anchored_style = Style {
            position: Position::Absolute,
            display: Display::Flex,
            ..Style::default()
        };

        let layout_id = cx.request_layout(anchored_style, child_layout_ids.iter().copied());

        (layout_id, AnchoredState { child_layout_ids })
    }

    fn prepaint(
        &mut self,
        _id: Option<&GlobalElementId>,
        bounds: Bounds<Pixels>,
        request_layout: &mut Self::RequestLayoutState,
        cx: &mut WindowContext,
    ) {
        if request_layout.child_layout_ids.is_empty() {
            return;
        }

        let mut child_min = point(Pixels::MAX, Pixels::MAX);
        let mut child_max = Point::default();
        for child_layout_id in &request_layout.child_layout_ids {
            let child_bounds = cx.layout_bounds(*child_layout_id);
            child_min = child_min.min(&child_bounds.origin);
            child_max = child_max.max(&child_bounds.lower_right());
        }
        let size: Size<Pixels> = (child_max - child_min).into();

        let (origin, mut desired) = self.position_mode.get_position_and_bounds(
            self.anchor_position,
            self.anchor_corner,
            size,
            bounds,
        );

        let limits = Bounds {
            origin: Point::default(),
            size: cx.viewport_size(),
        };

        if self.fit_mode == AnchoredFitMode::SwitchAnchor {
            let mut anchor_corner = self.anchor_corner;

            if desired.left() < limits.left() || desired.right() > limits.right() {
                let switched = anchor_corner
                    .switch_axis(Axis::Horizontal)
                    .get_bounds(origin, size);
                if !(switched.left() < limits.left() || switched.right() > limits.right()) {
                    anchor_corner = anchor_corner.switch_axis(Axis::Horizontal);
                    desired = switched
                }
            }

            if desired.top() < limits.top() || desired.bottom() > limits.bottom() {
                let switched = anchor_corner
                    .switch_axis(Axis::Vertical)
                    .get_bounds(origin, size);
                if !(switched.top() < limits.top() || switched.bottom() > limits.bottom()) {
                    desired = switched;
                }
            }
        }

        let edges = match self.fit_mode {
            AnchoredFitMode::SnapToWindowWithMargin(edges) => edges,
            _ => Edges::default(),
        };

        // Snap the horizontal edges of the anchored element to the horizontal edges of the window if
        // its horizontal bounds overflow, aligning to the left if it is wider than the limits.
        if desired.right() > limits.right() {
            desired.origin.x -= desired.right() - limits.right() + edges.right;
        }
        if desired.left() < limits.left() {
            desired.origin.x = limits.origin.x + edges.left;
        }

        // Snap the vertical edges of the anchored element to the vertical edges of the window if
        // its vertical bounds overflow, aligning to the top if it is taller than the limits.
        if desired.bottom() > limits.bottom() {
            desired.origin.y -= desired.bottom() - limits.bottom() + edges.bottom;
        }
        if desired.top() < limits.top() {
            desired.origin.y = limits.origin.y + edges.top;
        }

        let offset = desired.origin - bounds.origin;
        let offset = point(offset.x.round(), offset.y.round());

        cx.with_element_offset(offset, |cx| {
            for child in &mut self.children {
                child.prepaint(cx);
            }
        })
    }

    fn paint(
        &mut self,
        _id: Option<&GlobalElementId>,
        _bounds: crate::Bounds<crate::Pixels>,
        _request_layout: &mut Self::RequestLayoutState,
        _prepaint: &mut Self::PrepaintState,
        cx: &mut WindowContext,
    ) {
        for child in &mut self.children {
            child.paint(cx);
        }
    }
}

impl IntoElement for Anchored {
    type Element = Self;

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

enum Axis {
    Horizontal,
    Vertical,
}

/// Which algorithm to use when fitting the anchored element to be inside the window.
#[derive(Copy, Clone, PartialEq)]
pub enum AnchoredFitMode {
    /// Snap the anchored element to the window edge.
    SnapToWindow,
    /// Snap to window edge and leave some margins.
    SnapToWindowWithMargin(Edges<Pixels>),
    /// Switch which corner anchor this anchored element is attached to.
    SwitchAnchor,
}

/// Which algorithm to use when positioning the anchored element.
#[derive(Copy, Clone, PartialEq)]
pub enum AnchoredPositionMode {
    /// Position the anchored element relative to the window.
    Window,
    /// Position the anchored element relative to its parent.
    Local,
}

impl AnchoredPositionMode {
    fn get_position_and_bounds(
        &self,
        anchor_position: Option<Point<Pixels>>,
        anchor_corner: AnchorCorner,
        size: Size<Pixels>,
        bounds: Bounds<Pixels>,
    ) -> (Point<Pixels>, Bounds<Pixels>) {
        match self {
            AnchoredPositionMode::Window => {
                let anchor_position = anchor_position.unwrap_or(bounds.origin);
                let bounds = anchor_corner.get_bounds(anchor_position, size);
                (anchor_position, bounds)
            }
            AnchoredPositionMode::Local => {
                let anchor_position = anchor_position.unwrap_or_default();
                let bounds = anchor_corner.get_bounds(bounds.origin + anchor_position, size);
                (anchor_position, bounds)
            }
        }
    }
}

/// Which corner of the anchored element should be considered the anchor.
#[derive(Clone, Copy, PartialEq, Eq)]
pub enum AnchorCorner {
    /// The top left corner
    TopLeft,
    /// The top right corner
    TopRight,
    /// The bottom left corner
    BottomLeft,
    /// The bottom right corner
    BottomRight,
}

impl AnchorCorner {
    fn get_bounds(&self, origin: Point<Pixels>, size: Size<Pixels>) -> Bounds<Pixels> {
        let origin = match self {
            Self::TopLeft => origin,
            Self::TopRight => Point {
                x: origin.x - size.width,
                y: origin.y,
            },
            Self::BottomLeft => Point {
                x: origin.x,
                y: origin.y - size.height,
            },
            Self::BottomRight => Point {
                x: origin.x - size.width,
                y: origin.y - size.height,
            },
        };

        Bounds { origin, size }
    }

    /// Get the point corresponding to this anchor corner in `bounds`.
    pub fn corner(&self, bounds: Bounds<Pixels>) -> Point<Pixels> {
        match self {
            Self::TopLeft => bounds.origin,
            Self::TopRight => bounds.upper_right(),
            Self::BottomLeft => bounds.lower_left(),
            Self::BottomRight => bounds.lower_right(),
        }
    }

    fn switch_axis(self, axis: Axis) -> Self {
        match axis {
            Axis::Vertical => match self {
                AnchorCorner::TopLeft => AnchorCorner::BottomLeft,
                AnchorCorner::TopRight => AnchorCorner::BottomRight,
                AnchorCorner::BottomLeft => AnchorCorner::TopLeft,
                AnchorCorner::BottomRight => AnchorCorner::TopRight,
            },
            Axis::Horizontal => match self {
                AnchorCorner::TopLeft => AnchorCorner::TopRight,
                AnchorCorner::TopRight => AnchorCorner::TopLeft,
                AnchorCorner::BottomLeft => AnchorCorner::BottomRight,
                AnchorCorner::BottomRight => AnchorCorner::BottomLeft,
            },
        }
    }
}